Information

How is oxygen and carbon dioxide exchange mediated by hemoglobin?


Oxygen is transferred by hemoglobin from the lungs to tissues, while carbon dioxide is transferred by hemoglobin from tissues to lungs. How is this regulated bidirectional transfer mediated?


Under conditions of high CO2 (in tissues such as muscle) and hence high acidity hemoglobin binds preferentially CO2, thereby mediating CO2 removal. Under conditions of low CO2 (high pH) and high O2 (conditions met in the lungs), it preferably binds O2, thereby releasing CO2. Hence, the specific characteristics of hemoglobin allow for CO2 uptake in the body, and O2 uptake and CO2 release in the lungs. Info from wikipedia on hemoglobin


Outline-4, BIO 3360, Respiration V – Oxygen and Carbon Dioxide Transport in Blood

—Affinity is the oxygen being attracted to hemoglobin and once they attach to each other, we use the term saturation and say that hemoglobin is saturated with oxygen. Oxygen will saturate hemoglobin where oxygen levels are high – such as in the lungs.

—Dissociation is oxygen pulling away from the hemoglobin and that way it is not attached and free to go into a tissue that requires oxygen. Oxygen will dissociate from hemoglobin in tissues were oxygen levels are low – such as an exercising leg muscle.

1. Partial pressure of oxygen is the greatest determining factor on oxygen and hemoglobin dissociation (letting go of each other ) or saturation (attaching to each other)

2. Tissues needing the oxygen get it, while oxygen and hemoglobin attach where tissues do not need the oxygen

3. Shifting the curve to the left –higher affinity between oxygen and hemoglobin higher pH, lower carbon dioxide level, lower temperature

4. Bohr Effect: Shifting the curve to the right –lower affinity between oxygen and hemoglobin lower pH, higher carbon dioxide levels, higher temperature, higher levels of metabolic byproducts (such as DPG)

5. Root Effect:increase in carbon dioxide plus decrease in pH not only cause a Bohr Effect but also the Root Effect when there is a reduction in the oxygen carrying capacity of the respiratory pigment (hemoglobin). This can release oxygen into solution and is the mechanism important in filling swim bladders with oxygen. Seen in some fish, cephalopods and crustaceans

II. Carbon Dioxide Transport in Body Fluids

A. Carbon dioxide diffuses from an area of higher pressure to lower pressure, but pressure gradient is not as great as with oxygen. Frogs compensate for this by having a huge skin surface area for carbon dioxide exchange.

70%) carried as bicarbonate ions in the plasma (some free in plasma, and some attached to the globin portion of hemoglobin)

CO 2 +H 2O <–> H 2CO3 <–> H + + HCO3

Enzyme for the formation of carbonic acid is carbonic anhydrase.

D. Carbon Dioxide equilibrium curve –rapid increase in CO2 content at relatively low PCO2in blood and a continued, but slower increase as PCO2 rises. Blood does not become saturated with CO2as it does with O2

E. Haldane Effect –Deoxygenated blood can carry more CO2 than oxygenated blood. The Haldane effect is that deoxygenation of hemoglobin at the tissues promotes CO2 uptake by the blood whereas oxygenation of the hemoglobin at the respiratory surface promotes CO2 unloading. Deoxygenated hemoglobin pushes the equation shown above in B. to the right and increases the amount of carbon dioxide that can be carried in the form of bicarbonate ions. Oxygenated hemoglobin pushes the equation shown above in B. to the left so that carbon dioxide is produced and may be exhaled.


BIO 140 - Human Biology I - Textbook

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Chapter 30

Gas Exchange

  • Compare the composition of atmospheric air and alveolar air
  • Describe the mechanisms that drive gas exchange
  • Discuss the importance of sufficient ventilation and perfusion, and how the body adapts when they are insufficient
  • Discuss the process of external respiration
  • Describe the process of internal respiration

The purpose of the respiratory system is to perform gas exchange. Pulmonary ventilation provides air to the alveoli for this gas exchange process. At the respiratory membrane, where the alveolar and capillary walls meet, gases move across the membranes, with oxygen entering the bloodstream and carbon dioxide exiting. It is through this mechanism that blood is oxygenated and carbon dioxide, the waste product of cellular respiration, is removed from the body.

Gas Exchange

In order to understand the mechanisms of gas exchange in the lung, it is important to understand the underlying principles of gases and their behavior. In addition to Boyle&rsquos law, several other gas laws help to describe the behavior of gases.

Gas Laws and Air Composition

Gas molecules exert force on the surfaces with which they are in contact this force is called pressure. In natural systems, gases are normally present as a mixture of different types of molecules. For example, the atmosphere consists of oxygen, nitrogen, carbon dioxide, and other gaseous molecules, and this gaseous mixture exerts a certain pressure referred to as atmospheric pressure (Table 1). Partial pressure (Px) is the pressure of a single type of gas in a mixture of gases. For example, in the atmosphere, oxygen exerts a partial pressure, and nitrogen exerts another partial pressure, independent of the partial pressure of oxygen (Figure 1). Total pressure is the sum of all the partial pressures of a gaseous mixture. Dalton&rsquos law describes the behavior of nonreactive gases in a gaseous mixture and states that a specific gas type in a mixture exerts its own pressure thus, the total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases in the mixture.

Table 1: Partial Pressures of Atmospheric Gases

Gas Percent of total composition Partial pressure
(mm Hg)
Nitrogen (N2) 78.6 597.4
Oxygen (O2) 20.9 158.8
Water (H2O) 0.4 3.0
Carbon dioxide (CO2) 0.04 0.3
Others 0.06 0.5
Total composition/total atmospheric pressure 100% 760.0

Figure 1: Partial pressure is the force exerted by a gas. The sum of the partial pressures of all the gases in a mixture equals the total pressure.

Partial pressure is extremely important in predicting the movement of gases. Recall that gases tend to equalize their pressure in two regions that are connected. A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower. In addition, the greater the partial pressure difference between the two areas, the more rapid is the movement of gases.

Solubility of Gases in Liquids

Henry&rsquos law describes the behavior of gases when they come into contact with a liquid, such as blood. Henry&rsquos law states that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas. The greater the partial pressure of the gas, the greater the number of gas molecules that will dissolve in the liquid. The concentration of the gas in a liquid is also dependent on the solubility of the gas in the liquid. For example, although nitrogen is present in the atmosphere, very little nitrogen dissolves into the blood, because the solubility of nitrogen in blood is very low. The exception to this occurs in scuba divers the composition of the compressed air that divers breathe causes nitrogen to have a higher partial pressure than normal, causing it to dissolve in the blood in greater amounts than normal. Too much nitrogen in the bloodstream results in a serious condition that can be fatal if not corrected. Gas molecules establish an equilibrium between those molecules dissolved in liquid and those in air.

The composition of air in the atmosphere and in the alveoli differs. In both cases, the relative concentration of gases is nitrogen > oxygen > water vapor > carbon dioxide. The amount of water vapor present in alveolar air is greater than that in atmospheric air (Table 2). Recall that the respiratory system works to humidify incoming air, thereby causing the air present in the alveoli to have a greater amount of water vapor than atmospheric air. In addition, alveolar air contains a greater amount of carbon dioxide and less oxygen than atmospheric air. This is no surprise, as gas exchange removes oxygen from and adds carbon dioxide to alveolar air. Both deep and forced breathing cause the alveolar air composition to be changed more rapidly than during quiet breathing. As a result, the partial pressures of oxygen and carbon dioxide change, affecting the diffusion process that moves these materials across the membrane. This will cause oxygen to enter and carbon dioxide to leave the blood more quickly.

Table 2: Composition and Partial Pressures of Alveolar Air

Composition and Partial Pressures of Alveolar Air
Gas Percent of total composition Partial pressure
(mm Hg)
Nitrogen (N2) 74.9 569
Oxygen (O2) 13.7 104
Water (H2O) 6.2 40
Carbon dioxide (CO2) 5.2 47
Total composition/total alveolar pressure 100% 760.0
Ventilation and Perfusion

Two important aspects of gas exchange in the lung are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible. However, factors such as regional gravity effects on blood, blocked alveolar ducts, or disease can cause ventilation and perfusion to be imbalanced.

The partial pressure of oxygen in alveolar air is about 104 mm Hg, whereas the partial pressure of oxygenated blood in pulmonary veins is about 100 mm Hg. When ventilation is sufficient, oxygen enters the alveoli at a high rate, and the partial pressure of oxygen in the alveoli remains high. In contrast, when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane. The body has mechanisms that counteract this problem. In cases when ventilation is not sufficient for an alveolus, the body redirects blood flow to alveoli that are receiving sufficient ventilation. This is achieved by constricting the pulmonary arterioles that serves the dysfunctional alveolus, which redirects blood to other alveoli that have sufficient ventilation. At the same time, the pulmonary arterioles that serve alveoli receiving sufficient ventilation vasodilate, which brings in greater blood flow. Factors such as carbon dioxide, oxygen, and pH levels can all serve as stimuli for adjusting blood flow in the capillary networks associated with the alveoli.

Ventilation is regulated by the diameter of the airways, whereas perfusion is regulated by the diameter of the blood vessels. The diameter of the bronchioles is sensitive to the partial pressure of carbon dioxide in the alveoli. A greater partial pressure of carbon dioxide in the alveoli causes the bronchioles to increase their diameter as will a decreased level of oxygen in the blood supply, allowing carbon dioxide to be exhaled from the body at a greater rate. As mentioned above, a greater partial pressure of oxygen in the alveoli causes the pulmonary arterioles to dilate, increasing blood flow.

Gas Exchange

Gas exchange occurs at two sites in the body: in the lungs, where oxygen is picked up and carbon dioxide is released at the respiratory membrane, and at the tissues, where oxygen is released and carbon dioxide is picked up. External respiration is the exchange of gases with the external environment, and occurs in the alveoli of the lungs. Internal respiration is the exchange of gases with the internal environment, and occurs in the tissues. The actual exchange of gases occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes. Instead, these gases follow pressure gradients that allow them to diffuse. The anatomy of the lung maximizes the diffusion of gases: The respiratory membrane is highly permeable to gases the respiratory and blood capillary membranes are very thin and there is a large surface area throughout the lungs.

External Respiration

The pulmonary artery carries deoxygenated blood into the lungs from the heart, where it branches and eventually becomes the capillary network composed of pulmonary capillaries. These pulmonary capillaries create the respiratory membrane with the alveoli (Figure 2). As the blood is pumped through this capillary network, gas exchange occurs. Although a small amount of the oxygen is able to dissolve directly into plasma from the alveoli, most of the oxygen is picked up by erythrocytes (red blood cells) and binds to a protein called hemoglobin, a process described later in this chapter. Oxygenated hemoglobin is red, causing the overall appearance of bright red oxygenated blood, which returns to the heart through the pulmonary veins. Carbon dioxide is released in the opposite direction of oxygen, from the blood to the alveoli. Some of the carbon dioxide is returned on hemoglobin, but can also be dissolved in plasma or is present as a converted form, also explained in greater detail later in this chapter.

External respiration occurs as a function of partial pressure differences in oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries.

Figure 2: In external respiration, oxygen diffuses across the respiratory membrane from the alveolus to the capillary, whereas carbon dioxide diffuses out of the capillary into the alveolus.

Although the solubility of oxygen in blood is not high, there is a drastic difference in the partial pressure of oxygen in the alveoli versus in the blood of the pulmonary capillaries. This difference is about 64 mm Hg: The partial pressure of oxygen in the alveoli is about 104 mm Hg, whereas its partial pressure in the blood of the capillary is about 40 mm Hg. This large difference in partial pressure creates a very strong pressure gradient that causes oxygen to rapidly cross the respiratory membrane from the alveoli into the blood.

The partial pressure of carbon dioxide is also different between the alveolar air and the blood of the capillary. However, the partial pressure difference is less than that of oxygen, about 5 mm Hg. The partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg. However, the solubility of carbon dioxide is much greater than that of oxygen&mdashby a factor of about 20&mdashin both blood and alveolar fluids. As a result, the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar.

Internal Respiration

Internal respiration is gas exchange that occurs at the level of body tissues (Figure 3). Similar to external respiration, internal respiration also occurs as simple diffusion due to a partial pressure gradient. However, the partial pressure gradients are opposite of those present at the respiratory membrane. The partial pressure of oxygen in tissues is low, about 40 mm Hg, because oxygen is continuously used for cellular respiration. In contrast, the partial pressure of oxygen in the blood is about 100 mm Hg. This creates a pressure gradient that causes oxygen to dissociate from hemoglobin, diffuse out of the blood, cross the interstitial space, and enter the tissue. Hemoglobin that has little oxygen bound to it loses much of its brightness, so that blood returning to the heart is more burgundy in color.

Considering that cellular respiration continuously produces carbon dioxide, the partial pressure of carbon dioxide is lower in the blood than it is in the tissue, causing carbon dioxide to diffuse out of the tissue, cross the interstitial fluid, and enter the blood. It is then carried back to the lungs either bound to hemoglobin, dissolved in plasma, or in a converted form. By the time blood returns to the heart, the partial pressure of oxygen has returned to about 40 mm Hg, and the partial pressure of carbon dioxide has returned to about 45 mm Hg. The blood is then pumped back to the lungs to be oxygenated once again during external respiration.

Figure 3: Oxygen diffuses out of the capillary and into cells, whereas carbon dioxide diffuses out of cells and into the capillary.

Everyday Connection

Hyperbaric Chamber Treatment

A type of device used in some areas of medicine that exploits the behavior of gases is hyperbaric chamber treatment. A hyperbaric chamber is a unit that can be sealed and expose a patient to either 100 percent oxygen with increased pressure or a mixture of gases that includes a higher concentration of oxygen than normal atmospheric air, also at a higher partial pressure than the atmosphere. There are two major types of chambers: monoplace and multiplace. Monoplace chambers are typically for one patient, and the staff tending to the patient observes the patient from outside of the chamber (Figure 4). Some facilities have special monoplace hyperbaric chambers that allow multiple patients to be treated at once, usually in a sitting or reclining position, to help ease feelings of isolation or claustrophobia. Multiplace chambers are large enough for multiple patients to be treated at one time, and the staff attending these patients is present inside the chamber. In a multiplace chamber, patients are often treated with air via a mask or hood, and the chamber is pressurized.

Figure 4: (credit: &ldquokomunews&rdquo/flickr.com)

Hyperbaric chamber treatment is based on the behavior of gases. As you recall, gases move from a region of higher partial pressure to a region of lower partial pressure. In a hyperbaric chamber, the atmospheric pressure is increased, causing a greater amount of oxygen than normal to diffuse into the bloodstream of the patient. Hyperbaric chamber therapy is used to treat a variety of medical problems, such as wound and graft healing, anaerobic bacterial infections, and carbon monoxide poisoning. Exposure to and poisoning by carbon monoxide is difficult to reverse, because hemoglobin&rsquos affinity for carbon monoxide is much stronger than its affinity for oxygen, causing carbon monoxide to replace oxygen in the blood. Hyperbaric chamber therapy can treat carbon monoxide poisoning, because the increased atmospheric pressure causes more oxygen to diffuse into the bloodstream. At this increased pressure and increased concentration of oxygen, carbon monoxide is displaced from hemoglobin. Another example is the treatment of anaerobic bacterial infections, which are created by bacteria that cannot or prefer not to live in the presence of oxygen. An increase in blood and tissue levels of oxygen helps to kill the anaerobic bacteria that are responsible for the infection, as oxygen is toxic to anaerobic bacteria. For wounds and grafts, the chamber stimulates the healing process by increasing energy production needed for repair. Increasing oxygen transport allows cells to ramp up cellular respiration and thus ATP production, the energy needed to build new structures.

Chapter Review

The behavior of gases can be explained by the principles of Dalton&rsquos law and Henry&rsquos law, both of which describe aspects of gas exchange. Dalton&rsquos law states that each specific gas in a mixture of gases exerts force (its partial pressure) independently of the other gases in the mixture. Henry&rsquos law states that the amount of a specific gas that dissolves in a liquid is a function of its partial pressure. The greater the partial pressure of a gas, the more of that gas will dissolve in a liquid, as the gas moves toward equilibrium. Gas molecules move down a pressure gradient in other words, gas moves from a region of high pressure to a region of low pressure. The partial pressure of oxygen is high in the alveoli and low in the blood of the pulmonary capillaries. As a result, oxygen diffuses across the respiratory membrane from the alveoli into the blood. In contrast, the partial pressure of carbon dioxide is high in the pulmonary capillaries and low in the alveoli. Therefore, carbon dioxide diffuses across the respiratory membrane from the blood into the alveoli. The amount of oxygen and carbon dioxide that diffuses across the respiratory membrane is similar.

Ventilation is the process that moves air into and out of the alveoli, and perfusion affects the flow of blood in the capillaries. Both are important in gas exchange, as ventilation must be sufficient to create a high partial pressure of oxygen in the alveoli. If ventilation is insufficient and the partial pressure of oxygen drops in the alveolar air, the capillary is constricted and blood flow is redirected to alveoli with sufficient ventilation. External respiration refers to gas exchange that occurs in the alveoli, whereas internal respiration refers to gas exchange that occurs in the tissue. Both are driven by partial pressure differences.


Physiological factors that can shift the oxyhemoglobin

Changes in pH and the Bohr effect

Changes in the position of the curve with changes in red blood cell (RBC) intracellular hydrogen ion concentration constitute the Bohr effect. Decreases in pH shift the curve to the right, while increases shift the curve to the left.

Figure 4. Changes in pH are associated with changes in hemoglobin’s oxygen affinity. Decreases in pH shift the curve to the right, while increases shift the curve to the left.

Carbon dioxide

Carbon dioxide increases hydrogen ion concentration and lowers tissue pH. As a consequence, hemoglobin’s affinity for oxygen decreases and oxygen release to tissues is facilitated. Opposite changes occur in the lung.

Figure 5. Changes in carbon dioxide (CO2) are associated with shifts in hemoglobin’s oxygen affinity. Increases in CO2 decrease hemoglobin saturation, while decreases in CO2 increase hemoglobin saturation.

Organophosphates

During glycolysis, red blood cells generate organophosphates, particularly 2,3-diphosphoglycerate (2,3-DPG). In red cells, due to the absence of mitochondria, 2,3-diphosphoglycerate is used for energy generation. In the setting of diminished oxygen availability (e.g., anemia, blood loss, chronic lung disease, high altitude, or right-to-left shunts), organophosphate production in red cells is increased, shifting the oxyhemoglobin curve to the right, thereby facilitating unloading of oxygen in peripheral tissues.

Figure 6. Increased organophosphates shift the oxyhemoglobin curve to the right, which facilitates oxygen unloading into peripheral tissues.

Changes in temperature

Hyperthermia shifts the curve to the right. Opposite changes occur with hypothermia.

Figure 7. Changes in temperature are associated with changes in hemoglobin’s oxygen affinity. Hyperthermia shifts the curve to the right, while hypothermia shifts the curve to the left.

Carbon monoxide levels

Carbon monoxide shifts the oxyhemoglobin dissociation curve to the left, impeding oxygen unloading in peripheral tissues. This effect is in addition to the effect of carbon monoxide in binding to hemoglobin and preventing oxygen loading in the lungs.

Figure 8. Carbon monoxide shifts the oxyhemoglobin dissociation curve to the left, preventing oxygen unloading in peripheral tissues.

Methemoglobin

Methemoglobin is the result of oxidation of the iron moiety of hemoglobin from the ferrous to the ferric state. Intracellular enzymatic reductive pathways normally maintain methemoglobin levels of less than three percent.

Figure 9. Oxidation of the iron moiety of hemoglobin from the ferrous to ferric state results in methemoglobin.

In the presence of congenital deficiencies of reductive enzymes, or in the presence of oxidant drugs (e.g., antimalarials, dapsone, local anesthetics), methemoglobinemia may develop.

Methemoglobin shifts the oxyhemoglobin curve to the left, impairing oxygen release in peripheral tissues.

Figure 10. Methemoglobinemia shifts the oxyhemoglobin curve to the left, impairing oxygen release in peripheral tissues.

Presence of abnormal hemoglobins

Finally, the presence of abnormal hemoglobins—such as fetal hemoglobin in an adult—can have an effect on the oxygen-hemoglobin binding curve. Fetal hemoglobin, hemoglobin F, consists of two gamma chains replacing the normal two beta chains.

The oxyhemoglobin curve is shifted to the left in the presence of hemoglobin F, enhancing hemoglobin’s affinity for oxygen, an advantage during fetal life when arterial oxygen tension is low.

Figure 11. Abnormal hemoglobin shifts the oxyhemoglobin curve to the left, enhancing hemoglobin’s affinity for oxygen.


Transport of Carbon Dioxide in the Blood

Dissolution, hemoglobin binding, and the bicarbonate buffer system are ways in which carbon dioxide is transported throughout the body.

Learning Objectives

Explain how carbon dioxide is transported from body tissues to the lungs

Key Takeaways

Key Points

  • Carbon dioxide is more soluble in blood than is oxygen about 5 to 7 percent of all carbon dioxide is dissolved in the plasma.
  • Carbon dioxide has the ability to attach to hemoglobin molecules it will be removed from the body once they become dissociated from one another.
  • In the bicarbonate buffer system, the most common form of carbon dioxide transportation in the blood, carbon dioxide is finally expelled from the body through the lungs during exhalation.
  • Importantly, the bicarbonate buffer system allows little change to the pH of the body system it allows for people to travel and live at high altitudes because the system can adjust itself to regulate carbon dioxide while maintaining the correct pH in the body.

Key Terms

  • carbaminohemoglobin: a compound made up of hemoglobin and carbon dioxide one of the forms in which carbon dioxide exists in the blood
  • carbonic anhydrase: a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons
  • carbon monoxide: a colorless, odourless, flammable, highly toxic gas

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:

  1. Dissolution directly into the blood
  2. Binding to hemoglobin
  3. 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 is 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.

Third, the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H2CO3). Carbonic acid is an unstable, intermediate molecule that immediately dissociates into bicarbonate ions (HCO3 − ) and hydrogen (H + ) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood, down its concentration gradient. It also results in the production of H + ions. If too much H + is produced, it can alter blood pH. However, hemoglobin binds to the free H + ions, limiting shifts in pH. The newly-synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (Cl-) this is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H + ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation.

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 does 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 throughout the body. Carbon monoxide is a colorless, odorless gas which is 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 as it speeds up the separation of carbon monoxide from hemoglobin.

Carbon monoxide poisoning: When carbon monoxide (CO) in the body increases, the oxygen saturation of hemoglobin decreases since hemoglobin will bind more readily to CO than to oxygen. Therefore, CO exposure leads to death due to a decreased transportation of oxygen in the body.


The Role of Blood in the Body

Blood, like the human blood illustrated in Figure (PageIndex<1>) is important for regulation of the body&rsquos systems and homeostasis. Blood helps maintain homeostasis by stabilizing pH, temperature, osmotic pressure, and by eliminating excess heat. Blood supports growth by distributing nutrients and hormones, and by removing waste. Blood plays a protective role by transporting clotting factors and platelets to prevent blood loss and transporting the disease-fighting agents or white blood cells to sites of infection.

Figure (PageIndex<1>): The cells and cellular components of human blood are shown. Red blood cells deliver oxygen to the cells and remove carbon dioxide. White blood cells&mdashincluding neutrophils, monocytes, lymphocytes, eosinophils, and basophils&mdashare involved in the immune response. Platelets form clots that prevent blood loss after injury.

Red Blood Cells

Red blood cells , or erythrocytes (erythro- = &ldquored&rdquo -cyte = &ldquocell&rdquo), are specialized cells that circulate through the body delivering oxygen to cells they are formed from stem cells in the bone marrow. In mammals, red blood cells are small biconcave cells that at maturity do not contain a nucleus or mitochondria and are only 7&ndash8 µm in size. In birds and non-avian reptiles, a nucleus is still maintained in red blood cells.

The red coloring of blood comes from the iron-containing protein hemoglobin, illustrated in Figure (PageIndex<2>)a. The principal job of this protein is to carry oxygen, but it also transports carbon dioxide as well. Hemoglobin is packed into red blood cells at a rate of about 250 million molecules of hemoglobin per cell. Each hemoglobin molecule binds four oxygen molecules so that each red blood cell carries one billion molecules of oxygen. There are approximately 25 trillion red blood cells in the five liters of blood in the human body, which could carry up to 25 sextillion (25 × 10 21 ) molecules of oxygen in the body at any time. In mammals, the lack of organelles in erythrocytes leaves more room for the hemoglobin molecules, and the lack of mitochondria also prevents use of the oxygen for metabolic respiration. Only mammals have anucleated red blood cells, and some mammals (camels, for instance) even have nucleated red blood cells. The advantage of nucleated red blood cells is that these cells can undergo mitosis. Anucleated red blood cells metabolize anaerobically (without oxygen), making use of a primitive metabolic pathway to produce ATP and increase the efficiency of oxygen transport.

Not all organisms use hemoglobin as the method of oxygen transport. Invertebrates that utilize hemolymph rather than blood use different pigments to bind to the oxygen. These pigments use copper or iron to the oxygen. Invertebrates have a variety of other respiratory pigments. Hemocyanin, a blue-green, copper-containing protein, illustrated in Figure (PageIndex<2>)b is found in mollusks, crustaceans, and some of the arthropods. Chlorocruorin, a green-colored, iron-containing pigment is found in four families of polychaete tubeworms. Hemerythrin, a red, iron-containing protein is found in some polychaete worms and annelids and is illustrated in Figure (PageIndex<2>)c. Despite the name, hemerythrin does not contain a heme group and its oxygen-carrying capacity is poor compared to hemoglobin.

Figure (PageIndex<2>): In most vertebrates, (a) hemoglobin delivers oxygen to the body and removes some carbon dioxide. Hemoglobin is composed of four protein subunits, two alpha chains and two beta chains, and a heme group that has iron associated with it. The iron reversibly associates with oxygen, and in so doing is oxidized from Fe 2+ to Fe 3+ . In most mollusks and some arthropods, (b) hemocyanin delivers oxygen. Unlike hemoglobin, hemolymph is not carried in blood cells, but floats free in the hemolymph. Copper instead of iron binds the oxygen, giving the hemolymph a blue-green color. In annelids, such as the earthworm, and some other invertebrates, (c) hemerythrin carries oxygen. Like hemoglobin, hemerythrin is carried in blood cells and has iron associated with it, but despite its name, hemerythrin does not contain heme.

The small size and large surface area of red blood cells allows for rapid diffusion of oxygen and carbon dioxide across the plasma membrane. In the lungs, carbon dioxide is released and oxygen is taken in by the blood. In the tissues, oxygen is released from the blood and carbon dioxide is bound for transport back to the lungs. Studies have found that hemoglobin also binds nitrous oxide (NO). NO is a vasodilator that relaxes the blood vessels and capillaries and may help with gas exchange and the passage of red blood cells through narrow vessels. Nitroglycerin, a heart medication for angina and heart attacks, is converted to NO to help relax the blood vessels and increase oxygen flow through the body.

A characteristic of red blood cells is their glycolipid and glycoprotein coating these are lipids and proteins that have carbohydrate molecules attached. In humans, the surface glycoproteins and glycolipids on red blood cells vary between individuals, producing the different blood types, such as A, B, and O. Red blood cells have an average life span of 120 days, at which time they are broken down and recycled in the liver and spleen by phagocytic macrophages, a type of white blood cell.

White Blood Cells

White blood cells, also called leukocytes (leuko = white), make up approximately one percent by volume of the cells in blood. The role of white blood cells is very different than that of red blood cells: they are primarily involved in the immune response to identify and target pathogens, such as invading bacteria, viruses, and other foreign organisms. White blood cells are formed continually some only live for hours or days, but some live for years.

The morphology of white blood cells differs significantly from red blood cells. They have nuclei and do not contain hemoglobin. The different types of white blood cells are identified by their microscopic appearance after histologic staining, and each has a different specialized function. The two main groups, both illustrated in Figure (PageIndex<3>) are the granulocytes, which include the neutrophils, eosinophils, and basophils, and the agranulocytes, which include the monocytes and lymphocytes.

Figure (PageIndex<3>): (a) Granulocytes&mdashincluding neutrophils, eosinophils and basophils&mdashare characterized by a lobed nucleus and granular inclusions in the cytoplasm. Granulocytes are typically first-responders during injury or infection. (b) Agranulocytes include lymphocytes and monocytes. Lymphocytes, including B and T cells, are responsible for adaptive immune response. Monocytes differentiate into macrophages and dendritic cells, which in turn respond to infection or injury.

Granulocytes contain granules in their cytoplasm the agranulocytes are so named because of the lack of granules in their cytoplasm. Some leukocytes become macrophages that either stay at the same site or move through the blood stream and gather at sites of infection or inflammation where they are attracted by chemical signals from foreign particles and damaged cells. Lymphocytes are the primary cells of the immune system and include B cells, T cells, and natural killer cells. B cells destroy bacteria and inactivate their toxins. They also produce antibodies. T cells attack viruses, fungi, some bacteria, transplanted cells, and cancer cells. T cells attack viruses by releasing toxins that kill the viruses. Natural killer cells attack a variety of infectious microbes and certain tumor cells.

One reason that HIV poses significant management challenges is because the virus directly targets T cells by gaining entry through a receptor. Once inside the cell, HIV then multiplies using the T cell&rsquos own genetic machinery. After the HIV virus replicates, it is transmitted directly from the infected T cell to macrophages. The presence of HIV can remain unrecognized for an extensive period of time before full disease symptoms develop.

Platelets and Coagulation Factors

Blood must clot to heal wounds and prevent excess blood loss. Small cell fragments called platelets (thrombocytes) are attracted to the wound site where they adhere by extending many projections and releasing their contents. These contents activate other platelets and also interact with other coagulation factors, which convert fibrinogen, a water-soluble protein present in blood serum into fibrin (a non-water soluble protein), causing the blood to clot. Many of the clotting factors require vitamin K to work, and vitamin K deficiency can lead to problems with blood clotting. Many platelets converge and stick together at the wound site forming a platelet plug (also called a fibrin clot), as illustrated in Figure (PageIndex<4>)b. The plug or clot lasts for a number of days and stops the loss of blood. Platelets are formed from the disintegration of larger cells called megakaryocytes, like that shown in Figure (PageIndex<4>)a. For each megakaryocyte, 2000&ndash3000 platelets are formed with 150,000 to 400,000 platelets present in each cubic millimeter of blood. Each platelet is disc shaped and 2&ndash4 &mum in diameter. They contain many small vesicles but do not contain a nucleus.

Figure (PageIndex<4>): (a) Platelets are formed from large cells called megakaryocytes. The megakaryocyte breaks up into thousands of fragments that become platelets. (b) Platelets are required for clotting of the blood. The platelets collect at a wound site in conjunction with other clotting factors, such as fibrinogen, to form a fibrin clot that prevents blood loss and allows the wound to heal.

Plasma and Serum

The liquid component of blood is called plasma, and it is separated by spinning or centrifuging the blood at high rotations (3000 rpm or higher). The blood cells and platelets are separated by centrifugal forces to the bottom of a specimen tube. The upper liquid layer, the plasma, consists of 90 percent water along with various substances required for maintaining the body&rsquos pH, osmotic load, and for protecting the body. The plasma also contains the coagulation factors and antibodies.

The plasma component of blood without the coagulation factors is called the serum . Serum is similar to interstitial fluid in which the correct composition of key ions acting as electrolytes is essential for normal functioning of muscles and nerves. Other components in the serum include proteins that assist with maintaining pH and osmotic balance while giving viscosity to the blood. The serum also contains antibodies, specialized proteins that are important for defense against viruses and bacteria. Lipids, including cholesterol, are also transported in the serum, along with various other substances including nutrients, hormones, metabolic waste, plus external substances, such as, drugs, viruses, and bacteria.

Human serum albumin is the most abundant protein in human blood plasma and is synthesized in the liver. Albumin, which constitutes about half of the blood serum protein, transports hormones and fatty acids, buffers pH, and maintains osmotic pressures. Immunoglobin is a protein antibody produced in the mucosal lining and plays an important role in antibody mediated immunity.

Evolution Connection: Blood Types

Related to Proteins on the Surface of the Red Blood Cells Red blood cells are coated in antigens made of glycolipids and glycoproteins. The composition of these molecules is determined by genetics, which have evolved over time. In humans, the different surface antigens are grouped into 24 different blood groups with more than 100 different antigens on each red blood cell. The two most well known blood groups are the ABO, shown in Figure (PageIndex<5>), and Rh systems. The surface antigens in the ABO blood group are glycolipids, called antigen A and antigen B. People with blood type A have antigen A, those with blood type B have antigen B, those with blood type AB have both antigens, and people with blood type O have neither antigen. Antibodies called agglutinougens are found in the blood plasma and react with the A or B antigens, if the two are mixed. When type A and type B blood are combined, agglutination (clumping) of the blood occurs because of antibodies in the plasma that bind with the opposing antigen this causes clots that coagulate in the kidney causing kidney failure. Type O blood has neither A or B antigens, and therefore, type O blood can be given to all blood types. Type O negative blood is the universal donor. Type AB positive blood is the universal acceptor because it has both A and B antigen. The ABO blood groups were discovered in 1900 and 1901 by Karl Landsteiner at the University of Vienna.

The Rh blood group was first discovered in Rhesus monkeys. Most people have the Rh antigen (Rh+) and do not have anti-Rh antibodies in their blood. The few people who do not have the Rh antigen and are Rh&ndash can develop anti-Rh antibodies if exposed to Rh+ blood. This can happen after a blood transfusion or after an Rh&ndash woman has an Rh+ baby. The first exposure does not usually cause a reaction however, at the second exposure, enough antibodies have built up in the blood to produce a reaction that causes agglutination and breakdown of red blood cells. An injection can prevent this reaction.

Figure (PageIndex<5>): Human red blood cells may have either type A or B glycoproteins on their surface, both glycoproteins combined (AB), or neither (O). The glycoproteins serve as antigens and can elicit an immune response in a person who receives a transfusion containing unfamiliar antigens. Type O blood, which has no A or B antigens, does not elicit an immune response when injected into a person of any blood type. Thus, O is considered the universal donor. Persons with type AB blood can accept blood from any blood type, and type AB is considered the universal acceptor.

Play a blood typing game on the Nobel Prize website to solidify your understanding of blood types.


Hypocapnia and Hypercapnia

Transport of CO2 in the Blood

The vast bulk of CO2 is produced in the mitochondria, where cellular CO2 concentrations are highest. The pathway for transportation, involving step-wise decreases in CO2 partial pressure gradients, originates in the mitochondria and proceeds through the cytoplasm, cell membranes, capillaries, venules, larger veins, and ultimately into the mixed venous blood before elimination through the alveoli.

Transportation of CO2 in the blood is accomplished via three different mechanisms with the exact proportions carried by each mechanism varying depending on whether it is arterial or venous blood. 32 Dissolved CO2 in plasma, reported as arterial P co 2 (i.e., partial pressure) accounts for only 5% to 10% of the total CO2 transported in blood. Almost 90% of total CO2 in the blood is converted to bicarbonate ions ( ), almost all catalyzed by carbonic anhydrase within the red blood cells. The remainder (5% to 10%) is transported as carbamino-hemoglobin, in which CO2 is bound to terminal amino groups in hemoglobin (Hb) molecules. 32 The usual amount of CO2 in the arterial blood is 21.5 mmol per liter of blood, with slightly more (23.3 mmol/L) in venous blood. Overall, more than 80% of the CO2 is carried within the red blood cells.

Oxygen-Induced Hypercapnia

CO2 transport in the blood is altered by oxygen, leading to an elevated P co 2 this oxygen-induced hypercapnia is seen in patients with end-stage lung disease who inhale supplemental O2. The mechanism of oxygen-induced hypercapnia was formerly thought to be oxygen-induced inhibition of ventilatory drive in patients thought to be critically dependent on hypoxic ventilatory drive. In fact, minute ventilation is not diminished in such patients. 33,34 The mechanism is now better understood as having three key components: the Haldane effect , impaired hypoxic pulmonary vasoconstriction, and inability to increase minute ventilation. 35

The Haldane effect 36 is the term given to the phenomenon whereby increasing arterial P o 2 reduces the ability of the blood to store CO2 (as Hb-bound, carbamino Hb or as ), thereby increasing the CO2 partial pressure. There are two elements to the Haldane effect. First, increased arterial P o 2 decreases formation of carbamino compounds this reduces the quantity of CO2 bound to Hb, thereby elevating the dissolved CO2 (P co 2). Second, histidine is important for the buffering properties of Hb it contains an imidazole group that, at physiologic pH, is an effective buffer of H + ions but is also an important molecular link between heme groups and the Hb chains. Elevated P o 2 results in greater quantities of O2 bound to Hb, which causes allosteric modifications of the Hb confirmation. These conformational changes impact the heme-linked histidine and reduce its ability to buffer H + ion with less H + buffering by Hb, there is more H + binding to and release of stored CO2.

In patients with end-stage lung disease, hypoxic pulmonary vasoconstriction is an important mechanism to divert pulmonary artery blood from poorly ventilated regions (see Chapters 4 and 6 ). Increasing arterial P o 2 inhibits hypoxic pulmonary vasoconstriction, thus pulmonary artery blood containing CO2 is diverted to less well-ventilated regions, and the efficiency of CO2 excretion is impaired. Finally, while most patients would easily compensate for the increased P co 2 with minimal increases in minute ventilation, this is not possible in many patients with end-stage lung disease.


Exchanging Oxygen and Carbon Dioxide

The primary function of the respiratory system is to take in oxygen and eliminate carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli. The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier between air and blood averages about 1 micron ( 1 /10,000 of a centimeter, or 0.000039 inch) in thickness. Oxygen passes quickly through this air-blood barrier into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled.

Oxygenated blood travels from the lungs through the pulmonary veins and into the left side of the heart, which pumps the blood to the rest of the body (see Function of the Heart). Oxygen-deficient, carbon dioxide-rich blood returns to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. Then the blood is pumped through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide.

The function of the respiratory system is to add oxygen to the blood and remove carbon dioxide. The microscopically thin walls of the alveoli allow inhaled oxygen to move quickly and easily from the lungs to the red blood cells in the surrounding capillaries. At the same time, carbon dioxide moves from the blood in the capillaries into the alveoli.

To support the absorption of oxygen and release of carbon dioxide, about 5 to 8 liters (about 1.3 to 2.1 gallons) of air per minute are brought in and out of the lungs, and about three tenths of a liter (about three tenths of a quart) of oxygen is transferred from the alveoli to the blood each minute, even when the person is at rest. At the same time, a similar volume of carbon dioxide moves from the blood to the alveoli and is exhaled. During exercise, it is possible to breathe in and out more than 100 liters (about 26 gallons) of air per minute and extract 3 liters (a little less than 1 gallon) of oxygen from this air per minute. The rate at which oxygen is used by the body is one measure of the rate of energy expended by the body. Breathing in and out is accomplished by respiratory muscles.

Gas Exchange Between Alveolar Spaces and Capillaries

The function of the respiratory system is to move two gases: oxygen and carbon dioxide. Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.


Xx.2 Pulmonary Ventilation (Breathing)

Breathing can be described as the movement of air into (inspiration/inhalation) and out of the lungs (expiration/exhalation). The major mechanism that drive breathing is differences between atmospheric pressure and the air pressure within the lungs.

Relationship Between Pressure and Volume

Inspiration (or inhalation) and expiration (or exhalation) are dependent on the differences in pressure between the atmosphere and the lungs. In a gas, pressure is a force created by the movement of gas molecules that are confined. For example, a certain number of gas molecules in a two-liter container has more room than the same number of gas molecules in a one-liter container (Figure). In this case, the force exerted by the movement of the gas molecules against the walls of the two-liter container is lower than the force exerted by the gas molecules in the one-liter container. Therefore, the pressure is lower in the two-liter container and higher in the one-liter container. At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules. Boyle’s law describes the relationship between volume and pressure in a gas at a constant temperature. Boyle discovered that the pressure of a gas is inversely proportional to its volume: If volume increases, pressure decreases. Likewise, if volume decreases, pressure increases. Pressure and volume are inversely related (P = k/V). Therefore, the pressure in the one-liter container (one-half the volume of the two-liter container) would be twice the pressure in the two-liter container. Boyle’s law is expressed by the following formula:

In this formula, P1 represents the initial pressure and V1 represents the initial volume, whereas the final pressure and volume are represented by P2 and V2, respectively. If the two- and one-liter containers were connected by a tube and the volume of one of the containers were changed, then the gases would move from higher pressure (lower volume) to lower pressure (higher volume).

In a gas, pressure increases as volume decreases.

Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body. Atmospheric pressure can be expressed in millimeters of mercury (mm Hg), which is similar to the phrase “inches of mercury” used to describe atmospheric pressure on weather reports. 760 mm Hg is the atmospheric pressure at sea level under highly specific parameters of latitude and temperature.

How Changes in Volume and Pressure are Accomplished During Breathing

In addition to the differences in pressures, breathing is also dependent upon the contraction and relaxation of muscle fibers of both the diaphragm and thorax. The lungs themselves are passive during breathing, meaning they are not involved in creating the movement that helps inspiration and expiration. Contraction and relaxation of the diaphragm and intercostal muscles (found between the ribs) cause most of the pressure changes that result in inspiration and expiration. These muscle movements and subsequent pressure changes cause air to either rush in or be forced out of the lungs.

During inspiration, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs. During expiration, the diaphragm and intercostals relax, causing the thorax and lungs to recoil. The air pressure within the lungs increases to above the pressure of the atmosphere, causing air to be forced out of the lungs.

Respiratory Rate

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 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.

Chapter Review

The process of breathing is driven by pressure differences between the lungs and the atmosphere. Atmospheric pressure is the force exerted by gases present in the atmosphere. Pressure is determined by the volume of the space occupied by a gas. Air flows when a pressure gradient is created, from a space of higher pressure to a space of lower pressure. Boyle’s law describes the relationship between volume and pressure. A gas is at lower pressure in a larger volume because the gas molecules have more space to in which to move. The same quantity of gas in a smaller volume results in gas molecules crowding together, producing increased pressure.

Pulmonary ventilation consists of the process of inspiration (or inhalation), where air enters the lungs, and expiration (or exhalation), where air leaves the lungs. During inspiration, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs. During expiration, the diaphragm and intercostals relax, causing the thorax and lungs to recoil. The air pressure within the lungs increases to above the pressure of the atmosphere, causing air to be forced out of the lungs.

Both respiratory rate and depth are controlled by the respiratory centers of the brain, which are stimulated by factors such as chemical and pH changes in the blood. A rise in carbon dioxide or a decline in oxygen levels in the blood stimulates an increase in respiratory rate and depth.


Exchanging Oxygen and Carbon Dioxide

The primary function of the respiratory system is to take in oxygen and eliminate carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli. The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier between air and blood averages about 1 micron ( 1 /10,000 of a centimeter, or 0.000039 inch) in thickness. Oxygen passes quickly through this air-blood barrier into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled.

Oxygenated blood travels from the lungs through the pulmonary veins and into the left side of the heart, which pumps the blood to the rest of the body (see Function of the Heart). Oxygen-deficient, carbon dioxide-rich blood returns to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. Then the blood is pumped through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide.

The function of the respiratory system is to add oxygen to the blood and remove carbon dioxide. The microscopically thin walls of the alveoli allow inhaled oxygen to move quickly and easily from the lungs to the red blood cells in the surrounding capillaries. At the same time, carbon dioxide moves from the blood in the capillaries into the alveoli.

To support the absorption of oxygen and release of carbon dioxide, about 5 to 8 liters (about 1.3 to 2.1 gallons) of air per minute are brought in and out of the lungs, and about three tenths of a liter (about three tenths of a quart) of oxygen is transferred from the alveoli to the blood each minute, even when the person is at rest. At the same time, a similar volume of carbon dioxide moves from the blood to the alveoli and is exhaled. During exercise, it is possible to breathe in and out more than 100 liters (about 26 gallons) of air per minute and extract 3 liters (a little less than 1 gallon) of oxygen from this air per minute. The rate at which oxygen is used by the body is one measure of the rate of energy expended by the body. Breathing in and out is accomplished by respiratory muscles.

Gas Exchange Between Alveolar Spaces and Capillaries

The function of the respiratory system is to move two gases: oxygen and carbon dioxide. Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.