In: Anatomy and Physiology
Answer) There are three different stages in the formation of urine by the filtration of blood through the nephrons. The process of urine formation in kidneys include the following steps:
i) Glomerular filtration -
Urine formation begins when the blood is filtered by the
glomerulus then enters the Bowman's capsule and the glomerular
filtrate is formed. The afferent arteriole entering the glomerulus
is wider than the efferent arteriole in diameter. This increases
the blood pressure within the glomerulus helps in the filtration.
Glomerular filtrate is formed of all blood constituents except
corpuscles and plasma proteins.
(ii) Tubular reabsorption -
When the glomerular filtrate comes to the proximal tubule then
the essential nutrients are reabsorbed. Glucose, amino acids,
water, ions like sodium, potassium, chloride, bicarbonate,
magnesium, calcium are reabsorbed from the glomerular filtrate.
When the filtrate finally comes to the collecting duct ADH
(anti-diuretic hormone) acts and reabsorbs the excess water and
decreases the urine volume.
(iii) Tubular secretion -
When the glomerular filtrate is in the proximal tubule creatinine, uric acid are secreted in it. When the filtrate goes to the distal part urea, potasium ion and some hydrogen ions are secreted in the form of urine.