Tubular function in the kidneys

Proximal convulated tubule(PCT)

-          Approximately 120ml/min of glomerular filtrate are generated in glomerulus.

-          In the PCTmost  of the filtered load reabsorb again which are water by osmotic activity (80%) most of the Na and under normal condition almost all K+ by passive diffusion(driven by  the positive tubule electrical potential present along the S2 and S3 segments and by paracellular solvent drag and glucose actively co transpot with Na from the tubular fluid.  Also have Na-H antiporter.

-          Number of substances also secreted by into tubular fluid for example creatinine, histamine many drugs and toxic.

Loop of Henle

-          Normally about 30ml/min of isotonic filtrate is delivered to loop oh Henle where counter multiplier mechanism can achieves concentration of the urine.

-          Loop of Henle important to make sure osmotic gradient for facultative water reabsorption from collecting tubule to prevent water loss by kidney. Along the descending limb of the loop of Henle, K is secreted into the tubule lumen from the interstitium. Along the thick ascending limb, K is reabsorbed via Na-K-2 Cl cotransport. In the loop, there is net K reabsorption of 25% of the filtered K. Na reabsorption is controlled by sympathetic system and aldosterone.

Distal convulated tubule(DCT)

-          In the distal convulated tubules there are facultative N and Ca reabsorption and K and P excretion controlled by aldosterone and parathyroid hormone.

Collecting duct

-          It contain 2 types of cells: principal cell and intercalated cell.

-          The principal cell mediates the collecting duct's influence on sodium and potassium balance via sodium channels and potassium channels located on the cell's apical membrane. Aldosterone determines expression of sodium channels with increased aldosterone causing increased expression of luminal sodium channels. Aldosterone increases the number of Na⁺/K⁺-ATPase pumps  that allow increased sodium reabsorption and potassium secretion. Vasopressin determines the expression of aquaporin channels on the cell surface. Together, aldosterone and vasopressin let the principal cell control the quantity of water that is reabsorbed.

-          Intercalated cells come in α and β varieties and participate in acid-base homeostasis

Type of cell	Secretes	Reabsorbs
α-intercalated cells	acid (via an apical H+-ATPase and H+/K+ exchanger) in the form ofhydrogen ions	bicarbonate (via band 3, a basolat-eral Cl-/HCO3-exchanger)[5]
β-intercalated cells	bicarbonate (via pendrin a specialised api-cal Cl-/HCO3-)	acid (via a basal H+-ATPase)

For their contribution to acid-base homeostasis, the intercalated cells play important roles in the kid-ney's response to acidosis and alkalosis.