Handout 2: Physiological Control Systems
Draw a control system for each of the physiological systems described below. Examples are adapted from (Milhorn 1966).
1. Regulation of sodium
Sodium ions need to be regulated because small variations in their concentration can cause drastic changes in bodily function. Sodium seems to be actively reabsorbed by all segments of the renal tubules. It may be almost completely reabsorbed or, on the other hand, it may fail to be reabsorbed and large portions pass into the urine. The variability of sodium reabsorption is controlled by the aldosterone concentration of the extracellular fluid. According to one theory, if sodium concentration falls it causes the diencephalon (which contains the hypothalamus, thalamus, etc.) to increase the secretion rate of glomerulotropin. Increased glomerulotropin levels cause an increase in the secretion rate of aldosterone from the adrenal cortex, thereby raising the extracellular fluid concentation of this hormone. This, then, acts on the kidney tubules to increase the reabsorption rate of sodium ions.
2. Regulation of calcium
A low level of calcium ion concentration in the extracellular fluid can lead to tetanic contraction of muscles and death due to respiratory paralysis. As calcium ion concentration in the extracellular fluid decreases, parathyroid hormone (PTH) is released which acts on the bone salts and releases stored calcium. Increased PTH also results in increased calcium ion reabsorption by the renal tubules which increases extracellular concentration of calcium. Conversely, a high calcium concentration acts through the parathyroid hormone system to reduce the extracellular calcium concentration.
3. Regulation of extracellular fluid osmolarity
The regulation of extracellular fluid (ECF) osmolarity is vested in the so-called osmoreceptor-antidiuretic hormone system. A high ECF osmolarity causes the hypothalamus to send nerve impulses to the neurohypophysis (posterior pituitary gland), which causes secretion of antidiuretic hormone (ADH). ADH promotes increased water reabsorption from the distal tubules and collecting ducts. This, in turn, results in a reduced ECF osmolarity.
Special ganglion cells, known as osmoreceptors, are located in the anterior hypothalamus and respond to changes in ECF osmolarity. These cells supposedly contain large fluid chambers which are filled with ECF and each of which continually emits nerve impulses. When the ECF osmolarity becomes very low, osmosis of the water into the fluid chambers of the osmoreceptors causes them to swell, which decreases their rate of impulse discharge. The impulses are transmitted through the pituitary stalk into the neurohypophysis where they cause the release of ADH.
4. Regulation of arterial pressure
There are three basic mechanisms by which the arterial pressure is normally regulated. (1) If all pressures throughout the circulatory system become very high, fluid begins to leak from the capillaries into the interstitial spaces. This decreases the blood volume and lowers the arterial pressure until the capillary pressure returns almost to normal. (2) The autonomic nervous system regulates the activity of the heart and the degree of constriction of the blood vessesls throughout the body, thus affecting arterial pressure. High heart rate and an increase in total peripheral resistance increase arterial pressure. (3) The kidneys help regulate arterial pressure. When the arterial pressure becomes too low, the kidneys, either by altering the body fluids or by causing constrition of arterioles throughout the body, elevate the arterial pressure back to normal. Conversely, if the pressure rises too high, the kidneys promote appropriate circulatory changes to decrease the pressure to normal.