Download Osmoregulation And Excretion In Animals

STD: XIITH
Jacob Thomas M.Sc.M.Ed.
Osmoregulation And
Excretion In Animals
Introduction:
Excretion is the elimination of nitrogenous waste products from the body. The waste
products are formed in the body by following ways:
(i) Some part of ingested food may remain undigested in the alimentary canal, (ii)
Due to the cellular catabolism of glucose, fatty acids and glycerol, the waste products
formed are CO2 and H2O. (iii) Due to the cellular catabolism of amino acids, the waste
products formed are CO2, H2O and nitrogenous wastes, (iv) The excess of inorganic salts,
hormones and vitamins are also the wastes. (v) The organism gains water along with
food. It also drinks water. If the skin is permeable, it may gain water through skin. This
excess water may dilute the body fluids. Hence, the excess water must be given out so
that the osmotic contents of the body are maintained at an appropriate level. This is called
osmoregulation.
Thus, the wastes are undigested food, CO2, H2O, excess salts, hormones and vitamins.
The undigested food is given out through anus or cloacal aperture. Much of the CO2 and
some amount of water are given out through expiration. The CO2 is given out in gaseous
state and water is given out in the form of vapour during expiration. Hence, these are
called volatile wastes.
The remaining excess water, nitrogenous wastes, excess inorganic salts, hormones and
vitamins are called non-volatile wastes. To eliminate these things, there is a development
of an excretory system in the animal body. Thus, the separation, collection and
elimination of nitrogenous wastes from the body is called excretion.
Osmoregulation:
Osmoregulation is nothing but regulation of water and ionic contents in the body or the
regulation of solute and solvent movement between body and the surrounding medium is
also called osmoregulation.
Osmoconformers are the animals, which do not actively control the osmotic condition of
their body fluids. They change the concentration of their own body fluid and make it
similar to that of the surrounding fluid.
All marine invertebrates and some fresh water invertebrates are strictly osmoconformers.
The only vertebrate which is osmoconfirmer is Myxine (Hag fish). It is a member of
Agnatha. A character of a solution with respect to its solute concentration is also termed
as osmolarity. It is expressed as molarity or moles of solute per litre of solution. The unit
of measurement for osmolarity is milliosmole per litre (mosm L-1).
When two solutions have the same osmolarity, these two solutions are said to be isotonic.
When one of the solutions has higher osmolarity, then it is said to be hypertonic and the
other is said to be hypotonic.
(A) Water and solute regulation in fresh water environment
A fresh water has a very low osmolarity i.e. less than 50 mosmL-1. The fresh water
vertebrates have their blood osmolarities in the range of 200-300 mosmL-1.
The body fluids of fresh water animals are generally hypertonic to the surrounding water.
Therefore, the fresh water animals constantly face two kinds of osmoregulatory problems
(i) Endosmosis: The surrounding fresh water constantly enters the body because of
osmotic gradient, (ii) Loss of body salts: As body fluids have more salts as compared to
outside water, salts move out of body.
Therefore, fresh water animals have (i) To get rid of excess water and (ii) To retain salts
in body.
This is performed by various organisms in following ways.
(a) Amoeba and Paramoecium have contractile vacuoles that pump out
excess water.
(b) Some other fresh water invertebrates like crab, bivalve excrete large
volume of very dilute urine.
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STD: XIITH
Jacob Thomas M.Sc.M.Ed.
(c) Fresh water animals including fish do not drink water so as to reduce
the need to expel water. In fresh water fish, water enters inside through skin, gill
membranes etc. To some extent, entry through skin is prevented by scales. If
scales are absent, the subcutaneous fat serves the same purpose. Thus, the entry of
water into body is prevented to some extent and whatever excess water is present
in the body, is given out in the form a large volume of dilute urine.
Fresh water fishes have specialised cells called ionocyles or chloride cells,
in their gill membranes. These cells can actively absorb Na+ and Cl- from the
surrounding water. Energy is required for this process as the process is against
concentration gradient. The fish has a body fluid concentration of about 100
mosmL-1 NaCl.
Nature of Excretory Products
The nitrogen containing wastes are produced from the metabolism of proteins and nucleic
acids.
The nature of nitrogen containing wastes and their excretion varies among the species.
The variation mainly depends upon the availability of water. Animals mainly excrete
either ammonia, urea or uric acid.
(A) Ammonotelism: It is the elimination of nitrogenous wastes in the form of ammonia.
Ammonia is formed directly in the process of catabolism of amino acids. Ammonia is
highly toxic and highly soluble in water. To give out ammonia, large amount of water is
required. If plenty of water is available, then ammonia can be given out by diffusion and
hence for such an excretion, there is no expenditure of energy. Aquatic animals like bony
fish, frog larvae, aquatic insects, aquatic invertebrates excrete ammonia.
In soft bodied invertebrates, ammonia diffuses out across the whole body surface into the
surrounding water. In fish, most of the ammonia (NH3) is lost as ammonium ions (NH4+)
across the gill epithelium. Kidneys play minor role in excretion of ammonia.
(B) Ureotelism: It is the urinary elimination of nitrogenous wastes in the form of urea.
Certain animals cannot get sufficient amount of water to excrete ammonia, and as
ammonia is very toxic, it must be converted into some other less toxic form.
In the liver of such animals, ammonia is combined with CO2 to form urea.
2NH3 + CO2 → CO (NH2)2 + H2O. Urea is 100,000 times less toxic than ammonia. This
conversion requires expenditure of energy; even then these animals can afford it because
urea can be stored for long and can be excreted at a slower rate with the availability of
water. Urea is highly soluble in water and needs a considerable volume of water for its
elimination. Urea is carried by circulatory system from liver to kidney for the
elimination. However, in mammals all the urea produced is not excreted immediately.
Some portion of it is retained in the kidneys for osmoregulation.
The animals excreting urea are called ureotelic animals. These are mainly semiaquatic or
terrestrial animals like frogs, toads and mammals
respectively.
(C) Uricotelism: It is the urinary elimination of nitrogenous wastes in the form of uric
acid.
Synthesis of uric acid from ammonia requires far higher expenditure of energy than the
synthesis of urea. It is advantageous to land animals as it is far less toxic and insoluble in
water. Hence, it does not require much water for its elimination. It can be excreted in the
form of a paste.
Organisms excreting uric acid are called uricotelic animals. These are terrestrial animals
like land snails, land reptiles, birds etc.
Uricotelism is particularly advantageous to the oviparous land vertebrates like birds.
Birds lay shelled eggs on land. Ammonia and Urea can pass out of shell-less eggs of
pisces and amphibia which are in water. Egg shell of bird has pores, only for gaseous
exchange. Had an embryo produced ammonia or urea within the shelled egg, the soluble
wastes would have accumulated to the level of toxic concentration. But uric acid
precipitates out of the solution and can be stored in the shell as a solid waste. It is left
behind, when the youngone hatches.
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STD: XIITH
Jacob Thomas M.Sc.M.Ed.
Excretory System of Man
It is made up following parts.
(A) Kidneys: These are two in number. These are somewhat flattened, bean shaped
structures located in abdominal cavity, one on either side of lumbar vertebrae.
Each kidney is covered by a tough capsule. It is about 10 cms. in length, 5 cms. in
breadth and about 9 cms. in thickness.
The kidney has outer convex surface and inner concave surface
Renal column
Cortex
Minor calyx
Major calyx
Artery
Vein
Hilium
Renal
pelvis
Ureter
Papilla
Pyramid
Fig. Longitudinal Selection of Kidney of Human Being
On the concave side, there is a longitudinal opening called hilum. Ureter starts at
hilum.
The upper end of the ureter is expanded into a funnel shaped structure called renal
pelvis.
The kidney consists of an outer layer called renal cortex and an inner tissue called
renal medulla. The medulla is in the form of conical structures called pyramids.
The cortical tissue between the pyramids is called renal columns of Bertini.
The apex of each pyramid fits into a cup shaped depression called minor calyx.
The minor calyces lead into a major calyx. Each major calyx in turn
communicates with the renal pelvis.
(B) Ureters: From a pelvis of a kidney, arises a thin muscular tube called ureter. The
urine is conveyed to urinary bladder by peristalsis. The ureter is lined by transitional
epithelium. This epithelium allows stretching without getting torn off.
The two ureters enter near rear corners of the upper surface of urinary bladder. This
opening is guarded by a valve so that backflow of urine from urinary bladder to the ureter
is prevented.
(C) Urethra: From the lowest part of the urinary bladder arises a thin canal called
urethra. The opening of bladder in the urethra is guarded by a sphincter.
The urethra opens to outside by a urinary aperture.
Structure of Nephron: Nephron is a structural and functional unit of a kidney. A human
kidney contains about 1.2 million nephrons in it.
The nephron is a microscopic, thin, long structure which is coiled at certain places. It is
about 3 cm long and 20-60 m m in diameter.
Each nephron begins as a cup shaped, dilated, double walled structure called Bowman's
capsule. It encloses a capillary knot called glomerulus.
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STD: XIITH
Jacob Thomas M.Sc.M.Ed.
Fig. Structure & Arrangement of Nephron in
a kidney
The inner wall of the capsule is closely applied to the wall of the glomerulus. The outer wall is
continuous with the wall of the next segment of the nephron. The outer wall is made up of squamous
epith-elium. The inner wall has specialised cells called podo-cytes or foot cells. These cells have foot like
appen-dages, hence the name.
The Bowman's capsule, along with the glomerulus is called a renal corpuscle or a Malpighian body.
Blood enters the glomerulus through afferent renal arteriole and leaves it through efferent renal
arteriole.
In the lumen of the Bowman's capsule, due to a process called ultrafiltration, the glomerular
filtrate is formed.
The renal corpuscle is followed by a neck. The neck continues into a long, highly coiled and
twisted part called proximal convoluted tubule (PCT). The renal corpuscle and PCT are located in
the cortex. The PCT continues into a thin straight tubule which descends from cortex to the renal
medulla. It again turns back towards cortex as a straight, thick tubule, parallel to the thin limb.
Thus, a U shaped segment is formed and is called Henle's loop. The Henle's loop continues into
a coiled and twisted tubule called the distal convoluted tubule (DCT). The terminal part of the
DCT is relatively straight and short. This part opens into a collecting tubule.
The collecting tubules unite with each other in the medulla to form still larger ducts called
— Efferent arteriole
Afferent artericte
Bowman's
capsul
Glomerulus
Neck
Renal tubule
Fig. Malpighian body / corpuscle
duct of Bellini. These duct pass through renal pyramids to open in a renal calyx.
The efferent arteriole produces a capillary net-work around the tubule in the cortex. It is called
peritubular capillary net-work. It also produces some parallel, wide, thin walled straight capillaries called
vasa rectae.
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STD: XIITH
Jacob Thomas M.Sc.M.Ed.
Each vasa recta descends into medulla and then turns back to the cortex and then opens in a venule.
The loops of vasa rectae lie close to Henle's loop, producing an infrastructure for counter current
mechanism.
Mechanism of Urine Formation
The urine is formed due to the collective effect of following three sub-processes.
(A) Ultrafiltration: This is also called glomerular filtration. It is a filtration at microscopic level. Every
minute, about 1300 ml of blood is through both the kidneys. The filtrate formed is called
ultrafiltrate; and it comes to lie in the lumen of the Bowman's capsule, lumen of the capillary and the
lumen of the Bowman's capsule are separated by three layers namely:
(i) Wall of capillary, which is nothing but endothelium. This layer is having pores called endothelial pores
which are 50 to 100 nm in size. (ii) a basement membrane, (iii) podocytes which form the inner wall of
Bowman’s capsule.
The podocytes meet the basement membrane by their foot like processes, which have 25 nm gaps called
filtration slits. The nt membranes serves as dialysing membrane. Through the slits, blood gets filtered.
The inner wall of Bowman's capsule is neither plain nor uniform.
Every part of glomerular capillary is covered by podocytes. The slits do not allow the cells of
blood and proteins to pass through them. Rest all the substances of plasma pass out as a
glomerular filtrate contains water, arnino acids, glucose, salts, nitrogenous exactly in the same
proportion as are found in plasma of blood, glomerular filtrate resembles protein free plasma and it
is having same osmolarity with plasma.
Every minute, 1300 ml of blood passes through both the kidneys. The quantity of
glomerular filtrate formed per minute is called, filtration rate (GFR). In a normal person, it is
about 125 ml. Thus, in an hour about 7.5 litres or in a day, 180 litres of glomerular
filtrate is formed.
The autoregulation of GFR is carried out by two factors namely Myogenic mechanism
and luxtaglomerular apparatus (IGA).
(i)
Myogenic Mechanism: When blood pressure becomes high, there would be
more blood flowing to the glomerulus. But the wall of afferent arteriole,
responds to strech by increased blood pressure in a reverse way i.e. when it is
stretched beyond particular limit, it undergoes contraction and its diameter gets
reduced. Thus any how, blood entering glomeruli is regulated.
(ii) Juxtaglomerular apparatus: It is a structure formed by some cells of DCT,
some cells of glomerulus, some cells of walls of afferent and efferent vessels. Juxta
means near. The structure is present near the glomerulus, hence the name. The JGA
secretes enzyme renin which ultimately leads to constriction of arteries.
(B) Selective reabsorption: It is also called tubular reabsorption. This is the process by
which the useful components of the glomerular filtrate are absorbed back by the cells of
the renal tubule. The reabsorption involves osmosis, passive transport, as well as active
transport.
The proximal convoluted tubule is the main site of selective reabsorption. Therefore, this
area is elongated. The internal surface area is further increased many fold by the presence
of finger like projections called microvilli. These microvilli give an appearance of a brush
border.
In the cells of PCT, large number of mitochondria are seen. These provide energy for
active transport.
PCT reabsorbs all the useful components from the glomerular filtrate. It reabsorbs water,
water soluble vitamins, glucose, amino acids etc. It also reabsorbs Na+, K+ etc. HCO-3 is
an important buffer of blood. Therefore PCT reabsorbs HCO-3 also.
DCT has an interval lining of columnar epithelium, but it is without microvilli. It also
reabsorbs some amount of HCO-3, Na etc.
HCO-3 is an important buffer of blood.
(C) Tabular secretion: The cells of the tubule, especially of PC and DCT have a
capacity to extract wastes from neighboring interstitial fluid. This fluid is present inside
the Kidney between neighboring nephrons.
If H+, K+ etc. are excess in the interstitial fluid, then the cells of the tubule extract them
and release them into the glomerular filtrate. To maintain pH of blood, controlled
excretion of H+ ions is of great importance.
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STD: XIITH
Jacob Thomas M.Sc.M.Ed.
Nitrogenous wastes like NH3, uric acid, creatinine are also carried from interstitial fluid
to the glomerular filtrate by tubular secretion. Uric acid is formed due to catabolism of
nitrogenous bases of nucleic acid
Role of Lungs in Excretion:
Lungs of human being can eliminate about 18L of CO2 per hour, and about 400 ml of
water per day in normal resting condition.
In hot humid climate, the water loss via lungs is less and it is more cold and dry climates.
The rate of ventilation and ventilation pattern i.e. breathing through mouth or nose or
both also affect the water loss through the lungs.
Many volatile materials are also readily eliminated through the lungs.
In an expired air of a drunk person, alcohol can be detected.
Role of Skin in Excretion:
Human skin has two types of glands namely sweat glands and sebaceous glands.
The sweat glands secrete sweat and sebaceous glands sea sebum.
The sweat has 99.5%, water. Thus, it is an aqueous fluid. It contains NaCl, lactic acid,
urea, amino acids and glucose.
The sweat, volume varies from negligible to 14 liters a day, rising with activity and
temperature.
The main function of sweat is to lower down the body temperature by evaporation.
Sebum is an oily protective secretion to keep the hair soft, pliable waterproof. This
secretion eliminates some liquids like waxes, sterols,
other hydrocarbons and fatty acids. In aquatic animals, where skin is permeable,
ammonia is given out through skin.
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