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Unit 6 Excretory Products & their Elimination


 EXCRETORY PRODUCTS AND THEIR ELIMINATION

DEFINITION

(i) Elimination of metabolic waste from body is called excretion.
(ii) Due to metabolic activity in the body numerous waste substances are produced. 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.
Carbohydrate metabolism, produces CO2 and H2O. Protein metabolism produces nitrogenous wastes–ammonia, urea and uric acid.

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.

Osmoregulation:
Osmoregulation : The regulation of solute movement and hence water movement (which follows solutes by osmosis) is called osmoregulation.
On the basis of osmoregulation, animals are either osmoconformer or ormoregulators.
a. 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 marine cyclostome fish, is the only vertebrate osmoconformer.

Osmoconformes show an excellent ability to tolerate a wide range of cellular osmotic environments.


b. Osmoregulators :

Osmoregulators are those who animals maintain an osmolarity internally 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 m osm L–1) are hypertonic to surrounding medium (osmolarity 50 m osm L–1). Due to this, the freshwater animals constantly face two problems :
(i) They gain water passively due to osmotic gradient
(ii) Continuous loss of body salts to surrounding low salt containing medium occurs.

To encounter these problems the fresh water fishes perform following acts :
 They do not drink water
 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.



 Ammonia, Urea and Uric acid are the major forms of nitrogenous wastes excreted by the animals.
 Toxicity sequence = Ammonia > Urea > Uric acid.
 Ammonotelic animals = Many bony fishes, aquatic amphibians, aquatic insects.
 Ureotelic animals = Mammals, Marine fishes, terrestrial amphibians.
 Uricotelic animals = Reptiles, birds, insects, land snails.
 A survey of animal kingdom presents a variety of excretory structures
 In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
 The outer layer of kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex and an inner medulla.
 The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces.
 The cortex extends in between the medullary pyramids as renal columns called "Columns of Bertini".
 Each kidney has nearly one million complex tubular structures called nephrons which are the functional units .
 Glomerulus + Bowman's capsule = Malpighian body or Renal corpuscle.
 The malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region of the kidney whereas the loop of Henle dip into the medulla.
 Glomerular filtration takes place in Bowman's capsule and glomerular filtrate is formed which is protein less plasma.
 On an average 1100-1200 ml of blood is filtered by the kidney per minute.
 Glomerular filtration rate (GFR) in a healthy individual is 125 ml/minute or 180 litres per day.
 Nearly 99 percent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption.
 Substances like glucose, amino acids, Na+ etc. in the filtrate are reabsorbed actively whereas the nitrogenous wastes and water are absorbed by passive transport.
 During urine formation, the tubular cells secrete substances like H+, K+ and ammonia into the filtrate. This tubular secretion is an active process.
Functions of the tubules :
1. PCT:
Maximum reabsorption occur
All the essential nutrients, 70-80% of electrolytes and water, 3– HCO are reabsorbed.
Selective secretion of hydrogen ions, ammonia and potassium ions.
2. Henle's loop:
Descending limb reabsorb water passively and ascending limb reabsorbed electrolytes actively or passively.

 

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