Transport Mechanisms 1 Flashcards
plasma is filtered by the glomerulus into
- bowman’s space
what lines the lumen of the nephron
- polarized epithelial cells
what is the transcellular pathway
- transport through tubular epithelial cells across apical and basolateral membrane
what is the paracellular pathway
- transport between tubular epithelial cells
in the trans cellular pathway, solutes require what
- tranporters
in the transcellular pathway, water moves along osmotic gradients through
- aquaporins
in the paracellular pathway, solutes require what
- leaky epithelia
where are the leaky epithelias located
- proximal tubule
- descending limb of LOH
where are the tight epithelias located
- ascending limb of LOH
- distal tubule
- collecting duct
the proximal tubule receives ultra filtrate that is
- isosmolar with plasma
what does the proximal tubule reabsorb
- 67% NaCL
- 67% water
- all glucose
- all amino acids
- most bicarb
- 67% K+
what constitutes the majority of the total solute of the tubular fluid
- Na+ and Cl-
how does Na+ get through the glomerulus
- freely filtered
where is the Na/K ATPase located
purpose
- in the basolateral membrane of literally all cells in all portions of the nephron
- keeps Na low inside the epithelial cell and in interstitial fluid so it can move out of the tubular membrane and into the epithelial cell and get reabsorbed
what transporters are located on the lumen of the PCT
- Na/Cl cotransporter
- Na/glucose cotransporter
- Na/amino acid cotransporter
- Na/H+ exchanger
- aquaporin 1
what transporters are located on the basolateral membrane of the PCT
- Na/K ATPase
- glucose transporters
- amino acid transporters
- Na/HCO3 cotransporters
- aquaporin 1
what is the concentration of Na+ inside the epithelial cell
why
what is the importance of this
What happens to hydrogen?
- low
- due to the basolateral Na/K ATPase
- drives the reabsorption of sodium into the epithelial cell along with glucose and amino acids.
H+ pumped out of the cell and into the tubular lumen.
amino acid reabsorption in the proximal tubule. How much reabsorbed?
on which membranes
- 99% reabsorbed
- secondary active transport on lumen membrane
- facilitated diffusion on basolateral membrane
glucose is freely filtered where
- at the glomerulus
glucose reabsorption by the proximal tubules
- 100% reabsorbed
what glucose transporters are located on the lumen membrane of the PCT
by what mechanism
what are the numbers of glucose transporters on the lumen membrane of the PCT
- sodium dependent glucose transporter (SGLUT)
- secondary active transport
- finite number
what glucose transporters are located on the basolateral membrane
by what mechanism
- glucose transporter (not using sodium)
- via facilitated diffusion
plasma glucose in uncontrolled diabetes
result
- plasma glucose concentration rises
- elevated plasma glucose = elevated ultra filtrate glucose
- transport maximum of SGLT is exceeded
- glucose appears in the urine
what is the capacity of the SGLUT transporters in the lumen
- 180 mg/dL
what happens when excess glucose remains in the PCT
- excess glucose in urine
- water retain due to osmotic action of unabsorbed glucose
- high urine flow
plasma glucose after a meal in uncontrolled diabetics
> 180 mg/dl
big symptom of diabetes
- thirst and excessive drinking
carbonic anhydrase enzyme is located where
- brush border
- cytoplasmic face
reclaiming filtered bicarb mechanism in PCT
- filtered HCO3 exposed to acidic tubular lumen due ot Na/H exhcnager
- HCO3 + H2O -> H2CO3
- brush border carbonic anhydrase catalyzes conversion of H2CO3 -> H2O and CO2
- H2O and CO2 diffuse across luminal membrane
- cytoplasmic carbonic anhydrase converts CO2 + H2O -> H2CO3
- H2CO3 breaks down into H+ and HCO3-
- Na/HCO3 symporter
role of Na/HCO3 symporter
how many of each one
- transports 3 HCO3 and 1 Na across basolateral membrane and into peritubular capillaries
mechanism for generation of new bicarbonate from glutamine
- glutamine metabolism by epithelial cells
- transport into epithelial cell by Na/AA cotransporter
- breaks down into HCO3 and NH4
- HCO3 transporter across basolateral membrane by Na/HCO3 symporter
- NH4 secreted into tubular fluid by Na/H+ exchanger
how is K+ reabsorbs in the PCT
- 80% by solvent drag
solvent drag of K+
- water flowing through paracellular route pulls K+ along with it
how much urea is reabsorbed in the PCT
- about 50%
what does H2O reabsorption in the early PCT do the urea
- concentrates
urea moves out of PCT lumen by
- diffusion along concentration gradient
- tubules are permeable to urea
comparison of water permeability to urea permeability
- urea permeability is slower than water
what happens to urea if GFR decreases
why
- it increases
- decreased flow rate gives more time for urea to be reabsorbed
- increased urea there plasma BUN increases
what is the definition of secretion into the proximal tubule
- from peritubular capillary into tubular lumen
what is secreted into the PCT
mechanism
- end products of metabolism
- pollutants
- drugs
- nonspecific transporters
which drugs can compete for the same transporter on the PCT
- cimetidine and procainamide
what creates a hyper osmotic medullary interstitium
- countercurrent multiplication
the proximal tubules reabsorb
- salt and water together
which limb of the LOH is leaky
result
- descending limb
- permeable to H2O and slightly to NaCl
is there active transport in the descending limb of LOH
- no
ascending limb of LOH
what is reabsorbed
- impermeable to H2O
- reabsorbs Na, Cl, K
what transporter is located in the ascending limb of the LOH
role
- Na/K/2Cl transporter
- brings those solutes into the epithelial cell
what in the ascending limb allows K+ secretion into the tubular lumen
importance
- lumenal ROMK (renal outer medullary) channel
- tubular K+ helps drive Na/K/2Cl transporter
what is the osmolarity of the late proximal tubule
- 300 mOsm
what is the osmolarity of the bend of the LOH
- 1200 mOsm
what is the osmolarity of the early distal tubule
- 100 mOSM
what is the osmolarity of the outer medulla
- 300-600 mOsm
what is the osmolarity of the inner medulla
- 600-1200 mOsm
what is a condition where you would have the absence of ADH
- full hydration
result of absence of ADH
- late distal tubule and collecting duct impermeable to water
- excretion of a large volume of dilute urine
what is a condition where you would have ADH present
- dehydration
result of ADH present
- late distal tubule and collecting duct permeable to water
- excretion of small volume of concentrated urine
what is the urea cycle in response to ADH
- ADH stimulates insertion of urea transporter making the medullary collecting duct permeable to urea
- urea diffuses out into the interstitium and increases the osmotic gradient
result of low protein diet on urea
result on osmotic gradient and urine
- low urea
- decreased osmotic gradient
- inability to form concentrated urine
END PRODUCT OF PROTEIN METABOLISM IS UREA
effect of long loops of henle on osmotic gradient and concentration of urine
- larger osmotic gradient
- more concentrated urine
effect of short loops of henle on osmotic gradient and concentration of urine
- smaller osmotic gradient
- less concentrated urine
result of high rates of flow on osmotic gradient and concentration of urine
- prevents complete equilibrium and NaCl trapping in medulla
- prevents full hyper osmotic concentration
role of the vasa recta
- prevents destruction/dissipation of longitudinal osmotic gradients in medullary interstitium
