Oxygen Transport and Exchange
Oxygen transport and exchange provide necessary oxygen to the body's cells through the process of respiration. Oxygen enters the respiratory system through the mouth and nose, and travels through the pharynx, larynx, trachea, and bronchi during inspiration (inhalation) to reach the lungs. The bronchi enter into the lungs and branch to form bronchioles. The bronchioles further divide to form alveolar ducts. These ducts lead to tiny sacs called alveoli that are surrounded by pulmonary capillaries where oxygen exchange takes place. There are approximately 300 million alveoli that greatly increase the surface area of the lungs. Oxygen exchange occurs by the process of diffusion across the alveolar-capillary wall. During diffusion, oxygen travels down its concentration gradient into the blood and is exchanged for carbon dioxide. The partial pressure of oxygen within the body is denoted by pO2 and is measured in millimeters of mercury (mm Hg). The pO2 of oxygen in the alveoli is 105 mm Hg and the pO2 of deoxygenated blood entering the pulmonary capillaries at rest is 40 mm Hg. Because of this difference in pO2, oxygen is exchanged for carbon dioxide until the pO2 of the capillaries equals the pO2 of the alveoli. At equilibrium the pO2 of oxygen equals 105 mm Hg in the now oxygenated blood of the capillaries. Likewise, carbon dioxide will diffuse down its concentration gradient from the deoxygenated blood of the capillaries, with a pCO2 of 45 mm Hg, into the alveoli until it reaches equilibrium. Once equilibrium is reached, the pCO2 in the capillaries will be 40 mm Hg, equal to the pCO2 in the alveoli.
The rate of oxygen exchange is dependent upon the partial pressure difference, surface area of gas exchange, diffusion distance, and the rate and depth of respiration. In order for diffusion to take place, there must be a difference in the partial pressure of oxygen between the alveoli and the pulmonary capillaries. At higher altitudes, the partial pressure of atmospheric oxygen decreases, which decreases the pO2 in the alveoli and thus impairs the diffusion process. Oxygen exchange is largely dependent on the available surface area of the alveoli. For example, patients who have pulmonary disease have their alveolar-capillary membrane destroyed, thereby decreasing the surface area available for oxygen exchange. The effectiveness of oxygen exchange is in part due to the short distance that oxygen has to diffuse across the alveolar-capillary membrane. Additionally, the diameter of the capillaries is only wide enough to allow one red blood cell to pass through at a time. If the capillaries swell due to a buildup of fluid, the diffusion distance is increased and oxygen exchange is hindered. Oxygen exchange is also dependent on the amount of oxygen that is available for diffusion. The respiratory rate and depth can affect the amount of oxygen available for exchange.
Once the red blood cells have received the oxygen, the oxygenated blood travels from the pulmonary capillaries to the pulmonary veins, and to the heart where it is pumped through the systemic arteries to the systemic capillaries and then to tissues. Oxygen is exchanged from the oxygenated systemic capillaries to the deoxygenated tissues. The pO2 of the oxygenated systemic capillaries is 105 mm Hg and the pO2 of the tissues is 40 mm Hg; therefore, oxygen diffuses down its concentration gradient until it reaches equilibrium. During oxygen diffusion, carbon dioxide is also being exchanged from the tissues to the systemic capillaries until equilibrium is reached. Diffusion of carbon dioxide occurs because the pCO2 in the tissues is 45 mm Hg while the pCO2 in the systemic capillaries is 40 mm Hg. Once oxygen and carbon dioxide exchange is complete, the deoxygenated blood returns to the heart where it is pumped to the lungs as respiration continues.
About 98% of oxygen is transported by molecules of hemoglobin in red blood cells. One molecule of hemoglobin contains four heme groups and one globin protein. A heme group is an iron containing pigment that is able to bind one oxygen molecule. Only 2% of oxygen dissolves in the blood plasma because oxygen is relatively insoluble in water. The most important factor that determines the ability of oxygen to bind to hemoglobin is the partial pressure of oxygen in the blood. However, the oxygen bound to hemoglobin has no effect on the pO2 of the blood, but rather is determined by the amount of dissolved oxygen. Hemoglobin is able to bind dissolved oxygen from the blood and maintain a low partial pressure of oxygen. However, the higher the pO2, the greater affinity hemoglobin has for oxygen. When the pO2 is 105 mm Hg, hemoglobin becomes fully saturated. When the pO2 is 40 mm Hg, hemoglobin will contain about 75% of its total oxygen capacity. The hemoglobin molecule is said to be 75% saturated, meaning that three of the four heme groups will have a bound oxygen molecule. Therefore, under resting conditions, the tissues only utilize 25% of the available oxygen from hemoglobin.
Other factors that influence oxygen's ability to bind to hemoglobin include acidity, partial pressure of carbon dioxide, temperature, and 2,3-biphosphoglycerate (BPG). In acid environments, oxygen has a decreased ability to bind to hemoglobin; this is called the Bohr effect. Carbon dioxide can displace oxygen from hemoglobin; thus as pCO2 increases, hemoglobin has a decreased affinity for oxygen. Additionally, high pCO2 can increase the blood acidity due to the conversion of carbon dioxide to carbonic acid in blood. An increase in temperature can also increase the release of oxygen from hemoglobin. BPG is a byproduct of glycolysis found in red blood cells that decreases hemoglobin's ability to bind oxygen.
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