How the Body Controls Its Internal Environment

How the Body Controls Its Internal Environment

A competitive tennis match or working out hard in the gym can pose a challenge to various of the body’s control systems since exercise increases metabolic demands (e.g. lactic acid build up) thereby challenging homeostasis. Therefore, if one understands how things are being processed inside the body then one can understand what needs to be adjusted to make the athlete stronger or to prevent the ill effects and maximize the positive effects.

© Mark Howard Photography
© Mark Howard Photography

Homeostasis vs. Steady-State

When the body’s biological control systems maintain physiological variables at manageable constant values at rest, it is called homeostasis.

More specifically, the term homeostasis describes the maintenance of an unchanging/constant “normal” internal environment by a biological control system, where a physiological variable (e.g. body temperature) remains within a normal range when the body is not under any stress (e.g. at rest).

On the other hand, the term steady state describes the maintenance of an internal environment by a biological control system, where a physiological variable (e.g. body temperature) remains relatively constant yet deviates from its normal value, which occurs when the body is experiencing stress (e.g. exercise).

Due to the external stress the body cannot maintain a normal constant internal environment and has to make adjustments in order to retain a constant environment under stress, where the body’s response to the external stress is equal to its magnitude.

In other words, the term homeostasis is being used to describe the body’s maintenance during resting conditions whereas the term steady state refers to the body’s maintenance during exercise; exercise is a stressor which disrupts homeostasis, also called homeostatic disturbance (e.g. an increase in temperature).

More specifically, homeostatic disturbances occur at the cellular level causing a temporary decrease in O2 levels, whereas the CO2 levels go up, which causes acidity levels to increase.

The body’s metabolic changes are regulated by biological control systems in the same manner an air conditioning unit responds to changes in outside temperature to maintain indoor temperature at a certain constant level.

Biological Control Systems

In order to track metabolic changes the body has numerous biological control systems that monitor the body’s internal environment.

The biological control systems’ task is to maintain a physiological variable (e.g. blood pressure or body temperature) at a manageable constant value.

How exactly these control systems operate remains under investigation but each biological control system consists of a(n):

  1. Receptor
  2. Control center
  3. Effector

When a homeostatic disturbance in the internal environment occurs a signal arises, called stimulus. The stimulus excites the receptors, which send a message to the control center.

The control center assesses the strength of the stimulus (message, situation) and sends out an appropriate response to the effector to correct the disturbance, thereby returning the internal environment back to” normal” (manageable constant values), hence removing the stimulus.

Negative Feedback Loop

The response procedure of the control system to a stimulus is called negative feedback loop because the response of the control system decreases the initial stimulus, which initially caused the control system to respond, in order to return the internal environment back to normal.

In other words, the control system’s response is opposite to the stimulus. If the stimulus increases a variable then the control system will respond by decreasing the variable back to normal values and vice versa.

For example, a homeostatic disturbance such as an increase in the cell’s acidity provokes an integrative system response designed to restore (decrease) acidity/alkalinity values (normal blood pH level of 7.4) back to normal, thus the nature of the response is in an opposite direction to the homeostatic disturbance.

The precision and capability by which the control systems operate is called the gain of the control system.

The higher the gain, the higher the capabilities of the control system.

Summary of control system operation:

  1. Homeostatic disturbance occurs causing a stimulus
  2. Stimulus excited receptors
  3. Receptors forward message to control center
  4. Control center assesses situation
  5. Control center sends message to effector (messenger; e.g. hormones)
  6. Effector corrects disturbance and removes stimulus

Example of Control System Operations: Regulation of Blood Glucose

Blood glucoselevels are being regulated by a control system, called the endocrine system.

The endocrine system consists of eight (8) major glands (hypothalamus, pineal gland, pituitary gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, testes [male] & ovaries [female]) throughout the body that can make (synthesize) and release chemical substances, called hormones (messengers), into the blood stream.

Then the hormones are being transported via the blood stream (circulatory system) to an effector, signaling the effector to cause a certain metabolic response.

Let’s assume you eat a high-carbohydrate (glucose) meal. Once the food is digested blood glucose levels rise above normal.

The rise in blood glucose levels causes a disturbance, which stimulate receptors in the pancreas.

The receptors send a message to the control center in the pancreas, which analyses the situation and orders the synthesis of a hormone, called insulin, which is then being released into the blood stream and transports glucose to cells throughout the body.

Insulin then signals the cells to absorb glucose from the blood stream into the cells, which causes blood glucose levels to return to “normal”.

Failure of the blood glucose control system causes a disturbance in homeostasis, which results in disease, called diabetes mellitus (type I & II).

Both types of diabetes are caused by high blood glucose concentrations, called hyperglycemia.

In type I diabetes, the beta cells in the pancreas that synthesize the hormone insulin are damaged, which means that the body cannot produce any more insulin to decrease blood glucose levels.

This example shows a failure of the effector component.

In type II diabetes, insulin is being produced but the cells have become resistant to insulin, which means that the cell receptors don’t respond to absorb glucose from the blood stream.

Picture Credit

© Mark Howard Photography @Flickr