Homeostasis

Homeostasis is one of the fundamental properties shared by all organisms on earth. It can be defined as ‘the ability to maintain relatively constant internal conditions (i.e. different from their environment)’. It is an example of the self-regulating process by which every organism maintain stability. It is an approach to encounter environmental variation and to maintain steady state internal environment. In one line, it is said that if homeostasis is successful then life continues. Homeostasis is a part of maintaining dynamic equilibrium. As a result of dynamic equilibrium, the organism maintains steady state condition i.e. the condition which resists change in outside condition. 

Upon the change in environmental condition, body system (for example, feedback control) would be activated and they respond in such a way that establishes a balance in the body. All process that is responsible for maintaining inner balance, whether mediated by nervous and/ or hormonal systems are examples of homeostatic regulation. There are different mechanisms to maintain homeostasis such as physiological, morphological, or behavioral mechanisms. For example, using a behavioral mechanism by beetle to cope with drastic changes in water availability. Many organisms tend to conform to the different aspects of surroundings like temperature, salinity etc. 

Different organisms use different mechanisms to maintain homeostasis inside their body. These responses can be seen over the short term as well as long term. The time period in the short term is often a few minutes and primarily expressed through a variety of coping mechanisms. In long term response, natural selection can take place and it might result in the population having better adaptation to the environment.

Mechanisms of homeostasis

1. Physiological Mechanism

Many organisms maintain homeostasis by making physiological adjustments. For example, some insects have glycerol in their blood as an antifreeze in order to maintain body temperature. Humans, who are at high altitude, may initially suffer from symptoms like heart palpitation, nausea, fatigue, mental impairment, and in a severe case, pulmonary edema due to low availability of oxygen at high altitude. After several days, these symptoms will overcome due to several physiological changes that take place in the human body at high altitude in order to increase the amount of oxygen delivered to body tissues:

• Increased rate of breathing,

• Increased red blood cell (RBC) production and hemoglobin (Hb) in the blood,

• The density of mitochondria, capillaries, and muscle myoglobin increase at higher altitude.

• At high altitude, oxygen binding capacity of Hb decreases, and hence, the rate of oxygen unloading in body tissues will increase.

2. Morphological Mechanism

Endothermic animals (that maintain a constant internal temperature in a cold environment) are adapted such that they tend to minimize energy expenditure. During winter, certain mammals tend to hibernate themselves and in this way, they maintain constant body temperature without utilizing body energy. Insulation of body (e.g. wolf) is also an example of this mechanism in which some animal grows thicker fur to avoid energy loss while maintaining body temperature. Wolf has three times thicker fur in winter as that of in summer.

3. Behavioral Mechanism

Tropical lizard shows behavioral changes to maintain homeostasis. Many animals tend to migrate from one habitat to another habitat, from unfavorable habitat to favorable and suitable habitat. Tropical lizard maintains uniform body temperature in an open habitat by exposing in sunlight followed by entering into the shade when their body temperature rises. This adaptation can be very extreme. The spadefoot toad (Scapbiopbus) lives in deserts of North America. They tend to live nearly a meter below the surface for 9 months every year, however, they emerge and breed when moist condition cool condition return.

4. Long-term mechanism to environmental variation

This mechanism involves evolutionary responses to environmental variation and it is a result of natural selection. The ability to maintain homeostasis through physiology, morphology, or behavior is a part of an evolutionary adaptation. The effect of natural selection can be seen by comparing closely related species that live in different environments, in which significant variation in adaption can be seen. For example, mammals live in colder climates have shorter ears and limbs (Allen’s Rule) and larger bodies (Bergmann’s Rule). Hence, in this manner, they reduce the surface area through which animals lose heat. An example of this organism is the lizard, which is able to adopt different temperatures. Desert lizards are not affected by high temperature but lizards from northern Europe cannot survive by high temperature. Similarly, northern lizards are capable of running, capturing prey, and digest food at cooler temperatures and at this temperature, desert lizards would be immobilized. Another example is camel and other desert animals. They live in areas where water is scarce. These desert animals can survive for a long period without drinking water. Desert adaptation is also seen in frogs. The skin of the frog who lives in the desert is moist so that water can readily permeate. These types of organisms could not survive in a dry environment as they would quickly dehydrate and dry. In this condition also, adaptation is seen in several types of frogs. Some frogs highly tend to reduce water loss through the skin. Some species prevents water loss by secreting a waxy substance from specialized glands of the skin. This waxy substance covers the skin, insulate it and reduces water loss by around 95%. 

The adaptation to these types of environmental changes can understand experimentally. At 42˚C (i.e. at high temperature), Escherichia coli (E. coli) tends to utilize resources at a high rate. However, after 2000 generations, the ability to utilize resources at high speed would decrease by 30% as compared to the first generation. The mechanism by which an increase in resource utilization takes place is still unknown.

Management of water shortage in the human body through hormone system

During dehydration, blood volume decreases and hence, remaining blood plasma becomes highly concentrated. This physiological changes in the blood volume provide stimulation to the osmoreceptors. These receptors are present in the hypothalamus of the brain, located immediately above the pituitary gland. These osmoreceptors are important in promoting the feeling of thirst and also, they promote the release of antidiuretic hormone (ADH) from pituitary gland. ADH is a hormone secret from posterior pituitary gland. It is also called vasopressin. Generally, in response to an increased osmotic concentration of blood plasma, its secrets. In turn, ADH stimulates kidney to retain more amount of water to keep the body hydrated and to prevent more water loss by excretion. Hence, less amount of water is excreted in urine and due to the feeling of thirst, a dehydrated person drinks more water. 

Control of body temperature in humans

This is a good example of maintaining homeostasis in a biological system. The temperature of the body is regulated by the hypothalamus, a region of the brain. The normal body temperature of a human is around 37°Cor 98.6°F. This value is affected by various factors such as exposure to sunlight, hormones level in the body, metabolic rate and disease condition. Based on these factors, body temperature can go to an excessively high or low value depending upon the condition. This fluctuating body temperature triggers the body's feedback mechanism. This feedback mechanism is carried out through the bloodstream to the brain and that ultimately results in adjustments in breathing rate, the blood sugar level, and the metabolic rate. This is a part of compensatory mechanisms or adjustments. Increase sweating is also a part of this feedback adjustment. Heat loss is also aided by a decrease in activity and by heat exchange mechanism that permits a high amount of blood to circulate near the skin surface. In the case of winter, the body's heat loss is high. This heat loss can be prevented by insulation and also decreased blood circulation to the skin. Homeostatic condition lies between this high and low level of temperature and it is the normal range that sustains life. This is an example of a body's feedback mechanism and it is activated whenever condition approaches any side of extremes.

The concept of homeostasis also exists in the ecological systems, and its existence was proposed by Robert MacArthur in 1955. According to the proposal, homeostasis in the ecosystem is maintained by the combined effect of biodiversity and the large numbers of ecological interaction take place among varied species. This type of homeostasis is a part of maintaining the ecosystem’s stability and it is also responsible for the persistence of a particular ecosystem type over a long period of time. This concept is called as ecological resilience in which fundamentals of homeostasis mechanisms plays a quite important role. This concept has been used to describe the reciprocation that occurs between the living and nonliving or abiotic parts of the ecosystem to maintain the ecological balance.