Physiology 4 & 5 Flashcards
where does most of tubular reabsorption occur
the proximal tubule of the nephron
why is tubular reabsorption necessary
because the kidneys filter 180 litres per day so without it we would pee out our plasma volume 65 times a day
what do the kidneys reabsorbed (important ones) (and percentages)
99% of fluid 99% of salt 100% of glucose 50% of urea 0% of creatinine
why is reabsorption specific for different molecules
because it relies on specific transporter proteins for specific molecules
how much filtered fluid is reabsorbed in the proximal tubule
80ml/min
what substances are reabsorbed in the proximal tubule
sugar amino acids phosphate sulphate lactate
what substances are secreted into the proximal tubule
H+ ions Hippurates Neurotransmitters Bile pigments Uric acid Drugs (atropine, morphine, penicillin) toxins
what is transcellular reabsorption
reabsorption which involves the molecule traveling through the epithelial cells of the tubule and out the other side into the interstitial fluid and then into the capillary
at what points does transcellular reabsorption need transporter proteins
between lumen of the tubule and the epithelial cell membrane
between the epithelial cell membrane and the interstitial fluid
what is paracellular reabsorption
when the molecule being absorbed travels through the gaps in between the epithelial cells
(depends on how tightly they cells are pushed together, some segments of the tubule are tighter than others)
what are the 3 types of carrier mediated membrane transport
primary active transport
secondary active transport
facilitated diffusion
what is primary active transport
when a transporter protein uses ATP to transport a molecule against its concentration gradient (Na/K pump)
what is secondary active transport
couples the movement of an ion (Na) down its concentration gradient with another ion needing moved
can be symport - same direction or antiport - opposite direction
what is facilitated diffusion
movement of a substrate down its existing concentration gradient
uses a transport protein instead of relying on diffusion across the membrane
what transport allows salt reabsorption in the proximal tubule
primary active transport with a Na/K pump
how is salt moved from the tubule into the blood
Na/k pump keeps the Na level inside the cell less than out in the insterstitial fluid
this concentration gradient is used to drive sodium out the capillary and into the cell
Na/k pump pumps it out of the cell into the interstitial fluid
this sets up an electrical gradient allowing Cl- ions and water to pass in paracellularly
oncotic drag then pulls the salt and water into the capillary
what route of reabsorption does water take out of the tubule and into the interstitial fluid
transcellular route following the concentration gradient set up by Na in the interstitial fluid
what is oncotic drag
when the capillary is full of plasma proteins because it has lost all of its plasma to filtration it has a v high osmolarity
this causes it to pull salt and water out of the interstitial fluid in
what transporter moves glucose in reabsorption
Glucose Na transporter - symporter (couples movement of glucose to sodium as it comes into the cell)
water then follows this paracellularly
water and glucose then pulled into the capillary via oncotic drag
why is there glucose in the urine of diabetics
because when blood glucose is v high the the transport membranes become saturated and cannot reabsorb all the glucose coming through
it then passes through the tubules and is excreted in the urine
what does para-aminohippurate acid (PAH) measure
renal plasma flow
what is the cortico-medullary concentration gradient
a gradient that exists within the interstitial fluid - becoming more concentrated as you follow the loop of Henle down into the medulla
what is the purpose of the cortico-medullary concentration gradient
to allow concentrated urine to be produced
what allows juxtamedullary nephrons to produce more concentrated urine
their loop of Henle is longer so they are more exposed to the aortic-medullary concentration gradient
what is reabsorbed in the descending limb of the loop of Henle
water
NOT SALT
what is reabsorbed in the ascending limb of the look of Henle
SALT
not water
what does the triple co-transporter on the luminal membrane of the ascending tubule do
moves Na, K and Cl ions from the tubular fluid into the epithelial cell
what stops water from being absorbed in the ascending tubule
the epithelial cell junctions are too tight so it can’t cross paracellularly
what happens to potassium ions than come into the epithelial cell via the triple co-transporter
some are recycles and put back into lumen
excess k and Cl move into interstitial fluid via K Cl transporter
what do loop diuretics do
stop the activity of the triple co-transporter so stop salt reabsorption in the kidneys (so you have less salt and K in the blood)
what causes water to leave the descending limb
the high osmolarity of the interstitial fluid caused by the NaCl from the ascending limb being pumped in?
what happens to the interstitial fluid the further down the loop of Henle you go
it gets more concentrated
What is the ideal concentration gradient between the ascending limb and the interstitial fluid
200 mili osmols
how does the ascending limb achieve this gradient
by pumping out sodium into the interstitial fluid (as soon as new filtrate comes in)
what does the descending limb do in response to the increases osmolarity of the interstitial fluid (due to the increase in salt put there by the ascending limb)
matches it by loosing water via osmosis
in the steady state what is the highest concentration achieved by the cortico-medullary concentration gradient
1200 mili osmols
What is the ideal concentration gradient between the ascending limb and the interstitial fluid
200 mili osmols
how does the ascending limb achieve this gradient
by pumping out sodium into the interstitial fluid (as soon as new filtrate comes in)
what does the descending limb do in response to the increases osmolarity of the interstitial fluid (due to the increase in salt put there by the ascending limb)
matches it by loosing water via osmosis
in the steady state what is the highest concentration achieved by the cortico-medullary concentration gradient
1200 mili osmols
