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13: The Urinary System - Biology

13: The Urinary System - Biology



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13: The Urinary System

13: The Urinary System - Biology

16. The Urinary System

In Chapter 15a we learned that water is perhaps the most important nutrient. Indeed, while we can survive for several weeks without food and its component nutrients (such as fats, proteins, and carbohydrates), most people survive only a few days without water. In this chapter we focus on the urinary system and maintenance of fluid balance in the body. Specifically, we consider the critical roles that kidneys play in maintaining homeostasis by removing wastes from the body and regulating the volume, solute concentration, and pH of blood.

Excretion is the elimination of wastes and excess substances from the body. Metabolic wastes include carbon dioxide, water, heat, salts, and nitrogen-containing molecules such as ammonia, urea, uric acid, and creatinine. Kidneys and other organs that eliminate metabolic wastes help to maintain homeostasis.

The organs that eliminate wastes and excess essential ions—such as hydrogen (H + ), sodium (Na + ), potassium (K + ), and chloride (Cl - )—are shown in Figure 16.1. Lungs and skin eliminate heat, water, and carbon dioxide (as the gas CO2 from the lungs and as bicarbonate ions, HCO3 - , from the skin). Skin also excretes salts and small amounts of urea and uric acid.

FIGURE 16.1. Organs from several systems eliminate wastes of different kinds from the body. The kidneys, organs of the urinary system, play a major role in the excretion of nitrogen- containing wastes, water, salts, and excess ions.

Minor amounts of other substances, such as alcohol, are also eliminated from the skin and lungs. (Alcohol excretion by the lungs forms the basis of the breathalyzer test.) Organs of our gastrointestinal tract eliminate solid wastes and some metabolic wastes. Defecation is the elimination of feces from the digestive tract. Feces contain undigested food, bacteria, water, bile pigments, and sloughed-off epithelial cells (see Chapter 15).

· Proper fluid balance is critical to homeostasis. We challenge the balance of fluids in our bodies through diet and exercise. In the face of such challenges, our kidneys help to achieve the correct balance by filtering the blood and regulating its volume and solute concentration.

Our focus in this chapter is the kidneys, which are the organs of the urinary system that form urine. Urine, a yellowish fluid, is a mix of water and solutes. Through urine, the body excretes water, salts, excess ions, urea, uric acid, and creatinine. Ammonia is formed in the liver during the breakdown of amino acids. Because ammonia is toxic, the liver converts it to urea, a less harmful waste. Uric acid is formed during the recycling of nitrogen-containing bases of nucleotides (see Chapter 2). In excess, uric acid may form crystals in the synovial fluid of joints (particularly in the big toe), causing the painful condition called gout. Creatinine is generated in skeletal muscle from the breakdown of creatine phosphate, a compound that serves as an alternative energy source for muscle contraction.

Components of the Urinary System

The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra (Figure 16.2). The main function of this system is to regulate the volume, pressure, and composition of the blood. The kidneys are the organs of the urinary system that accomplish this task by regulating the amount of water and dissolved substances that are removed from and returned to the blood. Substances not returned to the blood form urine.

FIGURE 16.2. Organs of the urinary system (labeled on the left side) and their relation to major blood vessels (identified on the right side)

Urine from the kidneys travels down muscular tubes called ureters to the urinary bladder. Each ureter is 25 to 30 cm in length. Peristaltic (wavelike) contractions begin where the ureters leave the kidneys and travel down the ureters, pushing urine toward the urinary bladder. Depending on the rate of urine formation, up to five peristaltic waves occur per minute. Each ureter enters the urinary bladder through a slit-like opening. The urinary bladder is a muscular, expandable organ that temporarily stores urine until it is excreted from the body. Urine leaves the body through the urethra, a tube that transports urine from the urinary bladder to the outside of the body through an external opening. The female urethra transports only urine. The male urethra, however, carries urine and reproductive fluids (but not simultaneously). The reproductive function of the male urethra is discussed in Chapter 17.

In the United States, bladder cancer is the fifth most common cancer and one of the 10 most deadly. Because it tends to recur, long-term monitoring of patients is essential. This makes bladder cancer costly in fact, it ranks fifth among all cancers in total costs. Despite these statistics, bladder cancer receives relatively little national attention. It also receives comparatively few resources. When compared with breast, colorectal, prostate, and lung cancer, bladder cancer ranked last in research funding per case in 2007. If you were given the responsibility of deciding how to allocate funds for research on different diseases, what would you base your decision on? Would you consider incidence, risk of death, or cost? What about the potential for improvements in prevention, diagnosis, and treatment?

Kidneys and Homeostasis

Our kidneys are reddish brown in color and shaped like kidney beans. Each one is about the size of a fist. They are located just above the waist, sandwiched between the parietal peritoneum (the membrane that lines the abdominal cavity) and the muscles of the dorsal body wall. The slightly indented, or concave, border of each kidney faces the midline of the body. Perched on top of each kidney is an adrenal gland.

The kidneys are covered and supported by several layers of connective tissue (Figure 16.3). The outermost layer is a tough, fibrous layer that anchors each kidney and its adrenal gland to the abdominal wall and surrounding tissues. Beneath this layer is a protective cushion of fat (the middle connective tissue layer, also called the adipose capsule). The innermost layer covering the kidneys is a layer of collagen fibers that protects the kidneys from trauma.

FIGURE 16.3. Structure of the kidney

Structure of the Kidneys

One ureter leaves each kidney at a notch in the concave border, as shown in Figure 16.3. This notch is also the area where blood vessels enter and exit the kidney. The renal arteries branch off the aorta and carry blood to the kidneys. The renal veins carry filtered blood away from the kidneys to the inferior vena cava, which transports the blood to the heart.

Each kidney has three regions: an outer region, the renal cortexa region enclosed by the cortex, the renal medulla and an inner chamber, the renal pelvis (see Figure 16.3). The renal cortex begins at the outer border of the kidney, and portions of it, called renal columns, extend inward between the pyramid-shaped subdivisions (renal pyramids) of the renal medulla. The narrow end of each renal pyramid joins a cuplike extension of the renal pelvis. As we will soon see, urine produced by the kidneys eventually drains into the renal pelvis and out the ureter to the urinary bladder.

Nephrons are the microscopic functional units of the kidneys and are responsible for the formation of urine. Each kidney contains 1 million to 2 million nephrons.

Structure of nephrons . A nephron has two basic parts: the renal corpuscle and the renal tubule, as shown in Figure 16.4. The renal corpuscle is the portion of the nephron where fluid is filtered from the blood. It consists of a tuft of capillaries, the glomerulusand a surrounding cuplike structure, the glomerular capsule (sometimes called Bowman's capsule). Within the glomerular capsule is a space that is continuous with the lumen of the renal tubule. Blood enters a glomerulus by an afferent (incoming) arteriole. Inside the glomerular capillaries, water and small solutes move from the blood into the space within the glomerular capsule and then into the renal tubule, where they are considered filtrate (a fluid similar to blood plasma but typically lacking proteins). The blood then leaves the glomerulus by an efferent (outgoing) arteriole. The renal tubule is the site where substances are removed from and added to the filtrate. This tubule has three sections: the proximal convoluted tubule, the loop of the nephron (sometimes called the loop ofHenle), and the distal convoluted tubule. The proximal convoluted tubule, its name reflecting its location nearest the glomerular capsule, has cells with many tiny projections called microvilli. Microvilli allow the efficient removal (reabsorption) of useful substances from the filtrate, which are eventually returned to the blood (see later in this discussion). The loop of the nephron resembles a hairpin turn, with a descending limb and an ascending limb. Next comes the distal convoluted tubule, its name reflecting this section's more distant location from the glomerular capsule. The loop of the nephron and the distal convoluted tubule have cells with few or no microvilli. The distal convoluted tubules of many nephrons empty into a single collecting duct many collecting ducts eventually drain into the renal pelvis. The renal pelvis is connected to the ureter. Urine exits the kidney by way of the ureter and moves into the urinary bladder.

FIGURE 16.4. Structure of the nephron

About 80% of the nephrons in our kidneys are confined almost entirely to the renal cortex they have short loops that dip only a short distance into the renal medulla. The remaining 20% have long loops that extend from the cortex deep into the renal medulla. Once in the medulla, the loops of these nephrons turn abruptly upward, back into the cortex, where they lead into distal convoluted tubules. As we will see, the nephrons whose loops extend deep into the medulla play an important role in water conservation.

Functions of nephrons . The kidneys are crucial for maintaining homeostasis. For one thing, they filter wastes and excess materials from the blood. Indeed, over the course of one day, two healthy kidneys filter all the blood in the body 30 times! They also assist the respiratory system in the regulation of blood pH. Finally, the kidneys maintain fluid balance by regulating the volume and composition of blood and urine.

To understand what occurs in the kidneys, we must examine the work of nephrons. Nephrons perform three functions: (1) glomerular filtration, (2) tubular reabsorption, and (3) tubular secretion.

We can compare these functions to the steps you might take during a selective cleaning of small items from your bedroom closet. First, you might remove almost all of the small items—the valuable along with the unwanted. This activity is analogous to glomerular filtration, which removes from the blood all materials small enough to fit through the pores of the kidney's filter (discussed next). The next step in cleaning your closet might be to look through the various items you have removed and put back "the good stuff," the items worth saving. Returning valuable materials to the closet is analogous to tubular reabsorption, which returns useful materials to the blood. The final step in cleaning the closet might be to once again scan what you have in your closet and to selectively remove items, such as those in excess. This last step is analogous to tubular secretion, in which wastes and excess materials are removed from the blood and added to the filtrate that will eventually leave the body as urine. Secretion also removes from the blood substances not naturally found in the body, such as pesticides and certain drugs. Here are the three steps in more detail.

1. Glomerular filtration occurs as blood pressure forces water and small solutes from the blood in the glomerulus to the space inside the glomerular capsule (Figure 16.5). Reaching the space inside the glomerular capsule requires passing sequentially through the following three layers: (1) the single layer of endothelial cells that forms the walls of the capillaries, (2) the basement membrane just outside the capillary walls, and (3) the epithelial lining of the glomerular capsule.

FIGURE 16.5. The renal corpuscle is the site of glomerular filtration.

Water and small solutes in the blood first move through the walls of the glomerular capillaries. The capillary walls consist of a single layer of endothelial cells the walls of these capillaries are fenestrated, meaning they have many pores. The pores allow some substances to move out of the capillaries, but prevent red blood cells from doing so. Next, the filtered substances cross the basement membrane, a layer of protein fibers in a glycoprotein matrix. This layer restricts the passage of large proteins. Finally, the filtered substances pass through slits in the inner lining of the glomerular capsule these slits occur in between extensions of the epithelial cells of the capsule (refer, again, to Figure 16.5). These substances are known collectively as glomerular filtrate. The concentrations of the molecules dissolved in the glomerular filtrate are approximately the same as in the blood plasma.

Several things can change the rate of filtration by the glomerulus. An increase in the diameter of afferent (incoming) arterioles brings more blood into the glomerulus and produces higher pressure in the glomerular capillaries. Higher pressure results in higher filtration rates (more filtrate produced by the kidneys each minute). A reduction in the diameter of efferent (outgoing) arterioles also produces higher pressure in glomerular capillaries and higher filtration rates. General (systemic) increases in blood pressure can also produce higher filtration rates.

2. Tubular reabsorption is the process that removes useful materials from the filtrate and returns them to the blood. This process occurs in the renal tubule, primarily in the proximal convoluted tubule. The characteristics of the cells lining the proximal convoluted tubule make it an ideal location for reabsorption (Figure 16.6). As mentioned, these epithelial cells have numerous microvilli (projections of the plasma membrane) that reach into the lumen of the tubule. Similar in function to the microvilli in the small intestine, these microvilli dramatically increase the surface area for the reabsorption of materials. Reabsorption returns water, essential ions, and nutrients to the blood (see Table 16.1). Remarkably, as the glomerular filtrate passes through the renal tubule, about 99% of it is returned to the blood in the surrounding capillaries. Thus, only about 1% of the glomerular filtrate is eventually excreted as urine. Put another way, of the approximately 180 liters (48 gal) of filtrate that enter the glomerular capsule each day, between 178 and 179 liters (47 gal) are returned to the blood by reabsorption. The remaining 1 to 2 liters (about 0.5 to 1 gal) are excreted as urine. Imagine how much water and food we would have to consume if we did not have reabsorption to offset the losses from glomerular filtration! Some wastes are not reabsorbed at all, and will eventually be excreted others, such as urea, are partially reabsorbed. We will see that antidiuretic hormone (ADH), manufactured by the hypothalamus and released by the posterior pituitary gland, regulates the amount of water reabsorbed in parts of the renal tubule.

FIGURE 16.6. The proximal convoluted tubule is the site of tubular reabsorption.

TABLE 16.1. Reabsorption by Nephrons of Some Substances

Amount filtered per day (in liters or grams)

% Reabsorbed (removed from filtrate and returned to blood)

3. Tubular secretion removes additional wastes and excess ions from the blood. For example, hydrogen ions (H + ), potassium ions (K + ), and ammonium ions (NH4 + ) in the blood are actively transported into the renal tubule, where they become part of the filtrate to be excreted. Tubular secretion also removes foreign substances from the blood, including pesticides and drugs such as penicillin, cocaine, and marijuana. These substances are added to the filtered fluid that will become urine. Tubular secretion occurs along the proximal and distal convoluted tubules and collecting duct.

The regions of the nephron and their roles in filtration, reabsorption, and secretion are shown in Figure 16.7 and reviewed in Table 16.2. When reviewing these, keep in mind the directions that substances move in each of the three processes: (1) during glomerular filtration, substances move from the blood into the nephron to form filtrate (2) during tubular reabsorption, useful substances move from the filtrate within the nephron back into the blood (3) during tubular secretion, drugs and substances in excess move from the blood into the filtrate.

TABLE 16.2. Review of Nephron Regions and Their Roles

Renal corpuscle (glomerular capsule and glomerulus)

Filters the blood, removing water, glucose, amino acids, ions, nitrogen-containing wastes, and other small molecules

Proximal convoluted tubule

Reabsorbs water, glucose, amino acids, some urea, Na + , Cl - , and HCO3 -

Reabsorbs water, Na + , Cl , and K +

Reabsorbs water, Na + , Cl - , and HCO3 -

Secretes drugs, H + , K + , and NH4 +

*Major reabsorbed or secreted substances are listed here.

By the end of glomerular filtration, tubular reabsorption, and tubular secretion, blood leaving the kidneys contains most of the water, nutrients, and essential ions that it contained upon entering the kidneys. Wastes and excess materials have been removed, leaving the blood cleansed. This purified blood now moves from the capillaries surrounding the nephron into small veins that eventually join the renal vein.

The final filtrate, now called urine, contains all of the materials that were filtered from the blood and not reabsorbed, plus substances that were secreted from the blood. Urine empties from the distal convoluted tubules into collecting ducts. There, more water may be reabsorbed, and some additional removal of excess substances, such as hydrogen and potassium ions, also occurs. From the collecting ducts, urine moves into the renal pelvis and leaves each kidney through a ureter. It then travels down the ureters to the urinary bladder, which stores the urine until it is eliminated from the body through the urethra. The precise composition of urine can be examined in a medical laboratory. See the Health Issue essay, Urinalysis.

In addition to removing wastes and regulating the volume and solute concentration of blood plasma, the kidneys help regulate the pH of blood. Recall from Chapters 2 and 14 that blood pH must be regulated precisely for proper functioning of the body. This precise regulation is achieved through the actions of the kidneys, through buffer systems in the blood, and through respiration. Buffer systems regulate pH by picking up hydrogen ions (H + ) when their concentrations are high and releasing hydrogen ions when their concentrations are low. Chapter 2 described the importance of carbonic acid as such a buffer in the blood. The role of the kidneys in maintaining pH is twofold. First, by secreting hydrogen ions into the urine, the kidneys remove excess hydrogen ions from the blood, thereby increasing blood pH. Second, the kidneys help sustain the carbonic acid buffer system by returning bicarbonate to the blood. Because of their role in regulating blood pH, the kidneys ultimately influence breathing rate. Recall from Chapter 14 that chemoreceptors in the medulla of the brain respond to changes in the pH of blood (and cerebrospinal fluid) by adjusting breathing rate.

Our kidneys enable us to conserve water through the production of concentrated urine. Because of their role in water conservation, the kidneys participate in the maintenance of cardiac output and blood pressure. Production of concentrated urine is performed by the 20% of our nephrons with long loops that dip deep into the renal medulla. The special ability of these nephrons to concentrate urine derives from an increasing concentration of solutes in the interstitial fluid (the fluid that fills the spaces between cells) from the cortex to the medulla of the kidneys. The most important of these solutes are sodium chloride (NaCl) and urea. Let's explore this mechanism for urine concentration by tracing the path of the filtrate as it flows through these long loops (Figure 16.8).

FIGURE 16.7. Overview of glomerular filtration, tubular reabsorption, and tubular secretion along the nephron

How does the structure of glomerular capillaries aid filtration?

The walls of glomerular capillaries have pores that allow many substances (but not red blood cells) to move out of the capillaries.

The kidneys are our body's filtering system, so the urine they produce contains substances that originate from almost all of our organs. A detailed analysis of our urine, therefore, tells us not only how the organs of our urinary tract are functioning but also about the general health of our other organs. In other words, a typical urinalysis, which assesses the physical and chemical properties of urine and evaluates whether microorganisms are present, provides an overview of a person's basic health.

Healthy urine exiting the body typically contains no microorganisms. The presence of bacteria in a properly collected urine sample usually signals infection of the urinary system. Bacteria found in a sample may be cultured to determine their identity, which can help in diagnosing the infection. Urine also may be screened for fungi or protozoans that cause inflammation within the urinary tract.

Sometimes a urine sample is contaminated because of improper collection. For example, if the perineal area has not been cleaned and the collection cup or urine is allowed to contact that area, bacteria may be introduced. (In males, this is the area between the anus and scrotum in females, it is between the anus and vulva.) It is therefore important to be careful when collecting a urine sample.

The following physical characteristics are routinely checked in urine analyses: color turbidity (cloudiness) pH and specific gravity, or density, of urine (the ratio of its weight to the weight of an equal volume of distilled water). The color of urine comes from urochrome, a yellow pigment produced as a waste product by the liver during the breakdown of hemoglobin in red blood cells. The urochrome travels in the bloodstream from the liver to the kidneys, where it is filtered from the blood and excreted with urine. The color of urine varies somewhat according to diet. Beets, for example, lend urine a red color, and asparagus causes a green tinge. The color of urine also varies with concentration. More concentrated urine, such as that collected first thing in the morning, is darker than more dilute urine. An abnormal color of urine—particularly red, when followed by microscopic confirmation of the presence of red blood cells—can indicate trauma to urinary organs.

Freshly voided urine is usually transparent. Cloudy, or turbid, urine may indicate a urinary tract infection, particularly if white blood cells are detected. Healthy urine has a pH of about 6, although considerable variation may occur in response to diet. High-protein diets produce acidic urine, and vegetarian diets produce alkaline urine. An alkaline pH also is associated with some bacterial infections. Specific gravity is a measure of the concentration of solutes in urine and therefore of the concentrating ability of the nephrons within our kidneys. When urine becomes highly concentrated for long periods, substances such as calcium and uric acid may precipitate out and form kidney stones.

Quantitative chemical tests are run to assess the levels of specific chemical constituents of urine. The major constituent is water, making up about 95% of the total volume. The remaining 5% consists of solutes from the metabolic activities of our cells or from outside sources (such as drugs). Tests run on urine samples look for certain abnormal constituents of urine (such as glucose, red blood cells, or white blood cells) and for normal constituents present in abnormal amounts (such as higher-than-normal amounts of protein).

Questions to Consider

• Why would it be important for a woman to inform her doctor that she is menstruating at the time of urine collection?

• Urinalysis is often preferred over blood testing in workplace drug-testing programs. What might explain this preference?

The solute concentration of the filtrate passing from the glomerular capsule to the proximal tubule is about the same as that of blood. As the filtrate moves through the proximal convoluted tubule, large amounts of water and salt are reabsorbed (recall from our earlier discussion of reabsorption that nutrients are also reabsorbed at this time). This reabsorption produces dramatic reductions in the volume of filtrate but little change in its solute concentration (because both water and salt are reabsorbed). However, when the filtrate enters the descending limb of the loop of the nephron, major changes in solute concentration begin (Figure 16.8). This path takes it from the cortex to the medulla.

Along the descending limb, water leaves the filtrate by osmosis (a special type of diffusion described in Chapter 3). The departure of water creates an increase in the concentration of solutes, including sodium chloride, within the filtrate. The concentration of salt in the filtrate peaks at the curve of the loop, setting the stage for the next step in the process of urine concentration. Now the filtrate moves up the ascending limb of the loop. As it does so, large amounts of sodium chloride are actively transported out of the filtrate into the interstitial fluid of the medulla. Water, however, remains in the filtrate because the ascending limb is not permeable to water. When the filtrate reaches the distal convoluted tubule in the cortex, it is quite dilute. In fact, the filtrate is hypotonic to body fluids. (Recall from Chapter 3 that hypotonic means having a lower solute concentration than another fluid. Hypertonic means having a greater solute concentration than another fluid. Isotonic means having the same solute concentration as another fluid.) The filtrate then moves into a collecting duct and begins to descend once again toward the medulla. This pathway is one of increasing salt concentration in the interstitial fluid because of all the salt that was transported out of the filtrate as it ascended the loop. Collecting ducts are permeable to water but not to salt (antidiuretic hormone increases the permeability of the collecting duct to water see Figure 16.8). Thus, as the filtrate encounters increasing concentrations of salt in the fluid of the inner medulla, water leaves the filtrate by osmosis. With this departure of large amounts of water, urea is now concentrated in the filtrate. In the lower regions of the collecting duct, some of the urea moves into the interstitial fluid of the medulla. This leakage of urea contributes to the high solute concentration of the inner medulla and thus aids in concentrating the filtrate. The remaining urea is excreted.

FIGURE 16.8. Some nephrons have loops that extend deep into the medulla. These nephrons are responsible for water conservation. The steps by which these nephrons concentrate urine and conserve water are shown here. Stippling indicates solute concentration of the filtrate within the nephron and collecting duct. The color gradient behind the nephron and collecting duct indicates solute concentration in the interstitial fluid of different regions of the kidney (renal cortex, and outer and inner renal medulla darker is more concentrated).

At its most concentrated, urine is hypertonic to blood and interstitial fluid from any other part of the body except the inner medulla, where it is isotonic. Together, the loop of the nephron and collecting duct maintain extraordinarily high solute concentrations in the interstitial fluid of the kidneys, making possible the concentration of urine and conservation of water by the kidneys.

Hormones and Kidney Function

Our health depends on our keeping the salt and water levels in our body near certain optimum values. This, as we have seen, is an important job of the kidneys. It is also a challenging job, because our activities produce constant fluctuations in those levels. For example, on a hot day or after exercise, we may lose body water and salts through perspiration. In contrast, eating a tub of salted popcorn at the movies can boost our salt intake. The kidneys must deal with these challenges and adjust the concentration of solutes in the urine and in the blood to keep water and salt levels in our body relatively constant.

Three hormones—ADH, aldosterone, and atrial natriuretic peptide—adjust kidney function to meet the body's needs (Table 16.3). Antidiuretic hormone (ADH) is manufactured by the hypothalamus and then travels to the posterior pituitary for storage and release. This hormone regulates the amount of water reabsorbed by the collecting ducts. The hypothalamus responds to changes in the concentration of water in the blood by increasing or decreasing secretion of ADH. Decreases in the concentration of water in the blood stimulate increased secretion of ADH, as shown in Figure 16.9. Higher levels of ADH in the bloodstream then increase the permeability to water of the collecting ducts of nephrons, resulting in more water being reabsorbed from the filtrate. The movement of increased amounts of water from the filtrate back into the blood results in increased blood volume and pressure and production of small amounts of concentrated urine.


13: The Urinary System - Biology

The urinary system maintains blood homeostasis by filtering out excess fluid and other substances from the bloodstream and secreting waste.

Learning Objectives

Review the urinary system

Key Takeaways

Key Points

  • The renal system eliminate wastes from the body, controls levels of electrolytes and metabolites, controls the osmoregulation of blood volume and pressure, and regulates blood pH.
  • The renal system organs include the kidneys, ureter, bladder, and urethra. Nephrons are the main functional component of the kidneys.
  • The respiratory and cardiovascular systems have certain functions that overlap with renal system functions.
  • Metabolic wastes and excess ions are filtered out of the blood, combined with water, and leave the body in the form of urine.
  • A complex network of hormones controls the renal system to maintain homeostasis.

Key Terms

  • ureter: These are two long, narrow ducts that carry urine from the kidneys to the urinary bladder.
  • osmoregulation: The most important function of the renal system, in which blood volume, blood pressure, and blood osmolarity (ion concentration) is maintained in homeostasis.

The Renal System

The renal system, which is also called the urinary system, is a group of organs in the body that filters out excess fluid and other substances from the bloodstream. The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH.

The renal system organs include the kidneys, ureters, bladder, and urethra. Metabolic wastes and excess ions are filtered out of the blood, along with water, and leave the body in the form of urine.

Components of the renal system: Here are the major organs of the renal system.

Renal System Functions

The renal system has many functions. Many of these functions are interrelated with the physiological mechanisms in the cardiovascular and respiratory systems.

  1. Removal of metabolic waste products from the body (mainly urea and uric acid).
  2. Regulation of electrolyte balance (e.g., sodium, potassium, and calcium).
  3. Osmoregulation controls the blood volume and body water contents.
  4. Blood pressure homeostasis: The renal system alters water retention and thirst to slowly change blood volume and keep blood pressure in a normal range.
  5. Regulation of acid-base homeostasis and blood pH, a function shared with the respiratory system.

Many of these functions are related to one another as well. For example, water follows ions via an osmotic gradient, so mechanisms that alter sodium levels or sodium retention in the renal system will alter water retention levels as well.

Organs of the Renal System

Kidneys and Nephrons

Kidneys are the most complex and critical part of the urinary system. The primary function of the kidneys is to maintain a stable internal environment (homeostasis) for optimal cell and tissue metabolism. The kidneys have an extensive blood supply from the renal arteries that leave the kidneys via the renal vein.

Nephrons are the main functional component inside the parenchyma of the kidneys, which filter blood to remove urea, a waste product formed by the oxidation of proteins, as well as ions like potassium and sodium. The nephrons are made up of a capsule capillaries (the glomerulus) and a small renal tube.

The renal tube of the nephron consists of a network of tubules and loops that are selectively permeable to water and ions. Many hormones involved in homeostasis will alter the permeability of these tubules to change the amount of water that is retained by the body.

Ureter

Urine passes from the renal tube through tubes called ureters and into the bladder.

Bladder

The bladder is flexible and is used as storage until the urine is allowed to pass through the urethra and out of the body.

Urethra

The female and male renal system are very similar, differing only in the length of the urethra.

Human Osmoregulation

The kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from the glomerular filtrate in kidney tubules, which is controlled by hormones such as antidiuretic hormone (ADH), renin, aldosterone, and angiotensin I and II.

A basic example is that a decrease in water concentration of blood is detected by osmoreceptors in the hypothalamus, which stimulates ADH release from the pituitary gland to increase the permeability of the wall of the collecting ducts and tubules in the nephrons. Therefore, a large proportion of water is reabsorbed from fluid to prevent a fair proportion of water from being excreted.

The extent of blood volume and blood pressure regulation facilitated by the kidneys is a complex process. Besides ADH secretion, the renin-angiotensin feedback system is critically important to maintain blood volume and blood pressure homeostasis.


Kidney Failure Treatment

Kidney Dialysis is when blood is taken out of a vein, and then it is pumped through a machine which cleans the blood.

During the cleaning process, the machine gets rid of waste materials such as urea. The patient will then have cleaner blood without the toxic materials.

However this is an expensive and time consuming treatment, and the patient has to return to the hospital quite regularly to get this done.

A__ __kidney transplant can be used in order to help someone with a kidney failure. A donor kidney can be used to replace the original kidney that has failed.

There is a relatively high success rate of around 80%, and it is vital that the tissue types that are involved with this procedure have to be the same between the donor and the patient. This is why it can take quite a long time to find the correct and appropriate donor for the patient.

Patients who have kidney transplants will also require antibiotics and anti-rejection drugs for their whole life to ensure that their body does not reject the new organ, and that it can cope with the new organ.

What do kidney transplant patients require after the surgery? Your answer should include: antibiotics / anti-rejection
Explanation: Antibiotics and anti-rejection drugs for their whole life to ensure that their body does not reject the new organ, and that it can cope with the new organ. What does ADH stand for? antidiuretic hormone What are kidneys needed for? Your answer should include: Remove / Urea / Blood / Control / Ions / Water
Explanation: To remove urea from the blood, control the amount of ions in the blood, and control the water content of blood.


Contents

The urinary system refers to the structures that produce and transport urine to the point of excretion. In the human urinary system there are two kidneys that are located between the dorsal body wall and parietal peritoneum on both the left and right sides.

The formation of urine begins within the functional unit of the kidney, the nephrons. Urine then flows through the nephrons, through a system of converging tubules called collecting ducts. These collecting ducts then join together to form the minor calyces, followed by the major calyces that ultimately join the renal pelvis. From here, urine continues its flow from the renal pelvis into the ureter, transporting urine into the urinary bladder. The anatomy of the human urinary system differs between males and females at the level of the urinary bladder. In males, the urethra begins at the internal urethral orifice in the trigone of the bladder, continues through the external urethral orifice, and then becomes the prostatic, membranous, bulbar, and penile urethra. Urine exits through the external urethral meatus. The female urethra is much shorter, beginning at the bladder neck and terminating in the vaginal vestibule.

Development Edit

Microanatomy Edit

Under microscopy, the urinary system is covered in a unique lining called urothelium, a type of transitional epithelium. Unlike the epithelial lining of most organs, transitional epithelium can flatten and distend. Urothelium covers most of the urinary system, including the renal pelvis, ureters, and bladder.

The main functions of the urinary system and its components are to:

  • Regulate blood volume and composition (e.g. sodium, potassium and calcium)
  • Regulate blood pressure.
  • Regulate pHhomeostasis of the blood.
  • Contributes to the production of red blood cells by the kidney.
  • Helps synthesize calcitriol (the active form of Vitamin D).
  • Stores waste product (mainly urea and uric acid) before it and other products are removed from the body.

Urine formation Edit

Average urine production in adult humans is about 1–2 litres (L) per day, depending on state of hydration, activity level, environmental factors, weight, and the individual's health. Producing too much or too little urine requires medical attention. Polyuria is a condition of excessive urine production (> 2.5 L/day). Oliguria when < 400 mL (millilitres) are produced, and anuria one of < 100 mL per day.

The first step in urine formation is the filtration of blood in the kidneys. In a healthy human the kidney receives between 12 and 30% of cardiac output, but it averages about 20% or about 1.25 L/min.

The basic structural and functional unit of the kidney is the nephron. Its chief function is to regulate the concentration of water and soluble substances like sodium by filtering the blood, reabsorbing what is needed and excreting the rest as urine.

In the first part of the nephron, Bowman's capsule filters blood from the circulatory system into the tubules. Hydrostatic and osmotic pressure gradients facilitate filtration across a semipermeable membrane. The filtrate includes water, small molecules, and ions that easily pass through the filtration membrane. However larger molecules such as proteins and blood cells are prevented from passing through the filtration membrane. The amount of filtrate produced every minute is called the glomerular filtration rate or GFR and amounts to 180 litres per day. About 99% of this filtrate is reabsorbed as it passes through the nephron and the remaining 1% becomes urine.

Regulation of concentration and volume Edit

The urinary system is under influence of the circulatory system, nervous system, and endocrine system.

Aldosterone plays a central role in regulating blood pressure through its effects on the kidney. It acts on the distal tubules and collecting ducts of the nephron and increases reabsorption of sodium from the glomerular filtrate. Reabsorption of sodium results in retention of water, which increases blood pressure and blood volume. Antidiuretic hormone (ADH), is a neurohypophysial hormone found in most mammals. Its two primary functions are to retain water in the body and vasoconstriction. Vasopressin regulates the body's retention of water by increasing water reabsorption in the collecting ducts of the kidney nephron. [4] Vasopressin increases water permeability of the kidney's collecting duct and distal convoluted tubule by inducing translocation of aquaporin-CD water channels in the kidney nephron collecting duct plasma membrane. [5]

Urination Edit

Urination, also sometimes referred to as micturition, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans (and many other animals), the process of urination is under voluntary control. In infants, some elderly individuals, and those with neurological injury, urination may occur as an involuntary reflex. Physiologically, micturition involves coordination between the central, autonomic, and somatic nervous systems. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex. In placental mammals the male ejects urine through the penis, and the female through the vulva.

Urologic disease can involve congenital or acquired dysfunction of the urinary system. As an example, urinary tract obstruction is a urologic disease that can cause urinary retention.

Diseases of the kidney tissue are normally treated by nephrologists, while diseases of the urinary tract are treated by urologists. Gynecologists may also treat female urinary incontinence.

Diseases of other bodily systems also have a direct effect on urogenital function. For instance, it has been shown that protein released by the kidneys in diabetes mellitus sensitizes the kidney to the damaging effects of hypertension. [6]

Diabetes also can have a direct effect in urination due to peripheral neuropathies, which occur in some individuals with poorly controlled blood sugar levels. [7]

Urinary incontinence can result from a weakening of the pelvic floor muscles caused by factors such as pregnancy, childbirth, aging, and being overweight. Pelvic floor exercises known as Kegel exercises can help in this condition by strengthening the pelvic floor. There can also be underlying medical reasons for urinary incontinence which are often treatable. In children, the condition is called enuresis.

Some cancers also target the urinary system, including bladder cancer, kidney cancer, ureteral cancer, and urethral cancer. Due to the role and location of these organs, treatment is often complicated. [ citation needed ]

Kidney stones have been identified and recorded about as long as written historical records exist. [8] The urinary tract including the ureters, as well as their function to drain urine from the kidneys, has been described by Galen in the second century AD. [9]

The first to examine the ureter through an internal approach, called ureteroscopy, rather than surgery was Hampton Young in 1929. [8] This was improved on by VF Marshall who is the first published use of a flexible endoscope based on fiber optics, which occurred in 1964. [8] The insertion of a drainage tube into the renal pelvis, bypassing the uterers and urinary tract, called nephrostomy, was first described in 1941. Such an approach differed greatly from the open surgical approaches within the urinary system employed during the preceding two millennia. [8]


The Urinary System

The urinary system has roles you may be well aware of: cleansing the blood and ridding the body of wastes probably come to mind. However, there are additional, equally important functions played by the system. Take for example, regulation of pH, a function shared with the lungs and the buffers in the blood. Additionally, the regulation of blood pressure is a role shared with the heart and blood vessels. What about regulating the concentration of solutes in the blood? Did you know that the kidney is important in determining the concentration of red blood cells? Eighty-five percent of the erythropoietin (EPO) produced to stimulate red blood cell production is produced in the kidneys. The kidneys also perform the final synthesis step of vitamin D production, converting calcidiol to calcitriol, the active form of vitamin D.

If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis. The affected individual might experience weakness, lethargy, shortness of breath, anemia, widespread edema (swelling), metabolic acidosis, rising potassium levels, heart arrhythmias, and more. Each of these functions is vital to your well-being and survival. The urinary system, controlled by the nervous system, also stores urine until a convenient time for disposal and then provides the anatomical structures to transport this waste liquid to the outside of the body. Failure of nervous control or the anatomical structures leading to a loss of control of urination results in a condition called incontinence.

Characteristics of the urine change, depending on influences such as water intake, exercise, environmental temperature, nutrient intake, and other factors . Some of the characteristics such as color and odor are rough descriptors of your state of hydration. For example, if you exercise or work outside, and sweat a great deal, your urine will turn darker and produce a slight odor, even if you drink plenty of water. Athletes are often advised to consume water until their urine is clear. This is good advice however, it takes time for the kidneys to process body fluids and store it in the bladder. Another way of looking at this is that the quality of the urine produced is an average over the time it takes to make that urine. Producing clear urine may take only a few minutes if you are drinking a lot of water or several hours if you are working outside and not drinking much.

“Urine Color” by OpenStax College / CC BY 3.0

Urine volume varies considerably. The normal range is one to two liters per day. The kidneys must produce a minimum urine volume of about 500 mL/day to rid the body of wastes. Output below this level may be caused by severe dehydration or renal disease and is termed oliguria. The virtual absence of urine production is termed anuria. Excessive urine production is polyuria, which may occur in diabetes mellitus when blood glucose levels exceed the filtration capacity of the kidneys and glucose appears in the urine. The osmotic nature of glucose attracts water, leading to increased water loss in the urine.

Urine is a fluid of variable composition that requires specialized structures to remove it from the body safely and efficiently. Blood is filtered, and the filtrate is transformed into urine at a relatively constant rate throughout the day. This processed liquid is stored until a convenient time for excretion. All structures involved in the transport and storage of the urine are large enough to be visible to the naked eye. This transport and storage system not only stores the waste, but it protects the tissues from damage due to the wide range of pH and osmolarity of the urine, prevents infection by foreign organisms, and for the male, provides reproductive functions. The urinary bladder collects urine from both ureters (Figure 2.21 “Urinary System Location”).

Figure 2.21 Urinary System Location

“Illu Urinary System” by Thstehle / Public Domain

“The Bladder” by OpenStax College / CC BY 3.0

The kidneys lie on either side of the spine in the retroperitoneal space behind the main body cavity that contains the intestines. The kidneys are well protected by muscle, fat, and the lower ribs. They are roughly the size of your fist, and the male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest.

“Kidney Position in Abdomen” by OpenStax College / CC BY 3.0

The kidneys (as viewed from the back of the body) are slightly protected by the ribs and are surrounded by fat for protection (not shown).

The effects of failure of parts of the urinary system may range from inconvenient (incontinence) to fatal (loss of filtration and many other functions). The kidneys catalyze the final reaction in the synthesis of active vitamin D that in turn helps regulate Ca++. The kidney hormone EPO stimulates erythrocyte development and promotes adequate O2 transport. The kidneys help regulate blood pressure through Na+ and water retention and loss. The kidneys work with the adrenal cortex, lungs, and liver in the renin–angiotensin–aldosterone system to regulate blood pressure. They regulate osmolarity of the blood by regulating both solutes and water. Three electrolytes are more closely regulated than others: Na+, Ca++, and K+. The kidneys share pH regulation with the lungs and plasma buffers, so that proteins can preserve their three-dimensional conformation and thus their function.


Sex differences in lower urinary tract biology and physiology

Females and males differ significantly in gross anatomy and physiology of the lower urinary tract, and these differences are commonly discussed in the medical and scientific literature. However, less attention is dedicated to investigating the varied development, function, and biology between females and males on a cellular level. Recognizing that cell biology is not uniform, especially in the lower urinary tract of females and males, is crucial for providing context and relevance for diverse fields of biomedical investigation. This review serves to characterize the current understanding of biological sex differences between female and male lower urinary tracts, while identifying areas for future research. First, the differences in overall cell populations are discussed in the detrusor smooth muscle, urothelium, and trigone. Second, the urethra is discussed, including anatomic discussions of the female and male urethra followed by discussions of cellular differences in the urothelial and muscular layers. The pelvic floor is then reviewed, followed by an examination of the sex differences in hormonal regulation, the urinary tract microbiome, and the reticuloendothelial system. Understanding the complex and dynamic development, anatomy, and physiology of the lower urinary tract should be contextualized by the sex differences described in this review.

Keywords: Cell biology Lower urinary tract Sex differences Urology.

Conflict of interest statement

Ethics approval and consent to participate
Consent for publication
Competing interests

Dr. Wolfe has received research funding from Astellas Scientific Medical Affairs and Kimberly Clark Corporation. All other authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


16.5 Summary

  • Ureters are tube-like structures that connect the kidneys with the urinary bladder . Each ureter arises at the renal pelvis of a kidney and travels down through the abdomen to the urinary bladder. The walls of the ureter contain smooth muscle that can contract to push urine through the ureter by peristalsis . The walls are lined with transitional epithelium that can expand and stretch.
  • The urinary bladder is a hollow, muscular organ that rests on the pelvic floor. It is also lined with transitional epithelium. The function of the bladder is to collect and store urine from the kidneys before the urine is eliminated through urination. Filling of the bladder triggers the sensation of needing to urinate. When a conscious decision to urinate is made, the detrusor muscle in the bladder wall contracts and forces urine out of the bladder and into the urethra.
  • The urethra is a tube that connects the urinary bladder to the external urethral orifice. Somatic nerves control the sphincter at the distal end of the urethra. This allows the opening of the sphincter for urination to be under voluntary control.

NORMAL URINE TRANSPORT AND MICTURITION

Urine production is a function of both renal-glomerular filtration and tubular reabsorption and is tightly regulated by systemic hydration state and electrolyte balance. Urinary filtrate is passed through the nephron as it winds through the cortex and medulla and is concentrated via a counter-current mechanism. Urine exits the kidney at the renal papillae and is transported through the upper collecting system. The smooth muscle surrounding the calyces, renal pelvis, and ureters is of the syncytial type without discrete neuromuscular junctions. Instead, smooth-muscle excitation is spread from one muscle cell to the next. In humans, atypical smooth-muscle cells, located near the pelvicalyceal border, are thought to act as the pacemakers of urinary-tract peristalsis (49, 50). These cells initiate unidirectional peristaltic contractions which, in turn, promote the forward flow of urine. Recently, Hurato et al. demonstrated that disruption of the pelvicalyceal region from the more distal urinary-tract segments prevented downstream peristalsis. Furthermore, hyperpolarization-activated cation-3 (HCN3), an isoform of a channel family known for initiating electrical activity in the brain and heart, was isolated in the same spatial distribution as the atypical smooth-muscle cells of the pelvicalyceal junction. Inhibition of this channel protein caused a loss of electrical activity in the pelvicalyceal junction and led to randomized electrical activity and loss of coordinated peristalsis (51). Whether HCN3-positive cells are the same as the atypical smooth muscle remains to be seen. Normal ureteral contractions occur two to six times per minute and it is the advancing contraction wave that forces the urine bolus down the length of the ureters and then into the bladder (52). Some uropathogenic bacteria appear to have evolved a way to overcome the normally protective forward flow of urine that results from the peristaltic ureteral contractions. Recent studies demonstrated that most UPEC have the ability to impair ureteric contractility via a calcium-dependent mechanism and that mechanism is dependent upon FimH-urothelial interaction (53, 54).

The micturition cycle is best thought of as two distinct phases: urine storage/bladder filling and voiding/bladder emptying (55). The viscoelastic properties of the bladder allow for increases in bladder volume with little change in detrusor or intravesical pressures. Additionally, during bladder filling, spinal sympathetic reflexes (T12–L2) are activated that, through modulation of parasympathetic-ganglionic transmission, inhibit bladder contractions and increase bladder-outlet resistance via smooth-muscle activation (56). Bladder-outlet resistance also increases during filling secondary to increased external urethral-sphincter activity via a spinalsomatic reflex (guarding reflex) (57). As the bladder reaches its capacity, afferent activity from tension, volume, and nociceptive receptors are conveyed via Aδ and C fibers through the pelvic and pudenal nerves to the sacral spinal cord (56). Afferent signals ascend in the spinal cord to the pontine micturition center in the rostral brainstem. Here signals are processed under the strong influence of the cerebral cortex and other areas of the brain. If voiding is deemed appropriate, the voiding/bladder-emptying reflex is initiated. The pattern of efferent activity that follows is completely reversed, producing sacral parasympathetic outflow and inhibition of sympathetic and somatic pathways. First the external urethral-sphincter relaxes and shortly thereafter a coordinated contraction of the bladder causes the expulsion of urine (56, 58) ( Fig. 4 ).

Mechanism of storage and voiding. A. Storage of urine. Low-level bladder afferent firing, secondary to bladder distension, increases sympathetic outflow to the bladder outlet and external urethral sphincter (‘guarding reflex’). Sympathetic signaling also acts to inhibit detrusor-muscle contractions. B. Voiding. At bladder capacity, high-level bladder afferent activity activates the pontine-micturition center. This, in turn, inhibits the guarding reflex. The activated pontine-micturition center, under appropriate conditions, will lead to parasympathetic outflow to the bladder and internal-sphincter smooth muscle. Urinary sphincter relaxation is soon followed by a large, coordinated detrusor contraction leading to expulsion of urine from the bladder. (Reprinted and adapted from reference 58 with permission of the publisher.) doi:10.1128/microbiolspec.UTI-0016-2012.f4

The forward flow of urine is imperative to the maintenance of a healthy urinary tract. Any structural or functional process that impedes the flow of urine has the potential to promote urine stasis, hence UTI pathogenesis. In the next few sections, we will elaborate upon those anatomic and physiologic abnormalities that can affect either storage or emptying of urine and, in turn, promote UTI pathogenesis.


Urinary System

Urinary System Regulates Body Fluids  Excretion: processes that remove waste and excess materials from the body  Digestive System: excretes food residues and waste produced by the liver  Respiratory System (lungs): excretes carbon dioxide  Integumentary System (skin): excretes water and salt  Urinary System (kidneys): excretes nitrogenous wastes, excess solutes and water

The Kidneys Regulate Water Levels:  To maintain homeostasis, Water input = water output  Kidneys adjust water output as necessary -water input: food, drink, metabolism -water output: lungs, skin, feces, kidneys -kidneys modify output based on intake and loss -output varies from ½ liter/day to 1 liter/hour

The Kidneys Regulate Nitrogenous Wastes and Other Solutes:  Protein metabolism produces nitrogenous wastes  Initially, NH 3 (ammonia) is produced during breakdown of amino acids  Liver detoxifies NH 3 , producing urea  Urea is transported from liver to kidneys for disposal  Other solutes regulated by kidneys -Sodium, chloride, potassium, calcium, hydrogen ions, creatinine

Organs and Urinary System:

 Kidneys - Principal organ of urinary system - Cortex: outer portion of the kidney - Medulla: inner region of the kidney - Renal pelvis: hollow space in center of kidney where urine collects  Ureters - Muscular tubes that transport urine from kidneys to bladder  Urinary bladder - Three layers of smooth muscle, lined with epithelial cells - Stores urine (600–1,000 ml)  Urethra - Carries urine from bladder to outside of body - Two sphincters control urination

The Internal Structure of the Kidney:  Nephron: functional unit of the kidney

  • Two functional parts:
  • Tubule
  • Associated blood supply  1 million nephrons per kidney  Each nephron consists of a long thin hollow tube (tubule) plus associated blood supply  Role of nephrons: remove approximately 180 liters of fluid from the blood daily, and return most of it, minus the wastes that are excreted  Nephron structure
  • Glomerular capsule: cuplike end of nephron tubule surrounding glomerulus (network of capillaries)—this is where filtration occurs
  • Four distinct regions of tubule
  • Proximal tubule: extends from glomerular capsule to renal medulla

 Large volume filtration, yet highly selective - Impermeable to large proteins and cells  Filtration is driven by high blood pressure in glomerular capillaries  Rate of filtration is regulated in two ways - Resting rate under local control that adjusts diameter of afferent arterioles - Stress causes sympathetic nervous system to reduce blood flow to kidneys

Tubular Reabsorption Returns Filtered Water and Solutes to Blood:  One hundred percent of filtered glucose, amino acids, and bicarbonate and 50% of urea are reabsorbed  Most tubular reabsorption occurs in proximal tubule  Water reabsorption

  • Sixty-five to seventy percent occurs in proximal tubule
  • Twenty-five percent occurs in loop of Henle
  • Less than 10% occurs in distal tubule and collecting duct—but this is where water excretion is regulated  Brush border of microvilli on proximal tubule cells facilitate reabsorption  Reabsorption process starts in proximal tubule
  1. Sodium moved by active transport out of tubule cell into interstitial fluid (on capillary side)
  2. Sodium diffuses from interstitial fluid into capillary
  3. This establishes a concentration gradient favoring facilitated diffusion of sodium from lumen of tubule into proximal tubule cell
  4. Chloride passively accompanies sodium (balanced charge)
  5. Water diffuses through aquaporins from lumen to proximal tubule cell to interstitial fluid to capillary
  6. Movement of sodium provides energy to cotransport glucose and amino acids from tubule into surrounding cells
  1. Glucose, amino acids then diffuse to the interstitial fluid and capillaries Tubular Secretion Removes Other Substances from Blood: Involves the movement of materials from the peritubular capillaries or vasa recta to the tubule  Purpose: Substances secreted:

-Penicillin, cocaine, marijuana, pesticides, preservatives, hydrogen ions, ammonium, potassium

Producing a Dilute Urine: Excreting Excess Water:

 Kidneys respond to excess water by excreting it  Mechanism: -distal tubile is impermeable to water, so water is not reabsorbed here -NaCl is reabsorbed without the concurrent reabsorption of water -high-volume dilute urine is produced

Producing Concentrated Urine: Conserving Water  Too little water can lead to lower blood volume, declining blood pressure, risk of dehydration of body cells  Kidneys respond by conserving water and producing a more concentrated urine  Mechanism: -mediated by ADH (antidiuretic hormone) from the posterior pituitary gland -ADH increases permeability of the collecting ducts to water and increases conservation of water

Urination Depends on a Reflex  Micturition reflex: neural reflex that enables emptying of the bladder

  • Responds to stretch receptors in bladder wall
  • Increase in water reabsorbed by kidney
  • Decrease in urine production
  • Increase in thirst  If blood solute concentration is too low (water concentration too high), ADH secretion is reduced
  • Decrease in permeability of collecting duct to water
  • Decrease in water reabsorbed by kidney
  • Increase in urine production Decrease in thirst  Diuresis: high urine flow rate  Diuretic: any substance that increases the formation and excretion of urine
  • Lasix (furosemide): medication that reduces blood volume and blood pressure
  • Used in treatment of congestive heart failure and hypertension
  • Caffeine: inhibits sodium reabsorption
  • Alcohol: inhibits ADH release

Aldosterone Regulates Salt Balance  Blood volume control is dependent on salt balance  Aldosterone: adrenal hormone that regulates sodium excretion

  • Mechanism: increases Na+ reabsorption from distal tubule and collecting duct  Aldosterone secretion is controlled by the renin-angiotensin system

The Renin-Angiotensin System Controls Blood Volume and Blood Pressure  Aldosterone release is stimulated indirectly by decreased blood volume or blood pressure  Decreased blood volume/blood pressure causes release of renin (enzyme) from juxtaglomerular apparatus (region where afferent and efferent arterioles are in close contact with distal tubule)

 Renin cleaves inactive angiotensinogen (produced by liver), releasing angiotensin I  Angiotensin converting enzyme (ACE) in lungs converts antiotensin I (inactive peptide) to angiotensin II (biologically active peptide)

 Effects of angiotensin II: - Constricts arterioles, which raises blood pressure - Stimulates release of aldosterone from adrenal glands - Aldosterone: increases sodium reabsorption by distal tubules and collection ducts  ACE inhibitors: medication for blood pressure control  Inhibit angiotensin converting enzyme (ACE) in the lungs - Block normal production of angiotensin II - Aldosterone concentration falls - Sodium and water excretion increase - Blood volume reduced slightly - Blood vessels dilate, lowering blood pressure

Atrial Natriuretic Hormone Protects Against Blood Volume Excess  Another controller of renal sodium excretion  High blood volume stretches atria of heart  Atria secrete ANH (atrial natriuretic hormone) in response to stretching  ANH inhibits Na+ reabsorption in distal tubules and collecting ducts  Na+ excretion increases  Water follows the Na+  Effect of ANH is opposite to that of aldosterone

Kidneys Help Maintain Acid-Base Balance and Blood pH  Blood pH must stay between 7.35 and 7.  pH regulated by kidneys, buffers, lungs

 Chronic renal failure - Also known as end stage renal disease (ESRD) - ESRD: long-term, irreversible damage leading to &ampgt60% reduction in functioning nephrons - Patients may have &amplt10% normal filtering capacity - Results when  Renal tubular cells do not receive the nutrients they need  Glomerular filtration is blocked for too long - Diabetes may lead to diabetic nephropathy, which often progresses to ESRD Dialysis Cleanses Blood Artificially  Dialysis: attempts to duplicate function of healthy kidneys  CAPD: continuous ambulatory peritoneal dialysis - Can be done at home - Uses peritoneal cavity for waste and ion removal - Risk of infection  Hemodialysis - Requires several visits/week to a dialysis center - Blood is circulated through a kidney machine

Dialysis Cleanses Blood Artificially  Problems with dialysis

  • Dialysis cannot achieve complete homeostasis of ions and wastes
  • Dialysis does not replace renal hormones Kidney Transplants Are a Permanent Solution to Renal Failure  Best hope for many chronic renal failure patients  Improvements in transplant protocols/processes have improved outcomes
  • Better tissue-matching techniques
  • Improved anti-rejection medications
  • National data banks  Shortage of donated kidneys

Urinary Incontinence Is a Loss of Bladder Control  Develops with age due to aging bladder muscles  More common in women


Watch the video: WCLN -AP 12 Unit 8- Urinary System- Question 13 Explanation (August 2022).