The process of urine formation enables the kidney to purify blood of the toxic materials and

The process of urine formation enables the kidney to purify blood of the toxic materials and
balance the salt and water levels in it.
It occurs in two stages.
a. Ultra filtration.
b. Selective Re-absorption.
Ultra filtration.
It takes place in the glomerulus and requires two conditions
I. A very high pressure.
II. A filtering barrier.
The pressure is built up in two ways.
1) The usual high pressure of the blood in the arteries due to the pumping action of the heart and
the renal artery takes blood from the dorsal aorta at a point close to the heart which has blood
at a very high pressure.
2) Due to the fact that the afferent vessel that brings blood to the glomerulus is much wider than
the efferent vessel that takes blood away from the glomerulus
The capillary walls of the glomerulus are permeable and act as the filtering barrier.

Because of the permeability of the capillary walls and very high pressure, smaller components of
blood such as water, mineral salts, urea, glucose, vitamins, etc. are passed from blood into the the
capsular space of the Bowman’s capsule. The liquid which collects in the Bowman’s capsule is
known as glomerular filtrate. It is similar to tissue fluid in chemical composition. The process
leading to the formation of the glomelular filtrate is known as pressure filtration or ultra
filtration. The filtering barrier retains all blood components which are bigger than pores e.g the
blood cells and the plasma protein like fibrinogen and globulin.
Selective reabsorption: this ensures that useful materials are not lost from the body.
As the glomerular filtrate passes from the Bowman’s capsule along each tubule, substances in it
which are still useful to the body are selectively reabsorbed. All the glucose, much of the water
and small amounts of mineral salts are reabsorbed by active transport, osmosis and diffusion. In
the proximal convoluted tubule all the glucose and some amino acids are re-absorbed into blood
by active transport and some water is re-absorbed by osmosis. Also mineral salts are re-absorbed
from here. In the loop of henle, more re-absorption of water takes place, large percentage of salts
are re-absorbed here. The remaining glomerular filtrate passes to the distal convoluted tubules
where salts and water are further re-absorbed. It moves to the collecting duct where more water
is re-absorbed regulating the concentration of blood. The end product is urine which is carried by
the collecting ducts to ureter then to the bladder.
All urea and small amounts of mineral salts remain in the tubules dissolved in water and are
passed on to the bladder as urine.
substance %age cone in the following fluids
Plasma glomerular filtrate.
urine Appr.increse in cone
of urine / plasma
Plasma protein 7.5 0 0 -
Glucose 0.1 0.1 0 -
Sodium ion 0.32 0.32 0.35 Χ 1
Chloride ion 0.37 0.37 0.60 Χ 2
Urea 0.03 0.03 2.0 Χ 60
Water 91.9 91.9 95.0 Χ 1