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The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion, whose sum is called renal clearance or renal excretion. That is: Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate [1]
The collecting duct system is the final component of the kidney to influence the body's electrolyte and fluid balance. In humans, the system accounts for 4–5% of the kidney's reabsorption of sodium and 5% of the kidney's reabsorption of water. At times of extreme dehydration, over 24% of the filtered water may be reabsorbed in the collecting ...
Excessive ADH causes an inappropriate increase in the reabsorption in the kidneys of solute-free water ("free water"): excess water moves from the distal convoluted tubules (DCTs) and collecting tubules of the nephrons – via activation of aquaporins, the site of the ADH receptors – back into the circulation. This has two consequences.
Reabsorption allows many useful solutes (primarily glucose and amino acids), salts and water that have passed through Bowman's capsule, to return to 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.
With less blood being filtered, the kidneys also activate the renin-angiotensin system, which causes aldosterone release by the adrenal glands, and tells the kidneys to reabsorb sodium, which ultimately results in water reabsorption. Water and sodium reabsorption is tied to urea reabsorption, so in a prerenal situation, more urea gets ...
An incidental consequence of this renal reabsorption of water is concentrated urine and reduced urine volume. AVP released in high concentrations may also raise blood pressure by inducing moderate vasoconstriction. [12] AVP also may have a variety of neurological effects on the brain. It may influence pair-bonding in voles.
Aldosterone causes the renal tubules to increase the reabsorption of sodium which in consequence causes the reabsorption of water into the blood, while at the same time causing the excretion of potassium (to maintain electrolyte balance). This increases the volume of extracellular fluid in the body, which also increases blood pressure.
The net result of these 4 mechanisms of salt and water retention is decreased output and decreased urinary excretion of sodium (< 20mEq/L). The increased reabsorption of Na leads to increased water and urea reabsorption from the proximal tubules of the kidney back into the blood. In contrast, creatinine is actually secreted in the proximal tubule.