Homeostasis is the state of living beings to maintain around a predetermined level the value of some internal parameters, disturbed continuously by various external and internal factors. The ordered set of subsystems that make up the human organism has a network of control systems, whose simultaneous intervention regulates the flow of energy and metabolites. It keeps the internal environment unchanged or almost unchanged, regardless of the modifications of the external one. Self-regulation of living organisms is a fundamental concept of modern biology, formulated at the end of the 19th century. Here, we will discuss the role of homeostasis.
Self-regulation mechanisms operate at all levels of system organization. At the molecular level, feedback inhibition (or feedback) limits the number of final products that are formed by the action of an enzymatic system. At the cellular level, the phenomenon of contact inhibition intervenes, for which in a reproduction of cells, the process of mitosis stops when they become so numerous that they touch each other. The close physical relationship would allow an inhibiting chemical messenger to pass from one cellular element to another to prevent further division. In tumors, this homeostatic mechanism is lost, and this explains the unstoppable reproduction of neoplastic cellular elements.
At an organismic level, the various mechanisms operate in different ways; the hormonal synthesis activity of the endocrine glands is governed by the events that occur in the systems regulated by the hormones. For instance, the increase in blood sugar stimulates the secretion of insulin, which in turn increases the glucose, with a consequent decrease in its blood concentration. Hunger and thirst are also sensations aimed at maintaining optimal levels of energy, nutrients, and water. Also, at the reproduction level, an example of homeostatic regulation is provided by the relationship between predators and prey.
The optimal functioning of each control system takes place only in a specific area, and its adaptability is therefore limited. However, within any sphere of control, a disturbance can exceed the compensation capacity of a given system, altering the transfer of energy and metabolites through that subsystem. If other control systems are able to compensate for the insufficient one, the stability of the organism is maintained, but with the loss of a part of reserve energy. On the contrary, the other systems cannot exercise this substitute; the entire network becomes unstable and insufficient to ensure control.
All the systems of the organism contribute to the maintenance of the, but the main control center is represented by the central nervous system which determines the most appropriate type of response (endocrine, immune, etc.). Particularly the role of the endocrine system is important because it controls and regulates the other systems of the organism. However, its response is slow (minutes, hours, days). Unlike it is implemented by the nervous system, which instead reacts promptly (fractions of a second or seconds), but whose effects are short-lived. Therefore, cooperation between the two systems provides complementary control methods.
The stimuli represented by the modifications of the external and internal environment are recognized and conveyed, through the afferent nerves to the spinal cord and brain which analyze them, associate them and compare them using an integration process.
A feedback mechanism makes a further modification of a troubled system. Most control systems use negative feedback, other positive feedback. The first consists of compensatory changes that bring the system back to its previous state, thus canceling or limiting the effects of the disturbances. Therefore, it opposes the changes and tends to maintain stability. Negative feedback systems are inherently unstable but are commonly found in endocrine and metabolic regulation. Some of the most typical examples are represented by the control of body temperature and weight. With positive feedback, however, there is a further increase in noise, which, however, allows us to carry out processes that are inactive in rest conditions, amplifying the starting signal (cascade mechanism). Examples are blood coagulation and glycolysis, which constitute self-limiting processes because the availability of substrates (fibrinogen and other coagulation factors, glycogen) is limited.
Sometimes, for a single variable (such as, for example, blood pressure and temperature), there are multiple control systems, which serve to guarantee the maintenance of a certain level even when not all the receptors and effectors of which they consist they are functional. This redundancy, therefore, represents a greater guarantee of control—a series of other baroreceptors located at different levels in the circulatory system. The atria and large vessels can send information to the brain.
Redundancy can also occur concerning effector mechanisms. Referring once again to the control of blood pressure, this can occur through changes in both peripheral resistance (contraction or relaxation of the vascular musculature), and in cardiac output (increase or decrease in heart rate or contraction energy). The effect of the abolition of one of these mechanisms is, therefore only transitory, because the other exerts a substitute. Redundancy guarantees the stability of the variable, despite many perturbations of opposite sign. The limits within which specific variables (temperature, blood sugar, blood pressure, bodyweight, etc.) are controllable by the negative feedback processes are usually set point). The oscillations around this point depend on the delay (phase shift) between the recognition, by the receptors, of the modification, and the response of the negative feedback system to that modification.
The frequency and depth of breathing in response to an increase in the partial pressure of CO2 in the arterial blood can vary rapidly due to the information that reaches the brain. They send the information to the brain via the vagus nerve and glossopharyngeal nerves from the peripheral chemoreceptors of the aortic arch and through the stimulation of the central chemoreceptors. Almost all physiological processes are regulated, at all levels, by these control systems. They allow the living being to adapt its biological individuality to preserve its constants against the stresses of the environment. Pathological changes occur when the stimulus is excessive and /or the response is not suitable to satisfy this need for balance and stability of the organism.