Tubular Reabsorption & Counter-Current Concentration Flashcards
what is the second step of urine production?
tubular reabsorption
where does tubular reabsorption occur?
PCT
what percent of water and ions are reabsorbed in the PCT?
65% (cannot be altered)
what percent of glucose and amino acids are reabsorbed in the PCT?
100%
describe the cells of the PCT
columnar epithelial with microvilli
what is reabsorbed in the PCT?
- 65% of all water
- Na, Cl, K, HCO3, all small molecules
- 100% of glucose & AAs
- 30-40% urea
what are the transport systems for glucose, amino acids, water, and sodium?
- glucose & AAs: secondary active Na co-transport (Na symport)
- H2O: osmosis (with small ions)
- Na: diffusion at luminal surface; primary active transport at anti-luminal surface
what can reach a transport maximum?
glucose
AAs
sulfate
phosphate
what is transport threshold?
some nephrons can no longer reabsorb and some molecules end up in the urine
what is transport maximum?
all nephrons can no longer reabsorb any more
what is the equation for excretion?
filtered load - transport maximum
what is the third step of urine production?
counter-current concentration
where does counter-current concentration occur?
loop of henle
what is the descending loop permeable to?
water only
what is the ascending loop permeable to?
salt only
what transport systems are used for NaCl at the thin and thick ascending loop?
thin: passive
thick: active
what is the thick ascending loop also permeable to?
Ca and Mg
what is the osmolarity at the bottom of the loop of henle?
1200 mmol
what is the osmolarity at the thick ascending loop of henle?
100 mmol
what is the collecting duct permeable to?
water with ADH only
(distal medullary part permeable to urea)
what is the approximate rate of urinary glucose excretion:
- GFR = 150mL/min
- plasma glucose = 300mg/dL
- normal renal transport maximum for glucose
filtered load = 150 x 300 = 450mg/min
excretion = FL - trans max.
450 - 300 = 150mg/min
what is the rate of glucose excretion:
- GFR = 50mL/min
- urine flow rate = 2.0mL/min
- plasma glucose concentration = 200mg/100mL
- kidney glucose trans max = 150mL/min
filtration = 50 x (200/100) = 100mg/min
trans max > filtration, all filtered glucose reabsorbed, renal excretion rate = 0
when dehydrated, assuming ADH levels are high, in what part of the renal tubule is the most water reabsorbed?
PCT
in a person on a very low potassium diet, which part of the nephron would be expected to reabsorb the most potassium?
PCT
which part of the nephron reabsorbs the most water?
PCT
what is the rate of glucose excretion:
- GFR = 100mL/min
- urine flow rate = 2.0mL/min
- plasma glucose = 200mg/100mL
- glucose transport max = 250mg/min
FL = 100 x 200/100 = 200mg/min
excretion = 0 (all filtered glucose reabsorbed)
which part of the tubule has the lowest permeability to water during antidiuresis?
thick ascending loop of henle is relatively impermeable to H2O even under conditions of max antidiuresis
what is the rate of glucose reabsorption:
- glucose transport max = 350mg/min
- GFR = 100mL/min
- plasma glucose = 150mL/dL
- urine flow rate = 2mL/min
- no glucose in urine
FL = 100mL/min x 150mL/dL = 100 x 1.5 = 150mg/min
which nephron segment is the primary site of magnesium reabsorption under normal conditions?
ascending loop of henle
what is the rate of urinary glucose excretion:
- GFR = 150mL/min
- plasma glucose = 400mg/100mL
- glucose transport max = 300mg/min
FL = 150 x 400/100 = 600
excretion = 600 - 300 = 300mg/min
