Blood Functions. Respiration

In terms of immediate urgency, the respiratory function of the blood is vital. A continuous supply of oxygen is required by living cells, in particularthose of the brain since deprivation is followed in minutes by unconsciousness and death. A normal male at rest uses about 250 millilitres of oxygen per minute, a requirement increased manyfold during vigorous exertion. All of this oxygen is transported by the blood, most of it bound to the hemoglobin of the red cells. The minute blood vessels of the lungs bring the blood into close apposition with the pulmonary air spaces (alveoli), where the pressure of oxygen is relatively high. Oxygen diffuses through the plasma and into the red cell, combining with hemoglobin, which is about 95 percent saturated with oxygen on leaving the lungs. One gram of hemoglobin can bind 1.35 millilitres of oxygen, and about 50 times as much oxygen is combined with hemoglobin as is dissolved in the plasma. In tissues where the oxygen tension is relatively low, hemoglobin releases the bound oxygen.

The two main regulators of oxygen uptake and delivery are the pH of tissues and the content of 2,3-diphosphoglycerate (2,3-DPG) in red cells. The effect of pH on the ability of hemoglobin to bind oxygen is called the Bohr effect: when pH is low, hemoglobin binds oxygen less strongly, and when pH is high (as in the lungs), hemoglobin binds more tightly to oxygen. The Bohr effect is due to changes in the shape of the hemoglobin molecule as the pH of its environment changes. The oxygen affinity of hemoglobin is also regulated by 2,3-DPG, a simple molecule produced by the red cell when it metabolizes glucose. The effect of 2,3-DPG is to reduce the oxygen affinity of hemoglobin. When the availability of oxygen to tissues is reduced, the red cell responds by synthesizing more 2,3-DPG, a process that occurs over a period of hours to days. By contrast, tissue pH mediates minute-to-minute changes in oxygen handling.

Carbon dioxide, a waste product of cellular metabolism, is found in relatively high concentration in the tissues. It diffuses into the blood and is carried to the lungs to be eliminated with the expired air. Carbon dioxide is much more soluble than oxygen and readilydiffuses into red cells. It reacts with water to form carbonic acid, a weak acid that at the alkaline pH of the blood appears principally as bicarbonate.

The tension of carbon dioxide in the arterial blood is regulated with extraordinary precision through a sensing mechanism in the brain that controls the respiratory movements. Carbon dioxide is an acidic substance, and an increase in its concentration tends to lower the pH of the blood (i.e., becoming more acidic). This may be averted by the stimulus that causes increased depth and rate of breathing, a response that accelerates the loss of carbon dioxide.It is the tension of carbon dioxide, and not of oxygen, in the arterial blood that normally controls breathing. Inability to hold one's breath for more than a minute or so is the result of the rising tension of carbon dioxide, which produces the irresistible stimulus to breathe. Respiratory movements that ventilate the lungs sufficiently to maintain a normal tension of carbon dioxide are, under normal conditions, adequate to keep the blood fully oxygenated. Control of respiration is effective, therefore, in regulating the uptake of oxygen and disposal of carbon dioxide and in maintaining the constancy of blood pH.