Page 8 - LN- EXCRETION

GFR is regulated by one of the efficient mechanism carried out by Juxtaglomerular
Apparatus (JGA).
JGA is a special sensitive region formed by cellular modifications in the distal convoluted
tubule and the afferent arteriole at the location of their contact.
This apparatus includes
(i) granular juxtaglomerular cells in the afferent arteriole.
(ii) macula densa cells of DCT.
(iii) agranular lacis cells situated in between the above two.
A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular
blood flow and thereby, the GFR back to I normal.
2. Selective Reabsorption
This is the second step in the formation of urine from filtrate. The urine released is 1.5 L as
compared to the volume of the filtrate formed per day (180 L). It suggests that as much as
99% of the material in the filtrate is reabsorbed by the renal tubules. Thus, the process is
called reabsorption.
Depending upon the types of molecules being reabsorbed, movements into and out of
epithelial cells in different segments of nephron occur either by passive transport or active
transport.
These are described as follows
(i) Water and urea, are reabsorbed by passive transport (i.e., water is reabsorbed by
osmosis and urea by simple diffusion).
(ii) Glucose and amino acids are reabsorbed by active transport.
(iii) The reabsorption of Na+, occurs both by passive and active transport.
3. Tubular Secretion
It is also an important step in urine formation. Certain chemicals in the blood that are not
removed by filtration from the glomerular capillaries are removed by this process of tubular
secretion. It helps in the maintenance of ionic and acid-base balance of body fluids by
+
+
removing chemicals like foreign bodies, ions (K , H , NH ) and molecules (medicines), etc.,
–
that are toxic at elevated levels.
Difference between the Tubular Reabsorption and Tubular Secretion

Functions of the Tubules
When the glomerular filtrate/primary urine passes through renal tubule, water and different
materials of filtrate reabsorb at various places.
These are given below in the following manner