What gases are needed for humans to breathe?

What gases are needed for humans to breathe?

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I will be beginning a science project about sustaining life on space stations. I already know that pure oxygen is harmful for humans but would $O_2$ + $CO_2$ be enough for humans to breathe in or do humans need other gases as well?

Why not look at the existing space station as an example. The International Space Station (ISS), a starting article is "Breathing easy on the space station" (NASA), while this article does not discuss the amounts of various chemicals, it looks at the process about how the chemical balance is maintained - an important point related to your question is that carbon dioxide is filtered away.

The second article "Breathing on the Space Station" (e-missions), discusses the actual percentages of each component. It states that the chemical composition of the space station air is ideally similar to Earth's atmosphere. With 78% nitrogen, 21% oxygen and 1% water vapour on the space station (Argon is in the Earth's atmosphere, but appears to be unnecessary on the space station).

I hope this helps.

13.36: Processes of Breathing

  • Contributed by CK-12: Biology Concepts
  • Sourced from CK-12 Foundation

Grapes. Why? What do these have in common with a breath of air?

Below are the parts of the lungs where oxygen moves from the lungs into the blood. If the alveoli below were purple, they could resemble a bunch of grapes. Of course, as the alveoli are in the lungs, they must be very small to provide enough area for the exchange of gases. In fact, there are about 300 million alveoli in the adult lung.

30.4 Transport of Gases in Human Bodily Fluids

In this section, you will explore the following questions:

  • How is oxygen bound to hemoglobin and transported to body tissues?
  • How is carbon dioxide transported from body tissues to the lungs?

Connection for AP ® Courses

Gas exchange at the tissue level also occurs by diffusion. The majority of oxygen transported from the lungs to body tissue is bound to a protein called hemoglobin. Hemoglobin is a quaternary protein comprise of four iron-containing heme groups iron has a great affinity for oxygen. (We know this because iron rusts when exposed to air.)

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 4 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 4 Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A Interactions within biological systems lead to complex properties.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 6.1 The student can justify claims with evidence.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecule.

Once the oxygen diffuses across the alveoli, it enters the bloodstream and is transported to the tissues where it is unloaded, and carbon dioxide diffuses out of the blood and into the alveoli to be expelled from the body. Although gas exchange is a continuous process, the oxygen and carbon dioxide are transported by different mechanisms.

Transport of Oxygen in the Blood

Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent of oxygen in the blood is dissolved directly into the blood itself. Most oxygen—98.5 percent—is bound to a protein called hemoglobin and carried to the tissues.


Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits: two alpha subunits and two beta subunits (Figure 30.19). Each subunit surrounds a central heme group that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is deoxygenated is darker red.

It is easier to bind a second and third oxygen molecule to Hb than the first molecule. This is because the hemoglobin molecule changes its shape, or conformation, as oxygen binds. The fourth oxygen is then more difficult to bind. The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis). The resulting graph—an oxygen dissociation curve—is sigmoidal, or S-shaped (Figure 30.20). As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.

Visual Connection

  1. The blood pH will drop and hemoglobin affinity for oxygen will increase.
  2. The blood pH will increase and hemoglobin affinity for oxygen will drop.
  3. The blood pH will drop and hemoglobin affinity for oxygen will decrease.
  4. The blood pH will increase and hemoglobin affinity for oxygen will also increase.

Factors That Affect Oxygen Binding

The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition to P O 2 P O 2 , other environmental factors and diseases can affect oxygen carrying capacity and delivery.

Diseases like sickle cell anemia and thalassemia decrease the blood’s ability to deliver oxygen to tissues and its oxygen-carrying capacity. In sickle cell anemia, the shape of the red blood cell is crescent-shaped, elongated, and stiffened, reducing its ability to deliver oxygen (Figure 30.21). In this form, red blood cells cannot pass through the capillaries. This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood cells, but these cells have lower-than-normal levels of hemoglobin. Therefore, the oxygen-carrying capacity is diminished.

Transport of Carbon Dioxide in the Blood

Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Several properties of carbon dioxide in the blood affect its transport. First, carbon dioxide is more soluble in blood than oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body.

The benefit of the bicarbonate buffer system is that carbon dioxide is “soaked up” into the blood with little change to the pH of the system. This is important because it takes only a small change in the overall pH of the body for severe injury or death to result. The presence of this bicarbonate buffer system also allows for people to travel and live at high altitudes: When the partial pressure of oxygen and carbon dioxide change at high altitudes, the bicarbonate buffer system adjusts to regulate carbon dioxide while maintaining the correct pH in the body.

Carbon Monoxide Poisoning

While carbon dioxide can readily associate and dissociate from hemoglobin, other molecules such as carbon monoxide (CO) cannot. Carbon monoxide has a greater affinity for hemoglobin than oxygen. Therefore, when carbon monoxide is present, it binds to hemoglobin preferentially over oxygen. As a result, oxygen cannot bind to hemoglobin, so very little oxygen is transported through the body (Figure 30.22). Carbon monoxide is a colorless, odorless gas and is therefore difficult to detect. It is produced by gas-powered vehicles and tools. Carbon monoxide can cause headaches, confusion, and nausea long-term exposure can cause brain damage or death. Administering 100 percent (pure) oxygen is the usual treatment for carbon monoxide poisoning. Administration of pure oxygen speeds up the separation of carbon monoxide from hemoglobin.

What's in a Breath?

Nitrogen makes up the bulk (78 percent) of the air that humans breathe in and out, considering human bodies have no use for it. Second place belongs to oxygen (21 percent in, 16 percent out) and at a distant third carbon dioxide (0.04 percent in, four percent out). Other trace elements exist in exhaled air, such as argon (0.09 percent both ways, again because humans don't use it). Human beings also exhale water vapor, a byproduct of cellular respiration, at a rate that varies depending on the person, their health and other factors.

Other chemicals can exist in the air that humans inhale and exhale, some of which can be damaging to a human's health. Particulate matter from industries, smoke from cigarettes and other chemicals like sulphur and nitrogen oxides can cause harm to the lungs. Some forms of dangerous matter, like germs and particulates, get caught by the hair-like growths that line the passage into a person's throat. Called cilia, they help protect people from these elements in Earth's air, but it's not a perfect system and sometimes things can reach the rest of the lungs and get caught in the alveoli. Germs, for example, can potentially cause infections.

Protecting the Respiratory System

Figure (PageIndex<7>): The upward sweeping motion of cilia lining the respiratory tract helps keep it free from dust, pathogens, and other harmful substances.

You may be able to survive for weeks without food and for days without water, but you can survive without oxygen for only a matter of minutes except under exceptional circumstances. Therefore, protecting the respiratory system is vital. That&rsquos why making sure a patient has an open airway is the first step in treating many medical emergencies. Fortunately, the respiratory system is well protected by the ribcage of the skeletal system. However, the extensive surface area of the respiratory system is directly exposed to the outside world and all its potential dangers in inhaled air. Therefore, it should come as no surprise that the respiratory system has a variety of ways to protect itself from harmful substances such as dust and pathogens in the air.

The main way the respiratory system protects itself is called the mucociliary escalator. From the nose through the bronchi, the respiratory tract is covered in the epithelium that contains mucus-secreting goblet cells. The mucus traps particles and pathogens in the incoming air. The epithelium of the respiratory tract is also covered with tiny cell projections called cilia (singular, cilium), as shown in Figure (PageIndex<7>). The cilia constantly move in a sweeping motion upward toward the throat, moving the mucus and trapped particles and pathogens away from the lungs and toward the outside of the body.

What happens to the material that moves up the mucociliary escalator to the throat? It is generally removed from the respiratory tract by clearing the throat or coughing. Coughing is a largely involuntary response of the respiratory system that occurs when nerves lining the airways are irritated. The response causes air to be expelled forcefully from the trachea, helping to remove mucus and any debris it contains (called phlegm) from the upper respiratory tract to the mouth. The phlegm may spit out (expectorated), or it may be swallowed and destroyed by stomach acids.

Sneezing is a similar involuntary response that occurs when nerves lining the nasal passage are irritated. It results in forceful expulsion of air from the mouth, which sprays millions of tiny droplets of mucus and other debris out of the mouth and into the air, as shown in Figure (PageIndex<8>). This explains why it is so important to sneeze into a sleeve rather than the air to help prevent the transmission of respiratory pathogens.

Figure (PageIndex<8>): Sneezing results in tiny particles from the mouth being forcefully ejected into the air.

Respiratory Rate and Control of Ventilation

Breathing usually occurs without thought, although at times you can consciously control it, such as when you swim under water, sing a song, or blow bubbles. The respiratory rate is the total number of breaths, or respiratory cycles, that occur each minute. Respiratory rate can be an important indicator of disease, as the rate may increase or decrease during an illness or in a disease condition. The respiratory rate is controlled by the respiratory center located within the medulla oblongata in the brain, which responds primarily to changes in carbon dioxide, oxygen, and pH levels in the blood.

The normal respiratory rate of a child decreases from birth to adolescence. A child under 1 year of age has a normal respiratory rate between 30 and 60 breaths per minute, but by the time a child is about 10 years old, the normal rate is closer to 18 to 30. By adolescence, the normal respiratory rate is similar to that of adults, 12 to 18 breaths per minute.

Factors That Affect the Rate and Depth of Respiration

The respiratory rate and the depth of inspiration are regulated by the respiration centers of the brain: the medulla oblongata and pons. These centers respond to systemic stimuli utilizing a positive-feedback relationship in which the greater the stimulus, the greater the response. Thus, increasing stimuli results in forced breathing. Multiple systemic factors are involved in stimulating the brain to produce pulmonary ventilation.

The major factor that stimulates the medulla oblongata and pons to produce respiration is surprisingly not oxygen concentration, but rather the concentration of carbon dioxide in the blood. As you recall, carbon dioxide is a waste product of cellular respiration and can be toxic. Concentrations of chemicals are sensed by chemoreceptors. Central chemoreceptors are located in the brain and brainstem, whereas peripheral chemoreceptors are located in the carotid arteries and aortic arch. Concentration changes in certain substances, such as carbon dioxide or hydrogen ions, stimulate these receptors, which in turn signal the respiration centers of the brain. In the case of carbon dioxide, as the concentration of CO2 in the blood increases, it readily diffuses across the blood-brain barrier, where it collects in the extracellular fluid. As will be explained in more detail later, increased carbon dioxide levels lead to increased levels of hydrogen ions, decreasing pH. The increase in hydrogen ions in the brain triggers the central chemoreceptors to stimulate the respiratory centers to initiate contraction of the diaphragm and intercostal muscles. As a result, the rate and depth of respiration increase, allowing more carbon dioxide to be expelled, which brings more air into and out of the lungs promoting a reduction in the blood levels of carbon dioxide, and therefore hydrogen ions, in the blood. In contrast, low levels of carbon dioxide in the blood cause low levels of hydrogen ions in the brain, leading to a decrease in the rate and depth of pulmonary ventilation, producing shallow, slow breathing.

Another factor involved in influencing the respiratory activity of the brain is systemic arterial concentrations of hydrogen ions. Increasing carbon dioxide levels can lead to increased H+ levels, as mentioned above, as well as other metabolic activities, such as lactic acid accumulation after strenuous exercise. Peripheral chemoreceptors of the aortic arch and carotid arteries sense arterial levels of hydrogen ions. When peripheral chemoreceptors sense decreasing, or more acidic, pH levels, they stimulate an increase in ventilation to remove carbon dioxide from the blood at a quicker rate. Removal of carbon dioxide from the blood helps to reduce hydrogen ions, thus increasing systemic pH.

Blood levels of oxygen are also important in influencing respiratory rate. The peripheral chemoreceptors are responsible for sensing large changes in blood oxygen levels. If blood oxygen levels become quite low—about 60 mm Hg or less—then peripheral chemoreceptors stimulate an increase in respiratory activity. The chemoreceptors are only able to sense dissolved oxygen molecules, not the oxygen that is bound to hemoglobin. As you recall, the majority of oxygen is bound by hemoglobin when dissolved levels of oxygen drop, hemoglobin releases oxygen. Therefore, a large drop in oxygen levels is required to stimulate the chemoreceptors of the aortic arch and carotid arteries.

The hypothalamus and other brain regions associated with the limbic system also play roles in influencing the regulation of breathing by interacting with the respiratory centers. The hypothalamus and other regions associated with the limbic system are involved in regulating respiration in response to emotions, pain, and temperature. For example, an increase in body temperature causes an increase in respiratory rate. Feeling excited or the fight-or-flight response will also result in an increase in respiratory rate.

Disorders of the Respiratory System: Sleep Apnea

Sleep apnea is a chronic disorder that can occur in children or adults, and is characterized by the cessation of breathing during sleep. These episodes may last for several seconds or several minutes, and may differ in the frequency with which they are experienced. Sleep apnea leads to poor sleep, which is reflected in the symptoms of fatigue, evening napping, irritability, memory problems, and morning headaches. In addition, many individuals with sleep apnea experience a dry throat in the morning after waking from sleep, which may be due to excessive snoring.

There are two types of sleep apnea: obstructive sleep apnea and central sleep apnea. Obstructive sleep apnea is caused by an obstruction of the airway during sleep, which can occur at different points in the airway, depending on the underlying cause of the obstruction. For example, the tongue and throat muscles of some individuals with obstructive sleep apnea may relax excessively, causing the muscles to push into the airway. Another example is obesity, which is a known risk factor for sleep apnea, as excess adipose tissue in the neck region can push the soft tissues towards the lumen of the airway, causing the trachea to narrow.

In central sleep apnea, the respiratory centers of the brain do not respond properly to rising carbon dioxide levels and therefore do not stimulate the contraction of the diaphragm and intercostal muscles regularly. As a result, inspiration does not occur and breathing stops for a short period. In some cases, the cause of central sleep apnea is unknown. However, some medical conditions, such as stroke and congestive heart failure, may cause damage to the pons or medulla oblongata. In addition, some pharmacologic agents, such as morphine, can affect the respiratory centers, causing a decrease in the respiratory rate. The symptoms of central sleep apnea are similar to those of obstructive sleep apnea.

A diagnosis of sleep apnea is usually done during a sleep study, where the patient is monitored in a sleep laboratory for several nights. The patient’s blood oxygen levels, heart rate, respiratory rate, and blood pressure are monitored, as are brain activity and the volume of air that is inhaled and exhaled. Treatment of sleep apnea commonly includes the use of a device called a continuous positive airway pressure (CPAP) machine during sleep. The CPAP machine has a mask that covers the nose, or the nose and mouth, and forces air into the airway at regular intervals. This pressurized air can help to gently force the airway to remain open, allowing more normal ventilation to occur. Other treatments include lifestyle changes to decrease weight, eliminate alcohol and other sleep apnea–promoting drugs, and changes in sleep position. In addition to these treatments, patients with central sleep apnea may need supplemental oxygen during sleep.

Every breath you take: the process of breathing explained

Breathing is central to life, as it allows the human body to obtain the energy it needs to sustain itself and its activities. But how does it work?


Breathing uses chemical and mechanical processes to bring oxygen to every cell of the body and to get rid of carbon dioxide. Our body needs oxygen to obtain energy to fuel all our living processes. Carbon dioxide is a waste product of that process. The respiratory system, with its conduction and respiratory zones, brings air from the environment to the lungs and facilitates gas exchange both in the lungs and within the cells. Nurses need a solid understanding of how breathing works, and of vital signs of breathing and breathing patterns, to be able to care for patients with respiratory problems and potentially save lives in acute situations.

Citation: Cedar SH (2018) Every breath you take: the process of breathing explained. Nursing Times [online] 114: 1, 47-50.

Author: SH Cedar is associate professor and reader in human biology at the School of Health and Social Care, London South Bank University, and author of Biology for Health: Applying the Activities of Daily Living.


The first question asked in an emergency situation is: “Is the person breathing?”. It is also often the first question asked about newborns and the last one asked about the dying. Why is breathing so important? What is in the breath that we need so much? What happens when we stop breathing? These might seem obvious questions, but the mechanisms of respiration are often poorly understood, and their importance in health assessments and diagnostics often missed. This article describes the anatomy and physiology of breathing.

Collaborating with green plants

We need energy to fuel all the activities in our bodies, such as contracting muscles and maintaining a resting potential in our neurons, and we have to work to obtain the energy we use.

Green plants take their energy directly from sunlight and convert it into carbohydrates (sugars). We cannot do that, but we can use the energy stored in carbohydrates to fuel all other reactions in our bodies. To do this, we need to combine sugar with oxygen. We therefore need to accumulate both sugar and oxygen, which requires us to work. As a matter of fact, we spend much of our energy obtaining the sugar and oxygen we need to produce energy.

We source carbohydrates from green plants or animals that have eaten green plants, and we source oxygen from the air. Green plants release oxygen as a waste product of photosynthesis we use that oxygen to fuel our metabolic reactions, releasing carbon dioxide as a waste product. Plants use our waste product as the carbon source for carbohydrates.

Breaking chemical bonds

To obtain energy we must release the energy contained in the chemical bonds of molecules such as sugars. The foods we eat (such as carbohydrates and proteins) are digested in our gastrointestinal tract into molecules (such as sugars and amino acids) that are small enough to pass into the blood. The blood transports the sugars to the cells, where the mitochondria break up their chemical bonds to release the energy they contain. Cells need oxygen to be able to carry out that process. As every cell in our body needs energy, every one of them needs oxygen.

The energy released is stored in a chemical compound called adenosine triphosphate (ATP), which contains three phosphate groups. When we need energy to carry out an activity, ATP is broken down into adenosine diphosphate (ADP), containing only two phosphate groups. Breaking the chemical bond between the third phosphate group and ATP releases a high amount of energy.

Internal and external respiration

Our lungs supply oxygen from the outside air to the cells via the blood and cardiovascular system to enable us to obtain energy. As we breathe in, oxygen enters the lungs and diffuses into the blood. It is taken to the heart and pumped into the cells. At the same time, the carbon dioxide waste from the breakdown of sugars in the cells of the body diffuses into the blood and then diffuses from the blood into the lungs and is expelled as we breathe out. One gas (oxygen) is exchanged for another (carbon dioxide). This exchange of gases takes places both in the lungs (external respiration) and in the cells (internal respiration). Fig 1 summarises gas exchange in humans.

Bringing air into the lungs

Our respiratory system comprises a conduction zone and a respiratory zone. The conduction zone brings air from the external environment to the lungs via a series of tubes through which the air travels. These are the:

  • Nasal cavity
  • Pharynx (part of the throat behind the mouth and nasal cavity),
  • Larynx (voice box),
  • Trachea (windpipe)
  • Bronchi and bronchioles.

Aside from conducting air to the lungs, these tubes also:

  • Warm the incoming air
  • Filter out small particles from it
  • Moisten it to ease the gas exchange in the lungs.

The nasal cavity has a large number of tiny capillaries that bring warm blood to the cold nose. The warmth from the blood diffuses into the cold air entering the nose and warms it.

The lining of the pharynx and larynx (which form the upper respiratory tract) and the lining of the trachea (lower respiratory tract) have small cells with little hairs or cilia. These hairs trap small airborne particles, such as dust, and prevent them from reaching the lungs.

The lining of the nasal cavity, upper respiratory tract and lower respiratory tract contains goblet cells that secrete mucus. The mucus moistens the air as it comes in, making it more suitable for the body’s internal environment. It also traps particles, which the cilia then sweep upwards and away from the lungs so they are swallowed into the stomach for digestion, rather than getting trapped in the lungs. This mechanism of moving trapped particles in this way is known as the mucociliary escalator.

The lungs are a little like balloons: they do not inflate by themselves, but only do so if air is blown into them. We can blow into the lungs and inflate them – which is one of the two techniques used for cardiopulmonary resuscitation – but that does not happen in the normal daily life of healthy people. We have to inhale and exhale air by ourselves. How do we do that?

Controlling the volume of air in the lungs

We have two lungs (right and left) contained in the thoracic cavity (chest). Surrounding the lungs are ribs, which not only protect them from damage but also serve as anchors for the intercostal muscles. Beneath the lungs is a very large dome-shaped muscle, the diaphragm. All these muscles are attached to the lungs by the parietal and visceral membranes (also called parietal and visceral pleura).

The parietal membrane is attached to the muscles and the visceral membrane is attached to the lungs. The liquid between these two membranes, pleural fluid, sticks them together just as panes of glass become stuck together when wet.

As the visceral membrane covers, and is part of, the lungs and is stuck by pleural fluid to the parietal membrane, when the muscles in the thorax move, the lungs move with them. If air gets between the membranes, they become unstuck and, although the muscles can still contract and relax, they are no longer attached to the lung – as a result, the lung collapses. This abnormal collection of air in the pleural space is called a pneumothorax. If the pleural fluid liquid becomes infected, the person develops pleurisy.

When the intercostal muscles contract, they move up and away from the thoracic cavity. When the diaphragm contracts, it moves down towards the abdomen. This movement of the muscles causes the lungs to expand and fill with air, like a bellows (inhalation). Conversely, when the muscles relax, the thoracic cavity gets smaller, the volume of the lungs decreases, and air is expelled (exhalation).

Equalising pressure

When the thoracic muscles contract, the volume of the lungs expands so there is suddenly less pressure inside them. The air already in the lungs has more space, so it is not pushing against the lung walls with the same pressure. To equalise the pressure, air rushes in until the pressure is the same inside and outside. Conversely, when the muscles relax, the volume of the lungs decreases, the air in the lungs has less space and is now at high pressure, so the air is expelled until pressure is equalised. In short:

  • When volume (V) increases, pressure (P) decreases, resulting in air rushing into the lungs – we inhale
  • When V decreases, P increases, resulting in air being squeezed out of the lungs – we exhale.

Gas exchange

The job of the conduction zone is to get air into the lungs while warming, moistening and filtering it on the way. Once the air is in the respiratory zone (composed of the alveolar ducts and alveoli), external gas exchange can take place (Fig 2).

The lungs contain thin layers of cells forming air sacs called alveoli, each of which is surrounded by pulmonary blood capillaries that are linked to the pulmonary arteries coming out of the heart. The alveoli are kept open by liquid secretions (pulmonary surfactant) so they do not stick together when air is expelled from the lungs. Premature babies do not have enough pulmonary surfactant, so they need some sprayed into their lungs.

During inhalation, each alveoli receives air that contains various gases: nitrogen (almost 80%), oxygen (almost 20%) and other gases including 0.04% carbon dioxide. External gaseous exchange then takes place, using the principle of diffusion:

  • Oxygen diffuses from the alveoli into the pulmonary capillaries because there is a high concentration of oxygen in the lungs and a low concentration in the blood
  • Carbon dioxide diffuses from the pulmonary capillaries into the alveoli because there is a high concentration of carbon dioxide in the blood and a low concentration in the lungs
  • Nitrogen diffuses both ways.

In other words: we inhale, high concentrations of oxygen which then diffuses from the lungs into the blood, while high concentrations of carbon dioxide diffuses from the blood into the lungs, and we exhale. Once in the blood, the oxygen is bound to haemoglobin in red blood cells, taken through the pulmonary vein to the heart, pumped into the systemic vascular system and, finally, taken to all the cells of the body.

Controlling breathing

The main cue that we are not breathing is not so much the lack of oxygen as the accumulation of carbon dioxide. When our muscles carry out activities, oxygen is used up and carbon dioxide – the waste product – accumulates in the cells. Increased muscle activity means increased use of oxygen, increased production of glucose-forming ATP and, therefore, increased levels of carbon dioxide.

Carbon dioxide diffuses from the cells into the blood. Deoxygenated blood is carried by the veins towards the heart. It enters the right side of the heart and is pumped into the pulmonary system. Carbon dioxide diffuses into the lungs and is expelled as we exhale.

While the deoxygenated blood travels in the veins, detectors in the brain and blood vessels (chemoreceptors) measure the blood’s pH. The peripheral chemoreceptors – although sensitive to changes in carbon dioxide levels and pH, as well as oxygen levels – mainly monitor oxygen. The central chemoreceptors, located in the brain constitute the control centres for breathing, as they are especially sensitive to pH changes in the blood. As carbon dioxide levels rise, blood pH falls this is picked up by the central chemoreceptors and, through feedback mechanisms, signals are sent to alter breathing.

Altering breathing

We change our breathing to match our activity. When we move skeletal muscles, we use energy and therefore need more sugar and oxygen. Muscles have a good blood supply, bringing oxygen and glucose and taking away carbon dioxide. As muscles move more – for example, if we go from walking to running – the heart pumps faster (increased heart rate) to increase the blood supply and we breathe more quickly (increased respiratory rate) to get more oxygen into the blood.

The respiratory rate can be increased or decreased to suit the amount of oxygen needed. To increase the respiratory rate, effectors in the lungs are triggered to ventilate (inhale and exhale) faster, so carbon dioxide is removed and oxygen brought in more quickly. At the same time, the brain sends messages to the heart to beat faster, pumping oxygenated blood to the cells more quickly. The depth of breathing can also be altered so that a larger or smaller volume of air is taken into the lungs.

Respiratory rate is one of the respiratory vital signs (Box 1). To diagnose any respiratory problem, these vital signs need to be measured at rest and at work (Cedar, 2017). Respiratory rate is hard to measure, because when patients are told it is going to be measured, they usually start to breathe slower or faster than normal. It may be beneficial for nurses to tell patients that they are going to measure their temperature, and then measure their respiratory rate at the same time.

Box 1. Vital signs of breathing

  • Respiratory rate (RR) – number of breaths taken per minute. Adults breathe in and out approximately 12-18 times per minute
  • Tidal volume (TV) – amount of air inhaled and exhaled per breath (about 500ml in adults)
  • Expiratory reserve volume (ERV) – volume of air that can be exhaled after normal breathing
  • Inspiratory reserve volume (IRV) – volume of air that can be inhaled after normal breathing
  • Residual volume (RV) – the air that remains in the lungs the lungs are never completely empty, otherwise they would collapse and stick together
  • Lung capacities (depth and volume of breathing), which can be measured using a spirometer:
    • Vital capacity = ERV + TV + IRV
    • Inspiratory capacity = TV + IRV
    • Functional residual capacity = ERV + RV
    • Total lung capacity = RV + ERV + TV + IRV

    Accurately measuring breathing rate and depth at rest gives a key measure of pulmonary function and oxygen flow. Changes in breathing rate and depth at rest not only tell us about physical changes in the body, but also about mental and emotional changes, as our state of mind and our feelings have an effect on our breathing.

    A lifetime of breathing

    Our respiratory vital signs not only change during the course of one day according to our activities, but also during the course of our lifetimes.

    Before birth, the embryo and then the foetus draw oxygen from the mother’s blood through the placenta. Haemoglobin changes take place to enable the embryo/foetus to take oxygen from blood at lower concentration than it will find in the air after birth. Immediately after birth, the newborn has to switch from drawing oxygen from the blood to inflating its lungs and taking air into them (Schroeder and Matsuda, 1958 Rhinesmith et al, 1957).

    Babies have a much faster heart rate and respiratory rate than adults: they take about 40 breaths per minute because they have smaller lungs (Royal College of Nursing, 2017). Heart rate and respiratory rate slow down with advancing age, partly because the lungs become less able to expand and contract. Becoming less elastic with age, all our muscles – not only skeletal muscle but also smooth muscle and cardiac muscle – reduces the speed at which they expand and contract (Sharma and Goodwin, 2006).

    When we die, one of the signs of death is the cessation of breathing. Oxygen stops diffusing into the blood and, as ATP is used up and we are unable to synthesise more, we become cyanotic. We run out of energy and all of the body’s processes cease. In the brain, the potential difference (measured in volts) becomes the same inside and outside the neurons, and electrical activity stops. The brain ceases all activity, including the involuntary activity that is needed to sustain life.

    Respiratory conditions

    Health professionals are likely to encounter patients with breathing problems in any setting. Common respiratory conditions are:

    • Asthma – often caused by certain chemicals or pollution, asthma affects the bronchioles, which become chronically inflamed and hypersensitive
    • Chronic obstructive pulmonary disorder – often caused by smoking or pollution
    • Pneumonia – usually caused by a bacterial infection, pneumonia is the swelling of tissues in one or both lungs
    • Lung cancers – the predominant tissue in the lungs is epithelial tissue, so lung cancers are mostly carcinomas (squamous cell carcinomas, adenocarcinomas, small cell carcinomas), which are cancers of epithelial tissue.

    Lung disease can appear at any age but susceptibility increases with age because, as we age:

    • The elasticity of our lungs decreases
    • Our vital capacity decreases
    • Our blood-oxygen levels decrease
    • The stimulating effects of carbon dioxide decrease
    • There is an increased risk of respiratory tract infection.

    Respiratory emergencies

    Patients who are rapidly deteriorating or critically ill must be assessed immediately, and nursing interventions can go a long way to ensure recovery (Fournier, 2014). In an acute situation, one of the first interventions is to ensure the airways (upper respiratory tract) are clear so air can be drawn into the lungs. This is the first step of the ABCDE checklist. ABCDE stands for:

    The ABCDE approach is outlined in more detail here.

    An inability to breathe normally is extremely distressing and the more distressed a person becomes, the more likely it is that their breathing will be compromised. If one of our lungs collapses, we can manage without it, but we do need at least one functioning lung. We have about 90 seconds worth of ATP stored in our bodies, which we constantly use, so we need to be able to get oxygen.

    A solid understanding of vital respiratory signs, as well as human breathing patterns (Box 2) is key. Armed with such know-ledge, nurses can react quickly to acute changes, potentially saving lives and restoring health (Fletcher, 2007).

    Box 2. Breathing patterns

    • Regular breathing: breaths are similar in amplitude, duration, wave shape and frequency
    • Irregular breathing: breaths vary in one or more of the following: amplitude, duration, wave shape and frequency
    • Hypopnea: breathing at reduced breath (tidal) volume and/or frequency
    • Apnoea: cessation of breathing
    • Periodic breathing: a sequence of several breaths followed by apnoea, then a sequence of breaths, then apnoea, and so on
    • Cheyne-Stokes breathing: similar to periodic breathing breath amplitude starts low and gradually increases, then decreases to apnoea, and the pattern repeats

    Source: Adapted from Neuman (2011)

    Key points

    • Energy in our bodies is obtained by breaking the chemical bonds in molecules
    • Oxygen sourced from the air is a vital ingredient in the process of energy synthesis
    • The respiratory system is designed to facilitate gas exchange, so that cells receive oxygen and get rid of carbon dioxide
    • Breathing changes throughout the day according to our activities
    • In an acute situation, one of the first interventions is to check the airways are clear so air can be drawn into the lungs

    Cedar SH (2017) Homeostasis and vital signs: their role in health and its restoration. Nursing Times 113: 8, 32-35.

    Fletcher M (2007) Nurses lead the way in respiratory care. Nursing Times 103: 24, 42.

    Fournier M (2014) Caring for patients in respiratory failure. American Nurse Today 9: 11.

    Neuman MR (2011) Vital signs. IEEE Pulse 2: 1, 39-44.

    Rhinesmith HS et al (1957) A quantitative study of the hydrolysis of human dinitrophenyl(DNP)globin: the number and kind of polypeptide chains in normal adult human hemoglobin. Journal of the American Chemical Society 79: 17, 4682-4686.

    Royal College of Nursing (2017) Standards for Assessing, Measuring and Monitoring Vital Signs in Infants, Children and Young People. London: RCN.

    Schroeder WA, Matsuda G (1958) N-terminal residues of human fetal hemoglobin. Journal of the American Chemical Society 80: 6, 1521.

    Sharma G, Goodwin J (2006) Effect of aging on respiratory system physiology and immunology. Clinical Interventions in Aging 1: 3, 253-260.

    Show/hide words to know

    Concentration: in chemistry the ratio of the mass or volume of one substance (solute) in the mass or volume of a solvent. For example saline is a concentration of salt (solute) in water (solvent).

    Diffuse: to spread out.

    Molecule: a chemical structure that has two or more atoms held together by a chemical bond. Water is a molecule of two hydrogen atoms and one oxygen atom (H2O). more

    Paleozoic: period (era) in geological time from 544 million to 230 million years ago. more

    Respiratory: process related to respiration (the action of breathing). The respiratory system is responsible for movement of gases in and out of animals. more

    Trachea: in animals such as humans, a large tube that is the main passage for moving air to and from the lungs. The windpipe. In insects fine tubes that move air directly to tissues. more

    Ask a scientist: Why do we breathe out carbon dioxide?

    When we take a breath, we pull air into our lungs that contains mostly nitrogen and oxygen. When we exhale, we breathe out mostly carbon dioxide. Why do we do this?

    Our bodies need oxygen to function. After we take a breath, the lungs transfer oxygen to our blood to be transported all over our bodies to help our cells work. Think about it — when you are running, you breathe more heavily to get more oxygen. Oxygen helps our cells work harder by breaking down the nutrients we get from food like sugars. With sugars and oxygen, our cells can create the energy they need to function.

    This process also produces carbon dioxide. The carbon dioxide produced is a waste product and needs to be removed. Just like oxygen, carbon dioxide is transferred to blood to be carried to the lungs, where it is removed and we breathe it out. This is important because if we couldn’t remove carbon dioxide from our blood, it would take up all the carrying capacity of our blood and we wouldn’t be able to get oxygen to the rest of our body. This is another reason you breathe heavier when exercising — you produce carbon dioxide faster and need to get it out of your body to make room for more oxygen.

    Fun fact: Plants use the carbon dioxide we breathe out and create the oxygen we breathe in!


    Hemoglobin is a protein found in red blood cells that is comprised of two alpha and two beta subunits that surround an iron-containing heme group. Oxygen readily binds this heme group. The ability of oxygen to bind increases as more oxygen molecules are bound to heme. Disease states and altered conditions in the body can affect the binding ability of oxygen, and increase or decrease its ability to dissociate from hemoglobin.

    Carbon dioxide can be transported through the blood via three methods. It is dissolved directly in the blood, bound to plasma proteins or hemoglobin, or converted into bicarbonate. The majority of carbon dioxide is transported as part of the bicarbonate system. Carbon dioxide diffuses into red blood cells. Inside, carbonic anhydrase converts carbon dioxide into carbonic acid (H2CO3), which is subsequently hydrolyzed into bicarbonate (HCO − 3) and H + . The H + ion binds to hemoglobin in red blood cells, and bicarbonate is transported out of the red blood cells in exchange for a chloride ion. This is called the chloride shift. Bicarbonate leaves the red blood cells and enters the blood plasma. In the lungs, bicarbonate is transported back into the red blood cells in exchange for chloride. The H + dissociates from hemoglobin and combines with bicarbonate to form carbonic acid with the help of carbonic anhydrase, which further catalyzes the reaction to convert carbonic acid back into carbon dioxide and water. The carbon dioxide is then expelled from the lungs.

    Watch the video: Gas Exchange and Partial Pressures, Animation (August 2022).