HUMAN EXCRETORY SYSTEM

1. In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.

Kidneys: Kidneys are reddish brown, bean shaped structures situated between the level of last thoracic and 3rd Lumbar vertebra close to the dorsal inner wall of the abdominal cavity. Each kidney measures about 10 -12 cm long, 5 -7 cm in width and 2 -3 cm thick. It weighs about 120 -170 g in adults. Left kidney is little higher than the right one because of more space being occupied by the liver on right side.

2. Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessel and nerve enters. Inner to the hilum is a broad funnel shaped space called the renal pelvis.

Kidney is covered by 3 protective layers which are as foillows :

1. Renal Capsule: It is innermost, tough protective cover made up of white fibrous connective tissue, with few elastic fibres and few muscles.

2. Adipose Capsule: It is the middle cover involving adipose tissue and acts as shock absorber.

3. Renal Fascia: It is the outermost fibrous cover linking it with the abdominal wall.

As the kidneys are fused with the body wall on the dorsal side, peritoneal cover is present only on the ventral side. This arrangement is called Retroperitoneal arrangement.

Internal Structure of Kidney

A longitudinal section of the kidney shows two functional layers, outer renal cortex and inner renal medulla.

(a) Renal Cortex: It is the outer part which is dark in colour and granular in nature.

(b) Renal Medulla : It is the inner part which is lighter in colour and striated in nature. Medulla has 8 -18 conical renal pyramids. They actually represent multilobular condition of foetal kidney.

Each renal pyramid has a broad base towards the cortical side. Apex is pointed and is called renal papilla.

1-3 renal papillae project into a activity called minor calyx, which join up and form major calyces.

Cortex projects into medulla in the regions in between the pyramids and calyces, called renal columns of Bertini.

Interstitial fluid of medulla region has a higher osmotic concentration equal to some 1200 mosm/lt. due to higher quantity of two solutes, NaCl and urea.

Cortical region close to medulla is called juxtamedullary area.

The major calyces open into a broad funnel-shaped structure called renal pelvis placed inner to hilum.

It is lined by transitional epithelium.

It leads into ureter.

Structural and functional units of kidney are called nephrons or uriniferous tubules.

There are about 1 million nephrons in each kidney.

The number of uriniferous tubules decreases with age.

Ureters:

They are a pair of fine whitish distensible muscular tubes of 25 -30 cm length, about 3 mm in diameter.

Ureters develop from hilum part of the kidneys, descend along the abdominal wall, bend obliquely inwards and upwards to open in to urinary bladder in the region of trigone by oblique slits, one on each side.

Wall of ureter has 3 coats -extemal adventitia, middle muscular and inner mucosa, Muscular coat has three layers of smooth muscle fibres -outer longitudinal, middle circular and inner longitudinal.

Ureters are always undergoing peristalsis which helps in passing urine from kidney to urinary bladder.

Urinary Bladder:

It is a median pyriform sac which varies in shape, size and position according to the amount of urine contained in it.

The fully distended bladder becomes ovoid in outline.

Bladder has three parts -apex, fundus or body and neck. Body has a triangular area called trigone.

It has openings of ureters and an internal urethral orifice.

Neck region possesses two sphincters, involuntary internal sphincter and voluntary external sphincter.

Neck leads into urethra.

Wall of urinary bladder consists of three coats -outer adventitia, middle muscular and inner mucosa.

The muscle present in the middle muscular coat is also called detrusor muscle because it takes part in detrusion or pushing down of urine.

Muscular coat has involuntary circular muscles in the middle and involuntary longitudinal muscles on its either side.

Mucosa has loose connective tissue towards the side of muscular layer and transitional epithelium or urothelium towards lumen.

Adventitia is formed of soft connective tissue.

During the micturition, both the sphincters undergo relaxation.

Cerebral cortex direct the sphincter to relax and person undergoes urination.

Wall of urinary bladder is innervated by both sympathetic and parasympathetic nervous system.

Urethra:

It is present only in mammals. It starts from the neck of the urinary bladder and opens outside the body.

In female, it is short (2-4 cm), straight and concerned with the release of urine through an aperture called urethral orifice or urinary aperture present in the vulva in front of the vaginal aperture.

However, in male, it is quite long (20 cm), passes through the ejaculatory duct, prostate gland, cowper's glands and penis.

It is concerned with the release of urine as well as semen (sperms + glandular secretion) through an aperture called urinogenital aperture at the tip of the penis.

Blood Supply to Kidney

A renal artery enters each kidney and divides into many afferent arterioles which enter the Bowman's capsules and sub-divide to form glomerulus.

The glomerular capillaries rejoin to form efferent arteriole.

Efferent arteriole is narrower than afferent arteriole.

This raises blood pressure inside glomerulus.

Efferent arteriole further forms numerous peritubular capillaries in the cortex region around the proximal and distal convoluted tubules of the nephron.

It further forms vasa recta around the loop of Henle.

Blood from the vasa recta is released into the renal venule from where it escapes into renal vein.