Excretory Products and Their Elimination
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1. GLØMËRÜLÅR FÏLTRÅTÏØÑ (ÜLTRÅFÏLTRÅTÏØÑ)
Filtration is a non-selective process.
The glomerular capillary blood pressure causes filtration of blood
through 3 layers, i.e. endothelium of glomerular blood vessels, epithelium
of Bowman’s capsule & a basement membrane between these 2 layers.
The epithelial cells (podocytes) of the Bowman’s capsule are arranged
in an intricate manner leaving some minute spaces called filtration slits
(slit pores).
(AIPMT 2011)
Almost all constituents of the blood plasma except the proteins pass
onto the lumen of the Bowman’s capsule
About 1100-1200 ml of blood is filtered by the kidneys per minute. It
constitutes 1/5th of the blood pumped out by each ventricle of the heart
in a minute.
The amount of the filtrate formed per minute is called Glomerular
filtration rate (GFR).
Normal GFR = 125 ml/minute, i.e., 180 litres /day
2. RËÅBSØRPTÏØÑ
180 litres of glomerular filtrate is produced daily. But about 99% of this is reabsorbed
by the renal tubules.
(AIPMT 2010)
So normal volume of urine released is 1.5 litres.
From the filtrate, glucose, amino acids, Na+, etc. are reabsorbed actively and
nitrogenous wastes are absorbed passively. Passive reabsorption of water occurs in
the initial segments of the nephron.
PCT reabsorbs most of the nutrients, and 70-80% of electrolytes & water. Simple
cuboidal brush border epithelium of PCT increases surface area for reabsorption
(AIPMT 2012)
In loop of Henle , minimum reabsorption takes place. It maintains high osmolarity of
medullary interstitial fluid. The descending limb is permeable to water but almost
impermeable to electrolytes. This concentrates the filtrate. The ascending limb is
impermeable to water but allows transport of electrolytes. So, the filtrate gets diluted.
Collecting duct extends from cortex to inner parts of medulla. It reabsorbs large
amount of water to concentrate urine. It also allows passage of small amounts of
urea into medullary interstitium to keep up the osmolarity.
In DCT, conditional reabsorption of Na+ & water takes place.