Regulatory circuits in the human body maintain various vital variables and processes. The pH value, the blood hormone level, the body temperature or the oxygen tension of the blood are kept constant with the help of control circuits.
What is a control loop?
A control loop is a control system that can control various processes and functions in the organism. For example, pH is kept constant with the help of control loops. A control loop is a control system that can control various processes and functions in the organism. Most functions have their own control loop. A control loop can either run in the target organ itself or be controlled by a higher-level organ. Such superordinate organs are, for example, the central nervous system (CNS) or hormone glands. The aim of a control loop is to keep a controlled variable constant or to bring it to a desired setpoint. This target value is measured by various receptors and compared with the current actual value. The actuator in the control loop then corrects the actual value until it matches the setpoint. Most control loops in the human body operate on the principle of negative feedback.
Function and task
One well-known control loop in the human body is the thyrotropic control loop, which regulates the hormone activity of the thyroid gland. The thyroid gland (thyroid glandula) produces the hormones triiodothyronine (T3), thyroxine (T4), and calcitonin. The two iodine-containing hormones T3 and T4 are produced in the follicular epithelial cells of the thyroid gland. They play an important role in energy metabolism and influence the growth of the organism. The function of the thyroid gland is controlled by the hypothalamus and the pituitary gland via the thyrotropic regulatory circuit. The pituitary gland secretes thyroid stimulating hormone (TSH). This reaches the thyroid cells via the bloodstream. There, TSH promotes the production of T3 and T4 on the one hand and stimulates the growth of the thyroid gland on the other. A higher level of T3 and T4 in the blood in turn inhibits the release of TSH. Thus, thyroid levels in the blood are regulated according to need and are normally kept relatively constant. The thyrotropic control loop is an example of a negative feedback loop. However, the set point of the control loop is not given by the pituitary gland, but by the hypothalamus. This produces thyrotropin releasing hormone (TRH). The body’s heat balance is also regulated by a control circuit. The aim of this control circuit is to keep the temperature in the body constant at around 37°C. The ambient temperature influences the body temperature. In this process, the ambient temperature influences the body temperature. Intensive physical activity, for example, also has an influence on temperature. Temperature sensors are located throughout the body. However, heat sensors are particularly localized in the spinal cord, hypothalamus and skin. The hypothalamus plays an important role in the regulation of temperature. This is where all actual value information from the body is collected. The hypothalamus is also informed about all physical needs. From all these inputs, the control center in the hypothalamus now calculates the desired set point as well as the difference between this set point and the actual value. Normally, the set point is between 36°C and 37°C. The setpoint in the body adjusts during infections with fever, for example. Body temperature also changes during ovulation in women. If both values match, no regulation is necessary. However, if there is a difference in the comparison, the body initiates a reaction. It changes individual actuators in the control loop. One possible actuator in the regulation of temperature, for example, is the musculature. When it is cold, the muscles tremble and thus generate heat.
Diseases and ailments
Disruptions in the regulatory circuit can occur at any point. For example, the target organs, the sensing elements, or the actuators may be affected. These changes affect the entire regulatory circuit. Disorders in the thyrotropic control circuit usually result in either hypothyroidism or hyperthyroidism. In primary hypothyroidism, the cause is found in the target organ of the regulatory circuit, i.e. in the thyroid gland itself. Causes of such primary hypothyroidism include thyroid surgery, radioiodine therapy, thyrostatic drugs, or extreme selenium or iodine deficiency.In secondary hypothyroidism, the cause is found in the pituitary gland. Too little TSH is produced there. The control loop is therefore already impaired before the thyroid gland. The consequences of primary and secondary hyperthyroidism are similar. There is fatigue, loss of strength, depression, hair loss, constipation, erectile dysfunction and the typical myxedema. However, disturbances in the control circuit of the thyroid gland can also lead to hyperthyroidism. The cause is often autonomic or autoimmune processes. An example of a disorder in the thyrotropic control loop leading to hyperthyroidism is Graves’ disease. Graves’ disease is an autoimmune disease of unclear genesis. The body forms antibodies against receptors on the thyroid gland. These receptors are actually intended for TSH. However, the antibodies bind to the receptors and cause a similar effect as TSH. As a result, the thyroid gland produces more thyroid hormones. However, this occurs completely independently of the actual control loop. In Graves’ disease, the TSH level drops almost to 0 because there are too many thyroid hormones in the blood at any given time. Typical symptoms of hyperthyroidism are weight loss, diarrhea, irritability, nervousness, hair loss and heat intolerance. Pathological control loops are also called vicious circles or circulus vitiosus. In this case, two disturbed bodily functions influence each other and thereby reinforce already existing diseases or maintain diseases. Pathological control loops are found in diseases such as cardiac insufficiency or diabetes mellitus. They are usually based on a positive feedback loop.
