Question

Discuss the different mechanisms of reabsorption in the renal tubule. How are valuable nutrients reabsorbed from the filtrate? How are ions and water reabsorbed? In which parts of the renal tubule do these events occur? Distinguish between obligatory and facultative reabsorption.

Answer 1

The kidneys filter/excrete waste and reabsorb essential electrolytes, nutrients, and fluid (ultimately to maintain homeostasis). Tubular reabsorption occurs via multiple mechanisms of transport.

A. Mechanisms of tubular reabsorption

Passive transport

Passive transport is movement of a molecule without utilizing energy. In this form of transport, solutes move from a high concentration to low concentration (along their concentration gradient).

Active transport

Active transport requires energy in order to transfer a solute against an electrochemical gradient. There are two major forms of active transport in the tubules:

• Primary active transport involves movement of a solute against an electrochemical gradient at the expense of ATP (energy). An example of this type of transport is seen with the Na+/K+ ATPase pump on the basolateral cell membranes of the tubules. Sodium ( low intracellular concentration, high extracellular concentration)is pumped out of the cell into the interstitium. At the same time, potassium is moved into the cell from the interstitium, also against its concentration gradient.
• Secondary active transport is a form of active transport where two different molecules interact with a single membrane transporter to be translocated across the cell membrane. One molecule is transported against its electrochemical gradient, whereas the other molecule is transported along its electrochemical gradient. The SGLT2 carrier protein is an example, transporting Na+ with its gradient and glucose against its gradient. This type of secondary transport relies on the Na+/K+ ATPase pump described aboveto ensure that the intracellular Na+ is low enough to pull in additional Na+ and bring glucose along with it.

Pinocytosis

Pinocytosis is mechanism of transport where the cells take up large particles and fluids by “drinking” extracellular fluid. The cellular membrane invaginates to enclose filtrate within a vesicle that pinches off from the membrane and enters the cell cytoplasm. It then fuses with lysosomes to form endolysosomes where the larger molecules in the fluid are degraded and digested into molecular components.

Receptor-mediated endocytosis

Receptor mediated endocytosis is a variant of pinocytosis; however, in this process a larger protein will bind to a receptor in the membrane before the vesicle forms.

B. proximal convulated tubule reabsorbs about 65% of the filtered sodium , chloride, bicarbonate and potassium and essentially filtered glucose and amino acids. Because reabsorption of the solutes creates an osmotic gradient that promotes the reabsorption of water via osmosis.

calcium, bicarbonate, magnesium and other ions are reabsorbed in the thick segment ascending segment of loop of henle.  

Reabsorption of water and urea occur at medullary collecting duct . the reabsorption of water is controlled by the level of ADH.

C. Parts of renal tubule where reabsorption occur-

proximal convulated tubule, thin descending segment of loop of henle, thin ascending segment of loop of henle, early and late distal convulated tubule, medullary collecting duct.

D. Water reabsorbed with solutes in tubular fluid is termed as obligatory water reabsorption.

The facultative reabsorption occurs from the collecting tubule as it courses through the renal medulla. It is under the control of antidiuretic hormone (ADH) which controls the permeability of the collecting tubule to water.