Tubular transport, lect 4 Flashcards
give equation for filtered load
FLx = GFR x Px
- Px = plasma concentration of X
- equation gives the amount of material in glomerular filtrate
give equation for excretion rate (mg/min)
- amount of material lost in urine
- ER = (Ux)(V)
Give equation for transport rates
- amount of material added to (secreted) or removed (reabsorbed) from glomerular filtrate
-
Tx = FL - ER
- ER: excretion rate
- if positive: then some material was removed from filtrate by reabsorption
- if negative: then some material was added to the filtrate by secretion
name the two routes of reabsorption
- paracellular (Between cells)
- “leaky” epithelium
- transcellular
transcellular reabsorption must cross what two barriers
- luminal (apical) membrane
- basolateral membrane
- diffusion through interstitial fluid and capillary wall is fast
reabsorption of amino acids, glucose, lactate, citrate, phosphate etc. is done via what mechanism
- secondary active transport
2/3 of water is passively reabsorbed and 2/3 of sodium is actively reabsorbed where in the nephron
proximal tubule
- 2/3 is reabsorbed no matter how dehydrated a person is
sodium reabsorption in the distal tubule and collecting tubule is subjected to control by what
aldosterone
water reabsorption in the distal and collecting tubule is regulated by
ADH
fluid reabsorption is achieved through what mechanism?
a high oncotic pressure in the peritubular capillary
sodium enters proximal tubule cells via what transport system
via cotransport with organics and by Na+-H+ antiport (NHE)
sodium leaves the proximal tubule cell to enter interstitium via what transport mechanism
- Na+K+ ATPase or via cotransport with HCO3-
sodium potassium ATPase is always located on what side of proximal tubule cell
interstitium side (near blood capillary)
angiotensin II influence on Na+ reabsorption in the proximal tubule
- stimulates Na+H+ exchange (NHE) across apical membrane
- increases Na+ reabsorption and H+ secretion
sympathetic nerve activity on Na+ reabsorption in the proximal tubule
stimulates Na+ reabsorption
parathyroid hormone influence on Na+ reabsorption in the proximal tubule
- inhibits Na+-phosphate cotransport
- increases urinary excretion of phospate
- relates to calcium homeostasis; parathyroid hormone released when calcium levels in blood are low and function is break down bone to release calcium; bone releases ca2+ and phosphate but don’t want them to meet in blood so parathyroid inhibits reabsorption of phosphate
tubular fluid to plasma concentration (TF/P) = 1
what does this mean
reabsorption has been exactly proportional to the reabsorption of water
does osmolarity change in tubular fluid to plasma concentration ratio in proximal tubule
osmolarity DOES not change
tubular fluid to plasma concentration (TF/P) < 1
what does this mean
reabsorption of the substance has occurred to a greater extent than water
tubular fluid to plasma concentration (TF/P) > 1
what does this mean
reabsorption of the substance has been less than water OR there has been net secretion of the substance
how is tubular fluid to plasma concentration of inulin (TF/P) used
used as marker
- since inulin is only filtered, its concentration in the tube is solely determined by the movement of water
what is tubular maximum (TM)
the maximal rate at which renal transport systems can transport a particular solute
- maximum rate (mg/min) of a solute that can be transported
What is tubular maximum (TM) due to
due to saturation of membrane transport proteins
below tubular maximum (TM), all of the filtered load is
reabsorbed
above tubular maximum (TM), all of the filtered load is
excreted
tubular maxima are found primarily in which portion of the nephron
proximal tubule
how do you find transport maximum on blood X concentration vs rate (mg/min) graph
rate when reabsorption levels off
In a blood X concentration vs rate (mg/min) graph, what does decreasing the GFR do to the threshold
decreasing GFR increasing threshold
- threshold (x axis point when reabsorption line breaks off from filtered (GFR) line)
name some other solutes that have transport maxima
- sugars (fructose, galactose, glucose)
- amino acids
- metabolic intermediates (lactate)
- phosphate ions
- vitamins
Osmotic diuresis
is increased urination due to the presence of certain substances in the fluid filtered by the kidneys that cause water to come into the urine
what effect does excess unreabsorbed solute (e.g. mannitol) have in the proximal tubule
- manitol: non-reabsorbed carbohydrate can be given IV to induce osmotic diuresis
- inhibits osmotic water flow from lumen to basolateral spaces
- causes Na+ back diffusion into lumen of tubule with increased Na+ and water loss in urine
- osmotic diuresis
osmotic diuresis results in a rapid loss of what two substances
- sodium and water -> polyuria
name another commonly known disease that causes osmotic diuresis
diabetes: when glucose load exceeds TM , there is excess glucose in filtrate
tubular secretion occurs via what two routes
- paracellular
- transcellular
the two main transport mechanisms present in tubular secretion seperate what compounds
- one is for organic cations
- one is for organic anions
- **very non-specific
compounds to be secreted by the kidney are tagged for secretion in the liver via what
- glucuronic acid or sulfate
organic anions are secreted in the nephron via what type of transport
tertiary active transport
how is PAH, an organic anion, secreted across kidney cell
- PAH is taken up into the cell in exchange for alpha-KG
- PAH leaves the cell on the apical side via a PAH-anion antiporter
why does elevated plasma levels on one anion inhibit secretion of others?
all organic anions compete for the same transporter
- ex: infusion of PAH inhibits the secretion of Penicillin, extending its life
Why does the inulin concetration increases (Tubular fluid/plasma concentration) in the proximal tubule
inulin is neither secreted or reabsorbed but water is reabsorbed making the concentration of inulin in the proximal tubule increase
Why does the sodium concetration appear not to change (Tubular fluid/plasma concentration) in the proximal tubule even though you know 2/3 is reabsorbed?
equal amount of water are being reabsorbed with it
