EXCRETORY SYSTEM (Part -1) - Notes, Zoology, Class 11 PDF Download

EXCRETORY SYSTEM

Homeostasis : Maintenance of steady state (Walter Cannon).
Homeostatic mechanism are important for normal life as they maintain condition within a range in which, the animals metabolic processes can occur. The process which is concerned with removal of nitrogenous waste materials (e.g.. urea, uric acid, CO₂, Ammonia, salts, excess water etc.) is termed excretion.

OSMOCONFORMERS & OSMOREGULATORS

Osmoregulation : The regulation of solute movement and hence water movement (which follows solutes by osmosis) is called osmoregulation.
On the basis of osomoregulation, animals are either osmoconformer or osmoregulators.
Osmoconformers : These animals can not actively control the osmotic condition of their body fluids. Instead of this, they change or adapt the osmolarity of body fluids according to the osmolarity of the surrounding medium.
Example : 

– All marine invertebrates and some fresh water invertebrates.

– Hagfish (myxine) which is a marine cyclostome fish, is the only vertebrate osomoconformer.
Osmoconformes show an excellent ability to tolerate a wide range of cellular osmotic environments.

Osmoregulators : These animals maintain an internal osmolarity different from the surrounding medium in which they inhabit.
Osmoregulator animals must either eliminate excess water if they are in hypotonic medium or they should continously take in water to compensate for water loss if they are in hypertonic medium.
Due to this the osmoregulator animals have to spend energy Strict osmoregulators : Are animals which maintain the composition of body fluids within a narrow osmotic range. eg. most vertebrates (except Hag fish and elasmobranch like shark & rays fish)

Water and solute regulation in freshwater environment :

Body fluids of fresh water animals (osmolarity 200-300 mosm L^-1) are hypertonic to surrounding medium (osmolarity 50 mosm L^–1). Due to this, the freshwater animals constantly face two problems :

(1) They gain water passively due to osmotic gradient

(2) Continuous loss of body salts to surrounding low salt containing medium occurs.

To encounter these problems the fresh water fishes perform following acts :

(1) They do not drink water

(2) Specialised cells called ionocytes or chloride cells are present in the gill membrane of fresh water fish.
These cells can actively import Na+ & Cl– from surrounding water (containing less than 1mM NaCl) against concentration gradient.

Water and solute regulation in Marine environment : Body fluids of marine bony fishes is hypotonic to seawater (osmolarity 1000 mosm L^–1). So the osmoregulatory problems are opposite here :

(1) Marine fishes loose water from the body through permeable surfaces (like gill membranes, oral and anal membranes.

(2) To compensate this water loss the marine fishes have to drink water, this drinking results in gain of excess salts.

These problems are encountered by marine bony fishes by following acts : 

(1) The ionocytes or chloride cells of the gill membrane eject out excess of monovalent ions.

(2) Excess of Divalent cations are excreted with faeces.

•  When fishes like Hilsa & Salman (which live in both sea & fresh water) migrate from fresh water to sea water they drink water and excrete excess salts through gill membranes.
•  Body fluids of marine invertebrates, ascidians and hag fishes are isosmotic to sea water.
• Elasmobranch fishes (shark & rays) and coelacanths (lobe fin fish) reduce the osmoregulatory challenges by raising the osmolarity of body fluids. This is done by accumulation of osmolytes like urea & trimethylamine oxide (TMAO). This makes the body fluids of shark and coelacanths slightly hyperosmotic to sea water,
•  Osmolarity of human blood is about 300 mosm L^–1

Water and solute regulation in terrestrial environment : 

Land animals continuosly loose water through oral, nasal or respiratory surface during breathing. A loss of 12 percent of body water may lead to death in humans. The water loss is compensated by drinking & eating moist food. Some example of water conservation in different animals are as follows.

• Kangaroo rats loose very little water, because they can recover 90 percent of the loss by using metabolic water.
•  When water is not available, the camels do not produce urine but store urea is tissues and solely depend on metabolic water. When water is available they rehydrate themselves by drinking upto 80 litres of water in 10 minutes.

Water balance in humans and desert kangaroo rats

ELIMINATION OF NITROGENOUS WASTES
On the basis of type of excretory products (ammonia, urea or uric acid) three types of animals are present.

(1) Ammonotelics : Most aquatic animals excrete nitrogenous waste as ammonia, the water soluble ammonia molecules diffuse across the body surface into surrounding water. In fishes most of the ammonia (NH₃) is lost as ammonium ions (NH₄⁺) across the gill epithelium. eg of ammonotelic animals are teloest (modern bony fish), tadpoles and aquatic insects.

(2) Ureotelics : Animals like mammals, most adult amphibians living on land, marine fish and turtles, face the problem of conserving water. Excretion of urea is beneficial for these animals than ammonia because of following reasons.

(1) Urea can be tolerated in much more concentrated form because it is 100000 times less toxic than ammonia.

(2) Urea excretion helps to sacrifice less water while disposing off the nitrogenous wastes.
In mammals urea is excreted by kidney. However entire amount of urea produced is not excreted immediately but some portion of it is retained in the kidneys for osmoregulation. (important for water reabsorption)

•  Sharks retain some amount of urea produced to balance the osmolarity of body fluids with surrounding sea water. Thus urea here acts as an osmolyte.

Urea is produced in the liver by urea cycle.
Ornithine Cycle :-It is also termed as theKreb-Henseleit cycle. In this cycle, 2 molecules of NH₃ react with 1 molecule of CO₂, resultants a molecule of urea is formed. The formation of urea through this cycle takes place by the following steps -

1. Firstly - 2 molecule of NH₃, 1 molecule of CO₂  combine to form carbamyl - Phosphate. This reaction is catalysed by the carbamyl-phosphate synthetase enzyme. 2 ATP's are used in this reaction.

2. In next step - carbamyl-phosphate reacts with ornithine amino-acid to form the Citrulline amino-acid.

3. In next Step - Citrulline reacts with Aspartic acid to form Argino-succinic acid. This reaction is catalysed by Argino-succinic synthetase. 1 molecule of ATP is used in this process.

4. In next step - Argino-succinic acid converts into Arginine and Fumaric-acid in the presence of Arginosuccinase enzyme.

5. Arginine, now dissociates into ornithine and urea in presence of Arginase enzyme. Ornithine again enters the cycle.

(3) Uricotelics : These animals excrete uric acid as waste products. eg. Land snails, insects, birds & many reptiles.

Excretion of wastes in the form of uric acid is particularly advantageous for land vertebrates which lay shelled eggs. This is because shelled eggs of reptiles & birds possess many fine pores which are permeable to gases only.

If the embryo would have produced ammonia or urea inside the shelled egg, the soluble nitrogenous waste would have accumulated to toxic concentration levels. But because the wastes are in the form of uric acid which is thousand times less soluble than NH₃ or urea, this uric acid precipitates out of the solution and can be stored in the shell as a solid waste which is left behind when the animal hatches.

•  In birds and reptiles urine is eliminated in a paste like form along with faeces.
•  Tadpole and aquatic amphibians excrete ammonia, while their terrestrial adult forms excrete urea.
•  Aquatic turtles excrete both urea & ammonia. Some tortoise can change their nitrogenous wastes when the environment is changed.
•  Most terrestrial reptiles excrete uric acid but crocodiles excrete ammonia in addition to uric acid.

Some Other Excretory Products

Tri-methyl amine-oxide - Some animals convert the ammonia into non-toxic tri-methyl amine oxide and excrete it. It has a typical fishy-smell. e.g. Marine-fishes, Marine molluscans and Marine crustacians etc.

Guanine:- Spiders convert ammonia into guanine and then excrete it. It is similar to uric- acid; its structure is same as that of uric acid. It is insoluble in water. Guanine is excreted in the form of crystals. It is also an adaptation to check the water-loss.

Allantonin- Majority of mammals convert the Purines and Pyrimidines to Allantonin and then excrete it. In man purines are excreted in the form of uric-acid and pyrimidines in the form of alanine and Iso-butyric acid.

Hippuric-acid :- In mammals, the Benzoic-acid is excreted out in the form of Hippuric acid.

Benzoic - acid + Glycine→Hippuric - acid But in birds, the benzoic acid is treated with Ornithine amino-acid, and ornithuric acid is excreted.

Benzoic acid + Ornithine → Ornithuric acid

Creatine:- In normal urine, creatine is absent. But in new-born infants, pregnant and lactating females the urine contains creatine. Creatine is obtained in the liver from amino-acids.

Creatinine:-  Creatinine is the break down metabolic product of creatine. It is formed in the muscles from high energy compound creatinine phosphate.  It is excreted along with urine.

Animals on the basis of excretory matter are divided into three categories :-

Excretory organs in animals :

HUMAN EXCRETORY SYSTEM

Excretory organ are also termed as organs of homeostasis.

The main excretory organ in humans is kidney.

Other excretory organs are skin, liver, lungs & large intestine.

Human excretory system consists of :

– Two kidneys & their blood supplies.

– A pair of ureters.

– urinary bladder

– Urethra