Excretion is the elimination of metabolic wastes like ammonia, urea, uric acid etc. from the tissues.
Types of excretion
Process of excretion of NH3. Ammonotelic animals: Aquatic invertebrates, aquatic insects, bony fishes, aquatic amphibians, etc. NH3 is highly toxic.
So, excretion needs an excess of water.
NH3 is readily soluble in water and is excreted by diffusion through a body surface or gill surfaces (in fishes) as ammonium ions.
Kidneys do not play any significant role in its removal.
Process of excretion of urea.
Ureotelic animals: Cartilaginous fishes, terrestrial & semi-aquatic amphibians (frogs, toads, etc.), aquatic & semi-aquatic reptiles (alligators, turtles), mammals, etc.
In the liver, NH3 is converted into less toxic urea.
So, it needs only a moderate quantity of water for excretion. Some amount of urea may be retained in the kidney matrix of some animals to maintain the desired osmolarity.
Process of excretion of uric acid. It is water insoluble & less toxic. So, water is not needed for excretion.
Uricotelic animals: Insects, some land crustaceans, land snails, terrestrial reptiles & birds. Ureotelism & uricotelism are needed for water conservation.
Some excretory organs in animals
• Protonephridia (flame cells): In Flatworms, rotifers, some annelids & cephalochordate. Protonephridia are primarily for osmoregulation.
• Nephridia: In Annelids. Help in the removal of nitrogenous wastes and osmoregulation.
• Malpighian tubules: In Insects. Help in the removal of nitrogenous wastes and osmoregulation.
• Antennal or green glands: In Crustaceans (prawn etc.)
• Kidneys: In higher animals.
HUMAN EXCRETORY SYSTEM
It includes kidneys, ureters, urinary bladder & urethra.
Structure of Kidney
– Reddish brown, bean-shaped structures situated between the levels of last thoracic & 3rd lumbar vertebra.
– Length: 10-12 cm, width: 5-7 cm, thickness: 2-3 cm.
Average weight: 120-170 gm. – It is enclosed in a tough, 3-layered fibrous renal capsule.
– On the concave side of the kidney, there is an opening (hilum or hilus) through which blood vessels, nerves, lymphatic ducts, and ureters enter the kidney.
– Hilum leads to funnel shaped cavity called renal pelvis with projections called calyces.
– A kidney has outer cortex & inner medulla.
– Medulla has few conical projections called medullary pyramids (renal pyramids) projecting into the calyces.
– Cortex extends in between the medullary pyramids as renal columns (Columns of Bertini).
– Each kidney has nearly one million tubular nephrons.
– Nephrons are the structural & functional units of kidney.
– Each nephron has 2 parts: Glomerulus & Renal tubule.
A tuft of capillaries formed by afferent arteriole (a fine branch of the renal artery). Blood from the glomerulus is carried away by an efferent arteriole.
o Renal tubule:
It begins with a double-walled cup-like Bowman’s capsule, which encloses the glomerulus.
Glomerulus + Bowman’s capsule = Malpighian body
– The tubule continues with proximal convoluted tubule (PCT), Henle’s loop & distal convoluted tubule (DCT).
– Henle’s loop is hairpin-shaped. It has to descending and ascending limbs. – The DCTs of many nephrons open into a collecting duct.
The collecting duct extends from the cortex to the inner parts of the medulla. They converge and open into the renal pelvis through medullary pyramids in the calyces.
– Malpighian body (Renal corpuscle), PCT, and DCT are situated in the renal cortex. Loop of Henle dips into the medulla.
– The efferent arteriole forms a fine capillary network (peritubular capillaries) around the renal tubule.
A minute vessel of this network runs parallel to Henle’s loop forming a ‘U’ shaped vasa recta.
Types of nephrons
1. Cortical nephrons (85%): In this, the Henle’s loop is short and extends only very little into the medulla. Vasa recta is absent or highly reduced.
2. Juxtamedullary nephrons (15%): In this, Henle’s loop is long and runs deep into medulla. Vasa recta present.