Blood Cleaning by the Kidneys - Part (1)
This section is about processes performed by the kidneys in order
to filter (clean) blood.
Filtration also called "Ultra-filtration",
Reabsorption also called "Selective Re-Absorption"
As these occur in the kidney
nephrons, it is best to read this page after viewing
the page about the
structure of a kidney nephron.
(1) Glomerular Filtration
Blood enters the kidney via the renal
many times (Renal Artery -> Segmental Arteries -> Interlobar
Arteries -> Arcuate Arteries -> Interlobular Arteries -> Afferent
Arterioles), eventually forming many afferent arterioles,
each of which delivers blood to an individual
The diameter of the afferent (incoming) arteriole is greater
than the diameter of the efferent arteriole (by which
blood leaves the glomerulus). The pressure of the
blood inside the glomerulus is increased due to the
difference in diameter of the incoming and out-going
This increased blood pressure helps
to force the following components of the blood out of the glomerular
- Most of the water;
- Most / all of the salts;
- Most / all of the glucose;
- Most / all of the urea.
The above are filtered in preference to other components
of blood based on particle size. (Water and
solutes of relative molecular mass less than 68,000 form
the filtrate.) Blood cells and plasma proteins are not filtered through
the glomerular capillaries because they are relatively
larger in physical size.
The water and salts that have been forced out of the glomerular
capillaries pass into the Bowman's Capsule and are called
the glomerular filtrate.
This glomerular filtrate is formed at a rate of above 125
minute in humans. This volume is approx.
20% of the plasma delivered during that time. (Again:
It contains all
present in the
blood except blood cells and most proteins - which are
too large to cross the basement membrane of the glomerulus.)
Click here to read more about the Glomerular
Filtration Rate (GFR) - link coming soon.
The glomerular filtrate passes from the renal
corpuscle to the renal
Click here to view the next page: Blood
Cleaning by the Kidneys - Part
(2): Tubular Re-absorption.