Gas exchange

Definition

Gas exchange is the process by which oxygen is transferred from the atmosphere to bodily tissues for use in metabolism ; and the gas produced by metabolism, carbon dioxide, is transferred from tissues to the atmosphere.

Description and function

Overview of gas exchange

The process of gas exchange has several steps. The following is a summary of the steps:

• ventilation (breathing)
• interchange of CO₂ and O₂ between air in the lungs' alveoli and blood in lung capillaries by diffusion
• transport of CO₂ and O₂ through the bloodstream
• interchange of CO₂ and O₂ between blood in lung capillaries and alveolar air by diffusion
• use of O₂ and production of CO₂ by cells through metabolism

Ventilation

The transfer of oxygen from the atmosphere to the tissues starts with the inspiration of air into the lungs . The lungs consist mainly of tiny air-containing alveolar sacs. The alveoli are small hollow sacs connecting to the larger terminal bronchioles of the airways. The air adjacent to the surfaces of the alveolar wall are lined by a single cell layer of flat epithelial cells called type I alveolar cells. In between these type I cells are thicker and more rounded type II alveolar cells, which produce a detergent-like fluid. In the alveolar walls, the fluid and connective tissue fills the interstitial space and is interspersed with capillaries. In some places the interstitial space is nonexistent, and the epithelial cell membranes are in direct contact with the capillaries. The blood in the capillaries is separated from the air molecules by a single layer of flat epithelial cells. The surface area in a single alveolus, because of the undulating terrain of type I and II epithelial cells, is roughly the size of a medium-sized room. There are around 300 million alveoli in the adult male. Therefore, there is a large amount of surface area placing air and the blood stream in close proximity. This trait is needed for gas exchange to easily occur. The respiratory system also needs a continual supply of fresh air. This air is supplied to the lungs through the nose and mouth, trachea, and bronchi. Ventilation is the interchange of air between the atmosphere and the alveoli by bulk flow. Bulk flow is the movement of air from a region of high pressure to one of low pressure.

The physics of gas exchange

In order to understand why oxygen and carbon dioxide are able to diffuse from their respective areas of high concentration, Dalton's Law must first be presented. It states that in a mixture of gases, the pressure exerted by each gas is independent of the pressure exerted by the others. It is why carbon dioxide can move out of the bloodstream while oxygen is diffusing into the blood stream. The concentration of oxygen (O₂) will not affect the activity of carbon dioxide (CO₂).

Henry's law explains why CO₂ can move from the blood stream into the airspace of the lung, and O₂ can move from that airspace into the bloodstream. It states that the amount of gas dissolved will be directly proportional to the partial pressure of the gas with which the liquid is in equilibrium. At equilibrium, the partial pressures of the gas molecules in liquid and gaseous phases must be identical. Elemental gas can move from air into or out of a liquid where there is a pressure difference.

Interchange

During inspiration, the partial pressure of oxygen (PO₂) in the lung (105 mm Hg) is higher than that in the arteries of the alveoli (40 mmHg). This pressure difference allows O₂ to transfer into the blood stream. The partial pressure of carbon dioxide (PCO₂) in the lung (40 mmHg) is less than the arterial partial pressure of the alveoli (46 mmHg). This pressure difference allows carbon dioxide to diffuse into the lung and eventually into the atmosphere. The ventilation of the lungs allows for the continual renewal of imbalance and need for breathing and metabolism to continue.

Transport

The circulatory system continually supplies blood in need of oxygenation and the ventilation of CO₂ to the lungs. It arrives in the lungs with a PO₂ of 40 mmHg and a CO₂ of 46 mmHg and leaves the lungs with a PO₂ of 100 mmHg and a CO₂ of 40 mmHg. From the lungs, the oxygenated blood travels through the pulmonary veins to the left side of the heart and into the systemic arteries. The blood eventually flows to the tissue capillaries where another pressure difference occurs.

Use and transport

At the capillaries the PO₂ is 100 mmHg and the PCO₂ is 40 mmHg. In the tissues the PO₂ is less than 40 mmHg and the PCO₂ is greater than 46 mmHg. The O₂ in the capillaries diffuses into the tissue and the CO₂ produced by metabolism comes into the capillaries. Deoxygenated blood travels from the tissues to the right side of the heart via the systemic veins and then returns to the lungs for more O₂ through the pulmonary arteries, where the process begins again.

Hemoglobin

Each liter of oxygenated blood normally contains 200 ml of pure gaseous O₂ at atmospheric pressure (760 mmHg). It exists in the blood stream dissolved in the plasma and erythrocyte water or combined with hemoglobin molecules in the erythrocytes.

Oxygen is relatively insoluble in water and only 3 ml will be dissolved in one liter of blood at the arterial partial pressure of 100 mmHg. It is consistent with Henry's law because the amount of O₂ dissolved in the blood is directly proportional to the partial pressure of the blood. This leaves another 197 ml of O₂ in need of a way to be dissolved in the blood stream.

The hemoglobin molecule is a protein with four subunits. Each subunit is made up of a heme (a molecular group) with a polypeptide attached. Heme contains one

KEY TERMS

Chemoreceptor —A type of cell activated by a change in its chemical balance that results in a nerve impulse.

Metabolism —The sum of physical and chemical changes in the tissue including anabolism, catabolism, energy production and synthesis of mole-cules.

Ventilation —Movement of gases into and out of the lungs; tidal exchange between the lungs and the atmosphere.

atom of Iron (Fe) to which one O₂ molecule can bind. This means that every hemoglobin molecule can bind four O₂ molecules. The four polypeptides in hemoglobin are called globin. With O₂ attached to the molecule, it is known as oxyhemoglobin (HbO₂) and without it is known as deoxyhemoglobin (Hb). The bonding of O₂ to hemoglobin allows a full 200 ml of O₂ to dissolve completely in the blood. In reference to Dalton's law it allows for a greater difference in the concentration of O₂ between the O₂ in the lung and the bloodstream, and the blood. The O₂ tied up by the hemoglobin cannot be considered when looking at concentration differences. Conversely the concentration difference between the tissues and the capillaries at the tissue level causes the O₂ to dissociate from the hemoglobin, leaving the O₂ free to diffuse into tissue and the hemoglobin free to bond to the carbon dioxide leaving the tissue. By tying up carbon dioxide with hemoglobin, even more CO₂ can be carried to the lungs by the blood.

CO₂ is far more soluble in water than O₂. Only ten percent of the carbon dioxide that enters the blood is dissolved in water. Thirty percent of the carbon dioxide bonds with hemoglobin to form carbaminohemoglobin. Sixty percent of the carbon dioxide is converted to bicarbonate.

Deoxyhemoglobin serves as a buffer in the bloodstream as well. It has an affinity for acidic hydrogen atoms left in the bloodstream by the formation of bicarbonate. This allows the blood to maintain a pH of around 7.4 and explains why even venous blood maintains this pH.

Regulation of gas exchange

The exchange of gases in the body will occur with the respective differences of partial pressure between the blood and tissues, and the lung and alveoli. Respiratory rate must be controlled in order to suit the O₂ needs of the body and ensure a balanced supply of O₂ to the tissues. Respiratory rate is controlled by the peripheral chemoreceptors located high in the neck, where the common carotid arteries split, as well as on the arch of the aorta. They are called carotid bodies and aortic bodies respectively. These chemoreceptors are stimulated by the minor elevation of PCO₂ levels, causing an increase in ventilation. Chemoreceptor response to the PCO₂ level in the blood is the primary and most immediate indicator of gas deficiency and surplus in the blood stream. Elevated levels of PCO₂ normally cause an increase in breathing, and lower levels normally cause a decrease. This response system allows for a balance of gases available for use in metabolism.

Conversely, a decrease in arterial PO₂ levels and an increase in blood acidity do not affect a minimal increase in ventilation stimulated by chemoreceptors until PO₂ goes below 60 mmHg. Oxygen transport at the tissues will not be reduced until the blood PO₂ reaches 60 mmHg. Thus, the chemoreceptors are not triggered for lack of PO₂ in normal circumstances. If there is lung disease, or in high altitudes, these receptors can be stimulated and will affect an increase in the respiratory rate.

Role in human health

Gas exchange provides a needed fuel (O₂) for metabolism to occur and a means to expel the gaseous byproduct (CO₂) of metabolism from the body. Without gas exchange the body would not function. The hindrance of gas exchange by disease, disorder, or chemicals can slow body functions and even cause death.

Common diseases and disorders

The interference of gas exchange occurs when the function of a number of different organs and tissues is impaired. The most common form of impairment to gas exchange is hypoxia, which is the lack of oxygen in the tissues. Hypoxia can be caused by hypoventilation, diffusion impairment, shunt, and or ventilation-perfusion (the rate of ventilation relative to CO₂ production)inequality.

Hypoventilation is the reduced alveolar ventilation in comparison to the metabolic CO₂ production in which the PCO₂ levels increase above normal. It is caused by disease in the lung, abnormalities in the thoracic cage, or deficits in the respiratory control pathway from the medulla to the chemoreceptors.

Diffusion impairment occurs when there is a decrease in the surface area or thickening of the alveolar membranes. Diseases or disorders in the lung can cause this impairment.

Asthma is an intermittent disease characterized by a chronic inflammation of the airway, which causes smooth muscle contraction in the airway.

Chronic obstructive pulmonary disease (COPD) refers to emphysema , chronic bronchitis or a combination of the two. Cigarette smoking is a major cause of this and the following diseases associated to COPD. Chronic bronchitis is characterized by excessive mucus production in the bronchi and chronic inflammatory changes in the small airways. Emphysema is a major cause of hypoxia and is characterized by the destruction of the alveolar walls, and the atrophy and collapse of the lower airways. Pneumonia is normally caused by bacterial or viral infection . The alveolar spaces fill with mucus, inflammatory cells, and fibrin.

Other disorders that impact gas exchange are hyperventilation, in which ventilation is increased relative to the metabolic CO₂ production, and in which the PCO₂ drops below normal levels; and the effects of high altitude (called altitude sickness), in which the lack of O₂ in the atmosphere causes the body to compensate for that deficiency.

Resources

BOOKS

Bullock, John et. al. National Medical Series for Independent Study: Physiology, third edition. Philadelphia: Williams & Wilkins, 1995.

Vander, Arthur et. al. Human Physiology-the Mechanisms of Body Function, eighth edition. New York: McGraw-Hill, 2001.

ORGANIZATIONS

American Lung Association 1740 Broadway New York, NY 10019. (212) 315-8700. <http://www.lungusa.org>.

OTHER

"Gas Exchange I." John Carroll University. Accessed August 8, 2001. <http://www.jcu.edu/biology/resp1.htm>.

Sally C. McFarlane-Parrott

Gas Exchange

Definition

Gas exchange is the process by which oxygen is transferred from the atmosphere to bodily tissues for use in metabolism; and the gas produced by metabolism, carbon dioxide, is transferred from tissues to the atmosphere.

Description and function

Overview of gas exchange

The process of gas exchange has several steps. The following is a summary of the steps:

• ventilation (breathing)
• interchange of CO₂ and O₂ between air in the lungs' alveoli and blood in lung capillaries by diffusion
• transport of CO₂ and O₂ through the bloodstream
• interchange of CO₂ and O₂ between blood in lung capillaries and alveolar air by diffusion
• use of O₂ and production of CO₂ by cells through metabolism

Ventilation

The transfer of oxygen from the atmosphere to the tissues starts with the inspiration of air into the lungs. The lungs consist mainly of tiny air-containing alveolar sacs. The alveoli are small hollow sacs connecting to the larger terminal bronchioles of the airways. The air adjacent to the surfaces of the alveolar wall are lined by a single cell layer of flat epithelial cells called type I alveolar cells. In between these type I cells are thicker and more rounded type II alveolar cells, which produce a detergent-like fluid. In the alveolar walls, the fluid and connective tissue fills the interstitial space and is interspersed with capillaries. In some places the interstitial space is nonexistent, and the epithelial cell membranes are in direct contact with the capillaries. The blood in the capillaries is separated from the air molecules by a single layer of flat epithelial cells. The surface area in a single alveolus, because of the undulating terrain of type I and II epithelial cells, is roughly the size of a medium-sized room. There are around 300 million alveoli in the adult male. Therefore, there is a large amount of surface area placing air and the blood stream in close proximity. This trait is needed for gas exchange to easily occur. The respiratory system also needs a continual supply of fresh air. This air is supplied to the lungs through the nose and mouth, trachea, and bronchi. Ventilation is the interchange of air between the atmosphere and the alveoli by bulk flow. Bulk flow is the movement of air from a region of high pressure to one of low pressure.

The physics of gas exchange

In order to understand why oxygen and carbon dioxide are able to diffuse from their respective areas of high concentration, Dalton's law must first be presented. It states that in a mixture of gases, the pressure exerted by each gas is independent of the pressure exerted by the others. It is why carbon dioxide can move out of the bloodstream while oxygen is diffusing into the blood stream. The concentration of oxygen (O₂) will not affect the activity of carbon dioxide (CO₂).

Henry's law explains why CO₂ can move from the blood stream into the airspace of the lung and O₂ can move from that airspace into the bloodstream. It states that the amount of gas dissolved will be directly proportional to the partial pressure of the gas with which the liquid is in equilibrium. At equilibrium, the partial pressures of the gas molecules in liquid and gaseous phases must be identical. Elemental gas can move from air into or out of a liquid where there is a pressure difference.

Interchange

During inspiration, the partial pressure of oxygen (PO₂) in the lung (105 mmHg) is higher than that in the arteries of the alveoli (40 mmHg). This pressure difference allows O₂ to transfer into the blood stream. The partial pressure of carbon dioxide (PCO₂) in the lung (40 mmHg) is less than the arterial partial pressure of the alveoli (46 mmHg). This pressure difference allows carbon dioxide to diffuse into the lung and eventually into the atmosphere. The ventilation of the lungs allows for the continual renewal of imbalance and need for breathing and metabolism to continue.

Transport

The circulatory system continually supplies blood in need of oxygenation and the ventilation of CO₂ to the lungs. It arrives in the lungs with a PO₂ of 40 mmHg and a CO₂ of 46 mmHg and leaves the lungs with a PO₂ of 100 mmHg and a CO₂ of 40 mmHg. From the lungs, the oxygenated blood travels through the pulmonary veins to the left side of the heart and into the systemic arteries. The blood eventually flows to the tissue capillaries where another pressure difference occurs.

Use and transport

At the capillaries the PO₂ is 100 mmHg and the PCO₂ is 40 mmHg. In the tissues the PO₂ is less than 40 mmHg and the PCO₂ is greater than 46 mmHg. The O₂ in the capillaries diffuses into the tissue and the CO₂ produced by metabolism comes into the capillaries. Deoxygenated blood travels from the tissues to the right side of the heart via the systemic veins and then returns to the lungs for more O₂ through the pulmonary arteries, where the process begins again.

Hemoglobin

Each liter of oxygenated blood normally contains 200 ml of pure gaseous O₂ at atmospheric pressure (760 mmHg). It exists in the blood stream dissolved in the plasma and erythrocyte water or combined with hemoglobin molecules in the erythrocytes.

Oxygen is relatively insoluble in water and only 3 ml will be dissolved in one liter of blood at the arterial partial pressure of 100 mmHg. It is consistent with Henry's law because the amount of O₂ dissolved in the blood is directly proportional to the partial pressure of the blood. This leaves another 197 ml of O₂ in need of a way to be dissolved in the blood stream.

The hemoglobin molecule is a protein with four subunits. Each subunit is made up of a heme (a molecular group) with a polypeptide attached. Heme contains one atom of iron (Fe) to which one O₂ molecule can bind. This means that every hemoglobin molecule can bind four O₂ molecules. The four polypeptides in hemoglobin are called globin. With O₂ attached to the molecule, it is known as oxyhemoglobin (HbO₂) and without it is known as deoxyhemoglobin (Hb). The bonding of O₂ to hemoglobin allows a full 200 ml of O₂ to dissolve completely in the blood. In reference to Dalton's law it allows for a greater difference in the concentration of O₂ between the O₂ in the lung and the bloodstream, and the blood. The O₂ tied up by the hemoglobin cannot be considered when looking at concentration differences. Conversely the concentration difference between the tissues and the capillaries at the tissue level causes the O₂ to dissociate from the hemoglobin, leaving the O₂ free to diffuse into tissue and the hemoglobin free to bond to the carbon dioxide leaving the tissue. By tying up carbon dioxide with hemoglobin, even more CO₂ can be carried to the lungs by the blood.

CO₂ is far more soluble in water than O₂. Only ten percent of the carbon dioxide that enters the blood is dissolved in water. Thirty percent of the carbon dioxide bonds with hemoglobin to form carbamino-hemoglobin. Sixty percent of the carbon dioxide is converted to bicarbonate.

Deoxyhemoglobin serves as a buffer in the bloodstream as well. It has an affinity for acidic hydrogen atoms left in the bloodstream by the formation of bicarbonate. This allows the blood to maintain a pH of around 7.4 and explains why even venous blood maintains this pH.

Regulation of gas exchange

The exchange of gases in the body will occur with the respective differences of partial pressure between the blood and tissues, and the lung and alveoli. Respiratory rate must be controlled in order to suit the O₂ needs of the body and ensure a balanced supply of O₂ to the tissues. Respiratory rate is controlled by the peripheral chemoreceptors located high in the neck, where the common carotid arteries split, as well as on the arch of the aorta. They are called carotid bodies and aortic bodies respectively. These chemoreceptors are stimulated by the minor elevation of PCO₂ levels, causing an increase in ventilation. Chemoreceptor response to the PCO₂ level in the blood is the primary and most immediate indicator of gas deficiency and surplus in the blood stream. Elevated levels of PCO₂ normally cause an increase in breathing, and lower levels normally cause a decrease. This response system allows for a balance of gases available for use in metabolism.

Conversely, a decrease in arterial PO₂ levels and an increase in blood acidity do not affect a minimal increase in ventilation stimulated by chemoreceptors until PO₂ goes below 60 mmHg. Oxygen transport at the tissues will not be reduced until the blood PO₂ reaches 60 mmHg. Thus, the chemoreceptors are not triggered for lack of PO₂ in normal circumstances. If there is lung disease, or in high altitudes, these receptors can be stimulated and will affect an increase in the respiratory rate.

Role in human health

Gas exchange provides a needed fuel (O₂) for metabolism to occur and a means to expel the gaseous byproduct (CO₂) of metabolism from the body. Without gas exchange the body would not function. The hindrance of gas exchange by disease, disorder, or chemicals can slow body functions and even cause death.

Common diseases and disorders

The interference of gas exchange occurs when the function of a number of different organs and tissues is impaired. The most common form of impairment to gas exchange is hypoxia, which is the lack of oxygen in the tissues. Hypoxia can be caused by hypoventilation, diffusion impairment, shunt, and or ventilation-perfusion (the rate of ventilation relative to CO₂ production) inequality.

Hypoventilation is the reduced alveolar ventilation in comparison to the metabolic CO₂ production in which the PCO₂ levels increase above normal. It is caused by disease in the lung, abnormalities in the thoracic cage, or deficits in the respiratory control pathway from the medulla to the chemoreceptors.

Diffusion impairment occurs when there is a decrease in the surface area or thickening of the alveolar membranes. Diseases or disorders in the lung can cause this impairment.

Asthma is an intermittent disease characterized by a chronic inflammation of the airway, which causes smooth muscle contraction in the airway.

Chronic obstructive pulmonary disease (COPD) refers to emphysema, chronic bronchitis, or a combination of the two. Cigarette smoking is a major cause of this and the following diseases associated to COPD. Chronic bronchitis is characterized by excessive mucus production in the bronchi and chronic inflammatory changes in the small airways. Emphysema is a major cause of hypoxia and is characterized by the destruction of the alveolar walls, and the atrophy and collapse of the lower airways. Pneumonia is normally caused by bacterial or viral infection. The alveolar spaces fill with mucus, inflammatory cells, and fibrin.

Other disorders that impact gas exchange are hyperventilation, in which ventilation is increased relative to the metabolic CO₂ production, and in which the PCO₂ drops below normal levels; and the effects of high altitude (called altitude sickness), in which the lack of O₂ in the atmosphere causes the body to compensate for that deficiency.

KEY TERMS

Chemoreceptor— A type of cell activated by a change in its chemical balance that results in a nerve impulse.

Metabolism— The sum of physical and chemical changes in the tissue including anabolism, catabolism, energy production and synthesis of molecules.

Ventilation— Movement of gases into and out of the lungs; tidal exchange between the lungs and the atmosphere.

Resources

BOOKS

Bullock, John, et. al. National Medical Series for Independent Study: Physiology, 3rd edition. Philadelphia: Williams & Wilkins, 1995.

Vander, Arthur, et. al. Human Physiology-the Mechanisms of Body Function, 8th edition. New York: McGraw-Hill, 2001.

ORGANIZATIONS

American Lung Association. 1740 Broadway New York, NY 10019. (212) 315-8700. 〈http://www.lungusa.org〉.

OTHER

"Gas Exchange I." John Carroll University. Accessed August 8, 2001. 〈http://www.jcu.edu/biology/resp1.htm〉.

Gas Exchange

Gas exchange is the process by which oxygen and carbon dioxide (the respiratory gases) move in opposite directions across an organism's respiratory membranes, between the air or water of the external environment and the body fluids of the internal environment. Oxygen is needed by cells to extract energy from organic molecules, such as sugars, fatty acids, and amino acids. Carbon dioxide is produced in the process and must be disposed.

Principles of Gas Exchange

The random movement of molecules is called diffusion. Although individual molecules move randomly, a substance can have directed movement, or net diffusion. The net diffusion of a substance occurs because of a difference in its concentration, or gradient , along its course. Within an animal's body as oxygen is used up and carbon dioxide produced, the concentration gradient of the two gases provides the direction for their diffusion. For example, as air or water nears the respiratory membrane, the oxygen concentration on the outside of the membrane is higher than on the internal side so oxygen diffuses inward. The concentration gradient for carbon dioxide is in the opposite direction, and so net diffusion of carbon dioxide keeps it diffusing out of the body.

The solubility of the respiratory gases in water is low, and the solubility of oxygen is only about one-twentieth that of carbon dioxide. Special transport molecules within body fluids increase the oxygen content by holding oxygen molecules within circulating fluids. These molecules are called respiratory pigments and include hemoglobin , which is red, and hemocyanin, which is blue. These molecules combine with oxygen at the respiratory membrane, where oxygen concentrations are relatively high and easily release the oxygen in deeper tissues, which are low in oxygen.

Variety in the Animal Kingdom

Animals with small bodies exchange respiratory gases sufficiently through the body surface without specialized respiratory membranes. Even some vertebrates, such as small, slender salamanders, exchange respiratory gases solely across the skin, which is richly supplied with blood vessels. Larger animals require an extended surface for gas exchange. This specialized respiratory membrane is often folded to increase its surface area without occupying excessive space. For most fish, many aquatic invertebrates, and some terrestrial invertebrates the specialized respiratory organs are the gills. In crustaceans, gills are often found where the legs attach to the body; moving the legs sweeps water across the gill surfaces. In fish and some mollusks, gills are ventilated by muscular contractions that pump water across the respiratory surface.

Terrestrial animals must protect their respiratory membranes from drying out. Many spiders have book lungs, which are specialized, leaf-shaped, inward folds of the cuticle, surrounded by an air chamber that can be ventilated with muscular contractions. In larger terrestrial insects, the respiratory organs are inward, branching, tubular extensions of the body wall called tracheae. The system is so extensive that most cells are in close proximity to a tracheal branch and the tissues do not depend on blood circulation for gas transport.

Terrestrial vertebrates generally have lungs. The surface area for gas exchange is correlated with metabolic rate. Endotherms, such as birds and mammals, have a high metabolic rate and a correspondingly high respiratory surface area. Birds have one-way flow through their lungs, enabled by a complex system of air-storing sacs. Since fresh air is always flowing through the lung, the oxygen concentration can be maintained at a constant, high level.

Mammals, reptiles, and amphibians have saclike lungs with tidal (twoway) air flow. This results in residual air remaining in the lungs, reducing the concentration of available oxygen in comparison to bird lungs. Reptile lungs have fewer air sacs and less respiratory surface area than mammals, and amphibian lungs have less surface area than reptilian lungs.

see also Blood; Amphibian; Arthropod; Bird; Circulatory Systems; Insect; Krebs Cycle; Mammal; Oxidative Phosphorylation; Reptile; Respiration

Margaret G. Ott

Bibliography

Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology. Philadelphia, PA: W. B Saunders, Co., 2000.

Hickman, Cleveland P. Biology of the Invertebrates. St. Louis, MO: C. V. Mosby Co., 1973.

Schmidt-Nielsen, Knut. Animal Physiology Adaptation and Environment. New York: Cambridge University Press, 1997.

gas exchange In biology, the uptake and output of gases, especially oxygen and carbon dioxide, by living organisms. In animals and other organisms that obtain their energy by aerobic respiration, gas exchange involves the uptake of oxygen and the output of carbon dioxide. In plants, algae and bacteria that carry out photosynthesis, the opposite may occur, with a carbon dioxide uptake and oxygen output. At the cellular level, gas exchange takes place by diffusion across cell membranes in solution. See also breathing; circulatory system; respiration; respiratory system; ventilation

gaseous exchange The transfer of gases between an organism and the external environment in either direction. It occurs by diffusion across a concentration gradient and includes the exchange of oxygen and carbon dioxide in respiration and photosynthesis. Successful gaseous exchange requires a large surface area, as is provided by the alveoli of the lungs and the leaves of plants.