Tubular reabsorption (The Guyton and Hall physiology)

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 Source: The Guyton and Hall physiology Maryam Fida (o-1827)

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”.

3. 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

4. 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

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

6. Mechanism of Transport 1, Active Transport. Primary Active Transport Secondary Active Transport Pinocytosis 2, Passive Transport

7. 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.

8. 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.

9. CO TRANSPORT COUNTER TRANSPORT

10. 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.

11. 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.