Excretory System


Excretion- to filter out and removemetabolic wastes.




Cellular level






Excretory system


Platyhelminthes-nepridia containing flame cells, two lateral lines.

Nematoda-two lateral lines.

Mullusca- nepridia

Artropoda-green glands


Vertebrate Excretory system



1)     Osmoregulation-control of tissue water balance.

2)     Elimination of excess salts.

3)     Elimination of wastes formed during metabolism of amino acids, nitrogenous wastes.


Parts of Excretory System


1)     Kidney-paired bean-shaped organ which lies on each side of the dorsal body wall.

a)     Nephron-functional unit of the kidney.



1)     Filtration

2)     Reabsorption

3)     Secretion


a)     Renal corpuscle-Filtration unit, it consists of Bowmanís capsule and the glomerulus, a cluster of capillaries.The capillaries in the glomerulus have poresand are surrounded by cells that have filtration slits.Blood pressure forces water and small molecules out of the capillaries, through slits, and into the Bowmanís capsule. The filtrate then enters the proximal tubule.


b)    Proximal tubule-65% of filtrate is reabsorbed in the proximal tubule.1) Mircovilli from epithelial cells extend into the lumen of the proximal tubule increasing the surface area.2) Reabsorption of water and solutes occurs because a) Na+/K+APTase in the basolateral membrane of epithelial cells sets up a strong concentration gradient to bring Na+ into the cell.

†††† Na+contransporters in the apical membrane result in†††††††††† reabsorption of nutrients and electrolytes with water††††† following via osmosis.c) The solutes exit the cell through channels with water following, and d) enter a nearby blood vessel.


c)     The Loop of Henle-The fluid that emerges from the proximal tubule enters a long loop.This loop plunges from the cortex of the kidney deep into the medulla.Descending loop is permeable to water, water moves out of the loop via passive transport. Thick ascending loopis highly permeable to Na+ and Cl-, moderately permeable to urea, and almost completely impermeable to water. Na+, Cl- and urea remove through the membrane via passive transport.Thick ascending loop additional Na+ and Cl- are transported out of the nephronvia active transport.Distal tubule and collecting duct The amount of Na+, Cl-, and water that is reabsorbed varies with the organismís condition and is under hormonal control.If Na+ levels in the blood are low, the hormone aldosterone is released and leads to the reabsorption of Na+ and Cl- in the distal tubule.Water follows these ions via osmosis.If and organism is dehydrated, a molecule called ADH is released from the pituitary.It interacts with the cells lining the collecting duct, aquaporin channels are inserted into the membrane.As a result the cells become highly permeable to water and large amounts of water are reabsorbed producing a concentrated urine.In the absence of ADH less water is reabsorbed producing a more dilute urine.


2)     Ureters-carry the urine to the urinary bladder.


3)     Urinary bladder-stores the urine until it is expelled.


†††† 4) Urethra- carries the urine out of the body.