Week 1 Lectures Flashcards
Are RBCs and plasma proteins (i.e albumin) effective osmoles between plasma and ISF?
Yes
What is oncotic pressure
osmotic pressure exerted by proteins
Are blood vessels walls permeable to salt and water? What is the significance of this?
Yes–thus they do not create an osmotic gradient between intravascular and interstitial space
BUT vessels are not fully permeable to proteins therefore albumin stays in the plasma
in the end does the hydraulic/oncotic pressure balance favor retention in the plasma or net movement of fluid out?
balance results in slight net movement out–lymphatics return fluid to venous circulation
list causes of edema
CHALO (Capillary-Hydraulic–Albumin–Lymphatic–Oncotic)
- increased capillary permeability–may occur in sepsis or certain diseases
- increased hydraulic pressure–increased plasma volume due to primary sodium retention; venous obstruction; decreased arteriolar resistance (and thus increased volume moving into arterioles)
- decreased albumin–from protein loss, particularly in glomerular disease or reduced production
- lymphatic obstruction–more fluid trapped in interstitium
- increased interstitial oncotic pressure causes more fluid drawn into interstitium
describe the pathophysiology of edema
- -fluid moves out of the intravascular space and into the interstitial space
- -the issue with this is that the body is extremely protective against shock; as sodium and therefore water are extravasated, the baroreceptors pick this up
- -this results in the activation of the RAAS system and sodium is retained + vasoconstriction
- -the trouble with this is that even though sodium is retained for the purpose of restoring inravascular pressure, this does not solve the primary issue, which is whatever let more water get into the ISF anyway
- –in the case of hypoalbuminemia, the oncotic pressure is too low to keep fluid in the intravascular space
- -so even though the kidney is “doing the right thing”, water just keeps moving out of the intravascular space and into the interstitium
- -instead, the patients salt content remains high and thus their fluid load remains high as well
How is Na+ filtered in the golmerulus?
it is freely filtered
what is the concentration of the ultrafiltrate in the glomerulus compared to the blood?
they are the same (135-145 mmol/L)
What happens at the proximal tubule RE: Na+ and H2O reabsorption?
-65% of filtered Na+ and water are reabsorbed together
How is Na+ reabsorption accomplished in the proximal tubule once it has crossed into the cell (i.e how does it cross the basolateral membrane)?
Via Na+/K+ ATPase pump in the basolateral membrane
2 steps:
- Na+ moves across the apical (luminal) membrane from the lumen into the cell, down an electrochemical gradient established by the Na+/K+ ATPase pump
- Na+ moves across the basolateral membrane from the cell into the blood, against its electrochemical gradient, via the Na+/K+ ATPase pump
What else is reabsorbed in the PT?
small proteins that were filtered by the glomerulus (reabsorbed via endocytosis)
Describe the two mechanisms by which Na+ crosses the apical membrane (from lumen into cell) in EARLY proximal tubule
- Na+/H+ antiporter
- -Na+ entry coupled with H+ exit from cell; H+ secretion results in NaHCO3 reabsorption into blood - NA+/X- symporter–Na+ and X- both enter cell from lumen; X- crosses basolateral membrane into blood via passive transporter; X- = organic solute (glucose, aa’s, Pi, lactate); X- are almost completely removed from tubular fluid
What provides the driving force for reabsorption of H2O by osmosis in the early proximal tubule
reabsorption of NaHCO3 and NaX establishes a trans-tubular osmotic gradient that provides the driving force for passive reabsorption of H2O by osmisis
Because H2O is absorbed in excess of Cl- in the early segment of the PT, the Cl- concentration rises along the length of the segment
How does Na+ cross the apical membrane (from lumen into cell) in the LATE proximal tubule?
- coupled Na+/H+ and Cl-/HCO3- antiporters–Na+ entry coupled with H+ exit from cell; Cl- entry coupled with HCO3- exit from cell; H+ and HCO3- combine in the tubular fluid to form H2CO3 and reenter the cell
- paracellular reabsorption of Na+ and Cl- by passive diffusion– established a transcellular osmotic gradient that provides the driving force for the passive reabsorption of water by osmosis
driving force for both mechanisms is the Cl- gradient established in the EARLY proximal tubule
What does “paracellular reabsorption” refer to?
reabsorption through tight junction (rather than transcellularly through the cells lining the lumen of the PT)
Through which 2 pathways is water reabsorbed in the proximal tubule?
PT is highly permeable to water and it flows from the lumen into the blood via:
- transcellularly
- paracellularly (through tight junctions)
How much Na+ is reabsorbed in the loop of henle? Where?
25% approx (plus Cl- and K+)
Na+ mostly in the thick ascending but to lesser extent in other parts
How much water is reabsorbed in the loop of Henle? where?
about 15%
in the descending thin limb
Describe the 3 pathways by which Na+ is reabsorbed in the thick ascending limb of the loop of henle
*remember the Na+/K+ ATPase moves Na+ into the blood from the cell and thus sets up a concentration gradient along which Na+ can flow from the TAL
- Na+/K+/2Cl- symporter–uses the energy released by the downhill movement of Na+ and Cl- to move K+ uphill into the cell
- Na+/H+ antiporter–same as in early PT
- paracellular pathway–Na+ (and several other cations)
What is the significance of the TAL being impermeable to water?
reabsorption of NaCl and other solutes in the TAL thus reduces the osmolality of tubular fluid to less than 150mOsm/L
How much Na+ is reabsorbed in the distal tubule and collecting duct?
about 7% of filtered NaCl
About how much water is reabsorbed in the distal tubule and collecting duct? What does this depend on?
about 8-17%, depending on the concentration of ADH
Describe Na+ reabsorption in the early segments of the DT
- Na+ and Cl- enter the cell from the lumen via the Na+/Cl- symporter
- Na+ leaves the cell and goes into the blood via the Na+/K+ ATPase pump
- Cl- leaves by diffusion channels in the basolateral cell membrane and paracellularly
- impermeable to water
What are the 2 cells types relevant to Na+ reabsorption in the late DT and collecting duct?
- principal cells
2. intercalated cells