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Control of the internal environment of mammals

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Introduction

Control of the internal environment of mammals Homeostasis is the maintenance of a constant internal environment. The endocrine and nervous systems both contribute to coordination. Nerves and hormones have key roles in the maintenance of this steady internal state. Levels of pH, blood glucose, oxygen, carbon dioxide and temperature all need to be controlled. Homeostasis is essential in mammals since whilst the environmental conditions fluctuate, those inside the organism remain stable. Body cells only function normally within a narrow range of conditions. Their enzymes are sensitive to temperature and pH changes, they need materials for growth and respiration, the removal of waste products and a specific composition of surrounding tissue fluid. The control systems must have a sensor monitoring the environment, which relays information to a control centre that stimulates an effector to change the environment. The system works through negative feedback loops. The pituitary gland is the key control gland. It affects many areas of the body and stimulates other hormones by the production of tropic hormones. Thyroid stimulating hormone (TSH) ...read more.

Middle

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. ...read more.

Conclusion

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. Lottie U6C ...read more.

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