Homeostasis is the maintenance of a relatively constant internal environment of the body, the extracellular fluid, and the cells within it. It is essential in maintaining a constant and stable condition of properties like pH, temperature, water, ion concentrations and blood glucose concentrations. Homeostasis can be in either a closed or open system. The mechanisms for homeostatic control require a number of different components. Firstly, a receptor to measure fluctuations in the condition being controlled. For example, in the maintenance of water potential in the body these are osmoreceptors in the hypothalmus. Additionally, a negative feedback system is required to make corrections in order to maintain stable conditions.
Failure to maintain homeostasis can result in illness or disease.
Blood glucose level
The level of glucose in the blood naturally fluctuates throughout the day, however it is important that a range of 3.5-5.5 mM is maintained. Blood glucose level is detected by clustered cells in the pancreas known as the islets of Langerhans. Negative feedback loops are used to return glucose levels to the normal value.
When blood glucose is low, alpha cells promote the conversion of glycogen into glucose (glycogenolysis), allowing glucose to be released into the blood stream. When blood glucose is high, beta cells secrete insulin into the blood stream which enhances the uptake of glucose into the cell via the GLUT4 transporter and the conversion of glucose into glycogen (glycogenesis).
Insulin also activates hexokinase which phosphorylates glucose as it enters a cell to form glucose 6-phosphate. This ensures that cellular glucose levels are lower than blood glucose levels and therefore the concentration gradient of glucose is maintained so it continues to enter the cell.
- ↑ 1.0 1.1 Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2009) Human Physiology, An Integrated Approach, 5th edition, San Francisco: Pearson Benjamin Cummings