Oxygen & Carbon Dioxide: Gas Exchange and Transport in Animals

Learning Objectives

  1. Apply the Law of Partial Pressures to predict direction of gas movement in solution
  2. Explain the functional adaptations of gas exchange surfaces in animals using Fick’s Law (surface area, distance, concentration gradients and perfusion)
  3. Compare and contrast the structure/function of gills, tracheae, and lungs
  4. Describe the reversible binding of O2 to hemoglobin (dissociation curves)
  5. Predict the effects of pH, temperature, and CO2 concentrations on hemoglobin affinity for O2

The information below was adapted from OpenStax Biology 39.0

Gas Exchange across Respiratory Surfaces

The information below was adapted from OpenStax Biology 39.2

The structure of any respiratory surface (lungs, gills, tracheae), maximizes its surface area to increase gas diffusion. Because of the enormous number of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large (75 m2). Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs. Respiratory surfaces are also extremely thin (typically only one cell thick), minimizing the distance gas must diffuse across the surface.

Basic Principles of Gas Exchange

Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen than that of oxygen-depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange during respiration.

Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The total pressure exerted by the mixture is the sum of the partial pressures of the components in the mixture. The rate of diffusion of a gas is proportional to its partial pressure within the total gas mixture.

Gas Pressure and Respiration

The respiratory process can be better understood by examining the properties of gases. Gases move freely, but gas particles are constantly hitting the walls of their vessel, thereby producing gas pressure.

Air is a mixture of gases, primarily nitrogen (N2; 78.6 percent), oxygen (O2; 20.9 percent), water vapor (H2O; 0.5 percent), and carbon dioxide (CO2; 0.04 percent). Each gas component of that mixture exerts a pressure. The pressure for an individual gas in the mixture is the partial pressure of that gas. Approximately 21 percent of atmospheric gas is oxygen. Carbon dioxide, however, is found in relatively small amounts, 0.04 percent. The partial pressure for oxygen is much greater than that of carbon dioxide. The partial pressure of any gas can be calculated by:

P = (Patm) (percent content in mixture).

Patm, the atmospheric pressure, is the sum of all of the partial pressures of the atmospheric gases added together,

Patm = PN2 +PO2+ PH2O+ PCO2= 760 mm Hg

The pressure of the atmosphere at sea level is 760 mm Hg. Therefore, the partial pressure of oxygen is:

PO2= (760mm Hg) (0.21) =160 mm Hg

and for carbon dioxide:

PCO2=(760 mm Hg) (0.0004) = 0.3 mm Hg.

At high altitudes, Patm decreases but concentration does not change; the partial pressure decrease is due to the reduction in Patm.

When the air mixture reaches the lung, it has been humidified. The pressure of the water vapor in the lung does not change the pressure of the air, but it must be included in the partial pressure equation. For this calculation, the water pressure (47 mm Hg) is subtracted from the atmospheric pressure:

760 mm Hg – 47 mm Hg =713 mm Hg

and the partial pressure of oxygen is:

(760 mm Hg – 47 mm Hg)0.21 = 150 mm Hg.

These pressures determine the gas exchange, or the flow of gas, in the system. Oxygen and carbon dioxide will flow according to their pressure gradient from high to low. Therefore, understanding the partial pressure of each gas will aid in understanding how gases move in the respiratory system.

Fick’s Law of Diffusion: the Rules of Gas Exchange

The rate of diffusion of a gas across a surface is controlled by the following:

  • k, the gas diffusion constant
  • A, the area for gas exchange
  • P2-P1, the difference in partial pressure of gas on either side of diffusion barrier
  • D, the distance across which the gas must diffuse (thickness of diffusion barrier)

These terms are related by the following equation:

Rate of diffusion = k x A x (P2-P1)/D

Gasses move “down” their partial pressure gradient (from areas of high concentration to areas of low concentration.

To sum up the discussion of partial pressures above:

Partial pressure=

  • Pressure of a particular gas in a mixture of gasses
  • Fractional component of gas x total air pressure in mm Hg
  • Gas moves down its partial pressure gradient (high conc to low conc)
  • The atmosphere is always composed of 21% oxygen. Partial pressure is the pressure of a particular gas in a mixture of gasses, and is calculated by multiplying the fractional composition of the particular gas by the total air pressure in mm Hg
The illustration shows the movement of deoxygenated air into the lungs, and oxygenated air out of the lungs. Also shown is the circulation of blood through the body. Circulation begins when deoxygenated blood in arteries leaves the right side of the heart and enters the lungs. Oxygenated blood exits the lungs, and enters the left side of the heart, which pumps it to the rest of the body via arteries. The partial pressure of oxygen in the atmosphere is 160 millimeters of mercury, and the partial pressure of carbon dioxide is 0.2 millimeters of mercury. The partial pressure of oxygen in the arteries is 100 millimeters of mercury, and the partial pressure of carbon dioxide is 40 millimeters of mercury. The partial pressure of oxygen in the veins is 40 millimeters of mercury, and the partial pressure of carbon dioxide is 46 millimeters of mercury.

The partial pressures of oxygen and carbon dioxide change as blood moves through the body.

 

In short, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.

Types of Respiratory Surfaces

The information below was adapted from OpenStax Biology 39.1

Direct Diffusion

For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist and gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which “breathe” through diffusion across the outer membrane. The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen.

The photo shows a worm with a flat, ribbon-like body, resting on sand. The worm is black with white spots.

This flatworm’s process of respiration works by diffusion across the outer membrane. (credit: Stephen Childs)

 

Skin and Gills

Earthworms and amphibians use their skin (integument) as a respiratory organ. A dense network of capillaries lies just below the skin and facilitates gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.

Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but at a lower concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water, the oxygen concentration is much smaller than that. Fish and many other aquatic organisms have evolved gills (outgrowths of the body used for gas exchange) to take up the dissolved oxygen from water. Gills are made of thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. Because of the constant flow of gas across the gas-exchange membrane and the constant partial pressure differences, gills are the most efficient  respiratory system in exchanging gases. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans.

The photo shows a carp with a wedge of skin at the back of the head cut away, revealing pink gills.

This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water. (credit: “Guitardude012″/Wikimedia Commons)

 

The folded surfaces of the gills provide a large surface area to ensure that the fish gets sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration). Similarly, carbon dioxide molecules in the blood diffuse from the blood (high concentration) to water (low concentration).

The illustration shows a fish, with a box indicating the location of the gills, behind the head. A close-up image shows the gills, each of which resembles a feathery worm. Two stacks of gills attach to a structure called a columnar gill arch, forming a tall V. Water travels in from the outside of the V, between each gill, then travels out of the top of the V. Veins travel into the gill from the base of the gill arch, and arteries travel back out on the opposite side. A close-up image of a single gill shows that water travels over the gill, passing over deoxygenated veins first, then over oxygenated arteries.

As water flows over the gills, oxygen is transferred to blood via the veins. (credit “fish”: modification of work by Duane Raver, NOAA)

 

Tracheal Systems

Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in oxygen transport. Insects have a highly specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. Because the circulatory system is not used primarily to move gasses, but instead the gas passes directly to the needed tissues, the tracheal system is the most direct and efficient respiratory system for getting oxygen to respiratory sites. The tubes in the tracheal system are made of a polymeric material called chitin.

Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body and regulating the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements.

The illustration shows the tracheal system of a bee. Openings called spiracles appear along the side of the body. Vertical tubes lead from the spiracles to a tube that runs along the top of the body from front to back.

Insects perform respiration via a tracheal system.

 

Mammalian Systems

In mammals, pulmonary ventilation occurs via inhalation (breathing) to bring air into the lungs (infoldings of the throat or body surface that enclose respiratory surfaces). During inhalation, air enters the body through the nasal cavity located just inside the nose. As air passes through the nasal cavity, the air is warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces of the mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages via mucus and cilia. The processes of warming, humidifying, and removing particles are important protective mechanisms that prevent damage to the trachea and lungs. Thus, inhalation serves several purposes in addition to bringing oxygen into the respiratory system.

The illustration shows the flow of air through the human respiratory system. The nasal cavity is a wide cavity above and behind the nostrils, and the pharynx is the passageway behind the mouth. The nasal cavity and pharynx join and enter the trachea through the larynx. The larynx is somewhat wider than the trachea and flat. The trachea has concentric, ring-like grooves, giving it a bumpy appearance. The trachea bifurcates into two primary bronchi, which are also grooved. The primary bronchi enter the lungs, and branch into secondary bronchi. The secondary bronchi in turn branch into many tertiary bronchi. The tertiary bronchi branch into bronchioles, which branch into terminal bronchioles. Each terminal bronchiole ends in an alveolar sac. Each alveolar sac contains many alveoli clustered together, like bunches of grapes. The alveolar duct is the air passage into the alveolar sac. The alveoli are hollow, and air empties into them. Pulmonary arteries bring deoxygenated blood to the alveolar sac (and thus appear blue), and pulmonary veins return oxygenated blood (and thus appear red) to the heart. Capillaries form a web around each alveolus. The diaphragm is a membrane that pushes up against the lungs.

Air enters the respiratory system through the nasal cavity and pharynx, and then passes through the trachea and into the bronchi, which bring air into the lungs. (credit: modification of work by NCI)

 

From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makes its way to the trachea. The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm long and 2 cm in diameter that sits in front of the esophagus and extends from the larynx into the chest cavity where it divides into the two primary bronchi at the midthorax. It is made of incomplete rings of hyaline cartilage and smooth muscle. The trachea is lined with mucus-producing goblet cells and ciliated epithelia. The cilia propel foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The smooth muscle can contract, decreasing the trachea’s diameter, which causes expired air to rush upwards from the lungs at a great force. The forced exhalation helps expel mucus when we cough. Smooth muscle can contract or relax, depending on stimuli from the external environment or the body’s nervous system.

The illustration shows the trachea, or windpipe. The larynx is a wide collar at the top of the trachea. At the bottom, the trachea bifurcates into smaller tubes, called primary bronchi, which enter the right and left lungs. Inside the lungs, the bronchi branch into primary and secondary bronchi, then into bronchioles.

The trachea and bronchi are made of incomplete rings of cartilage. (credit: modification of work by Gray’s Anatomy)

 

Lungs: Bronchi and Alveoli

The end of the trachea bifurcates (divides) to the right and left lungs. The lungs are not identical. The right lung is larger and contains three lobes, whereas the smaller left lung contains two lobes. The muscular diaphragm, which facilitates breathing, is inferior to (below) the lungs and marks the end of the thoracic cavity.

The illustration shows the trachea, which starts at the top of the neck and continues down into the chest, where it branches into the bronchi, which enter the lungs. The left lung has two lobes. The upper lobe is located in front of and above the lower lobe. The right lung has three lobes. The upper lobe is on the top, the lower lobe is on the bottom, and the middle lobe is sandwiched between them. The diaphragm presses against the bottom of the lungs and has the appearance of skin stretched over the top of a drum. Wide flaps of the diaphragm extend downward on the front left and right sides of the body. On the back, thin flaps of diaphragm stretch downward on either side of the spine.

The trachea bifurcates into the right and left bronchi in the lungs. The right lung is made of three lobes and is larger. To accommodate the heart, the left lung is smaller and has only two lobes.

 

In the lungs, air is diverted into smaller and smaller passages, or bronchi. Air enters the lungs through the two primary (main) bronchi (singular: bronchus). Each bronchus divides into secondary bronchi, then into tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as they split and spread through the lung. Like the trachea, the bronchi are made of cartilage and smooth muscle. At the bronchioles, the cartilage is replaced with elastic fibers. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles, depending on the nervous system’s cues. In humans, bronchioles with a diameter smaller than 0.5 mm are the respiratory bronchioles. They lack cartilage and therefore rely on inhaled air to support their shape. As the passageways decrease in diameter, the relative amount of smooth muscle increases.

The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles. In the acinar region, the alveolar ducts are attached to the end of each bronchiole. At the end of each duct are approximately 100 alveolar sacs, each containing 20 to 30 alveoli that are 200 to 300 microns in diameter. Gas exchange occurs only in alveoli. Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that look like tiny bubbles within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Because there are so many alveoli (~300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange. The surface area of alveoli in the lungs is approximately 75 m2. This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells.

The illustration shows a terminal bronchial tube branching into three alveolar ducts. At the end of each duct is an alveolar sac made up of 20 to 30 alveoli clustered together, like grapes. The airspace in the middle of the alveolar sac, called the atrium, is continuous with the air space inside the alveolus so that air can circulate from the atrium to the alveolus. Capillaries surround each alveolus, and this is where gas exchange occurs. A pulmonary artery (shown in blue) runs along the terminal bronchiole, bringing deoxygenated blood from the heart to the alveoli. A pulmonary vein (shown in red) running along the bronchiole brings oxygenated blood back to the heart. Small, flat mucous glands are associated with the outside of the bronchial tubes.

Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs. Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes. Air flows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries. Mucous glands secrete mucous into the airways, keeping them moist and flexible. (credit: modification of work by Mariana Ruiz Villareal)

 

Avian Lungs

The information below was adapted from OpenStax Biology 39.3

Birds face a unique challenge with respect to breathing: They fly. Flying consumes a great amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs and expelled during exhalation. The details of breathing between birds and mammals differ substantially.

In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs.

Decades of research by paleontologists have shown that birds evolved from therapods, meat-eating dinosaurs. In fact, fossil evidence shows that meat-eating dinosaurs that lived more than 100 million years ago had a similar flow-through respiratory system with lungs and air sacs. Archaeopteryx and Xiaotingia, for example, were flying dinosaurs and are believed to be early precursors of birds.

 Illustration A shows the direction of airflow in both inhalation and exhalation in birds. During inhalation, air passes from the beak down the trachea to the posterior air sac located behind the lungs. From the posterior air sac, air enters the lungs, and the anterior air sac in front of the lungs. Air from both air sacs also enters hollows in bones. During exhalation air from hollows in the bones enters the air sacs, then the lungs, then the trachea, where it exits through the beaks. Illustration B compares a dinosaur and a bird. Both have anterior air sacs in front of the lungs, and posterior air sacs behind them. The air sacs connect to hollow openings in bones.

(a) Birds have a flow-through respiratory system in which air flows unidirectionally from the posterior sacs into the lungs, then into the anterior air sacs. The air sacs connect to openings in hollow bones. (b) Dinosaurs, from which birds descended, have similar hollow bones and are believed to have had a similar respiratory system. (credit b: modification of work by Zina Deretsky, National Science Foundation)

 

Most of us consider that dinosaurs are extinct. However, modern birds are descendants of avian dinosaurs. The respiratory system of modern birds has been evolving for hundreds of millions of years.

The video below provides an overview of the human respiratory system:

Gas Transport in the Human Body

The information below was adapted from OpenStax Biology 39.4

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, about 98.5 percent, is bound to a protein called hemoglobin and carried to the tissues.

Hemoglobin

Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits: two alpha subunits and two beta subunits. 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.

Part a shows disc-shaped red blood cells. An arrow points from a red blood cell to the hemoglobin in part b. Hemoglobin is made up of coiled helices. The left, right, bottom, and top parts of the molecule are symmetrical. Four small heme groups are associated with hemoglobin. Oxygen is bound to the heme.

The protein inside (a) red blood cells that carries oxygen to cells and carbon dioxide to the lungs is (b) hemoglobin. Hemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. It is the iron in hemoglobin that gives blood its red color.

 

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. **Due to some conformation changes, the fourth oxygen can be said to be slightly more difficult to bind, but generally, cooperative binding increases the ability of oxygen to bind to hemoglobin and achieve greater saturation.**

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. As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.

The graph plots percent oxygen saturation of hemoglobin as a function of oxygen partial pressure. Oxygen saturation increases in an S-shaped curve, from 0 to 100 percent. The curve shifts to the left under conditions of low carbon dioxide, high pH, and low temperature, and to the right in conditions of high carbon dioxide, low pH, or high temperature.

The oxygen dissociation curve demonstrates that, as the partial pressure of oxygen increases, more oxygen binds hemoglobin. However, the affinity of hemoglobin for oxygen may shift to the left or the right depending on environmental conditions.

The kidneys are responsible for removing excess H+ ions from the blood. If the kidneys fail, what would happen to blood pH and to hemoglobin affinity for oxygen?

Factors That Affect Oxygen Binding

The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition to PO2, other environmental factors and diseases can affect oxygen carrying capacity and delivery.

Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity. When carbon dioxide is in the blood, it reacts with water to form bicarbonate (HCO3-)

and hydrogen ions (H+). As the level of carbon dioxide in the blood increases, more H+ is produced and the pH decreases. This increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve also results from an increase in body temperature. Increased temperature, such as from increased activity of skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced.

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.

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 (HCO−3and 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.

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.

The video below provides an overview of the transport of oxygen and carbon dioxide in the human bloodstream: