Proximal Tubule and Loop of Henle Flashcards
how much of the fluid passing through the glomerulus is filtered through into the lumen of the tubules via bowmans capsule?
20%
where does most reabsorption occur in the nephron?
proximal tubule (however does occur to some extent along the whole nephron)
composition of initial tubular fluid?
similar composition to plasma
but doesn’t contain any plasma proteins
why is reabsorption needed?
entire circulation is filtered around 65 times per day so important substances must be retained
how much of each substance is reabsorbed in the tubules?
99% of fluid 99% of salt 100% of glucose 100% of amino acids 50% of urea 0% of creatinine
reabsorption is specific/non-specific and filtration is specific/non-specific?
reabsorption = specific (each substance needs specific transporters etc) filtration = non-specific (anything small enough to fit through the pores will pass through)
what is glomerular filtrate and how much of this is reabsorbed in the proximal tubule?
modified filtrate of blood containing ions and solute
around 80ml/min reabsorbed
how does flow rate change between proximal tubule and loop of henle?
GFR at proximal tubule = 125ml/min
GFR at loop of henle = 45ml/min
due to lower volume of fluid as some is reabsorbed
how is osmolality of glomerular filtrate change alone length of nephron?
stays the same
fluid reabsorbed in proximal tubule has same osmolality as filtrate
what 5 substances are reabsorbed in PT?
sugars amino acids phosphate sulphate lactate
what 7 substances are secreted by cells into the PT into the tubular fluid?
H+ hippurates Neurotransmitters bile pigments uric acid drugs toxins
what lines tubules?
single cell epithelium
what must a substance pass through in movement from tubular fluid to blood via transcellular route?
across apical membrane of epithelial cells
across cytoplasm
across basolateral membrane of epithelial cell
through interstitial fluid
across capillary endothelial wall
how does paracellular reabsorption occur?
passes between tight junctions of epithelial cells of tubule epithelim
what determined how much paracellualr reabsorption can take place?
how tightly packed the epithelial cells are
some are very tight
some are leaky
what is often needed for transcellular reabsorption?
membrane transport proteins to hep substance to cross apical/basolateral membrane of epithelial cells
what is a primary active transporter and give an example?
e.g sodium potassium pump (3 Na out and 2K out for every ATP hydrolysed)
energy is directly required to operate the carrier and move the substrate against its concentration gradient
(i.e needs energy as Na concentration is higher outside the cell to takes energy to pump sodium out of cell against conc gradient)
what is a secondary active transporter?
carrier molecule is transported coupled to the concentration gradient of an ion (usually Na)
can have symporters (moving substrate in same direction as Na)
antiporters = moving substrate in opposite direction to sodium
what is a facilitated diffusion transport protein?
passive carrier-mediated transport of a substance down its concentration gradient, bypassing the lipid bilayer (e.g glucose)
what drives salt reabsorption?
Na K+ pump at only expressed at basolateral membrane of epithelial cells
help to maintain sodium concentration gradient which is used to drive movement of Na ions through ion channels or 2ndary active transporters
what is transport maximum?
rate at which you can reabsorb a substance
applies to any substance which depends on transport proteins
rate of filtration is equal to what?
plasma conc X GFR (usually constant)
therefore increased plasma conc = increased filtration
how are rates of filtration, reabsorption and excretion related?
filtration increases linearly with increase in plasma glucose
rate of reabsorption plateaus at the point where transport proteins become saturated and cant filter any more
at point where reabsorption plateaus, if filtration keeps increasing, rate of excretion increases as the substance cant be resorbed and just spills out into the urine
transport proteins are used and can be saturated in both reabsorption and secretion, give an example of secretion?
PAH (para-aminohippurate) is completely secreted from the plasma into the tubuar fluid at glomerulus and then excreted into the urine
secretion into tubular fluid is dependant on transport proteins which can become saturated
describe how Na+ resorption occurs in the proximal tubule?
transport mechanisms at apical membrane of epithelial cell bring Na into the cell from tubular fluid
Na/K pump at basolateral membrane pumps sodium out of the cell into interstitial fluid (transcellular)
Na+ +ve charge sets up electrical gradient between tubular and interstitial fluid which attracts -vely charged Cl- from tubular into interstitial fluid (paracellular) = resorption of salt
water reabsorption from the proximal tubule is dependant on what 2 forces?
water follows salt
therefore passive water absorption down NaCl gradient occurs down osmotic gradient
oncotic drag of peritubular plasma
- blood in capillary has less fluid content as 20% was filtered out at the glomerulus, therefore there is a relative increased concentration of plasma proteins in the blood which increases the oncotic pressure gradient, pulling water out of the tubular fluid and out into the interstitial fluid, then the blood
how is glucose reabsorbed in the proximal tubule from tubular fluid?
glucose brought into cell coupled to Na transport (co-transporter) creating glucose concentration gradient
facilitated diffusion uses this concentration gradient to pump glucose out of cell at basolateral membrane
this sets up an osmotic gradient between tubular and interstitial fluid which water follows via paracellular route
100% glucose is resorbed
what is the function of the loop of henle?
generates a cortico-medullary solute concentration gradient between the interstitial fluid and tubular fluid (increases osmolarity of interstitial fluid)
this enables the formation of hypertonic urine
describe flow of fluid in the loop of henle
opposing flow in the descending/ascending limbs - termed the countercurrent flow
the entire loop functions as a countercurrent multiplier
which type of nephron creates a more hyper-osmotic interstitial fluid and why?
juxtamedullary
has a longer loop of henle and vasa recta
what is absorbed in the ascending limb?
active salt (NaCl) reabsorption
no water reabsorption
little to no water follows salt reabsorption as thigh junctions are very tight between epithelial cells (impermeable to water reabsorption)
what is reabsorbed in the descending limb?
highly permeable to water
no salt is reabsorbed
how does transport occur in the ascending limb?
triple co-transporter expressed at apical membrane of epithelial cell which brings Na, Cl and K into cell (K+ pumped straight back out)
net movement of Na and Cl across cell
Na leaves basolateral membrane via Na/K pump, Cl leaves via Cl/K co-transporter
(osmotic gradient is created but tight junctions to tight for water to follow the gradient)
how do loop diuretics affect transport in the ascending limb?
block the triple transporter
what allows the movement of water from the tubular fluid of the descending limb into the interstitial fluid?
salt is removed from ascending limb and pumped into interstitial fluid, increasing its osmolarity
this creates an osmotic gradient between interstitial fluid and tubular fluid in descending limb
this allows water to leave via osmotis
fluid in the descending limb is therefore concentrated
what is countercurrent multiplication?
process of using energy to create an energy gradient
how does countercurrent multiplication occur?
tubular fluid becomes more concentrated as it goes down ascending limb due to loss of water into increasingly hyperosmotic interstitial fluid
then becomes less concentrated as it goes up the ascending limb due to loss of salts
what is the osmolarity of the tubular fluid entering and leaving the loop of henle?
entering = 300 (iso-osmotic to interstitial fluid) leaving = 100 (hypoosmotic to interstitial fluid)
in the steady state, how does concentration change in different areas of the kidney?
increases in osmolarity down from cortex to medulla from 300-1200
what 2 solutes generate the corticomedullary concentration gradient?
salt and urea
why is countercurrent multiplication needed?
to concentrate the medullary interstitial fluid
to enable the kidney to function to produce urine of different volume and concentration according to the amounts of circulating ADH
normal urine production?
usually around 1ml/min
depends on fluid intake
- can be between 0.3-25ml/min
vasa recta acts as a countercurrent exchanger, what does this mean?
capillary blood equilibrates with interstitial fluid across leaky endothelium
blood osmolarity increases and decreases as it passes down and up into the cortex
purpose of vasa recta?
ensures corticomedullary concentration gradient stays in place by ensuring solute isn’t washed away
how does the vasa recta minimise the problem of NaCl and urea being washed away as blood passes through medulla?
vasa recta capillaries follow hairpin loops, are freely permeable to NaCl and water
blood flow to vasa recta is low (few juxtamedullary nephrons)
