Homeostasis

Homeostasis

The internal environment of the human body must stay relatively stable for the person to survive. Homeostasis is a term that describes a stable internal body environment. It requires a constant balance. There must be normal concentrations of nutrients, oxygen, and water. Heat and pressure must be regulated at tolerable levels. Homeostatic mechanisms regulate the body by neg-ative or positive feedback.

Homeostatic Control

For homeostasis to occur, the body primarily uses the nervous and endocrine systems. These systems allow forms of communication that control homeostasis to occur. The nervous system uses neural electrical impulses for these activities, whereas the endocrine system uses blood borne hormones. The nervous system handles rapid, short term, extremely specific responses. The endocrine system responds more slowly, but its effects last for a longer duration. The event or factor that is being controlled (regulated) is referred to as the variable.

The two basic components of homeostatic control are as follows:

■■ Autoregulation: Also known as intrinsic regula-tion, this occurs when a body structure or sys-tem adjusts its activities because of some change in its environment. For example, declining tissue oxygen levels cause cells to release chemicals that widen local blood vessels, thereby increasing blood flow rate, which provides more oxygen to the local area of the body.

■■ Extrinsic regulation: Related to nervous or endo-crine system activity, these systems influence activ-ities of many other body systems simultaneously. For example, increased exercise causes the ner-vous system to increase the heart rate and circulate blood more quickly. It also reduces blood flow to the digestive tract and other less active organs. Oxygen in the circulating blood is then available to the active muscles, which need it the most.

All mechanisms used for homeostatic control involve at least three components:

■■ Receptors: These are “sensors” that monitor theinternal body environment and respond to stim-uli. Receptors send information to the control cen-teralong the afferent pathway. You can rememberthis more easily because the afferent pathway car-ries information that is “approaching” the control center.

■■ Control center: This is a point in the body thatdetermines the set point (the range or level at which a variable must be maintained). It analyzes the input from the receptors to determine appro-priate responses or actions. It then sends informa-tion to effectors via the efferent pathway. You can remember this more easily because the efferent pathway carries information that is “exiting” the control center (FIGURE 1-2). The set point for the average body temperature, for example, is 98.6°F (37°C). Another set point is normal adult blood pressure, which is ideally below 120 (systolic) and under 80 (diastolic).

■■ Effectors: These are components of homeostaticcontrol that allow the control center to respond to stimuli. The control center’s response involves negative (reducing) or positive (enhancing) feed-back. Basically, negative feedback shuts off the control process, whereas positive feedback makes it occur at a faster rate.

Negative Feedback

A negative feedback mechanism is one that prevents the correction of deviations from doing too much (which could possibly harm the body). Most of the feedback mechanisms of the human body use nega-tive feedback. Examples of negative feedback are blood pressure regulation, erythropoiesis (the production of red blood cells), body temperature regulation (ther-moregulation), and control of blood glucose levels. The hypothalamus of the brain maintains homeostatic con-trol of body temperature. Information is received from temperature receptors in the skin and within the hypo-thalamus itself. The normal set point of body tempera-ture is approximately 98.6°F (37°C). When temperature rises above this normal, hypothalamic activity targets muscle tissue in walls of blood vessels that supply the skin, and also targets the sweat glands. This causes blood flow to increase near the body surface, and accel-eration of sweat gland secretion. The skin loses heat to the environment, and sweat evaporation speeds up this process. As temperature lowers back to normal, hypo-thalamic activity declines, and all processes reverse.

Negative feedback is the main controller of homeostasis, providing long-term control over inter-nal systems and body conditions. Minor variations are usually ignored, while normal body ranges are main-tained instead of exact, fixed values. The regulatory process works dynamically since set points vary with changes in environment and activity. While sleeping, thermoregulation has a lower set point than when you are awake and active. Therefore, temperature varies because of small fluctuations around the set point, or change in the set point. Similar variations occur throughout all body physiology.

Set points differ between individuals based on age, gender, genetic factors, overall health, and the envi-ronment. There are no actual precise homeostatic con-ditions. Basically, homeostatic values are either based on average between large amounts of people, or as a range including 95% (or more) of those people being sampled. While most healthy adults have body tem-perature between 98.1°F and 98.9°F (between 36.7°C and 37.2°C), 5% have resting body temperatures above or below this range.

Positive Feedback

A positive feedback mechanism is one that makes conditions move away from the normal state to stimulate further changes. They are usually short-lived and extremely specific actions, producing extreme responses. A positive feedback mechanism is defined as one that results in or responds in an enhanced way to the original stimulus, accelerating the result or response. Examples of positive feedback are the onset of contractions before childbirth, the process of blood clotting, lactation, the secretion of estrogen during the follicular phase of menstruation, and the generation of nerve signals.

In positive feedback, cycles escalate and are often referred to as being part of a positive feedback loop. These loops are usually found when a possibly stressful or dangerous body process must be completed quickly prior to homeostasis being restored. One example is a severe laceration, which may lower blood pressure and reduce the heart’s effectiveness. As the clotting process attempts to combat the loss of blood, a positive feed-back loop occurs, which increases the clotting activities.

Homeostatic Imbalance

Most diseases occur because of homeostatic imbalance (meaning the disturbance of homeostasis). Aging causes body systems to become less efficient and more uncontrollable, resulting in instability in the internal body environment and increasing the risk for illness. Also, when helpful negative feedback mechanisms become overwhelmed, certain destructive positive feedback mechanisms can dominate (such as those seen in some forms of heart failure). Additional examples of homeostatic imbalance include abdom-inal injury due to physical trauma (and lack of pro-tective bones in this body region), sepsis (resulting in severe pain, such as in peritonitis), and metabolic acidosis or alkalosis (which can affect all body systems and lead to death). Trauma may involve hemorrhage and perforation of abdominal organs. Any cause of homeostatic imbalance can result in death if untreated.

1. Define homeostasis.

2. Why is homeostasis essential to survival?

3. Describe two homeostatic mechanisms.