Concept of Regulation

The homeostatic control mechanism involves a regulator which compares the actual output with a set point. Then, it produces some sort of error signal, which sends information to the corrective mechanism or effector regarding the difference between the set point and the actual output. The error signal is usually in the form of nerve impulses or hormones in the body. The corrective mechanism may include one or more effectors that set the controlled system and restore the output to its set point. In some physiological processes, such as temperature regulation, separate but coordinated mechanisms control deviations in different directions from the set point such as the rise or fall in body temperature, and lead to a greater degree of control.

The corrective mechanism explained above is said to be the key component of homeostatic control mechanism. It varies the output so that it can be brought back to the set point. Homeostasis is a dynamic process which works by making continual adjustments to compensate for fluctuations of output. Thus, it is more accurate to describe the homeostatic system as being in a steady state or in a dynamic equilibrium rather than being constant. In addition, homeostatic controls can be either extrinsic or intrinsic. Extrinsic control is one which originates from outside of the body organ or tissue while the intrinsic control is the one which originates from within the body, organ or tissue.

Any change from the set point activates the control system to initiate a sequence of events so as to either restore conditions towards their normal state, or to make the system deviate further. Feedback requires the action of the system to be related to a reference point or set-point (optimum level) of the variable being controlled.

Two forms of feedback, namely negative and positive feedback are recognised.

Negative feedback

A negative feedback occurs in a situation where the disturbance in a system sets in motion a sequence of events which tends to restore the system to its original state. A negative feedback in homeostatic control mechanism keeps a variable, such as the blood glucose level close to a particular value or set point.

A change in the state of an internal factor in the body causes effectors to restore the internal environment to its original state. For instance, an increase in the level of glucose in the body triggers a sequence of events that tend to remove excess glucose from the blood by converting it into glycogen. In contrast, a decrease in the level of glucose in the body causes the liver to break the stored glycogen to glucose in order to supply more glucose to the cells of the body. This corrective measure allows blood glucose level to remain constant. This type of system in which change in the level of an internal factor causes a corrective mechanism is referred to as a self-adjusting system. The science of a self-regulating control system which operates via feedback mechanisms in organisms is known as cybernetics.

Positive feedback

Positive feedback is the self-regulatory mechanism which operates when the system is deviated from a set point which initiates a sequence of events that tends to deviate further the system. The positive feedback mechanism makes the system to be unstable; that is why it is not common in living organisms. An example of positive feedback occurs during labour, when the hormone oxytocin stimulates muscular contraction of the uterus; which in turn stimulates the release of more oxytocin. Positive feedback mechanism may also occur in the nerves where a small stimulus can bring about a large response to the effectors.

Most organisms can survive in a narrow range of temperature from 10 °C to 30 °C. In order to survive, most animals have to regulate their body temperature. The tendency of animals to adjust their body temperature to suit the living conditions of varying temperatures in the environment is termed as temperature regulation (thermoregulation). Animals with a varying body temperature according to the changes in the environmental temperature are called poikilotherms (poikilos means 'various' and thermo means 'heat'). These animals obtain most of their body heat from the sources outside their bodies; hence they are sometimes referred to as ectotherms. Examples of poikilothermic organisms include reptiles, amphibians and most fish. With exception of birds and mammals, most animals are ectotherms.

Maintenance of a constant body temperature in warm environments

When the environment is overheated, the animals use the following adaptive mechanisms to overcome the effects of overheating.

1. Vasodilation This is an increase in the diameter of superficial blood vessels near the body surface caused by nerve signals, resulting into the relaxation of the vessel's walls. The blood in the capillaries in the skin may take three alternative routes; through capillaries close to the skin surface, in the dermis, and beneath the layer of subcutaneous fat. In warm climates, superficial arterioles dilate in order to allow blood flow close to the skin surface. Heat from the blood is rapidly conducted through the epidermis to the skin surface from where it is radiated away from the body. Rise in blood pressure within the capillaries cause them to dilate, that facilitate heat loss due to radiation, convection and conduction resulting into an increase of blood flow near to the skin surface. In cold environments, the blood flow in the body escapes the skin through the shunt vessels, resulting into reduction in heat loss. Just a small amount of blood passes into the skin to keep the tissue alive.

2. Sweating Human beings have the ability to control body temperature through sweating because their skins are not covered by fur or feathers. They have sweat glands over the whole body that enable them to be more efficient at cooling through sweating. The human being can produce about 1000 ml of sweat per hour. Animals with fur have limited sweat glands which are confined to areas that do not have fur, for example pads of the feet in dogs and cats. Animals with feathers such as birds lack sweat glands. Their skins are covered by feathers which prevent evaporation through the skin (evaporation occurs from the surface of their lungs and air sacs).

3. Panting and licking In animals with few or no sweat glands such as dogs and birds, cooling by evaporation takes place through the mouth and the nose. Dogs hang out their tongues; this may result in an increase of breathing rate and excessive removal of carbon dioxide from the blood thereby reducing heat from the body. Some animals lick their bodies to deposit saliva onto their body surfaces, which provide similar means of evaporative cooling. Licking is common to some animals that do not sweat, instead they make use of saliva to cool their bodies. For example; rabbits lick their front legs and chests, cats lick inside of their front paws and spread the saliva across their ears and face, rats lick their testicle, and kangaroos lick their fore arms and wrists.

4. Use of body extremities When compared to related species from cold climates, animals in warm climates usually have large extremities such as ears and large bushy tail. They are well supplied with blood vessels and covered by relatively short hairs, making them good radiators of heat.

5. Large surface area to volume ratio Animals with a large surface area to volume ratio (relatively small animals) lose energy (temperature) faster than those with smaller surface to volume ratio. To compensate for this, small animals such as mice feed more frequently compared to large animals such as lions. The former animals also tend to utilize an energy-rich diet such as nuts rich in lipids.

Maintenance of a constant body temperature in cold climates

Endotherms living in cold environment and those in hot climates experiencing cold weather have the following adaptations which enable them to maintain constant body temperatures:

1. Vasoconstriction When the animal is subjected to cold conditions, the superficial arterioles are constricted. This reduces the quantity of blood reaching the skin surface. Much blood passes beneath the insulating layer of subcutaneous fat; therefore, little heat is lost to the outside.

2. Shivering In cold conditions, the skeletal muscles of the body may undergo rhythmic involuntary contractions which increase the amount of heat produced in the body.

3. Insulation It is achieved by an external covering of fur or feathers and or an internal layer of subcutaneous fat. Their thickness is related to the intensity of coldness to that environment. It is an effective means of reducing heat loss from the body.

4. Increased metabolic rate During cold conditions, the liver increases its metabolic rate. There is also increased activity of the adrenal, thyroid, and pituitary glands resulting into secretion of hormones that help to increase the body metabolic rate; hence, additional heat is produced in the body. The increased metabolic rate of the body requires increased food consumption; that is why animals feed on large amount of food in cold climates.

Temperature regulation by ectotherms

Ectotherms regulate their body temperature mainly by behavioural means, depending on external heat sources; since these organisms do not have temperature control center like endotherms. The exchange is controlled by three factors which are; radiation, conduction and flow. Their body temperature rises and falls along with the temperature of the surrounding environment. Although they generate metabolic heat like endotherms, they cannot increase heat production to maintain an internal body temperature.

Most of the adjustment mechanisms are by behavioural means such as: huddling, hibernation, burrowing, aestivation, clustering, migration, and exposing themselves to the sun in hot environment.

1. Huddling In cold regions, animals are usually active during the day. Huddling of individuals is also another common way of reducing heat loss. Some animals are able to crowd together in a tightly packed group to keep them warm and reduce much heat loss when an individual animal is exposed to cold open air. Therefore, this is also a means of conserving heat.

2. Hibernation Some animals in cold climates undergo a period of long sleep. During this time, the metabolic rate is reduced 20-100 times below normal which consequently reduces food and oxygen utilisation.

3. Sun basking Ectothermic animals may use radiant heat provided by the environment to warm their bodies. Solar radiation is the most common way, as many ectotherms use the sun's rays to warm up their bodies. Reptiles and some amphibians bask in the sun with their bodies spread out to increase the surface area for heat absorption. When it is too hot, they hide in the shade or near water bodies; allowing their bodies to cool.

4. Clustering Some animals exhibit group behavioral mechanisms to regulate their body temperatures. A good example is how honey bees cuddle together in large groups to retain and generate heat. A similar example is how some gregarious caterpillars bask in the sun in large groups to gather heat.

5. Burrowing Some ectotherms burrow themselves and hide deep in the ground. This helps them to survive in cold environments.

6. Aestivation Some ectothermic animals like earthworms, snails, frogs, crocodiles, lizards, and tortoise maintain their body temperature by reducing body metabolic activities and protecting themselves from very high temperature. During summer time, some animals usually tend to rest in shady or cool places. Normally, they take a sleep during the hot hours of daytime as a means of avoiding environmental stress