Question

A normal human body yields around 1.5 liters of urine per day althouvh the total urine filtrate is 1,800 liters per day. Can yoh explain the mechanisms that allow human body to return this significant amount of water to the body fluids?

Answer 1

The glomerular filterate entering the renel tubule is not urine. It is an extremely dilute solution containing a lot of useful materials including glucose and some salts such as those of sodium. As the filterate passes down the tubule much of the water is reabsorbed into the blood together with the useful substances. But this reabsorbtion is only to the extend that the normal concentration of the blood is not diluted. so this kind of absoption is called selective absorption.

substances reabsorbed back to blood from renel tubule are :

a) in proximal convoluted tubule - most of water ie, about 2/3rd and much glucose and sodium and chloride ions.

b) in loop of Henle - some absorption of water and sodium ions.

c) in distal convoluted tubule - Reabsoption of remaining chlorides and some water.

The filterate left after reabsorption and tubular secretion( k+ ions and some drugs are absorbed into the filterate in distal convoluted tubule) is called urine.

So most of the water from the glomerular filterate is reabsorbed back in the body. That is the reason for the decrease in volume of urine (1.5L per day) when compaired to the volume of filterate which is 1800L per day.