Homeostasis

An aquatic invertebrate organism maintains the same osmotic environment as that of the surrounding medium. On the other hand, mammals and birds maintain a constant internal environment that is independent of the external environment. The process by which animals maintain a stable internal environment with only minor fluctuations is called homeostasis.

Homeostasis enables animals maintain their metabolic activities at optimum level at all times. A state of homeostasis is achieved by three important processes – thermoregulation, excretion and osmoregulation. The liver, pancreas, skin, kidneys and lungs are the major organs of homeostasis in the mammal.

The liver

The liver is the largest gland and solid organ in the body, weighing about 1.8 kg in men and 1.3 kg in women. The liver has vital roles in regulating, synthesizing, storing, secreting, transforming, and breaking down many different substances in the body. The liver receives its blood supply via the hepatic artery (30%) and the hepatic portal vein (70%), which transports nutrients from the intestine.

The Liver

  Functions of the liver

These can be divided into three categories:

1. Regulation, Synthesis, and Secretion  

a. Glucose - The liver plays a key role in the homeostasis of glucose by storing and releasing it in response to the pancreatic hormones insulin and glucagon.

b. Proteins – with the exception of antibodies, the liver synthesises most blood proteins, including albumen and clotting factors.

c. Bile - Bile is a greenish fluid synthesised by liver cells (hepatocytes), secreted into the bile duct and stored in the gallbladder before being emptied into the duodenum. In addition to bile salts, it contains cholesterol, phospholipids, and bilirubin. Bile salts aid in the digestion of fatty foods.

d. Lipids – the liver synthesises cholesterol and lipoproteins. Cholesterol is an important component of cell membranes. Lipoproteins circulate in the blood and shuttle cholesterol and fatty acids between the liver and body tissues.

2. Storage

The liver stores glucose in the form of glycogen, and also fat-soluble vitamins (A, D, E and K), Vitamins B₆, and B₁₂, and minerals such as copper and iron. 

3. Purification, Transformation, and Clearance

 The liver removes harmful substances from the blood and breaks them down into less harmful compounds. It also converts most hormones and drugs to less active products.

a. Ammonia - the liver converts ammonia to urea, which is excreted in urine by the kidneys. This process is called deamination. The liver can also convert one amino acid into another by a process called transamination. In transamination, an amino group is transferred from an amino acid to a keto acid, to form a different amino acid. This enables scarce amino acids to be made from abundant ones. In adult humans only 11 of the 20 amino acids can be made by transamination, the rest are called essential amino acids, which must be supplied by the diet.

b. Bilirubin - bilirubin is a yellow pigment formed as a breakdown product of red blood cell haemoglobin. The spleen, which destroys old red cells, releases bilirubin into the blood, where it circulates to the liver, which excretes it in bile. Excess bilirubin results in jaundice, a yellow pigmentation of the skin and eyes.

c. Hormones - the liver plays an important role in hormonal modification and inactivation, e.g. the steroids testosterone and oestrogen are inactivated by the liver. Men with cirrhosis, especially those who abuse alcohol, have increased circulating oestrogen, which may lead to body feminisation.

d. Drugs - nearly all drugs are modified or degraded in the liver. In particular, oral drugs are absorbed by the gut and transported to the liver, where they may be modified or inactivated before they enter the blood.  Alcohol, in particular, is broken down by the liver, and long-term exposure to its products can lead to cirrhosis.

e. Toxins - the liver is generally responsible for detoxifying chemical agents and poisons. 

4.  Fighting infections                        

The liver plays a vital role in fighting infections, particularly infections arising in the bowel. It does so by mobilising part of the body’s defence mechanism called the macrophage system. The liver contains over half of the body’s supply of macrophages (known as Kuppfer cells), which destroy any bacteria that they meet.      

Thermal regulation

Animals gain temperature from metabolic activities that take place in the cells and from absorbing solar energy. Various body activities that involve enzymes act maximally at certain temperature limits. If the temperature is too low, the reactions become too slow or do not react at all. If the temperature is above a certain limit, the enzymes, which are protein, may denature. Therefore, the body temperature must be maintained within certain limits that permit maximum and efficient body functions to take place. The mechanism by which this is done is known as thermal regulation. 

 Thermal regulation in different organisms

 (a)  Endotherms

Endotherms are animals that maintain a constant body temperature, usually referred as warm-blooded animals. With a body temperature that is not influenced by the temperature of the external environment, endotherms are able to multiply and occupy much of the earth’s surface. A relatively high stable body temperature enables them to maintain an active metabolism that produces sufficient heat energy from the food they eat. Birds and mammals are the only endothermic animals. Human beings maintain a body temperature of between 37 ^oC and 38 ^oC and birds have relatively high temperature of about 40 ^oC.

 (b)  Ectotherms

  Invertebrates, fish, amphibians and reptiles are animals whose body temperatures are influenced by the temperature of the external environment. These animals have a low metabolism that produces little heat energy. The main source of energy does not come from the food they eat but from the sun. The body temperature is controlled by the amount of heat from the sun, which greatly limits the activities of the animal. 

If the weather is very cold, its body activities slow down and the animal becomes very sluggish. When the temperature rises, the animal becomes active as its internal and metabolic activities increase so that more energy is produced. However, if the temperature continues to rise, the animal must move to the shade and cool itself.

Reptile basking in the sun. When it get too hot, it moves to shade

The human skin  

The human skin is composed of an outer layer of epithelial cells, which form the epidermis. Epithelial cells forming the top layer on the surface of the body are dead and eventually are sloughed off. These cells are replaced from beneath by the deepest layer, which lies just above the thick inner skin layer, the dermis.

In addition to functioning as a barrier to harmful microorganisms and chemicals, the epidermis contains specialized cells that produce the pigment melanin. 

Human skin   

 Melanin protects the skin by absorbing much of the ultraviolet light of the sun. However, if the skin is overexposed to the sun melanin cannot absorb all of the ultraviolet rays and the skin becomes inflamed. Skin cells also use ultraviolet rays to produce vitamin D, a substance essential to the normal growth and maintenance of bone. 

The dermis consists primarily of tough connective tissue that has a network of small blood vessels. Under the control of the autonomic nervous system, the blood vessels can constrict and dilate depending upon the external temperature. When the body temperature is low, the blood vessels constrict keeping the warm blood deeper in the tissues away from the surface. This prevents heat loss. As the body temperature rises, the blood vessels dilate, blood flows to the surface of the skin and heat can escape from the body. This cools the body. The dermis also contains a variety of sensory nerve receptors that are sensitive to touch, pressure, heat, cold and pain.

Hair is a specialized structure of the skin that extends from the surface down into the dermis. Each hair on the body arises from a structure called a hair follicle. Attached to the hair follicle is a small erector muscle that when it contracts causes the hair to stand on end. The familiar goose pimples result from very tight contraction of the follicle muscles.

 Two major types of the glands are found in the skin: sebaceous or oil glands and sweat glands. Sebaceous glands secrete an oily substance called sebum, which helps keep the hair and skin soft and waterproof. There are over 2.5 million sweat glands in the body; these function in the regulation of body temperature and in the excretion of waste. An increase in body temperature, as when one is exercising, stimulates the release of sweat. As the sweat evaporates, it absorbs heat from the skin and the body becomes cooler.

 Sweat is mostly water in which there are dissolves salts and a nitrogen waste product called urea. To some extent, then skin functions as an organ of excretion by releasing the salts and urea from the body through perspiration. However, profuse seating may deplete the body of needed salts that must be replaced. This is why someone engaged in strenuous exercise occasionally takes salt tablets or a salt fortified liquid.

Thermal regulation in desert animals  

Animals living in hot deserts experience very hot temperatures during the day and cold temperatures at night. To survive under such conditions desert animals have developed physiological and cultural adaptations. The camel is one animal that is superbly adapted to the desert. It can go without water for 17 days and can lose 25 % of its body weight by dehydration. Once water is found, the camel will gain the lost weight within a short time by drinking as much as 100 litres of water.

Other unique adaptations include high temperature tolerance. For instance, the body temperature can rise by as much as 6 ^oC – 7 ^oC above the normal body temperature during the day and drop by as much as 4 ^oC to 5 ^oC at night without endangering the animal. The camel’s hump contains fat that can be metabolised to release water. The animal’s thick, pale coloured fur reflects heat while at the same time providing insulation against temperature extremes.

The came is well-adapted to the desert  

Other desert animals including the kangaroo rat lead a nocturnal life, spending the day holed up under the sand and under shady places, coming out at night to feed when the temperatures are cool. The kangaroo rat feeds on grains and dry plant material and never drinks water. Whatever little water it gets comes from what it eats and from its own metabolic reactions. To conserve water, it has reduced its sweat glands to a minimum, produces very little concentrated urine and dry faeces.

Cold climates

Hibernation

 Animals living in the cold climates are insulated again heat loss by having thick fur and thick fat or blubber under the skin.  Under very severe winters, however, some animals go into hibernation, a process during which their physiological activities are reduced to a minimum. To conserve energy, the animal reduces, among others, its metabolic rate, body temperature, heartbeat, breathing and goes into a kind of coma in which it is unable to respond to stimuli.  It only reawakens when the weather becomes warmer.

Arctic dwellers like the polar bear tend to be large and have smaller surface area over which temperature can be lost. They have thick fur and blubber under the belly, which act as insulators.

Thermoregulatory center

In humans, temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus and receptors in the skin. The hypothalamic receptors monitor the temperature of the blood as it passes through the brain (the core centre), while the skin receptors monitor the external temperature. Both pieces of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature.

The thermoregulatory centre is part of the autonomic nervous system, so the various responses are all involuntary. The exact responses to high and low temperatures are described in the table below. 

Effector	Response to low temperature	Response to high temperature
Smooth muscles in the peripheral arterioles in the skin	Muscle contract causing vasoconstriction. Less heat is carried from the core to the surface of the body maintaining core temperature	Muscles relax causing vasodilation, more heat is carried from the core to the surface, where it is lost by convectional radiation
Sweat glands	No sweat reduced	Glands secrete sweat on the surface of skin, where it evaporates. This is an endothermic process and water has a high latent heat of evaporation, so it takes heat from the body
Erector pili of the skin	Muscles contract, raising skin hairs and trapping an insulating layer of still, warm air next to the skin. Not very effective in humans, just causing ‘goose pimples.’	Muscles relax, lowering the skin hairs and allowing air to circulate over the skin, encouraging convection and evaporation.
Skeletal muscles	Muscles shiver by contracting and relaxing repeatedly , generating heat by friction and from metabolic reactions	No shivering
Adrenal and thyroid glands	Secret adrenalin and thyroxine which increase by metabolism, generating heat	Glands stop releasing adrenalin and thyroxine
Behaviour	Curling up, huddling, finding shelter, putting on clothes	Stretching out, finding shade, swimming , removing clothes

 The thermoregulatory centre normally maintains a set point of 37.5 ± 0.5 °C in most mammals. However the set point can be altered in special circumstances as when the animal has fever, hibernating or in torpor. 

Osmoregulation

In order for the cells of an animal to carry out their activities, the internal environment of the body must have the right balance of salts and water. This is done by osmoregulation, which is a process by which an animal adjusts the various concentrations of salts and quantities of water that enter and leave its body. Osmoregulation ensures that the body maintains an osmotic concentration that enables the cells to function efficiently.

Marine invertebrates maintain the same body osmotic concentration as that of the marine water. As a result, they are not affected by any changes in salt concentrations of the seawater. 

 Marine cartilaginous fishes such as sharks and rays maintain an osmotic body concentration that is lower than that of seawater. They face the danger of losing water by osmosis and becoming dehydrated. This problem has been overcome by the retention of urea, a waste product, in their blood, which raises the osmotic concentration of the body to that of the seawater.

 On the other hand, marine bony fish must drink lots of seawater in order to avoid dehydration. Excess salts (mainly sodium chloride) present in the water is removed by active transport from the body by special cells in the in their gills. Their kidneys produce a little amount of urine.

Freshwater fish maintain an internal osmotic environment that is hypertonic to that of the water.  Water will therefore tend to enter the animal’s body and dilute the blood. To prevent this happening, the fish rarely drinks water but excretes large quantities of dilute urine. To replace any salts lost, the fish picks up salts from the water through the gills by active transport.

Amphibians

Frogs, toads, and other amphibians live in freshwater or near freshwater. They have thin, loose moist skins that are rich in blood capillaries. They can easily lose water by evaporation. They also have special glands in the skin that produce a slimy substance that keeps the skin moist and prevents the animal from dehydration. The skin is very permeable to water and gases. Most of the osmoregulation takes place on the skin and very little in the lungs. Even salts are exchanged across the skin by active transport. The kidney produces very little urine and plays virtually no role in osmoregulation.

Reptiles

Unlike the amphibians whose skin is the main organ of osmoregulation, land dwelling reptiles have scaly, dry skins that are impermeable to water. Most of the water that is lost by the animal is through the lungs. Water conservation is ensured by reabsorption of water and urine in the faeces by the cloaca. Moreover, the faeces consists mainly of uric acid, which requires very little water to expel from the body.  

Nitrogenous Wastes

 The digestive system reduces complex food compounds into smaller molecules that the body can use as energy, repair material or building blocks. The breakdown of some of these molecules, particularly nucleic acids and amino acids, produces nitrogenous wastes. For example, amino acids can be used by cells to synthesise new body protein or other nitrogen-containing molecules. The amino acids not used for synthesis are oxidised to generate energy or converted to fats or carbohydrates that can be stored. In either case, the amino acid groups (-NH₂) must be removed from the body. Depending on the species, it is excreted as ammonia, urea or uric acid.

Deamination

The liver removes many of the amino acids from the blood temporarily storing small quantities. Excess amino acids are deaminated by converting the amino group (-NH₂) to ammonia (NH₃). The ammonia may be released directly into the water or converted to urea or uric acid.

In addition to its metabolic functions, the liver detoxifies many injurious chemicals, destroys red corpuscles, and excretes bile pigments.

Ammonia

Amino groups (-NH₂) removed from proteins and nucleic acids are immediately converted to ammonia by addition of a third hydrogen. Ammonia is quite toxic because it tends to raise the pH of body fluids and interfere with membrane transport functions. It is too toxic to be stored in the body. It is highly soluble in water and diffuses rapidly across cell membranes. Ammonia is the waste product for aquatic animals because it can be removed by the water as soon as it excreted.

Urea and Uric acid                                          

As terrestrial animals cannot get rid of ammonia fast enough to avoid poisoning, they convert the amino group (-NH₂) from nitrogen containing compounds into less toxic compounds such as urea or uric acid. The conversion occurs in the liver by a set of enzymatic reactions known as the urea cycle or ornithine cycle. Unlike the formation of ammonia, the formation of urea and uric acid requires energy.

Terrestrial amphibians and mammals excrete urea as the main excretory waste product. Urea is less toxic than ammonia and can be excreted in a moderately concentrated form. This allows the body to conserve water, an invaluable asset in land-based animals. Uric acid is excreted by insects, reptiles and birds. It is not very toxic and is poorly soluble. It occurs in a more concentrated form than even urea, which makes it an ideal waste product for animals that need to conserve water.