Homeostasis

Constant conditions inside

Homeostasis is the maintenance of a stable internal environment in an organism, and it mainly applies to animals, especially mammals.

The cells within the body have requirements for certain chemical substances and physical conditions, and these are needed 24 hours a day.

Even if we only eat at mealtimes, cells need glucose constantly.

We keep breathing without thinking much about it, and we need to be able to breathe faster or more deeply when necessary. This keeps an even concentration of oxygen in the blood, to be used by all the cells of the body.

We don't have to do anything to make our heart speed up or slow down.

Our body temperature needs to stay the same no matter what the weather is.

Likewise, we do not think about the pH of our blood!

Most of these changes take place automatically. We take most of these things for granted, unless something seems to go wrong and we need to visit the doctor!

Inputs and outputs

We take things into the body to provide raw materials for the everyday processes performed by cells, organs, and tissues, and we expect to pass out things which we do not want.

Some simple substances which we regularly take into the body - water, salt and other inorganic ions - are usually ingested in greater quantities than the body requires, and keeping their concentrations in balance - part of homeostasis - requires the removal of the excess.

The main classes of organic foods: carbohydrates, fats and proteins, are also likely to be consumed in larger amounts.

Some food items which are in excess of the body's requirements may be simply stored (possibly for future use!).

Others may be converted into classes of compound which the body does require. This may result in the buildup of certain chemical compounds which need to be removed. An example is urea, produced by deamination of excess amino acids.

As a matter of fact, the process of getting rid of waste formed within the body (excretion) - carried out mainly by the kidneys and the lungs and skin - is part of the homeostatic process.

Sometimes even heat may be considered as a waste from the processes of living, and removing the excess is part of homeostasis.

Physiological control systems

It is said that homeostasis involves physiological control systems, responding to variations in conditions within the body.

In other words the functioning of the body at a cellular or chemical level is fine-tuned by interaction with other cells and chemicals so as to minimise or counteract the effects of changes.

This can be achieved using either hormones or the nervous system.

Hormones are chemicals produced by cells (in endocrine or ductless glands) in one part of the body and distributed via the bloodstream to the rest of the body. They can trigger a response in target organs containing cells which have the appropriate receptors (the tertiary structure of which is complementary in shape to the hormone molecules) on their cell surface.

Hormones can have a general effect on organs in different parts of the body and the response is fairly slow but longer-lasting.

The nervous system consists of a number of different types of neurones (nerve cells).

Some cells (receptors) located in particular locations in the body respond to various chemical and physical factors. Chemoreceptors respond to chemicals such as hydrogen ions, and baroreceptors react to pressure. They generate a stream of information about any changes in the conditions. This takes the form of nerve impulses, which generally vary in frequency as a result.

Often these impulses are passed to a specific control centre which co-ordinates information and then relays impulses to an effector organ, as well as inhibiting the reverse action. This then influences organs which are at the end of the chain of neurones, and this is control is a result of special chemical substances which are released.

It is a more direct pathway and it can bring about a specific localised and speedy response which is short-lived.

Keeping things in proportion

In Biological systems control systems do not operate as an on-off system like a thermostat with a single cut-off point set to operate a particular level. They act more as proportional control systems in that they vary the degree of their response to match the deviations from the desired state of the body systems.




A small variation may be dealt with by the release of a small amount of a controlling hormone, whereas greater amounts will be released in response to a greater variation.






The nervous control of organs depends on a stream of impulses which vary in frequency. The more frequent impulses cause a more intense response from the organ under control.

Enzymes need to be mollycoddled to make the body run efficiently

Enzymes are key to all the biochemical processes that take place in the body.

Each enzyme has an optimum value for pH and temperature to which they are exposed, and departures from this would result in inefficient metabolism.

pH control

The homeostatic regulation of the pH of the body's blood and extracellular fluid (ECF) is called
acid-base homeostasis.

The blood is normally slightly alkaline: pH about 7.4.

Respiration inside cells produces carbon dioxide which is converted into carbonic acid, which would cause the blood's pH to fall.

Mechanisms which contribute to keeping the pH of blood within limits are the breathing out of air containing carbon dioxide at the lungs, and the excretion of hydrogen ions H⁺ into the urine by the kidneys.

The concentration i.e. partial pressure of carbon dioxide in the arterial blood is monitored by the central chemoreceptors of the medulla oblongata, part of the central nervous system, and they send information to the respiratory centres in the medulla oblongata.

The respiratory centres control the rate and depth of breathing. Impulses are sent along motor neurons which activate the muscles involved in breathing (in particular the diaphragm).

Keeping the partial pressure of carbon dioxide in the arterial blood constant effectively maintains the blood at the correct pH.

Temperature control - Thermoregulation

There are several separate processes which cool the body or conserve heat or cause the release of more heat, depending on the body's temperature. Most of these are brought about as a result of nervous impulses sent from the hypothalamus.

Vasodilation of blood capillaries running near to the surface of the skin allows heat to be lost by radiation.
Sweating causes heat to be lost as water evaporates from the surface of the skin.

In order to maintain heat within the body, hairs on the skin are made to stand on end by erector pili muscles, thus forming a thicker layer of insulation which traps air - a poor conductor of heat. This is accompanied by vasoconstriction - reduced blood flow in superficial capillaries.

Shivering - repeated muscular contractions generates heat internally in response to cold conditions.
Brown adipose tissue contains fat deposits and a large number of mitochondria. It produces heat by non-shivering thermogenesis.

A bit more detail

There are several ions in blood plasma which function as buffers by virtue of the ratio of their concentrations. The bicarbonate buffer system holds the blood's pH at this value with a HCO₃^- (bicarbonate) to H₂CO₃ (carbonic acid) ratio of 20:1.

Acid(a)emia is when blood pH drops below 7.35, and alkal(a)emia is when blood pH rises below 7.45.

The kidneys also play a role in retention of bicarbonate ions HCO₃^- in the blood.



A rise in the partial pressure of carbon dioxide in the arterial blood plasma above 5.3 kPa (40 mmHg) causes an increase in the rate and depth of breathing. Normal breathing is resumed when the partial pressure of carbon dioxide has returned to 5.3 kPa

Maintaining a stable blood glucose concentration

Again there are two sides to this process: ensuring the availability of glucose as a respiratory substrate, and keeping the water potential of blood constant.

Control of blood glucose is achieved by interactions between the pancreas and the liver (and muscles).

There are two hormones produced in (different) islet cells in the pancreas: insulin and glucagon. These act in different ways.

Insulin causes excess glucose to be taken out of the blood and stored in the form of glycogen in the liver, whereas glucagon causes the hydrolysis of glycogen, releasing glucose back into the bloodstream when it is needed.

Osmoregulation

Dissolving glucose in water reduces its water potential, and if the concentration of glucose is increased, then the water potential would fall even lower. This would tend to draw water out of body cells by osmosis. Converting some of the soluble glucose into insoluble glycogen reduces the effect on the water potential of blood.

Of course the intake of water by drinking or in food has the effect of raising the water potential of blood.

The kidneys vary the reabsorption of water into the blood stream under the influence of the hormone ADH, and effectively divert excess water into the urine so that excess is excreted.

Negative feedback restores systems to their original state

Negative feedback is not criticism!

Negative feedback control systems are often shown in a loop format based on a central line, which signifies the passage of time.





If the state remains the same, no action is required.

The diagram shows separate mechanisms above and below the central line, each involving a negative feedback control.

Cells or organs in one part of the body monitor the state of the system. This could be the blood glucose concentration, the pH of the blood, the body's temperature or the water potential of blood.

If a change in state is detected, the monitoring cells/organs send information in either a chemical way by releasing hormones into the blood or an electrochemical way by sending nerve impulses along neurones to target organs.

Target organs respond by bringing about actions which cause the state to change in the opposite direction to the change detected.

In this way departures in different directions from the original state are brought under control.

Positive feedback

On the other hand, positive feedback results in a more intense response, by producing a greater deviation from the normal state.