September 5, 2013

Inter Zoology Imp Question - Excretory System of Man

Describe the excretory system of man giving structure of nephron.
Ans: 1. Kidneys:
A pair of reddish brown, bean shape meta nephric kidneys is present in the abdominal cavity, one on each side
of vertebral column at the level of last thoracic and third lumbar vertebrae. Kidneys are found attached to the inner surface of dorsal abdominal wall and are retroperitoneal organs. Right kidney is placed at a lower level than left one because of the presence of liver or right side (Each kidney of man weighs 120 g
to 170 g). Each kidney is enclosed by a protective, tough fibrous capsule called renal
capsule. The outer surface of the kidney is convex and inner surface is concave.
The concavity in the middle of inner surface of kidney is known as hilum. Renal
artery enters the kidney and renal vein and ureter exit the kidney at hilum.
Internal structure: Vertical section of a kidney shows two distinct regions.. namely,
outer cortex and inner medulla. Inside the kidney, ureter expands as a funnel like
pelvis. The edges of pelvis have cup like concavities called calyces. Medulla is
formed into cone like renal pyramids and fit into the calyces. Among the pyramids,
cortex projects inward as columns of Bertin. Pointed tip of renal pyramid is called
renal papilla.
Microscopic structure of Kidney: Each kidney is formed by about one million
microscopic convoluted tubular structures called nephrons. In addition, kidneys
contain a net work of blood capillaries, lymph sinuses, nerves and interstitial fluid
(cortical fluid & medullary fluid).
Nephron: Nephron is the structural and functional unit of the kidney. A nephron has
a Malpighian body (formed by Bowman's capsule and glomerulus) or renal
corpuscle and a renal tubule. Malpighian body lies in cortex.
i. The proximal blind end of renal tubule forms the double layered Bowman's
capsule. It is formed by simple squamous cells called podocytes.

ii. A branch of renal artery- called afferent arteriole enters the cavity of
Bowman's capsule and splits into capillaries. They unite and form an
efferent arteriole and comes out of Bowman's capsule.
iii. The diameter of efferent arteriole is lesser than that of afferent arteriole.
iv. The capillary net formed by afferent and efferent arterioles in Bowman's
capsule is called glomerulus. Glomerular capillary walls are formed by
endothelial cells (single layer) and have minute pores - fenestrae.
v. Podocytes wrap around glomerular capillaries leaving minute spaces called
filtration slits or slit pores.

vi. Renal Tubule behind the Bowman's capsule is divisible into Proximal
Convoluted Tubule (PCT), Loop Of Henle (LOH) and Distal Convoluted
Tubule (DCT).
vii. PCT is long and convoluted that lies in cortex. It is formed by cuboidal
epithelial cells. Their inner surfaces have numerous microvilli (Brush
viii. PCT is continued as a hair pin like loop of Henle, which lies in medulla. It
has a descending limb and an ascending limb. The wall of descending limb
and lower part of ascending limb is thin and remaining part of ascending
limb is thick.
ix. Ascending limb is continued as DCT, which lies in cortex. It is formed by
cuboidal epithelium.
x. DCT ends as an initial collecting duct. Initial collecting ducts of many
nephrons unite to form a straight collecting duct. It extends through renal
pyramid (medulla), becomes duct of Bellini and opens into the renal pelvis
on renal papilla.
- In some nephrons, LOH is short and lies in cortex only. Such nephrons are
called cortical nephrons.
- In some nephrons, LOH is long and extends deep into medulla. They are
called juxta medullary nephrons.
- Urinary tract infections in women are more common and the urethral
opening is close to anus.
xi. The efferent arteriole reaches the renal tubule (PCT, LOH and DCT) and splits into
capillaries around it.
- The capillary net around PCT and DCT is known as peritubular net.
- The capillary net around LOH is known as vasarecta.
- Number of functional nephrons in each kidney decreases by 10% per
year after the age of 40 years in man.
- Each kidney filters about 1100 ml - 1200 ml blood per minute.

2. Ureters: From each kidney, a white muscular tube emerges out at hilum, called
ureter. Their walls are formed by transitional epithelium. Ureters extend
posteriorly and opens into urinary bladder.
3. Urinary bladder: Ureters open into a muscular and distensible sac called urinary
bladder. It is lined by transitional epithelium. The neck of urinary bladder continues as urethra. It is encircled by internal urethral sphincter (smooth muscles) and an external urethral sphincter (striated muscles). Urethra opens at the tip of penis in males and in females near the vaginal opening.

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