- Ammonotelic: elimination of nitrogenous waste in the form of ammonia.(fish)
- Ureotelic: elimination of nitrogenous waste in the form of urea.(Amphibia and mammalian)
- Uricotelic: elimination nitrogenous waste in the form of uric acid. (Reptilia, bird and insects)
Excretory organs :
- Protonephridia or flame cells – Platyhelminthes (Planaria), rotifers, some annelids and cephalochordates (Amphioxus)
- Nephridia: annelid.
- Malpighian tubules – insects
- Antennal gland or green glands – crustacean like prawn.
HUMAN EXCRETORY SYSTEM :
- Human excretory system consists of
- A pair of kidney
- A pair of ureters
- A urinary bladder
- A urethra
- Kidney is reddish brown, bean shaped structure situated between the levels of last thoracic vertebra close to dorsal inner wall of the abdominal cavity.
- Each kidney measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness.
- Towards the centre of inner concave surface is a notch, called hilum through which ureters, blood vessel and nerves enter into the kidney.
- Inner to hilum is a broad funnel shaped space called renal pelvis with projections called calyces.
- The outer wall of kidney is a tough capsule.
- Internally the kidney is differentiated into outer cortex and inner medulla.
- The medulla is divided into a few conical masses called medullary pyramids.
- Pyramids projected into the calyces.
- The cortex extended in-between the medullary pyramids as renal columns called columns of Bertini.
- Each kidney has nearly one million complex tubular structures called nephrons.
- Structural and functional unit of kidney is called nephron or uriniferous tubule.
- Each nephron has two parts:
- Glomerulus is a tuft of capillaries formed by the afferent renal arteriole (a branch of renal artery).
- Blood from the Glomerulus is collected by efferent renal arteriole.
- The renal tubule begins with a double walled cup-like structure called Bowman’s capsule, which encloses the Glomerulus.
- Glomerulus along with Bowman’s capsule is called Malpighian body or renal corpuscles.
- Bowman’s capsule followed by highly coiled proximal convoluted tubule (PCT).
- PCT followed by hairpin shaped Henle’s loop with ascending and descending limb.
- The ascending limb followed by another coiled tubular region called distal convoluted tubule (DCT).
- DCT of many nephron opens into a straight tube called collecting duct.
- All the collecting duct converges and opens into renal pelvis through medullary pyramids in the calyces.
- The malpighian corpuscles, PCT and DCT of the nephron are located in the cortex but the loop of Henle dips into the upper medulla.
- In some of the nephron, the loop of the Henle is very long and runs deep into the inner medulla. These nephrons are called juxta medullary nephrons.
- The efferent renal arteriole emerging from the Glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries.
- A minute vessel of this network runs parallel to the loop of Henle forming‘U’ shaped vasa recta.
- Vasa recta are absent or reduced in cortical nephron.
- The juxta medullary nephron has juxta-glomerular apparatus, in which the DCT run close to the afferent renal arteriole.
MECHANISM OF URINE FORMATION :
- Urine formation involves three main processes –
- Glomerular filtration
- Selective reabsorption
- Tubular secretion.
Glomerular filtration or ultra filtration :
- On an average 1120-1200 ml blood is filtered by the kidneys per minute.
- The glomerular capillary blood pressure caused filtration of through filtration membrane.
- The filtration membrane is formed by –
- Endothelium of glomerular blood vessel.
- The epithelium of Bowman’s capsule (podocytes)
- Basement membrane of these two layers.
- The epithelial cells of Bowman’s capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slit or slit pores.
- All constituent of plasma pass the filtration membrane except protein, hence it is called ultra filtration.
- The amount of filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR).
- GFR is about 125 ml/min. i.e. 180 liters per day.
