Thursday 30 April 2020
Describe the process of urine formation in the mammalian kidneys
The afferent arteriole which is a branch of the
renal artery supplies blood to the glomerulus; the afferent arteriole has a
wider lumen/diameter than the efferent arteriole; which takes away blood from
the glomerulus; the differences in the diameter of the afferent and the
afferent vessels causes high pressure; leading to ultrafiltration of blood; the
walls of the blood capillaries are one-cell thick; hence glucose, amino acids,
vitamins, hormones, salts, creatine, urea and water filter into the Bowman’s
capsule; to form glomerular filtrate; white blood cells, red blood cells,
plasma proteins such as globulin and platelets are too large to pass through
the capillary wall; hence remain in the blood capillaries; useful substances in
the human body are selectively reabsorbed; back into the blood stream at the
proximal convoluted tubule; the tubule is highly coiled; to increase the
surface area for reabsorption of the substances; the useful substances include
amino acids, glucose, vitamins, hormones, sodium chloride and water; many
mitochondria found at the proximal convoluted tubule; provide energy for
reabsorption of these substances against a concentration gradient; the
glomerular filtrate flows into the descending and the ascending limb of the
loop of Henle; blood in the capillaries and the glomerular filtrate in the loop
of Henle move in opposite directions/counter-current flow; this provides a
steep concentration gradient that leads to maximum absorption of water through
osmosis; sodium chloride is actively absorbed from the ascending limb into the
blood capillaries; under the influence of aldosterone hormone; the glomerular
filtrate flows into the collecting tubule from where, more water is reabsorbed
into the blood stream; antidiuretic hormone influences the amount of water to
be reabsorbed depending on the osmotic pressure of the blood; the glomerular
filtrate from several collecting tubules now referred to as urine; is emptied
into the collecting duct; the urine passes through pyramid, pelvis and ureter
into the bladder; where it is stored for some time. The sphincter on the
urethra relaxes to allow urine to be released from the body;
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