L27-kidney: regulation of water and salt balance

where does salt reabsorption occur in the nephron

• proximal convoluted tubule

• loop of Henle- ascending

• collecting duct

explain how salt reabsorption happens in the proximal convoluted tubules

• NA⁺ is moved from the tubules into the tubule cells through co- transport ( with other molecules like glucose ) and counter- transport ( hydrogen ions retuning into tubule )

• once in the tubule cells the sodium ions move into the interstitial space by active transport

• the sodium/potassium ATPase pump, pumps sodium into the space and potassium goes back into the cell down a concentration gradient

explain salt reabsorption in the ascending loop of Henle

• the sodium moves from inside the tubules into the cells through co transport with K⁺ and Cl^-

• this happens through NA-K-CL cotransporters

• potassium moves back into the tubule via ion channels down a concentration gradient

• once in the tubule sodium/ potassium ATPase moves the sodium ions out into the interstitial space and the potassium out into the cell down a concentration gradient

explain how salt reabsorbtion occurs in the colecting duct

• between the tubule and the tubule cells there are ion channels one specific for sodium ions and one for potassium ions

• the sodium moves into the cell whilst the potassium moves out down a concentration gradient

• the sodium/potassium ATPase moves sodium out to the interstitial space and potassium ions back into the cell

what is the function of aldosterone in salt reabsorbption 

• its a steroid hormone that regulates sodium ion reabsorption and blood pressure 

• it acts in the collecting duct and tubule cells 

• it is able to enter the tubule cells from the blood, bind to a receptor ( nuclear receptor) and enter the cell nucleus of the tubule cells 

• it cause the expression of the ion channels and sodium/potassium ATPase 

• its effects cause more salt reabsorption but it takes hours for the proteins to be made 

where does water reabsorption take place in the nephron

• descending loop of Henle

• collecting ducts

explain how water reabsorption occurs in the descending loop of Henle

• only the the descending loop of Henle is permeable to water

• as the ascending loop of Henle reabsorbs salts it creates a hypertonic solution in the fluid around the loop

• this causes water to diffuse out of the loop in the descending 

• the blood flow around the loop help drive the water abs option through counter current flow

explain counter current flow 

• freshly filtered blood  (low in salt ) first encounters the ascending loop of Henle

• which encourages salt reabsorption from a high concentration inside the loop to a low one in the blood  

• the salt rich blood then circulates around the descending loop and water then moves from high concentration in the loop to a lower one in the blood.

explain how water reabsorption occurs in the collecting duct

• the collecting duct tubule contains lots of water channels- aquaporins

• as the interstitials space is full of salt from the reabsorption

• this allows the water to diffuse down the concentration gradient into the fluid/space through the aquaporins

explain the function of vasopressin in water reabsorption

• its the hormone that controls water reabsorption

• it is released from the blood

• it acts on the receptors that’s on the collecting duct cells

• it causes aquaporin rich vesicles inside the cells to fuse with the cell membrane

• this increases the number of channels water can use to cross

• as the channels are already made and just waiitng its a qucik process