Hormones trigger a cell in a target organ as the hormone molecule binds to the specific receptor site on the specific receptor protein. This releases an enzyme that can be used many times. In turn, another hormone is produced which can also be used many times. This is a cascade effect.
Temperature control of a mammal is essential so that the enzymes can function at an optimum level. Warm-blooded or endothermic animals can maintain their core temperature at an optimum level so that internal processes are constant. Cold-blooded (ectothermic) animals have a body temperature that fluctuates with the environmental temperature. They might therefore be vulnerable at times due to the enzyme driven reactions being slow.
In mammals, the regulation of temperature involves thermoreceptors in the skin, body core and blood vessels supplying the brain which link to the hypothalamus. Once the blood temperature decreases, the heat gain centre of the hypothalamus is stimulated. This leads to a rise in blood temperature; the heat loss center is therefore stimulated. The combination of the two in both directions contributes to homeostasis. If there is a fall in temperature vasoconstriction occurs, arteriole control is initiated by the hypothalamus that results in efferent neurones stimulating constriction of the arteriole sphincters of skin capillary beds. This deviates blood to the core of the body. Contraction of the erector-pili muscles is also initiated in the hypothalamus and hairs on the skin stand on end and trap an insulating layer of air so less heat energy is lost from the skin. There is also less heat energy loss by sweat reduction. Shivering occurs, as muscular contraction is accompanied heat energy release. There is a behavioural response such as putting on more clothes or turning on the heating, there is also an increased metabolic rate. This occurs as the hypothalamus produces a release factor substance, this stimulates the anterior part of the pituitary gland to secrete TSH which travels in the blood to the thyroid which is stimulated to secrete thyroxine, this increases the rate of respiration in the tissues therefore increasing body temperature. An increase in body temperature results in the opposite of these responses – vasodilatation when arterioles of capillary beds dilate allowing more blood to skin capillary beds. Relaxation of the erector-pili muscles as hairs lie flat, therefore more heat loss occurs. Sweat increase and behavioural response, e.g. moving into the shade or consuming a cold drink.
The amount of glucose in the blood must be maintained, as high levels of glucose in the blood would cause great problems. Hypertonic blood plasma would result in water leaving the tissues by osmosis. A rise in blood glucose levels is detected by the cells in the islets of Langerhans, this stimulates the secretion of insulin by beta cells, more blood glucose is taken into cells including the liver and muscle cells, the level of blood glucose therefore decrease. A drop in blood glucose levels is again detected by the cells in the islets of Langerhans, glucagons is secreted converting glycogen into glucose in the liver. Glucose is released from the liver into the blood and blood glucose levels rise. This is again an example of negative feedback.
The amount of water in the blood is controlled by the kidneys, as they are able to intercept water before it can reach the uretes. However, too much water loss can lead to dehydration problems, for example, excessive sweating in hot conditions. In cold conditions less water is lost by sweating giving a potential problem if too much water is retained in the blood. In warm environments osmoreceptors in the hypothalamus detect an increase in the solute concentration of the blood plasma. The hypothalamus then produces the hormone ADH by neurosecretion, this is secreted into the posterior lobe of the pituitary gland, it passes into the blood and reaches the kidneys. Here it increases the permeability of the collecting ducts and the distal tubes. More water can then be reabsorbed back into the blood. In cold environmental conditions osmoreceptors in the hypothalamus produce less ADH, less ADH leaves the posterior lobe of the pituitary gland and less ADH reaches the kidneys, the target organs. The collecting ducts and the distal tubules are therefore not so permeable and less water can be reabsorbed back. The urine is of greater volume due to the greater water content.
