Excretion| urinary system of humans| structure of kidney& nephrons|3d animation|

The excretory sytem of human is called urinary system. it is formed of one pair of kidney, a pair of ureter, a urinary bladder and a urethra. kidney filter blood to produce urine and the ureters carry urine from kidney to urinary bladder. the bladder temporarily stores urine until it is released from body. urethra is the tube that carries urine from urinary bladder to the outside to body.
Kidney are dark red bean shaped organ. Each kidney is 10cm long, 5cm wide and 4cm thick. The longitudinal section of kidney shows two regions.
Renal cortex is the outer part of kidney and it is dark red in color.
Renal medulla is the inner part of kidney and is pale red in color. Renal medulla consists of several cone shaped areas called renal pyramids. Renal pyramids projects into funnel shaped cavity called renal pelvis which is base of ureter.
The functional unit of kidney is called nephrons.
There are two parts of nephrons
1. Renal corpuscle. Renal corpuscle is not tubular and has two parts i.e glomerulus ( which is network of capillaries) and Bowman’s capsule ( which is U shaped structure that encloses glomerulus.
2. Renal tubule. Renal tubule is the part of nephron which starts after Bowman’s capscule.
• its first portion is called proximal convulated tubule
• next shaped is U shaped and is called loop of Henle
• the last portion of renal tubule is distal convulated tubule.
• The distal convulated tubules of many nephrons open in single collecting duct.
Functions of Nephron
When blood enters the kidney via the renal artery it goes to many arterioles and then to the glomerulus. The pressure of blood is high and most of water, salts, glucose and urea of blood is forced out of glomerular capillaries this material passes into the Bowman’s capscule and is now called glomerular filterate.
The second step is selective reabsorption. In this step about 99% of the glomerular filterate is reabsorbed into the blood capillaries surrounding renal tubule. It occur through diffusion, osmosis and active transport. Some water and most of glucose is reabsorbed from the proximal convulated tubule. The descending limb of loop of henle allows the reabsorption of water while ascending limb of loop of henle allows the reabsorption of salts. The distal convulated tubule again allows the reabsorption of water into the blood.
The third step is the tubular secretion. Different ions, creatine, urea etc are secreted from blood into the filterate in renal tubule this is done to maintain blood at a normal ph. The filterate present in renal tubule is known as urine. It moves into collecting ducts towards pelvis.
Normal composition of urine are 95% are water, urea 9.3g/l, chloride ions 1.87g/l, sodium ions 1.17g/l, potassium ions 0.75g/l.

Osmoregulation function of kidney
Osmoregulation is defined as regulation of the concentration of water, salts in lood. Kidney play an important role in osmoregulation by regulating water content in blood.
When excess water in blood
-- brain detects pituitary glnds releases less ADH
--causes kidney tubule less permeable to water so less water is reabsorbed
--dilute urine is produced
-- brings down the volume of water in blood to normal.

When there is shortage of water in blood
--brains detects and pituitary glands more ADH
-- which causes kidney tubule move permeable to water so more water is reabsorbed
-- small amount of concentrated urine is produced
-- increase the volume of water in blood to normal.

Disorders of kidney
1. Kidney stones
2. Renal failure

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