Osmoregulation refers to the physiological processes organisms use to regulate water balance and maintain proper osmotic concentrations of body fluids. Terrestrial animals are more prone to water loss and have developed strategies like specialized excretory organs to prevent it. Freshwater animals often have contractile vacuoles that collect and expel excess water to prevent bursting from osmosis. Marine animals need to conserve water and actively expel excess salts through gills and kidneys to maintain hypotonic internal environments.

1. 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 osmolarity
of 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.

2. 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.

3. Heat Production in Mammals
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.
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.

4. 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.

5. 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.

6. 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.