Biology essay on feedback mechanisms

Homeostasis is the ‘maintenance of equilibrium in a biological system by means of an automatic mechanism that counteracts influences tending towards disequilibria’. Homeostatic mechanisms operate at all levels within living systems, including the molecular, cellular, and population levels. in humans homeostasis involves the constant monitoring and regulating of numerous factors including, oxygen and carbon dioxide levels, nutrient and hormone levels and inorganic and organic substances. The concentrations of these substances in the body fluid remain unchanged, within limits, despite changes in the external environment. There are two general ways in which the body can respond to changes, these being positive and negative feedback.

Negative Feedback

Negative feedback causes the body to respond in such a way as to reverse the direction of a change and this tends to keep the internal environment at a constant, thus maintaining homeostasis. Sensors and receptors are what bring about a reaction to ensure conditions within the body remain favourable. Examples of negative feedback are as follows.

Blood glucose levels

The receptors of the pancreas are responsible for monitoring the blood glucose levels. The alpha-cells of the Islets of Langerhans release glucagons when glucose levels are low. Glucagons promote the conversion of glycogen into glucose; the lack of glucose can then be compensated for by the new supply of glucose brought about from glucagons. The beta cells of the Islets of Langerhans release insulin when the levels of glucose in the blood is high. Insulin promotes the conversion of glucose into glycogen and this can be stored in the liver for later use.

Fight or flight

In emergencies adrenaline is released by the body to override the homeostatic control of glucose levels and promotes the breakdown of glycogen into glucose. Adrenaline is secreted by the adrenal gland and its secretion leads to increased heart rate, breathing and metabolism. Once the emergency is over adrenaline levels drop, and the homeostatic controls are back in place.

Water regulation

Homeostatic control of water is also controlled by negative feedback. Osmoreceptors capable of detecting water concentration are situated on the hypothalamus. The hypothalamus sends chemical signals to the pituitary gland, which secretes anti-diuretic hormone (ADH) that targets the kidney. ADH on reaching the kidney causes the tubules to become more or less permeable to water. If more water is required in the blood stream then high. Concentrations are released and this makes the tubules of the kidney more permeable to water. If less water is required in the blood stream then low. Concentrations are released and this makes the tubules of the kidney less permeable to water.

Temperature control

Animals, which are capable of temperature control, are called homiotherms. The hypothalamus acts as the temperature control centre and detects any change in the temperature with thermoreceptor. One of the most obvious physical responses to overheating is sweating; this cools the body by making more moisture on the skin available for evaporation. Other responses, perhaps less obvious, is vasodilation of blood vessels. The blood vessels close to the skins surface become more dilated meaning there is a larger surface area for heat loss to the external environment. If the body is cooled then sweating is reduced, body hair stands on end to trap air close to the skin, shivering occurs, the metabolic rate decreases and vasoconstriction of blood vessels occurs to prevent the further loss of heat.

Positive Feedback

Positive feedback mechanisms increase the departure from the normal even more. Positive feedback can have both beneficial and harmful consequences.

Digestion

Positive feedback occurs in some digestive enzymes such as pepsin. Pepsin is a protein-digesting enzyme that works in the stomach, the stomach does not secrete pepsin instead it secretes an inactive form, called pepsinogen. When one pepsinogen molecule becomes activated, it helps to activate other pepsinogen molecules nearby, which in turn can activate others. In this way, the number of active pepsin molecules can rapidly increase, by using positive feedback. The advantage of pepsin being present in an inactive form is the prevention of self-digestion.

Temperature

One harmful effect of positive feedback is if an individuals temperature is very high the negative feedback system ceases to work and the increase temperature speeds up the body chemistry, which causes the temperature to rise even more, which in turn increases the temperature and so forth. This is the vicious cycle of positive feedback and can only lead to death if not stopped

Overall, homeostasis is a very important mechanism, and complex systems must have homeostasis to maintain stability to survive. The thickening of fur in winter, the darkening of skin in sunlight, the seeking of shade in heat, and the production of more red blood cells at high altitude are all examples of adaptations animals can make in order to maintain homeostasis