19. EXCRETORY PRODUCTS & THEIR ELIMINATION
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 kidney, there is an opening (hilum or hilus) through which blood vessels, nerves, lymphatic ducts and ureter 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.
Cortex extends in between the medullary pyramids as renal columns (Columns of Bertini).
Each kidney has nearly one million tubular nephrons.
Nephron
Each nephron has 2 parts: Glomerulus & Renal tubule.
- Glomerulus: A tuft of capillaries formed by afferent arteriole (a fine branch of renal artery). Blood from glomerulus is carried away by efferent arteriole.
- 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 descending and ascending limbs.
The DCTs of many nephrons open into a collecting duct. Collecting duct extends from cortex to inner parts of 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 renal cortex. Loop of Henle dips into 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
- 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.
- Juxtamedullary nephrons (15%): In this, Henle’s loop is long and runs deep into medulla. Vasa recta present.
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