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Orientation to the Human Body

Homeostasis in the Body

Homeostasis is the ability of the body to maintain a relatively stable internal environment despite changes that occur in the external environment.

One of the fundamental physiological functions of an organism is its ability to maintain a consistent internal environment. There are many potential variations in environment, to which a body must control its reactions. For example, humans must maintain a relatively stable internal temperature, fluid level, pH level, and glucose level. The state of equilibrium in a system used to maintain a consistent internal environment in the body is referred to as homeostasis.

Maintaining homeostasis is a key function of many of the body's organ systems, and every organ in the human body works to reach this goal. Without a relatively stable internal environment many of the body's processes will not function correctly. Enzymes function optimally within a narrow temperature range. Cellular signaling and transport require fairly specific concentrations of electrolytes in the extracellular fluid. Glucose concentrations must be maintained within a certain range in order for cells to access the energy they need.

One way the human body achieves homeostasis is through the use of negative and positive feedback systems. In these feedback mechanisms, the body is responding to a stimulus.

Negative Feedback

A negative feedback mechanism lessens or counteracts the original action of a stimulus on the body.

In order to maintain homeostasis, organ systems make use of negative feedback. A negative feedback mechanism promotes stability within the system by counteracting changes in the body that were originally initiated by a stimulus. A stimulus is a change in environment that is detected by receptors. Examples of these variables can be a change in blood pressure or body temperature. The receptor is an element that detects a stimulus, or change from baseline. For example, thermoreceptor cells detect changes in temperature. Receptors send information to the integrating center. This integrating center is the control center that integrates information received from receptors to determine an appropriate response or set point. In humans, most integration centers are located in various regions of the brain. Irrespective of location, the integration center establishes a set point, which is a determined level or range for a particular variable in order to maintain homeostasis. Following analysis, the integration center determines the most appropriate response to be carried out by effectors, or organs that will respond to the stimulus. Information is sent to the effectors, which serve to carry out the response. Under negative feedback, the effector will continue to carry out this response until the variable is changed.

A thermostat provides a simple example of negative feedback. In this example, the thermostat functions as both the temperature-sensing receptor and the integrating center. When the temperature in a room falls below a programmed set point, the thermostat detects this change and, acting as the integrating center, switches on the heating system (the effector). When the thermostat detects that the temperature in the room has returned to the set point, it turns off the heating system.

Similarly, thermoregulation provides an example of the way negative feedback maintains homeostasis. Humans have a remarkable ability to live in a wide range of climates, including places that are very hot and places that are very cold. Thermoregulation is the process the body uses to maintain a stable internal temperature. When the temperature rises above or falls below a set point, it sends signals via the body's exocrine and nervous systems to counteract the changes. The average set point for body temperature is 37°C. If the body temperature rises above this set point, it will be detected by receptors in the body. The hypothalamus (integrating center), which is a structure found in the brain, will analyze this information. In doing so, the most appropriate response will be determined and carried out by the effectors. The end result of this negative feedback process is that the body's temperature will be restored to its set point.

Negative Feedback in Thermoregulation

A negative feedback loop is a regulatory mechanism that helps the body restore a body value (whether upper or lower) to the ideal body value. For example, this loop can be used in an animal's body to raise its internal temperature in response to a cold environmental stimulus.
Some signals from the hypothalamus activate sweat glands in the skin, which lower the body temperature through the evaporative cooling caused by sweating. Other signals from the hypothalamus trigger vasodilation, which is the dilation, or widening, of blood vessels. This increases blood flow into capillaries close to the skin, where excess heat is lost. On the other hand, if receptors detect a temperature that is too low, the hypothalamus releases signals to trigger the body's heating mechanisms. These signals decrease sweat production and increase vasoconstriction, which is the constriction, or narrowing, of blood vessels. This redirects blood flow away from the periphery and toward the body's core. Other signals initiate rapid muscle contractions, or shivering, which produce heat. Some signals even raise hair follicles. In animals with fur, this traps heat in an insulating layer; in humans, it produces goose bumps. The body starts to shiver and produces goose bumps as methods to raise the internal body temperature to the core body temperature, or set point. Shivering involves muscle contraction, which generates heat to warm the body. Goose bumps form as a result of tiny muscles that cause hair on the skin to stand upright. This position helps trap air, creating an insulating layer on the skin that functions to minimize heat loss from the body.

Positive Feedback

A positive feedback mechanism amplifies a stimulus, thereby increasing the original action of stimulus on the body.

In contrast to negative feedback, positive feedback is a mechanism that amplifies a physiological signal by increasing the body's response to a stimulus. In a positive feedback loop, a small variation from the baseline state triggers a signal to increase the variation. The squeal produced when a microphone is placed too close to a speaker is an example of positive feedback. When the microphone detects a sound, the speaker amplifies it. The microphone then detects the amplified sound and continues to produce the noise called feedback.

Although less common, there are positive feedback loops in the human body. For example, during childbirth, the pressure of the fetus's head on the cervix stimulates nerve impulses that are sent to the hypothalamus. The hypothalamus stimulates the posterior pituitary gland to release the hormone oxytocin. Oxytocin causes the uterus to contract more vigorously. Uterine contractions continue to push the fetus against the cervix, amplifying contractions until the baby is delivered and the positive feedback loop is broken. Another example of positive feedback occurs during blood clotting. When there is a break or cut in a blood vessel, blood clotting is a response to this injury. Chemicals are released from the blood vessel wall to initiate blood clot formation. Through the action of positive feedback mechanisms, more chemicals are released, which results in the formation of a blood clot. This process of more chemicals being released is a result of a cascading, or waterfall, effect that leads to certain chemicals initiating even more chemicals to be released. Once the clot is formed, the feedback process ends when an anticoagulant inactivates any excess chemicals that would have continued to form the blood clot. The blood clot will also absorb any excess chemicals that form the blood clot to prevent the clot from growing in size.

Positive Feedback During an Injury

When a person injures themselves and a blood vessel wall is damaged, the positive feedback system will function to recruit platelets (a component of blood that helps form a clot) to the site of injury as a way to facilitate wound healing. Positive feedback systems are implemented to increase the direction of the original stimulus.