METABOLISM AND HOMEOSTASIS
Metabolism is a collective noun; it is all of the chem-ical reactions and physical processes that take place within the body. Metabolism includes growing, repair-ing, reacting, and reproducing—all the characteristics of life. The pumping of the heart, the digestion of food in the stomach, the diffusion of gases in the lungs and tissues, and the production of energy in each cell of the body are just a few of the thousands of aspects of metabolism. Metabolism comes from a Greek word meaning “change,” and the body is always changing in visible ways (walking down the street), microscopic ways (cells dividing in the skin to produce new epider-mis), and submicroscopic or molecular ways (RNA and enzymes constructing new proteins). A related concept, metabolic rate, is most often used to mean the speed at which the body produces energy and heat, or, put another way, energy production per unit of time, such as 24 hours. Metabolic rate, therefore, is one aspect of metabolism.
A person who is in good health may be said to be in a state of homeostasis. Homeostasis reflects the abil-ity of the body to maintain a relatively stable metabo-lism and to function normally despite many constant changes. The changes that are part of normal metab-olism may be internal or external, and the body must respond appropriately.
Eating breakfast, for example, brings about an internal change. Suddenly there is food in the stom-ach, and something must be done with it. What hap-pens? The food is digested or broken down into simple chemicals that the body can use. The protein in a hard-boiled egg is digested into amino acids, its basic chemical building blocks; these amino acids can then be used by the cells of the body to produce their own specialized proteins.
An example of an external change is a rise in envi-ronmental temperature. On a hot day, the body tem-perature would also tend to rise. However, body temperature must be kept within its normal range of about 978 to 998F (368 to 388C) in order to support normal functioning. What happens? One of the body’s responses to the external temperature rise is to increase sweating so that excess body heat can be lost by the evaporation of sweat on the surface of the skin. This response, however, may bring about an undesir-able internal change, dehydration. What happens? As body water decreases, we feel the sensation of thirst and drink fluids to replace the water lost in sweating. Notice that when certain body responses occur, they reverse the event that triggered them. In the preced-ing example a rising body temperature stimulates increased sweating, which lowers body temperature, which in turn decreases sweating. Unnecessary sweat-ing that would be wasteful of water is prevented. This is an example of a negative feedback mechanism, in which the body’s response reverses the stimulus (in effect, turning it off for a while) and keeps some aspect of the body metabolism within its normal range.
Look at Fig. 1–3 for another negative feedback mechanism, one in which the hormone thyroxine reg-ulates the metabolic rate of the body. As metabolic rate decreases, the hypothalamus (part of the brain) and pituitary gland detect this decrease and secrete hormones to stimulate the thyroid gland (on the front of the neck just below the larynx) to secrete the hor-mone thyroxine. Thyroxine stimulates the cellular enzyme systems that produce energy from food, which increases the metabolic rate. The rise in energy and heat production is detected by the brain and pituitary gland. They then decrease secretion of their hor-mones, which in turn inhibits any further secretion of thyroxine until the metabolic rate decreases again. Metabolic rate does rise and fall, but is kept within normal limits.
Figure 1–3. Feedback mechanisms. (A) The negative feedback mechanism of regulation of metabolic rate by thyroxine. (B) The positive feedback mechanism triggered by a fever. See text for description.
QUESTION: For each mechanism, where is the source of the “brake” or inhibition?
You may be wondering if there is such a thing as a positive feedback mechanism. There is, but they are rare in the body and quite different from a negative feedback mechanism. In a positive feedback mecha-nism, the response to the stimulus does not stop orreverse the stimulus, but instead keeps the sequence of events going. A good example is childbirth, in which the sequence of events, simply stated, is as follows: Stretching of the uterine cervix stimulates secretion of the hormone oxytocin by the posterior pituitary gland. Oxytocin stimulates contraction of the uterine muscle, which causes more stretching, which stimulates more oxytocin and, hence, more contractions. The mecha-nism stops with the delivery of the baby and the pla-centa. This is the “brake,” the interrupting event.
Any positive feedback mechanism requires an external “brake,” something to interrupt it. Blood clotting is such a mechanism, and without external controls, clotting may become a vicious cycle of clot-ting and more clotting, doing far more harm than good. Inflammation follow-ing an injury is beneficial and necessary for repair to begin, but the process may evolve into a cycle of dam-age and more damage. The rise of a fever may also trigger a positive feedback mechanism. Notice in Fig. 1–3 that bacteria have affected the body’s thermostat in the hypothalamus and caused a fever. The rising body temperature increases the metabolic rate, which increases body temperature even more, becoming a cycle. Where is the inhibition, the brake? For this infection, the brake is white blood cells destroying the bacteria that caused the fever. An interruption from outside the cycle is necessary. It is for this reason, because positive feedback mechanisms have the poten-tial to be self-perpetuating and cause harm, that they are rare in the body.
Negative feedback mechanisms, however, contain their own brakes in that inhibition is a natural part of the cycle, and the body has many of them. The secre-tion of most hormones is regulated by negative feedback mechanisms. The regulation of heart rate and blood pressure involves several negative feedback mechanisms.
The result of all of these mechanisms working to-gether is that all aspects of body functioning, that is, of metabolism, are kept within normal limits, a steady state or equilibrium. This is homeostasis.
Keep in mind as well that what we call the normal values of metabolism are often ranges, not single numbers. Recall that normal body temperature is a range: 978 to 998F (368 to 388C). Normal pulse rate, another example, is 60 to 80 beats per minute; a normal respiratory rate is 12 to 20 breaths per minute. Variations within the normal range are part of normal metabolism.