Feedback Systems

Definition

A feedback system uses as input the total or partial output of the system. Feedback systems are used to control and regulate processes. They use the consequences of the process (i.e. too much or too little produced) to regulate the rate at which the process occurs (to decrease or increase the rate of the process).

Description

A typical feedback system consists of a sensor, a control center that receives the signals of the sensor, and an effector pathway, meaning a pathway for the commands of the control center as it reacts to the information received from the sensor.

In complex organisms, such as humans, feedback systems constantly monitor and regulate numerous factors, including the levels of gases like oxygen and carbon dioxide, nutrients, hormones, and chemical substances. The feedback systems ensure that the amounts of these substances in body tissues and fluids remain unchanged within appropriate limits, despite changes in the external environment.

Feedback systems function with the help of the nervous and endocrine systems. While nerve cells detect changes in the body and quickly respond to the brain for a rapid response, hormones of the endocrine system regulate the system more slowly. For example, hormones and nerves regulate the progress of labor contractions. When labor begins, nerve cells in the uterus alert the hypothalamus (a control center in the brain). A hormone called oxytocin is released into the blood, which causes the uterus to contract.

Feedback control or regulation can be either negative or positive. With negative feedback, the activity of a system is reduced, slowed down or minimized by the presence of high amounts of a substance it produces. An example is the increased breathing resulting from the presence of high levels of carbon dioxide in extracellular fluids. The purpose of breathing is to provide the oxygen required by the body for its functioning (metabolism ) and to eliminate waste carbon dioxide. In the presence of high levels of carbon dioxide, breathing increases, which increases the supply of oxygen present in the air of the lungs, thus counteracting the high amounts of carbon dioxide, which are harmful to the body. With positive feedback, the activity of a system is increased by the presence of high amounts of a substance it produces. Positive feedback systems thus speed up or maximize the rates of processes. For example, they are used in the production of nerve impulses and blood clotting. In the above example, labor contractions are a positive feedback system. As oxytocin reaches the uterus, causing contractions, more stretching causes an increase in oxytocin levels, which causes more forceful contractions.

Function

Almost all body functions and processes operate using feedback systems. The body maintains its overall chemical and physical internal environment constant (homeostasis) using a control system activated by several feedback systems. This is because normal cell function depends on maintaining relatively constant intracellular and extracellular environments, such as pH, temperature, and the amounts of all the substances needed by the body. The body keeps these quantities within the range required to maintain life and proper function with the help of feedback systems.

Feedback systems are active at all levels of body organization, whether molecular (e.g. regulation of enzymes); cellular (e.g. regulation of cell volume or chemical contents); organ (e.g. regulation of blood flow in the heart or lungs ); systemic (e.g. regulation of breathing in the respiratory system ); and whole body (e.g. regulation of body temperature through shivering and sweating). Some examples include:

• Enzymatic feedback inhibition. At the molecular level, a mechanism called feedback inhibition operates to limit the amount of chemical product produced by an enzyme system. An enzyme system consists of several enzymes that act one after the other to convert a substance into an end product the body needs. Overproduction of the end product is prevented by the inhibitory effect of the end product on the first enzyme in the sequence, which is called the regulatory enzyme. As the end product is used up in other chemical conversions, however, its inhibitory effect on the regulatory enzyme decreases, so that more end product can be formed by the enzyme system. In this way, the level of end product is maintained at a fairly constant level.
• The pH of body fluids. The pH is a measure of the acidity of a solution, or of its hydrogen ion concentration. The normal pH of body cells and fluids is 7.4. When this value steps out of the normal range as a result of the presence of chemical substances that either increase it or decrease it, a feedback control system is triggered to remove or increase the amount of substance so as to bring back the pH to its normal value.
• The amounts of small ions. The cells of the body consist of a nucleus surrounded by a fluid cytoplasm that contains several structures (organelles) required for functioning, all enclosed in a membrane. Cells can be thought of as little chemical powerhouses that require specific amounts of chemical substances, among which several small ions (elements that have a positive or negative charge) that need to be made available either inside or outside the cell membranes. For example, the potassium ion (K⁺) is the major intracellular positive ion (cation) and the sodium ion (Na⁺) is the major extracellular cation. Likewise, the major extracellular negative ions (anions) are Cland HCO₃⁻. Both the amounts of these ions and their transport across the cell membranes are controlled by feedback systems.
• Arterial oxygen partial pressure. Another feedback system regulates the oxygen partial pressure P(O₂) and carbon dioxide partial pressure P(CO₂) in the arteries. This is because the cells of the body obtain their energy by combining oxygen with various foodstuffs, producing carbon dioxide as a waste product, so they need a constant supply of oxygen and also need to have the carbon dioxide removed. Normally, if the arterial blood reaching a special chemical sensor (chemoreceptor), called the carotid body, has a partial pressure of oxygen lower than the normal range or an excessive carbon dioxide partial pressure, then there is an immediate and marked increase in breathing.
• Internal body temperature. Body temperature is regulated by a feedback system that maintains it nearly constant at 98.6 °F (37 °C) throughout the day, week, month, or year. As a consequence, a person can remain active and working in the hot summer months or on cold winter days.

Role in human health

The role of the many feedback systems of the human body is to maintain the homeostasis required for life.

Feedback systems are activated as soon as disturbances occur in the narrow range of conditions which cellular processes need to be able to function at a level required for the continuation of life. For example, the body temperature feedback system will act to make sure that the body's internal temperature is maintained constant in spite of the weather. The Na⁺ feedback regulatory system will adjust the body cells and fluids to the dietary intake of sodium (salt) from foodstuffs. The breathing feedback system will likewise seek to maintain proper oxygen and carbon dioxide pressures and amounts when disturbed, say by heavy exercise. Another feedback system will regulate blood flow and pressure whenever these quantities are increased or decreased by events such as disease, injury, or blood donation. In other words, whenever anything happens that changes the narrow range of the normal values of body parameters, feedback regulatory systems step in to counteract the effect of the change. And if the change is too drastic for the feedback system to handle, the result is disease and even death.

Common diseases and disorders

The function and development of all body components are controlled by a variety of feedback systems. Any malfunctioning of a given regulatory feedback system will not only affect the body part or process directly controlled by that system but will also affect those functions controlled by other related feedback systems. For example, at the organ level, it is known that many use hormones and cytokines as effectors in regulatory feedback loops. It is now believed that these feedback systems are involved in some forms of human cancer. For instance, the disruption of a negative feedback system by a cancer transformation can result in the loss of growth control or in increased malignant behavior of tumor cells. Also, abnormal positive feedback loops can develop that increase tumor growth by allowing the excessive release of stimulatory factors.

KEY TERMS

Acidity— Refers to a compound that is acid or sour. When dissolved in water, acids yield hydrogen ions.

Cell— The individual units from which all the tissues and fluids of the body are formed. They consist of a nucleus surrounded by a fluid cytoplasm enclosed in a membrane. Intracellular refers to all substances found inside the membrane and extracellular refers to all substances outside the cell membrane.

Chemoreceptors— Specialized cells that can detect chemical substances in the body and relay that information to the central nervous system. The substances detected may be external, such as when an individual smells or tastes something; or they can be internal, such as the carotid bodies located in the carotid arteries that detect the amounts of oxygen or carbon dioxide present in the blood.

Cytokine— A protein secreted by tissues or leukocytes (a type of white blood cell) that has a regulatory function.

Enzyme— A type of protein produced by the body that speeds up chemical reactions. Some enzymes regulate certain functions due to their ability to change their activity by modifying their structure.

Homeostasis— Stability of the body's internal environment, achieved by a system of integrated control systems activated by feedback systems. Homeostasis is thus the maintenance of a constant internal environment (the immediate surroundings of cells) in response to changes occurring in a) the conditions of the external environment and b) the conditions of the internal body environment.

Hormone— A substance secreted by specialized glands in the body that affects the behavior of other cells. Hormones serve as messengers to coordinate activities of various parts of the body.

Ion— Elements consist of positively charged nuclei surrounded by negatively charged electrons. These charges are balanced and the overall charge of an element is zero. An element becomes an ion, which is charged, if it gains or losses electrons.

Metabolism— The sum of all the physical and biochemical processes occurring in the body to produce what is required to maintain life. Metabolism includes the transformation of nutrients into energy and the use of energy by the body.

pH— A measure of the acidity of a solution, or of its hydrogen ion concentration. In the human body, the solution can be the blood or the cerebrospinal fluid. The normal pH of body fluids is 7.4.

Resources

BOOKS

Guyton, A. C. G. Guyton and F. Hall. Textbook of Medical Physiology, 9th ed. Philadelphia: W.B. Saunders Co., 1995.

Sherwood, L. Human Physiology: From Cells to Systems. Pacific Grove: Wadsworth, 1997.

PERIODICALS

Carl, A., Lee, H. K., Sanders, K. M., "Regulation of ion channels in smooth muscles by calcium." American Journal of Physiology 271 (July 1996): C9-34.

Freeman, M., "Feedback control of intercellular signalling in development." Nature 408 (November 2000): 313-319.

Singer, C. F., Kubista, E., Garmroudi, F., Cullen, K. J. "Local feedback mechanisms in human breast cancer." Breast Cancer Research & Treatment 63 (September 2000): 95-104.