Control loop

Control loops Various vital quantities and processes are maintained in the human body. 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 loops.

What is a control loop?

A control loop is a control system that can control various processes and functions in the organism. Most of the functions have their own control loop.

A control loop can either run in the target organ itself or it can be controlled by a higher-level organ. Such higher-level organs are, for example, the central nervous system (CNS) or hormonal glands.

The aim of a control loop is to keep a controlled variable constant or to bring it to a desired target value. 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 agrees with the target value. Most of the control loops in the human body operate on the principle of negative feedback.

Function & task

A well-known control circuit in the human body is the thyrotropic control circuit, which regulates the hormonal activity of the thyroid gland. The thyroid gland (glandula thyreoidea) 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 the energy metabolism and influence the growth of the organism.

The function of the thyroid gland is controlled by the thyrotropic control circuit of the hypothalamus and the pituitary gland (pituitary gland). 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 content of T3 and T4 in the blood in turn inhibits the release of TSH. As a result, the thyroid levels in the blood are regulated as required and usually kept relatively constant.

The thyrotropic control loop is an example of negative feedback. The setpoint of the control loop is not given by the pituitary gland, but by the hypothalamus. This produces the thyrotropin releasing hormone (TRH).

The body's heat balance is also regulated via a control circuit. The aim of this control loop is to keep the temperature in the body constant at around 37 ° C. The ambient temperature influences the body temperature. For example, intense physical activity also has an impact on temperature.

Temperature meters are located all over the body. The heat sensors are, however, particularly located in the spinal cord, hypothalamus and skin. The hypothalamus plays an important role in regulating temperature. All actual value information from the body is collected here. The hypothalamus is also instructed about all physical needs. From all these inputs, the control center in the hypothalamus now calculates the desired target value and the difference between this target value and the actual value. Typically the setpoint is 36 ° C to 37 ° C.

The setpoint in the body changes, for example, in the case of infections with fever. Body temperature also changes during ovulation in women. If both values ​​match, no regulation is necessary. However, if the comparison shows a difference, the body initiates a reaction. It changes individual actuators in the control loop. One possible control element in regulating temperature is, for example, the muscles. When it's cold, the muscles tremble and generate heat.

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Illnesses & ailments

Disturbances in the control loop can occur at any point. For example, the target organs, the feelers or the actuators can be impaired. These changes affect the entire control loop.

Disturbances in the thyroid control loop usually lead to either an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism). In the case of primary hypothyroidism, the cause is found in the target organ of the control loop, i.e. in the thyroid itself. The causes of such primary hypothyroidism include Thyroid surgery, radioiodine therapy, anti-thyroid 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 impaired before the thyroid gland. The consequences of primary and secondary hyperthyroidism are similar. Fatigue, loss of strength, depression, hair loss, constipation, erectile dysfunction and the typical myxedema occur.

Disruptions in the thyroid regulating system can, however, also lead to hyperthyroidism. Autonomy or autoimmune processes are often the cause. An example of a disorder in the thyrotropic control circuit that leads to an overactive thyroid is Graves' disease. Graves' disease is an autoimmune disease of unknown origin (development). 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 produce an effect similar to that of TSH there. As a result, the thyroid gland produces more thyroid hormones. However, this happens 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 all times. Typical symptoms of an overactive thyroid are weight loss, diarrhea, irritability, nervousness, hair loss and heat intolerance.

Pathological control loops are also referred to as vicious circle or vicious circle. In this case, two disturbed bodily functions influence each other and thereby reinforce existing diseases or maintain diseases. Pathological control loops can be found in diseases such as heart failure or diabetes mellitus. They are usually based on positive feedback.