Homeostasis is the process used by the body to maintain a stable internal environment. Either negative feedback or positive feedback can be used in homeostasis.
In negative feedback, the body changes an internal condition back to its normal state; this is the most common way that the body regulates itself. Examples of negative feedback: (1) when blood pressure rises, the heart slows down; (2) when glucose levels in the blood are too high, the pancreas secretes insulin to stimulate the absorption of glucose and the conversion of glucose into glycogen.
In positive feedback, the body reacts to a change from the normal state by causing an even greater change. Examples of positive feedback: (1) when constrictions in the uterus push a baby into the birth canal, contractions in the uterus increase; (2) when a blood vessel is damaged, the vessel constricts and chemicals are released to plug and repair the hole.
Body Temperature Homeostasis
In warm-blooded animals such as humans, body temperature must be maintained by various negative feedback reflex responses within the body.
Heat in the body is generated by the cellular respiration reaction, combining oxygen and ATP (a product of glucose) and creating heat, carbon dioxide and water.
The hypothalamus of the brain monitors body temperature and regulates that temperature by either increasing the rate of cellular respiration reactions, or decreasing the rate and otherwise causing heat loss from the skin.
If the body becomes cold, and more heat is needed, more ATP / glucose are “burned” to generate that heat.
If the body becomes too hot, the blood vessels near the skin dilate (open up) to allow more heat loss from the blood to the skin to the air, and perspiration from the skin causes evaporative cooling.
The body temperature must be maintained within a few degrees of the normal 98.6o F. If the core temperature falls below 90o or above 110o, death can result.
Glucose Level in Blood
Another example of negative feedback in the human body is the regulation of glucose levels in blood. Glucose is the form of sugar that is found in blood and that provides energy for cellular respiration.
When glucose levels are too high in the blood, a hormone called insulin is secreted by the pancreas. Insulin stimulates the absorption of glucose by cells and the liver’s conversion of glucose into glycogen. Glycogen is a sugar that can be stored in the liver and blood cells.
As glucose levels decrease, less insulin is produced. When glucose levels are too low, , another hormone called glucagon is produced , which in turn causes the liver to convert glycogen back to glucose.
Turgor Pressure in Cells
The pressure that exists in a cell is called turgor pressure. Turgor pressure causes water to move through the cell membrane, in a process called osmosis, either in or out of the cell to maintain a constant concentration of sodium and other minerals critical to their function.
If the concentration of some mineral becomes too high (as compared to the blood supply), additional water enters the cell and corrects the problem. If the concentration becomes too low, water passes out of the cell, again to correct the problem.
Mineral ions can also pass through the cell membrane to maintain the correct concentrations.
Lactates in Muscles
When a person exercises very hard, the muscles do not get enough oxygen to complete the full cellular respiration chemical reaction. To provide the needed energy, a different chemical reaction, called lactic acid fermentation, produces ATP from glucose.
A chemical called lactate builds up in the muscle as this chemical reaction occurs. Lactates can cause muscle fatigue and pain, a signal from your body to your brain to stop the exercise and allow more oxygen to reach the muscles.
Once the exercise stops, the lactates are carried away from the muscles to the liver, and the pain stops.