Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid. Because oxygen is one of the majorsubstances required for chemical reactions in the cells, it is fortunate that the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical char-acteristics of hemoglobin, which is present in all red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. But if the oxygen con-centration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate con-centration. Thus, regulation of oxygen concentration in the tissues is vested principally in the chemical characteristics of hemoglobin itself. This regulation is called the oxygen-buffering function of hemoglobin.
Carbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of the oxidative reac-tions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, the mass action of the carbon dioxide itself would soon halt all energy-giving reactions of the cells. Fortu-nately, a higher than normal carbon dioxide concen-tration in the blood excites the respiratory center, causing a person to breathe rapidly and deeply. This increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concen-tration returns to normal.
Regulation of Arterial Blood Pressure. Several systems con-tribute to the regulation of arterial blood pressure. One of these, the baroreceptor system, is a simple and excellent example of a rapidly acting control mecha-nism. In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called baroreceptors, which are stimulated by stretch of thearterial wall. When the arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here these impulses inhibit the vasomotor center, which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and also dilation of the peripheral blood vessels, allowing increased blood flow through the vessels. Both of these effects decrease the arterial pressure back toward normal.
Conversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping, and raising arterial pressure back toward normal.
Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents
Table 1–1 lists the more important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are usually caused by illness.
Most important are the limits beyond which abnor-malities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellu-lar metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Another important factor is the potassium ion concentration, because whenever it decreases to less than one third normal, a person is likely to be paralyzed as a result of the nerves’ inability to carry signals. Alternatively, if the potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion con-centration falls below about one half of normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the sponta-neous generation of excess nerve impulses in the peripheral nerves. When the glucose concentration falls below one half of normal, a person frequently develops extreme mental irritability and sometimes even convulsions.
These examples should give one an appreciation for the extreme value and even the necessity of the vast numbers of control systems that keep the body operating in health; in the absence of any one of these controls, serious body malfunction or death can result.