Describe the excretory system of man, giving the structure of nephron.

In humans, the excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and urethra.
Kidneys :
* Kidneys are located in the abdominal region which are situated on either side of the vertebral
column.
* They are reddish brown, bean shaped and retroperitoneal organs.
* Kidneys are enclosed in tough fibrous renal capsule.
* Kidney shows outer cortex & inner medullary regions.
* Medulla is divided into multiple cone shaped renal pyramids which are separated by columns of Bertin.
* The funnel shaped cavity pelvis opens into ureter.
Ureters :
* Slender whitish tubes run posteriorly and open into urinary bladder.
Urinary Bladder :
* It is median storage sac situated in the lower abdominal cavity.
* The neck of the bladder leads into the urethra, which has internal and external urethral sphincters.
Structure of nephron :
* Structural and functional unit of kidney, each kidney contains nearly 1 million nephrons.
* Each nephron has 2 parts, Malpighian body and renal tubule.
* Renal corpuscle composed of network of blood capillaries called glomerulus and a doubled
walled cup called Bowman’s capsule present in cortex.
Bowman’s capsule :
* Its inner wall has certain unique cells called podocytes which leave minute spaces called filtration slits or slit pores.
* The endothelial cells of the capillaries have numerous pores or fenestrations.
Renal tubule :
* It includes 3 segments.
i) PCT (Proximal convoluted tubule).
ii) Henle’s Loop.
iii) DCT (Distal convoluted tubule).
i. PCT :
* Present in cortex which is wide and highly
coiled, which continues into descending limb
of loop of Henle. It is lined by cuboidal cells
with brush border.
ii. Loop of Henle :
* It is present in medulla, which is hair pin shaped.
* It has descending and ascending limbs.
* Proximal part of ascending limb is thin and distal part is thick which continues into DCT.
iii. DCT :
* It is present in cortex, which continues as the 'initial collecting duct’ in the cortex.
* Initial collecting ducts unite to form straight collecting duct(CD), which passes through medullary pyramid.
Types of Nephrons :
* There are two types.
1. Cortical Nephrons
2. Juxtamedullary Nephrons