Blood Cleaning by the Kidneys - Part (1)

This section is about processes performed by the kidneys in order to filter (clean) blood.
These are:

1. Glomerular Filtration also called "Ultra-filtration",
2. Tubular Reabsorption also called "Selective Re-Absorption" and
3. Tubular Secretion.

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 artery.
This seperates 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 kidney nephron.

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 arterioles.

This increased blood pressure helps to force the following components of the blood out of the glomerular capillaries:

• 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 cm³ per minute in humans. This volume is approx. 20% of the plasma delivered during that time. (Again: It contains all the materials 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 tubule.

 

Click here to view the next page: Blood Cleaning by the Kidneys - Part (2): Tubular Re-absorption.