important stuff

• When aldosterone triggers Na+ recovery, a negative electrical gradient promotes secretion of K+ and Cl- into the lumen
• All filtered glucose should be reabsorbed in the PCT into the peritubular bloodstream. This occurs by secondary active transport with an apical Na+ symporter (SGLT2) then a basolateral uniporter. The Na+ gradient is created by Na/K/ATPase pumping sodium out at the basolateral surface.

  Glucose is impermeable to tight junctions so there is no backwards leak
• vasopressin binds to V2 receptors on the basolateral membrane of the collecting duct. This allows for insertion of water aquaporin 2 channels into the apical membrane of principle cells. Water entering the cells can then leave through basolateral aquaporin 3 and 4 channels.
• To allow for excretion of concentrated urine and water uptake in the CD there is a requirement for a hyperosmolar medullary interstitium in addition to vasopressin. This is produced by NaCl reaborption in the TAL of LoH and the DCT and by urea recycling
• When GFR rises and Na+ reabsorption decreases so filtrate [Na+] is high. The macula densa senses this and releases ATP and adenosine to cause constriction of the afferent arteriole via paracrine signalling
• When GFR lowers and Na+ reabsorption increases so filtrate [Na+] is low. The macula densa senses this and releases Nitric oxide and PGE2 to cause dilatation of the afferent arteriole and constriction of the efferent arteriole via paracrine signalling
• The PCT is the site of bulk aborption and uses the Na+/K+/ATPase pump to ensure transfer of material from the lumen to the blood. Uses ATP to pump 2 K+ into the tube lumen and 3 Na+ into the cell (then into the bloodstream). This is a form of active transport and is the backbone for the transport of every other solute
• main renal reabsorption mechanisms in the PCT: diffusion (passive), facilitated diffusion, coupled transport (via transporters and antiporters) and active transport
• paracellular transport is solutes moving through tight junctions between cells
• In the PCT, water also 'drags' some solutes into the interstitium instead of them being reabsorbed
• the PCT is lined by simple cuboidal epithelium with a prominent brush border which increase surface area for reabsorption. Its convoluted structure allows for more and closer contact with the peritubular capillary network. They are packed with mitochondria for ATP production
• glomerular filter:

  1. glomerular endothelium and glycocalyx. 30-40% fenestrated, major route for water. Glycocalyx provides resistance to escape of water and macromolecules
  2. glomerular basement membrane. Condensed extracellular matrix containing structural and regulatory components. size selctive and -ve charge stops plasma proteins
  3. podocytes. interdigitating vesceral epithelium. between foot processes are filtration slits