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