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USC BISC 307L - Water and Salt Balance
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Response to Changes in Blood Volume and PressureThere are 2 different types of baroreceptors in the body1. In carotid and aortic body- pressurehigh pressure arteriesreferred to as the “high pressure baroreceptors”2. Low pressure baroreceptors- blood volumein low pressure parts of the vascular systemstretch sensitive nerves in the atria, pulmonary blood vessels, and the big veinsthese are highly compliant parts of the vascular systemoften called volume receptors whereas the high pressure ones are called pressure receptorsWater Balance in the BodyThe water has to be in water balanceGain and loss of water every dayMain regulated route of water loss= in the urineNo mater how much water one drinks, 99% of that water is reclaimed (proximal tubule and loop of henle and first part of distale tubule)Osmolarity Within Kidneyjuxtamedullary nephronkidney has an osmotic gradient where the cortex and the outer medulla have 300mOsM but as you go deeper it gets very highly concentrated due to high amounts of salt and ureaFiltrate in bowmans capsule has the same osmolarity as bloodDescending loop of henle: epithelial cells are permeable to water but not saltOnly water is reabsorbed raising the osmolarity levelSalt cannot follow increasing concentration of saltAscending loop of henle: permeable to salt but not waterSalt reabsorbed and osmolarity becomes very low (100)In the thin part there is little active transport of salt just passive but in the thick part there is more active transport of salt- because not as big of a concentration gradientBecomes lower than body fluids – 100Reabsorption of Na+,K+,Cl- in Ascending LimbMechanism of reabsorption of salt, K+ and Cl- in ascending limb of loop of henleImportant thing here is the K+/Na+ pump in the basolateral membraneLow Na+ concentration inside drives the Na+/K+/2Cl- symporter in the apical membrane (unique to this area of the body)- loop diuretics inhibit this transportNa+ ions that get pumped in get pumped out across the basolateral membraneK+ ions get short circuited out or goes through channels in the basolateral membraneCl- goes out through the transporters in the basolateral membraneSince water cant come in, this creates a hypoosmotic fluid- one of the only places in the membraneCan produce a lot of dilute urine without losing too much salt- this is the mechanism since water cannot follow salt in reabsorptionCountercurrent Multiplier and ExchangerCreation of the osmotic gradientLoop of henle going through the medulla: Descending limb permeable to water not salt. Once you get to active transport section of ascending limb salt will leave despite there being no osmotic gradient (assuming there wasn’t one) so fluid is slightly saltygoes through again and there is a little osmotic gradient so water leaves and osmolarity keeps changing  this establishes the osmotic gradient- called the counter current multiplier: THE MORE SALT THAT IS EXCRUDED THE EASIER IT BECOMES TO EXCRUDE SALT2 problems with this system:1. Why doesn’t the water that comes out the descending limb dilute the salty inner medulla2. Why on the ascending limb isn’t the salt coming in washed out by bloodANSWER: IMPORTANCE OF THE VASORECTA AS COUNTERCURRENT EXCHANGERS- loops of capillaries that parallel the loop of henle (diagram on the right). Not going to be tested on this diagram above- look belowSimpler explanation: fluid in capilarries going in opposite direction as loop of henle fluid. Vasorecta is regular endothelial cells with pores and leaky. Diffusion: as the blood goes down on the right hand side it moves into an environment that is more and more saltywill gain salt and loses water so by the time the blood gets to the bottom this makes the blood very high osmolarity (shrivels the RBC) and then goes back up on the left and is moving through and environment that is becoming less osmotic so it will gain water and lose saltThe salt that the blood gains as it travels down is the same salt that it loses on the way up and vice versa for waterBut water doesn’t just move into capillaries by osmosis but also by bulk flowThese capillaries are very low pressure because very downstream- the balance of forces strongly favors reabsorption of water by bulk flow in addition which is important because that’s where a lot of the water that comes out gets reabsorbed into the blood- the volume coming out of the vaso recta on the left is twice the volume that came in – this is due to bulk flowReabsorption of H2O from Collecting DuctComing in the osmolarity is 100 and then comes through the collecting ductModification happens: if the walls are permeable to water then water will be reabsorbedsmall volume of high osmolarity urine (what happens when your body needs to conserve water)If not then the walls are not permeable to water excrete a large volume of urineControlled by vasopressin (increases water permeability of collecting duct to conserve water)Vasopressin (ADH) Increases H2O PermeabilityHow vasopressin works to conserve waterInserts aquaporin 2 channels into the apical membrane (retrieved from the apical membrane by endocytosis) so if you don’t have the ADH system then they will be removedThe basolateral membrane is always permeable to water thoughMost potent stimulus for this is dehydration- sensed as an increase in blood osmolarityAlso excitatory input form the volume receptors in atria and veins and the high pressure baroreceptors (less so)Strong circadian rhythm in secretion- don’t secrete much at night so urine in the morning is more concentrated- allows us to sleep without peeingAldosterone Stimulates Na+ ReabsorptionRegulation of Na+ contentAldosterone- stimulates reabsorption of Na+ because steroid hormone that binds to carrier and alters gene expression for mitochondria to produce ATP and Na+/K+ pumps to reabsorb Na+ and Cl- flows and then H2O followsRenin-Angiotensin-Aldosterone PathwayAldosterone is secreted as part of a bigger schemeIncreases volume and lowers osmolarity of the bloodGo through this on your ownRenin: enzyme that converts angiotensin form the liver to angiotensin 1causes vasocontriction and raises blood pressureAngiotensin II is produced from angiotensin I from ACE on the surface of capillaries and this is more potent of a signalstimulates the adrenal cortex to secrete aldosterone and causes vasoconstriction and raises blood pressure and stimulates brain stem to increase sympathetic stimulation in heart and stimulates


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