The kidneys filter/excrete waste and reabsorb essential electrolytes, nutrients, and fluid (ultimately to maintain homeostasis). Tubular reabsorption occurs via multiple mechanisms of transport.
Mechanisms of tubular reabsorption
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 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 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 is a variant of pinocytosis; however, in this process a larger protein will bind to a receptor in the membrane before the vesicle forms.