Osmoregulation means the physiological processes that an organism uses to maintain water balance; that is, to compensate for water loss, avoid excess water gain, and maintain the proper osmotic concentration osmolarityof the body fluids.

Osmoregulation in terrestrial animals:

Terrestrial animals are more likely to loose water by evaporation through their permeable surfaces exposed to atmosphere. Among various animal groups only Arthropods and vertebrates became the most successful land dwellers. They have developed number of strategies to maintain osmoregulation of their body fluid.

Osmoregulation in freshwater animals:

Osmoregulation by contractile vacuole: Fresh water protests like Amoeba,

Paramecium etc bear one or more membrane bound tiny sac called contractile

vacuole. Since such freshwater protests have higher osmotic pressure than their

surrounding water, so the water constantly comes in by osmosis. If it is not

regulated, the organism would burst. Therefore the excess water is stored in

contractile vacuole. After it is completely filled, water is discharged out of the cell

through a pore into the surrounding water.

Osmoregulation in marine animals:

Marine bony fishes have hypotonic internal environment; so they are liable to

lose water. Thus in order to conserve water, they constantly drink water. The

salts taken in along with water are actively excreted by special excretory cells

in the gills. Moreover, the filtration rate in their kidneys in very low, so small

quantity of concentrated urine in excreted.

Heat Production in Mammals:

In endotherms heat generation can warm the body as it dissipates throughout tissues

and organs. Birds and Mammals can generate heat (thermogenesis) by muscle

contraction. ATPase pump enzymes, oxidation of fatty acids in brown fat, and other

metabolic processes.

Heat Production in Mammals

Shivering thermogenesis:

Every time a muscle cell contracts and the hydrolysis of ATP molecules generate heat.

Both voluntary muscular work (e.g. running, flying, jumping) and involuntary muscular

work like shivering generate heat. Heat generation by shivering is called shivering

thermogenesis.

Non Shivering thermogenesis:

Birds and Mammals have unique capacity to generate heat by using specific enzyme the

ATPase pump enzymes in the plasma membrane, of most cells. When the body cools,

the thyroid gland releases the hormone thyroxine. Thyroxine increases the permeability

of many cells to sodium (Na+) ions, which leak into the cells. ATPase pump quickly and

pump these ions out. In the process ATP is hydrolyzed, releasing heat energy. Hormonal

triggering of heat production is called non shivering thermogenesis.

Hypothalamic control by thermogenesis:

In amphibians, reptiles, birds and mammals specialized cells in the hypothalamus

control thermoregulation. It has heating centre and cooling centre. Heating centre

controls vasoconstriction of superficial blood vessels, erection of hair and fur and

shivering or non shivering thermogenesis.

Hibernation:

During winter various endotherms (e.g. bats, wood chucks, chipmunks and ground

squirrels) go into hibernation. During hibernation metabolic rate slows, as do the heart

and breathing rates. Mammals prepare for hibernation by building up fat reserves and

growing long winter pelts. All hibernating animals have brown fat. Decreasing day length

stimulates both increased fat deposition and fur growth.

Excretion in earthworms:Earthworms have combined excretory and osmoregulatory organs called

metanephridia, which are arranged segmentally. Each metanephridium is highly coiled

tubule immersed in coelomic fluid and surrounded by a network of capillaries. It is

opened at both the ends. Its internal opening called nephrostome, which lies in the

coelom is a ciliated funnel like structure. While the external minute opening or

nephridiopore opens outside in the skin. The coiled tubular part dilates finally to form a

bladder before opening to outside through nephridopore. Due to beating of cilia of

nephrostome, coelomic fluid is pumped into excretory tubule. Some excretory

substances are also secreted by cells of tubule. Here selective re-absorption of useful

substances also occurs which are taken back by the blood into the circulation. Finally

the excretory fluid (urine) is emptied in the bladder which excretes it outside through

nephridiopore.

Excretion in Cockroach:Cockroach and other insects have excretory system which consists of long, thin blind

tubules called malpiglian tubules which arise from the junction of midgut and hindgut.

Malpiglian tubules are immersed in the haemolymph (fluid of body cavity), cells of

malpiglian tubules absorb excretory wastes along with some useful substances present

in haemolymph. In the latter part of the tubule selective re-absorption of useful

substances occurs and the uric acid is discharged into the rectum. Rectum stores uric

acid for re-absorption of salts and water, so the uric acid becomes almost dry and pass

out along with faeces.

Excretion in Plavaria:Free living flatworms like plavaria have developed a tubular excretory system. In

Plavaria the excretory system consists of two longitudinal branching tubules or excretory

canals lying on either lateral sides and extending along the entire length of the animal.

Internally in the mesenchyme, each tubule gives rise to numerous blind bulb like cells

called flame cells or pronephridia which are bathed in the tissue fluids. Each flame cell is

hollow inside and bears a tuft of cilia which beat in manner like flickering flame (hence

called flame cell), water along with ammonia diffuse from the tissue fluid into the lumen

of flame cells. The beating of cilia propels this solution into excretory canal where it is to

be excreted out by excretory pores.

During the movement of excretory fluid, water is being reabsorbed if required by the

animal and the rest of the excretory fluid (urine) is passed out in the form of hypotonic

solution. Thus it seems that flame cell functions mainly in osmoregulation and most

metabolic wastes are removed from body surface or excreted in the gut where they are

removed through the mouth along with undigested food. In some parasitic flatworms

which are isotonic to the body fluids of their hosts, the flame cell, perform excretion of

nitrogenous wastes.