Control of homeostasis.

HOMEOSTASIS

According to totora ………. Homeostasis can be defined as “ the condition in which the body’s internal environment remains constant, within physiological limits”. Homeostasis is a dynamic condition as the body’s equilibrium point can change over a narrow range that is compatible with maintaining life. E.g., the level of glucose in the blood does not normally fall below 70mg of glucose per 100ml of blood.

Each body structure helps to contribute to keep the internal environment within its normal limits. For example, if the level of blood glucose within the drops then the body compensates it by using fats stores as a reserve supply of glucose

Body fluids

An important aspect of homeostasis is maintaining the volume and composition of body fluid. The fluid within cells is called intracellular fluid (ICF). The fluid outside the body cell is called extracelluar fluid (ECF). Dissolved in the ICF and ECF are substances needed to maintain life such as O2, nutrients, proteins and electrally charged ions.

Bernard predicted – the proper functioning of the body depends on precise regulation of the composition of the surrounding fluids of cells, known as interstitial fluid (internal environment). The composition of interstitial fluid changes as substances move in and out of the cell. For example, there is an exchange of materials across the capillary walls. This exchange of movement in both directions provides the capillary with much needed ions such as glucose and O2 to tissue cells and removes wastes such as CO2 from interstitial fluid.

Control of homeostasis

Homeostasis in the body is continually being disrupted. The disruption may come from the external environment such as the lack of O2 or the lack of heat. In most cases the disruption of homeostasis is mild and temporary and the body cells can restore the internal environment. In other cases, the disruption of homeostasis may be prolonged and intense such as the overexposure to extreme temperatures leading to death. Under theses circumstances homeostasis has failed.

Fortunately, the body has many regulating systems that bring the internal environment back into balance. Most often the nervous system ...

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Control of homeostasis

Fortunately, the body has many regulating systems that bring the internal environment back into balance. Most often the nervous system and the endocrine system work together or independently to provide corrective measures when homeostasis is disrupted. For example, the nervous system regulates homeostasis by detecting deviations from the balanced state and then sending theses messages in the form of nerve impulses to organs that counteract the deviations. The endocrine system secrete hormones into the blood which also regulate homeostasis. For example, there is an increased calcium response when concentration in the blood rises above 9-11 mg/dl. This is done to obtain homeostasis.

Feed back systems

The body can regulate its internal environment through feedback systems. A feedback system is a cycle of events in which the condition of the body is monitored, changed, re-monitored and re-evaluated. Each monitored variable such as temperature, blood glucose and blood pressure etc is termed controlled condition. Any disruption that changes a controlled condition is called a stimulus. Three basic components make up the feed back system – a receptor, a control center and an effector.

Receptor – is a body structure that monitors changes in a controlled condition and sends input in the form of nerve impulses or chemical signals to the control center. E.g., nerve ending in the skin sense temperature.

Flow of the message

Control center – a control center in the body sets the range of values within which a controlled condition should be maintained, evaluated from its input received by the receptors and generate the output commands for when they are needed.

Effector – is a body structure which receives output from the control center and produces a response that changes the controlled condition. For example, when the temperature of the body drops your brain (control center) sends nerve impulses to your skeletal muscles (effectors) that cause you to shiver.

In order for a control system to function there must be receptors to monitor changes (stimuli) in the internal environment. These receptors send information to a control center which monitors the change and integrates the information. The control center then sends information to effectors which generate a response to the change in the internal environment.

The group of effectors and effectors communicating with their control center forms a feedback system that can regulate the controlled condition in the body’s internal environment. Feed back systems can either form positive or negative systems.

Negative feedback system

In negative feedback a change in the internal environment (stimulus), is monitored by the receptor and transmitted to the control center. The control center then evaluates the input and if necessary the issues output command to an effector generating a response which cancels the effect of the stimulus (change) and re-establishes the homeostatic condition. Negative feedback acts by negating or eliminating the stimulus (change). For example, (see endocrine example below)

1) Endocrine System

The concentration of calcium in the blood is maintained at 9-11 mg/dl. This is the homeostatic condition for Ca++. This homeostatic condition is maintained as follows:

A) Increased Ca++ -- Response when concentration in the blood rises above 9-11 mg/dl

B) Decreased Ca++ -- Response when concentration of blood calcium drops below 9-11 mg/dl

Positive feedback system

This system tends to strengthen or reinforce a change in one of the body’s controlled condition. A change in the homeostatic condition is detected by receptors and the information is transmitted to the control center. The control center activates effectors which generate a response which increases the stimulus further reinforcing the initial change.

An example of positive feedback is the action of the hormone oxytoxin on the uterus during birth. During normal conditions the uterine muscle is passive and not contracting. Dilation of the cervix triggers stretch receptors which transmit nerve impulses to the Brain (hypothalamus). Stimulation of the hypothalamus results in oxytoxin (OT) being released from the posterior pituitary. Oxytocin is carried by the blood to the uterus where it causes uterine contractions. During the birth process dilation of the cervix initiates the release of OT which causes uterine muscle contractions. The uterine contractions begin to force the fetus through the cervix. As the fetus is pushed through the cervix this further stretches the cervix which results in more oxytocin being released. This positive feedback will continue until the baby has cleared the birth canal and the cervix is no longer stretched.