Question

describe the reabsorption of NA: how it is achieved structurally and its importance to other substances?

Answer 1

Sodium is the major extracellular fluid in the blody. The amount of sodium in the fluid influences its volume, which in turn determines blood volume and blood pressure. The kidneys of a normal man filter approximately 24,000 meq sodium/day, reabsorb about 23,900,of sodium. bulk of the sodium is reabsorbed in the proximal tubule and ascending loop of Henle, about 70% of the sodium reabsorption occurs in proximal convoluted tuble,about 25 % is absorbed in the thick asscending tubule, in this section Three-ion cotransporter, sodium, potassium chloride and the sodium/potassium ATPase, maintains the sodium concentration gradient. Sodium is actively pumped out, while potassium and chloride diffuse down their electrochemical gradients through channels in the tubule wall and into the bloodstream

. Sodium reabsorption is tightly linked to passive water reabsorption so when sodium is reabsorbed water will follow, this is very important because the movement of water balances the osmotic pressure within or across the tubule walls, which maintains extracellular body fluid volume.

Most essential substance like glucose, are reabsorbes in the proximal tubule, these substances use sodium co-transporters, which are membrane proteins that link the movement of two or more specific solutes together, these co-transporters  move sodium down its electrochemical gradient into tubule epithelial cells. Sodium/proton exchanger, which enables reabsorption of bicarbonate. Glucose, amino acids and other substances diffuse out of the epithelial cell down their concentration gradients on passive transporters and are then reabsorbed by the blood capillaries.