Renal: Tubular absorption, salt balance Flashcards
what occures in the proximal tubule?
- bulk reabsorption
2. majority of tubular secretion
what is secreted from the tubule?
organic cations - neurotransmitters
orgainic anions - bile salts, fatty acids, foreign chemicals
what is the purpose of the distal tubule and collecting duct?
fine tuning reabsorption and secretion
to match excretion rates to intake or generation rates
what is filtered load?
total amount of any nonprotein or nonprotein-bound substance filtered into Bowman’s space
GFR x (substance concentration)
example: filtered load of glucose at a concentration of 1 g/L (180 g/day of glucose in 180 L/day of H2O)
GFR = 180 L/day
so
180 L/day x 1 g/L = 180 g/day
how much water is filtered per day? excreted? reabsorbed?
a. 180 L/day
b. 1.8 L/day
c. 99 %
how much Na is filtered per day? excreted? reabsorbed?
a. 630 g/day
b. 3.2 g/day
c. 99.5 %
how much glucose is filtered per day? excreted? reabsorbed?
a. 180 g/day
b. 0
c. 100 %
how much urea is filtered per day? excreted? reabsorbed?
a. 54 g/day
b. 30 g/day
c. 44%
what is true of proximal tubular reabsorption?
Na and H2O have iso-osmotic reabsorption (concentration stays constant)
HCO3, glucose, lactate are efficient reabsorded
Cl concentration increases, then remains constant thus it is delayed behind Na
General principles of reabsorption?
- transcellular (tubular epithelial cell) is driven by secondary active transporters
- paracellular via the tight junction, between basolateral membrane, driven by concentration gradient
how does transcelluar uptake/transport work?
intracellular Na is lower than tubular luminal Na, this forms a pressure gradient across the apical membrane. the tubule cell uses the Na gradient to transport the substance across (i.e. Na-glucose symporter) via active transport.
the substance leaves the cell into the blood via facilitated diffusion
what is true of glucose concentration and flow rate in the kidney?
- filtration is linear
- reabsorption is not linear, it saturates at higher levels
- reabsorption is transporter mediated
- what isn’t reabsorbed is excreted to urine.
- glucose is osmotically active and induces H2O excretion
what is the apporx. ammount of Na in the body?
55 grams
90% extracellular 10% intracellular
How is Na taken in? out put?
a. foods
b. urine (regulated), sweat, feces
place the following in the correct order: collecting tubule proximal tubule bowmans capsule loop of henle collecting duct dital tuble
bowmans capsule > proximal tubule > loop of henle > distal tubule > collecting tubule > collecting duct
how is Na reabsorption mediated in the proximal tubule?
Na - H exchanger (NHE) (Na in, H out)
Na coupled co-transporters
accounts for 65 %
how is Na reabsorption mediated in the TAL (thick ascending loop)
Na/K/Cl cotransport
accounts for 20 %
how is Na reabsorption mediated in the DT (distal tubule)
Na/Cl cotransport
accounts for 10 %
how is Na reabsorption mediated in the CD (collecting duct)
epithelial Na channels
accounts for 8 - 10 %
How is Na transport mediated on the basolateral side of the epithelial cell?
Na/K-ATPase transporter
True or False. The Na+ movement to the tubule cells leads to a downhill transport of Cl and water. They virtually follow Na exclusively via paracellular pathways.
False. they follow Na by both transcelluar and paracellular pathways
What is Na relation ship to blood pressue?
Extracellur volume = plasma + interstitial
so plasma volume is related to plasma Na.
this means an increase in plasma Na increases overall plasma volume thus increase B/P. (opposite effect for decrease plasma Na)
how is low blood pressure compensated for when blood volume is low due to Na and H2O loss (i.e. diarrhea)?
- decreased volume = a decrease in MAP
- baroreceptors sense the change
- this stimulates the sympathetic renal nerve activity
- the afferent arteriols are constricted
- this causes decrease glomerular filtration pressure
- finally, kidney decreases Na and H2O excretion to maintain plasma volume and pressure
True or False. In blood pressure compensation the afferent arteriole constriction/dilation alone is responsible for regulating glomerular filtration pressure.
False
afferent arteriole constriction/dilation indirectly lowers the pressure AND the low arterial pressue itself directly reduces the net glomerular filtration pressure
Which is best for short term regulation of B/P? Long-term regulation?
a. short - GFR control
b. long - control of Na reabsorption
how is Na reabsorption regulated?
renin-angiotensin-aldosteorone signaling
ANP (atrial natriuretic peptide)
which 3 structures, other than nephrons, play an important role in long term regulation of Na reabsorption?
liver - secretes angiotensinogen
kidneys - secrete renin
adrenal cortex - secretes aldosterone
summarize control of Na reabsorption by aldosterone
In response to a low blood pressure, the kidney produces renin from juxtaglomerular cells and secretes it to the blood. Renin converts angiotensinogen (produced from the liver) to Ang I, which then becomes Ang II by AEC. Ang II then triggers aldosterone secretion from the adrenal cortex to plasma. Finally, aldosterone stimulates Na+ reabsorption in the kidney
True or False. Ang I, ACE, and Renin are always present in the blood.
False. Renin is not always present in blood. release into blood depends on blood volume or pressure?
What is aldosterone?
steroid hormone produced by adrenal cortex
what triggers aldosterone’s synthesis and secrection?
Ang II
What is the function of aldosterone in regulating Na in the cell?
upregulates the number and activity of apical (tubular lumen side of cell) Na channels and basolateral (blood vessel side) Na/K-ATPase
How does aldosterone effect K in the cell?
increase K secrection by by upregulating K channels
how does alosterone effect the Na uptake in the CD (collecting duct)?
8% reabsorption without aldosterone
10% with
what is true of Renin secretion from the juxtaglomerular cells?
it is stimulated by
1 increased activity of renal sympathetic nerves,
- reduced stretch of intrarenal baroreceptors in the juxtaglomerular cells,
3 reduced salt concentration flowing to macula densa.
How is Na reabsorption regulated by ANP?
it inhibits Na reabsorption, inhibits aldosterone secrection, premotes afferent artiole dilation thus increase GFR
when blood volume or pressure in high
the stretch in the atria cause the atria to secrete ANP
what is result of ANP in regards to urine?
natriuresis (high Na levels in urine)
true or false. The kidneys can excrete H2O without excreting Na.
True via vasopressin (ADH)
What triggers the release of Vasopressin/ADH from the posterior pituitary gland?
low B/P - reduced firing of barorecptors
high plasma osmolarity (decrease H2O) concentration) – osmorecptors of hypothamalmus are activated
How does ADH enhance water uptake?
stimulating insertion of water channels aquaporins into the apical membrane of the collecting duct. (increased tubular permeability)
What is another role of vasopressin?
vasoconstrictor in peripheral circulation
what is the ultimate compensatory mechanism in regards to reduced blood volume?
ingestion
what makes us feel thirsty?
reduced blood volume and increased plasma osmolarities effect on the hypothalmus
how do barorecptors trigger thirst?
Ang II
