Levels of Organization: Introduction to Human Anatomy and Physiology.
negative feedback
A negative feedback mechanism is one that prevents the correction of deviations from doing too much (which could harm the body). Most of the feedback mechanisms in the human body use negative feedback. Examples of negative feedback are regulation of blood pressure, erythropoiesis (the production of red blood cells), regulation of body temperature (thermoregulation), and control of blood glucose levels.
The brain's hypothalamus maintains homeostatic control of body temperature. Information is received from temperature receptors in the skin and within the hypothalamus itself. The normal reference point for body temperature is approximately 37°C (98.6°F). When the temperature rises above normal, hypothalamic activity targets the muscle tissue in the walls of the blood vessels that supply the skin and also targets the sweat glands. This causes blood flow to increase near the surface of the body and speeds up secretion from the sweat glands. The skin loses heat to the environment and the evaporation of sweat speeds up this process. As the temperature returns to normal, hypothalamic activity decreases and all processes are reversed.
Negative feedback is the main controller of homeostasis and provides long-term control over internal systems and bodily conditions. Minor variations are generally ignored, while normal body ranges are maintained rather than exact fixed values. The regulatory process works dynamically as the set points vary with changes in the environment and activity. While sleeping, thermoregulation has a lower set point than when you are awake and active. Therefore, the temperature varies due to small fluctuations around the set point or change in the set point. Similar variations occur throughout the body's physiology.
Set points differ between individuals based on age, gender, genetic factors, general health, and environment. There are no real precise homeostatic conditions. Basically, homeostatic values are based on the average across large numbers of people or as a range that includes 95% (or more) of people being sampled. While most healthy adults have a body temperature between 98.1°F and 98.9°F (36.7°C and 37.2°C), 5% have a resting body temperature above or below this range.
positive feedback
A positive feedback mechanism is one that causes conditions to move away from the normal state to stimulate further changes. They are usually short-lived and extremely specific actions that produce extreme responses. A positive feedback mechanism is defined as one that results in or responds in an enhanced way to the original stimulus, accelerating the outcome or response. Examples of positive feedback are the onset of contractions before delivery, the blood clotting process, lactation, estrogen secretion during the follicular phase of menstruation, and the generation of nerve signals.
In positive feedback, the loops increase and are often referred to as part of a positive feedback loop. These loops are typically found when a potentially stressful or dangerous bodily process must be completed quickly before homeostasis is restored. An example is a severe laceration, which can lower blood pressure and reduce the efficiency of the heart. As the coagulation process attempts to combat blood loss, a positive feedback loop occurs that increases coagulation activities.
