The term homeostasis comes from the Greek and means equilibrium. It refers to a process that serves to maintain an equilibrium within dynamic systems. In the human body, homeostasis maintains the internal environment. Examples of homeostatic processes include thermoregulation or regulation of blood glucose levels.
What is homeostasis?
The term homeostasis refers to a process that serves to maintain an equilibrium within dynamic systems. In the human body, homeostasis maintains the internal environment. All regulatory processes in the body strive for equilibrium. States of equilibrium are the basis for many functions of organs and for the survivability of the whole organism. Homeostasis in the body is maintained by mechanisms such as regulatory circuits or redundancies. With these mechanisms, the body is given the ability to self-regulate. The goal of homeostasis can be the maintenance of equilibrium within a single cell, within cell assemblies, an organ, or the entire organism. Here, the maintenance processes may relate to anatomical structures, chemical or physical processes, or even mathematical conditions, such as the number of cells in a given structure.
Function and task
In many cases, homeostasis is maintained by regulatory systems with negative feedback. In this process, a target value is first determined. This is the value that guarantees optimal conditions for safety, survival, and well-being. A sensor, which can be the pituitary gland or the hypothalamus, for example, compares the current value with the target value. If a discrepancy between the target value and the actual value is detected, a regulating process starts. This typically ends only when the discrepancy between the two values has disappeared. An example of such a negative feedback system is thermoregulation. The target value of body temperature is usually between 36.5 and 37 °C. The current body temperature is registered by so-called thermoreceptors located in the hypothalamus in the brain. In the event of deviations from the desired temperature, the hypothalamus can initiate measures to bring the temperature in the desired direction. For example, it can induce sweating or shivering by altering the blood vessels. Also, the hypothalamus can cause a person to dress warmer or colder or to move from sun to shade. Similar homeostasis processes exist for numerous functions of the body. When blood sugar drops, the feeling of hunger follows relatively quickly; when the salt content of the blood is too high, the person feels thirsty. Sleep regulation is also based on a homeostatic process. Sleep duration and sleep intensity are regulated on the one hand by circadian rhythmicity and on the other hand by homeostatic sleep pressure. To a certain extent, circadian rhythmicity reflects the internal clock. It ensures that we are tired at approximately the same time every day. Homeostatic sleep pressure, on the other hand, depends on previous wakefulness. The longer and more strenuous the waking phase, the higher the homeostatic sleep pressure. One of the most important homeostasis of the human body is the homeostasis of the brain. To ensure that the environment in the brain is always kept in balance, there is a barrier between the blood circulation and the central nervous system. This is called the blood-brain barrier. The blood-brain barrier protects the brain from pathogens, hormones or toxins. They cannot pass through this filter. Other substances, such as nutrients, can cross the blood-brain barrier. This maintains homeostasis in the brain.
Diseases and ailments
Disturbances of homeostasis lead to dysfunctions in individual organs or even in the entire organism. Many homeostasis disorders originate in the hypothalamus. If central impairment occurs here, body temperature may be permanently too low or too high. Frequently, phases of fever alternate with phases of hypothermia. For example, sufferers freeze during the day and sweat so much at night that they have to change their night and bedclothes several times. Obesity and eating disorders are also frequently based on disturbed homeostasis.Researchers suspect that many diets negatively affect the regulatory system for satiety and hunger until normal regulation is no longer possible. Disturbed sleep homeostasis causes insomnia and difficulty falling asleep. Alcohol, in particular, appears to play an important role in disturbances of sleep homeostasis. Alcohol increases homeostatic sleep pressure, which means that the need to sleep increases. As a result, the sleep period is shifted and sleep is not as sound as normal. Alcohol thus decreases the quality of sleep by disturbing homeostatic pressure. Homeostasis of blood glucose is important for survival. Hypoglycemia leads to reduced brain performance, seizures, sweating and, in an emergency, shock. Hyperglycemia, on the other hand, is manifested by severe thirst, deepened breathing and later unconsciousness. A disturbance in the homeostasis of blood glucose can also lead to a regulatory disturbance in the pH value of the blood. The reference range of the pH value in humans is between 7.35 and 7.45. Outside these values, homeostasis is disturbed. A lower pH value is referred to as acidosis (hyperacidity), while a higher pH value is referred to as alkalosis. The homeostasis of the pH value is maintained by the kidneys and the lungs. If there is an increase in certain metabolic products or if the excretion capacity of the kidneys and lungs is limited, this can lead to hyperacidity or increased pH values. A homeostasis disorder is also suspected as the cause of Parkinson’s disease. Thus, disruption of ionized calcium homeostasis appears to have negative effects on the production of dopamine. In Parkinson’s disease, a lack of dopamine results in characteristic symptoms such as muscle rigidity, muscle tremors, or postural instability. If the homeostasis of the brain cannot be maintained due to impairments of the blood-brain barrier, diseases such as meningitis (an inflammation of the meninges) or encephalitis (an inflammation of the brain) occur. Alcohol, nicotine, and electromagnetic waves affect the blood-brain barrier and increase susceptibility to neurological diseases.
