It is the second step of urine formation. It is defined as; “ The process by which water and other substances are transported by renal tubules back to blood is called Tubular Reabsorption”. Tubular reabsorption is highly selective. Some substances like glucose and amino acids are completely absorbed from tubules. So, the urinary excretion is zero. Ions such as Na+, Cl-, HCO3- are highly absorbed but rate of absorption and excretion varies, according to body needs. Materials Not Reabsorbed Nitrogenous waste products Urea Uric acid Creatinine Excess water

1. Tubular Reabsorption

2. TUBULAR REABSORPTION
• It is the second step of urine formation.
• It is defined as;
• “ The process by which water and other substances are
transported by renal tubules back to blood is called Tubular
Reabsorption”.

5. •Tubular reabsorption is
highly selective.
• Some substances like glucose and
amino acids are completely
absorbed from tubules. So, the
urinary excretion is zero.
• Ions such as Na+, Cl-, HCO3- are
highly absorbed but rate of
absorption and excretion varies,
according to body needs.
• Materials Not Reabsorbed
• Nitrogenous waste products
• Urea
• Uric acid
• Creatinine
• Excess water

6. Routes Of Reabsorption
•TRANSCELLULAR ROUTE
•
• In this route the substances move
through the cell.
• Sodium is a substance that moves
through both routes, mostly it is
transported through the transcellular
pathway.
PARACELLULAR ROUTE
In this route the substances move
through the intracellular space

7. Two Routes of the absorption
Lumen
Plasma
Cells
Transcellular
Pathway
Paracellular
transport

8. Mechanism of Transport
1, Active Transport.
I. Primary Active Transport
II. Secondary Active Transport
III. Pinocytosis
2, Passive Transport

9. ACTIVE TRANSPORT: In active transport, a solute moves against an
electrochemical gradient and needs energy.
1. PRIMARY ACTIVE TRANSPORT
• Transport that is coupled directly to an energy source such
as the hydrolysis of adenosine triphosphate ( ATP )By ATP
ase IS REFFERED AS PRIMARY ACTIVE TRANSPORT.
• There are special primary active transporters which can
move solute against electro-chemical gradient.
 Primary active transporters, known, include sodium-
potassium ATP ase, Hydrogen ATP ase, Hydrogen-potassium
ATP ase and Calcium ATP ase.
 Re-absorption of Na+ is distinct example of primary active
transport.
2. SECONDARY ACTIVE TRANSPORT
Transport that is coupled indirectly to an energy source, such
as that due to an ion gradient is reffered as “SECONDARY
ACTIVE TRANSPORT “
• Secondary active transport does not need energy directly
from ATP or other energy rich compounds.
• Example: transport of glucose & amino acids in proximal
tubules.

10. Reabsorption of GLUCOSE
Sodium glucose co-transporters (SGLT2 and SGLT1) are located on the brush border of
proximal tubular cells and carry glucose in to the cell cytoplasm against a concentration
gradient.
90% of the filtered glucose is reabsorbed by SGLT2 in early part of the proximal tubule.
10% is transported by SGLT1 in the later segment of proximal tubule.
On the basolateral side of the membrane, glucose diffuses out of the cell into the interstitial
spaces with the help of glucose transporters (GLUT2 IN EARLY SEGMENT ) and GLUT1 in the
late part of the proximal tubule.
As glucose reabsorbption is the example of secondary active reabsorption, it does not
required direct energy. It depends on sodium potassium ATPase pump.

11. • CO TRANSPORT • COUNTER TRANSPORT

12. Pinocytosis
• Large molecules like proteins are re-absorbed by
pinocytosis, especially in proximal tubules.
• In pinocytosis, protein attaches to brush border of luminal
membrane.
• This portion of membrane invaginates to interior of cell till
it completely pinches off, forming a vesicle which contains
protein.
• As pinocytosis requires energy, so it supposed to
be a form of active transport.
• Pinocytosis: large molecules such as proteins
transport by pinocytosis.
• CREATININE is a bigger molecule than urea.
• None of filtered creatinine is re- absorbed and
almost all filtered creatinine is exreted in urine.

13. PASSIVE TRANSPORT :It can move the solute along with electrochemical gradient. It doesnot require energy.
• Water is passively re-absorbed by osmosis. This is coupled
with Na+ re-absorption.
• PCT are highly permeable to water and its re-absorption
occurs at a faster rate. 65%.
• In ascending limb of loop of Henle water permeability is
always low.
• In other parts of tubule, water permeability depends on
presence or absence of ADH.
• In more distal parts of nephron, tight junctions become
less permeable to water & other solutes.
• ADH increases water permeability greatly in distal and
collecting tubules.
• Urea and Cl- are re-absorbed by passive diffusion. Some of Cl- ions
may be re-absorbed by sec active transport.
• UREA is passively reabsorbed from tubule but to a lesser extant
than Cl-.
• Urea is not as permeable as water through tubules.
• In some parts of nephron, especially inner medullary collecting
ducts, passive re-absorption of urea is facilitated by special urea
transporter.
• In spite of these phenomenon, only about 50 % of filtered urea is
reabsorbed from tubules.