Homeostasis uses both the nervous system and the endocrine system. They are capable of working together or independently of each other. The nervous system detects deviations from the norm and responds by transmitting messages in the form of nerve impulses. Chemical changes are detected and impulse sent to the brain which responds in the appropriate way. Blood passes over receptor cells that detect changes in conditions such as carbon dioxide levels. These then send a message to the brain using nerve impulses. The brain then sends a message to effector cells which cause the relevant organs to respond. The nervous system results in quick changes whereas the endocrine system is slower.
The control of blood sugar can be used as an example of homeostasis involving the endocrine system. This is a group of specialised organs and tissues that produce, store and secrete chemical substances known as hormones. The endocrine glands are ductless glands that secrete the hormones direct into the blood stream. These hormones pass through the blood to arrive at a target organ which has cells possessing the appropriate receptor. Each receptor is designed to respond to a specific hormone which binds to the receptor on the surface of the cell setting off a series of events within the cell. The pancreatic endocrine system is involved in the control of blood sugar.
Blood sugar is also known as blood glucose. It is delivered by the blood stream around the body and is broken down in respiration releasing energy. Blood glucose is maintained in a healthy person at around 800mg per dm3. If the level drops below 600 mg per dm3 a person suffers from hypoglycaemia and can loose consciousness. On the other hand if it rises above the norm it can cause hyperglycaemia resulting in a lack of blood to the heart. The amount of glucose is controlled by the hormones, insulin and glucagons which are produced in the islets of Langerhans in the pancreas.
When a person eats carbohydrate rich food levels of glucose increase. If glucose levels are too high detector cells in the pancreas cause the production of insulin. The cells which produce the insulin are beta cells. Whilst these cells produce more insulin alpha cells stop producing glucagons. The insulin passes into the tissue fluid and is carried by blood plasma to all parts of the body. It then binds to glycoprotein receptors in the cell membranes of its target cells. These are the effector cells as once the receptors have received the hormone the cells respond effecting a change. The changes in cells include:
- an increased rate of glucose uptake by muscle and fat cells, achieved by increasing the number of glucose channels in the cell surface membranes.
- an increased rate of use of glucose by the cells by increasing respiration rate especially in muscles.
- in the liver the glucose is converted into glycogen which can be stored for when it is needed.
As glucose levels fall the production of insulin is inhibited preventing further take up of glucose. The beta cells stop production.
When blood glucose is low, detectors in the pancreas cause the release of more glucagon. The alpha cells secrete glucagon and the beta cells stop producing insulin. When the hormone arrives at glycoprotein receptors in the liver they bind on the surface and set off a chain of events opposite to those produced by insulin. Enzymes in the liver are activated to release the glucose the liver has stored in its cells. The glycogen previously created is broken down and released back into the blood stream as glucose so maintaining levels within a homeostatic range. The rate of respiration is also reduced.
Diagram to show negative feed back control of blood sugar levels within the body.
The cycle continues depending on the levels. The pancreatic endocrine system is more complicated than some as two hormones are involved. It is important that both respond as both have a positive effect to balance the other. It is necessary for insulin production to be decreased and glucagon be produced as if just insulin decreased there would be nothing to cause the liver to respond to breakdown glycogen. The homeostatic control would be lost with severe consequences.
Homeostatic control is extremely important. Failure could result in death. Detectors, receptors and effectors all play a crucial role in maintaining a constant environment around the cells. The negative feedback cycle maintains tissue fluid at its norm for a variety of things including temperature, water content, glucose levels, oxygen levels, carbon dioxide levels andb ammonia levels. Chemical changes produced by hormones are slower than changes produced by the central nervous system but allow homeostatic control within a small fluctuation.
