Question

what is the effect of high AVP concentration on water reabsorption in PT, CCD, TAL, tAL, tDL?

Answer 1

Proximal tubule(PT) is a part of nephron in kidneys.It is divided into proximal convoluted tubule (PCT) and the proximal straight tubule (PST).Reabsorption of water and other solutes in PT is isosmotic.About 65% of water is reabsorbed in PCT.AVp or arginine vasopressin is a hormone synthesized in hypothalamus.It plays important role in osmotic balance,regulation blood pressure,regulation of sodium and functioning of kidney.When this hormone is released excessively then it will result in increased water reabsorption.This senses kidney for increased salt excretion.Thus increased salt in urine causes attraction of water to be excreted with salts due to osmotic balance to stabilize the fluids.This increased salt excretion causes low sodium concentrations.This causes decreased ability to think,nausea,poor balance etc.

Cortical collecting duct(CCD) consists of ducts connecting nephrons in kidney to minor calyx where the urine from kidneys is passed to.In humans CCD is responsible for about 5% of water reabsorption.During dehydration this helps reabsorption of water of about 24%.AVP binds to V2 receptors in the CCD epithelium which in turn leads to Aquaporin 2 channels insert to apical plasma membrane.So AVP increases the water permeability and thus reducing the urine volume.This increases blood volume,cardiac output and arterial pressure.High AVP causes decreased water permeability causing dehydration and decrease in blood volume and arterial pressure and may cause heart failure.

Thick ascending limb(TAL) of Loop of Henle is impermeable to water and permeable to urea.Thus the prevention of water flow leads to high osmotic pressure in renal medulla and urine at the end of TAL is hypotonic.High AVP hormone increases the urea permeability of TAL.The urea concentration and water reabsorption because urea excretion needs significant volume of water.This causes dehydration.In dehydrated patients, increased reabsorption of urea may lead to an increase in blood urea nitrogen(BUN) . This contributes to the BUN/creatinine ratio tends to be higher.This in turn reduces the blood flow to kidneys causing heart failure,gastrointestinal bleeding.

Thin ascending limb (tAL)is impermeable to water, thus becomes concentrated with ions, increasing the osmolarity. This drives water reabsorption from the descending limb as water moves from areas of low osmolarity to areas of high osmolarity.High AVP decreases the osmolarity and thus reaches a point where it becomes equal to plasma osmolality causing hypotonic urine.The thin descending limb is passively permeable to water through aquaporin 1 channels.High avp causes increased permeability and causes dehydration.