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In physiology, reabsorption or tubular reabsorption is the flow of glomerular filtrate from the proximal tubule of the nephron into the peritubular capillaries, or from the urine into the blood. It is termed "reabsorption" because this is technically the second time that the nutrients in question are being absorbed into the blood, the first time being from the small intestine into the villi. This happens as a result of sodium transport from the lumen into the blood by the Na+/K+ ATPase in the basolateral membrane of the epithelial cells. Thus, the glomerular filtrate becomes more concentrated, which is one of the steps in forming urine. In this way, many useful solutes (primarily glucose and amino acids), salts and water that have passed in the proximal convoluted tubule through the Bowman's capsule, return in the circulation. These solutes are reabsorbed isotonically, in that the osmotic potential of the fluid leaving the proximal convoluted tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport channels driven by the sodium gradient out of the nephron.
Renin Angiotensin System:
1. The kidneys sense low blood pressure.
2. Release renin into the blood.
3. Renin causes production of Angiotensin I.
4. Angiotensin Converting Enzyme (ACE) converts Angiotensin I to Angiotensin II.
5. Angiotensin II stimulates the release of Aldosterone, ADH, and thirst.
6. Aldosterone causes kidneys to reabsorb sodium. ADH increases the uptake of water.
7. Water follows sodium.
8. As blood volume increases, pressure also increases.
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