Flashcards in Renal Part II Deck (20)
Distinguish between paracellular and transcellular transport of water and solutes.
2/3 NaCl is transported across the cell via Na/K pumps. 1/3 water is transported via aquaporins in the cell membrane. Small molecules go through paracellularly as well as the other 2/3 water. The water pulls ca2+ and K osmotically.
What is secondary active transport?
When Na diffuses across the luminal membrane on co-transporters with H+, HCO3-, K+, Cl-
What are aquaporins?
Membrane proteins permeable to water
Where in the tubule are water and Na reabsorbed? Where does most of it occur?
Proximal tubule: Na and water reabsorbed (65%)
Thick ascending loop and distal convoluted tubule: Na reabsorbed via ion pumps (10% each)
Collecting tubule and duct: water variably reabsorbed via aquaporins under regulation via ADH (<24%)
Thin descending limb: Water (10%)
Describe reabsorption of organic solutes such as glucose.
Organic solutes are cotransported with Na across the luminal side of the cell. Occurs in the proximal convoluted tube. Na actively transports via ATP-ase across the basolateral side. Na concentration gradient pulls in the Na and organic solute. Proteins and peptides are broken down into amino acids and reabsorbed in the proximal convoluted tube. Small amount of proteins reabsorbed via pinocytosis.
What is transport maximum?
Saturation level for a given solute. Limited by saturation of carrier proteins. In diabetes, excess glucose is excreted in the urine.
Where does most of reabsorption of Na and water occur?
Proximal convoluted tube
Give a general description of secretion.
While some organic anions and cations and drugs are filtered in the usual way, most organic ions and drugs are protein bound and not filtered. Instead, they must be secreted by peritubular capillaries into the proximal convoluted tube. In peritubular capillaries, blood flow is slow, allowing for more time for dissociation and secretion. Ions are then taken up by proximal tubular cells via secondary active transport. OA- are taken up into the cell against their concentration gradient in exchange for a-ketoglutarate via OAT 1,2,3 antiporters. OAs then go through facilitated diffusion into the tubules via MRP2 and OAT transport proteins.
How is secretion of organic ions and drugs maintained? Is it ATP dependent?
Maintained by a constant flow of proteins. Not ATP dependent-- however, need specificity of transporters.
What are some examples of endogenous anions?
Urate, hippurates (PAH), bile salts, prostaglandins, hydrozybenzoates, oxalate, fatty acids
(Do not filter nicely. Either they are too big or attached to proteins. Must use transporters)
What are some examples of endogenous cations?
Histamine, choline, acetylcholine, dopamine, epinephrine, norepinephrine
What are some examples of drugs secreted?
Atropine, chlorothiazide, furosemide, penicillin, saccharin, salicylates, morphine
(Bound to proteins)
Why do protein bound waste metabolites need to be secreted?
Either they are too big or attached to proteins and proteins are not filtered. Higher pressure in the glomerus = higher pressure. The pressure drops at the efferent and afferent arterioles and further drops in the peritubular capillaries. As the pressure drops, the flow decreases, allowing for more time for dissociation. This is also why the basal side is invaginated to allow for a greater surface area for secretion.
Give a general description of clearance.
Plasma clearance is an abstract concept that measures how well the kidneys are clearing of the blood of certain substances. It is the volume of plasma from which a substance is completely cleared by the kidneys per unit time (usually 24 hours).
What are some typical values of clearance?
Phosphate is partially reabsorbed, its clearance lies between inulin and glucose. It's clearance is 14 ml/min. The higher the number = more dumped into the urine. Inulin and creatinine have high numbers while glucose is close to 0. Na is around .9, K is around 12, urea is around 70, inulin is around 125 and creatinine is close to 140.
Why are inulin and PAH used to measure GFR and RPF, respectively?
Inulin clearance can help estimate the GFR because it is filtered but neither secreted or reabsorbed. Creatinine is filtered and minimally secreted, so it can also be used to estimate the GFR like inulin. There is a consistent overestimate of 12%, so GFR can be estimated by using a 12% reduction. Glucose is completely reabsorbed, so its clearance is 0.
PAH clearance estimates renal plasma flow. 90% is cleared from the plasma by both filtration and secretion. It estimates the amount of plasma flow into the kidney, not just the amount filtered. Clearance is 585 ml/min. RPF is then 650 ml/min
What is inulin?
Inulin is a fructose polysaccharide from the inula plant that is completely cleared from the blood.
What is creatinine?
Creatinine is a muscle metabolite. It is filtered and minimally secreted.
What is PAH?
Para-aminohippuric acid. It is a carboxylic acid found in the urine of horses and other herbivores.