Excretory System

Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g. Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter. Inner to the hilum is a broad funnel shaped space called the renal pelvis with projections called calyces. The outer layer of kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex and an inner medulla. The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces (sing.: calyx).

The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.

Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units. Each nephron has two parts - the glomerulus and the renal tubule. 

Glomerulus is a tuft of capillaries formed by the afferent arteriole – a fine branch of renal artery. Blood from the glomerulus is carried away by an efferent 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 the malpighian body or renal corpuscle.

The tubule continues further to form a highly coiled network - proximal convoluted tubule (Per). A hairpin shaped Henle's loop is the next part of the tubule which has a descending and an ascending limb. The ascending limb continues as another highly coiled tubular region called distal convoluted tubule (DCT). The DCTs of many nephrons open into a straight tube called collecting duct, many of which converge and open into the renal pelvis through medullary pyramids in the calyces.

The Malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region of the kidney whereas the loop of Henle dips into the medulla. In majority of nephrons, the loop of Henle is too short and extends only very little into the medulla: Such nephrons are called cortical nephrons. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla. These nephrons are called juxta medullary nephrons.

The efferent 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 Henle's loop forming a 'U' shaped vasa recta. Vasa recta is absent or highly reduced in cortical nephrons.

• URINARY BUDPER

 Urinary bladder has two parts.

(i) Body - It is made of involuntary muscles which is called detrusor muscle.

(ii) Trigon - It is a triangular part. Two ureter open on its, two comers.

There are two sphinctors on the lower end of trigon.

(a) Internal sphinctor - Made of unstriated muscles.

(b) External sphincter - Made of straited muscles.

Urinary bladder has capacity of 700-800 ml.

When it is filled upto 300-400 ml of urine then stretch receptors present on wall of urinary bladder, are stimulated and impulse is formed.

This impulse is carried to S_2-3-4 (Spinal segment) through pelvic nerve. Parasympathatic nerve fibers are exited and contraction detrusor muscle and relaxation in internal sphinctor occurs.

This causes urine come out. This process is called micturation because external sphinctor is made of striped muscles.

Micturation is unvoluntary in rabbit and children up to 2 years while it is voluntary in adult human.

Abdominal muscles and diaphragm help in micturation.

Lower part of urinary bladder is joint with urethra. 

Urethra: The urinary bladder leads into the urethra. In a female; it is quite short, only about 3 to 5 cm long, and carries only urine. It opens by urethral orifice, or urinary aperture in the vulva in front of the veginal or genital aperture. In a male urethra is much longer, about 20 cm and carries urine as well as spermatic fluid. It passes through the prostate gland and the penis. It opens out at the tip of the penis by urinogenital aperture. In males the epithelium of spongy urethra is stratified or pseudostratified columnar epithelia, except near external urethral orifice, which is non keratinized stratified. squamous epithelia. The prostatic urethra lined by transitional epithelia, while membranous urethra lined by pseudostratified columnar. 

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