Answer:
Urine is formed in the
kidneys in the nephron, i.e., the structural and functional unit of kidney.
Blood at high pressure travels into these tubules by the tuft of blood
capillaries called glomerulus.
The
following steps are involved in the process
(i) Filtration Blood enters the glomerulus through
the afferent arterioles, it passes under pressure that results in filtration of
blood. Water and small molecules are forced out of glomerular capillary walls
and Bowman's capsule. Large molecules remain in the blood of the glomerulus.
(ii) Selective reabsorption Some molecules are
selectively reabsorbed into the blood.
The glomerular filtrate flows through the proximal
convoluted tubule, the U-shaped Henle's loop and distal convoluted tubule.
The useful substances present such as glucose, amino acids
and salts .are reabsorbed by a process, which requires energy. The filtrate now
contains urea, some salts and water. Reabsorption of solutes increases the
water concentration of the filtrate.
The water is then reabsorbed into blood by osmosis
(osmoregulation).
(iii) Tubular secretion Some nitrogenous waste
products like creatinine and some other substances like are removed from
blood by DCT (Distal convoluted tubule), and are added to urine.
The urine formed is collected in the urinary bladder.
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