CONTROL OF THE STEADY STATE
The concept of the cell as existing on a continuum of function and structure includes the relationship of the cell to compen-satory mechanisms, which occur continuously in the body to maintain the steady state. Compensatory processes are regulated primarily by the autonomic nervous system and the endocrine system, with control achieved through negative feedback.
Negative feedback mechanisms throughout the body monitorthe internal environment and restore homeostasis when condi-tions shift out of the normal range. These mechanisms work by sensing deviations from a predetermined set point or range of adaptability and triggering a response aimed at offsetting the deviation. Blood pressure, acid–base balance, blood glucose level, body temperature, and fluid and electrolyte balance are examples of functions regulated through such compensatory mechanisms.
Most of the human body’s control systems are integrated by the brain and influenced by the nervous and endocrine systems. Control activities involve detecting deviations from the predeter-mined reference point and stimulating compensatory responses in the muscles and glands of the body. The major organs affected are the heart, lungs, kidneys, liver, gastrointestinal tract, and skin. When stimulated, these organs alter their rate of activity or the amount of secretions they produce. Because of this, they have been called the “organs of homeostasis or adjustment.”
In addition to the responses controlled by the nervous and en-docrine systems, local responses consisting of small feedback loops in a group of cells or tissues are possible. The cells detect a change in their immediate environment and initiate an action to counteract its effect. For example, the accumulation of lactic acid in an exercised muscle stimulates dilation of blood vessels in the area to increase blood flow and improve the delivery of oxygen and removal of waste products.
The net result of the activities of feedback loops is homeosta-sis. A steady state is achieved by the continuous, variable action of the organs involved in making the adjustments and by the con-tinuous small exchanges of chemical substances among cells, in-terstitial fluid, and blood. For example, an increase in the carbon dioxide concentration of the extracellular fluid leads to increased pulmonary ventilation, which decreases the carbon dioxide level. On a cellular level, increased carbon dioxide raises the hydrogen ion concentration of the blood. This is detected by chemosensi-tive receptors in the respiratory control center of the medulla of the brain. The chemoreceptors stimulate an increase in the rate of discharge of the neurons that innervate the diaphragm and in-tercostal muscles, which increases the rate of respiration. Excess carbon dioxide is exhaled, the hydrogen ion concentration returns to normal, and the chemically sensitive neurons are no longer stimulated.
Another type of feedback, positive feedback, perpetuates the chain of events set in motion by the original disturbance instead of compensating for it. As the system becomes more unbalanced, disorder and disintegration occur. There are some exceptions to this; blood clotting in humans, for example, is an important pos-itive feedback mechanism.
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