Intro to Renal Physiology Flashcards
What is a quick way to estimate osmolality?
What is a more accurate way?
Plasma osmolality = 2 * [Na+]
See image
E
In addition to the question posed on this slide, what are the implications of the venous reabsorption pressure gradient being lower than that of the arterial filtration gradient?
- The graph suggests that aterial net ultrafiltration pressure is higher than venous reabsorption. This indicates that we filter more than we reabsorb, which conceptually would lead one to think that we constantly have an accumulation of fluid in the interstitium (edema). However, the lymphatic system is responsible for draining this excess fluid so we don’t end up with tissue edema.
- For second question, see slide below.
D
Complete this table.
Total body water (TBW) = […] % of body weight
60%
1L for 1kg
Describe the concepts of ultrafiltration and reabsorption that occur in capillaries of the kidney.
Ultrafiltration is when nutrient rich fluid at the arterial end tends to leave the capillary and enter the interstitial fluid. This is driven by the fact that the net filtration pressure at the arterial end of capillary beds is positive, so there’s a force driving the fluid out.
Reabsorption occrs when waste-rich fluid from tissues tends to enter the capillary at the venous end, resulting in reabsorption. This is driven by the fact that the net filtration pressure at the venous end of the capillary bed is negative, so there’s a force driving the fluid into the capillary.
What are the 2 driving forces for fluid exchange across membranes?
Hydrostatic pressure is pressure exerted by the fluid onto something else (membrane)
Osmotic pressure is the pressure exerted by the solutes in the solution onto the water.
E
What are the criteria for being an effective osmole?
TBW = […] + […]
Intracellular fluid (ICF) + Extracellular fluid (ECF)
True or false: the movement of water when talking about osmotic pressures is sometimes active.
False - it is always passive
ICF = […] % of body weight
ECF = […] % of body weight
40%
20%
What makes a compound an effective osmole?
An effective osmole is an agent that increases osmotic pressure across a biological membrane. In order for it to do this, it must be impermeable to the membrane. Therefore, subtances that are non-polar and readily cross membranes can be freely exchanged back and forth and do not exert an osmotic pressure because they won’t cause water to move across the membrane.
What does it mean to say that body fluids between all these compartments (ICF, ECF, IF and plasma) are in osmotic equilibrium?
They have the same concentration of osmotically active solutes. Osmotically active solutes are those that have the ability to change the [water] in their environment to push water away or pull it in.
- Through what pathway is water exchanged bewteen the ICF and ECF across a cell membrane?
- What force drives this exchange?
Aquaporins
Osmotic pressure
- In the equation below, what is Kf?
- What is sigma?
- When would this be 1?
- When would this be 0?
sigma = 1 in glomerulus –> despite fenestrated capillaries, tightly regulated boundary (podocytes, basement membrane, etc.)
sigma = 0 in liver –> discontinuous capillaries
What is osmolality?
What is a normal value for this parameter?
Osmolality is the total number of particles in solution
290mmol/L = 290 mOsm/kg H2O
A
Potassium is not the best osmole because the cell membrane has K+ leak channels, so it’s semi-permeable to K+
+13
Fluid will exit capillary –> filtration
The ECF consists of interstitial fluid (fluid between cells) and blood plasma/lymph. ECF is 20% of body weight.
- What proportion of the ECF is IF?
- What proportion is plasma?
- 3/4
- 1/4
What is tonicity?
total # particles (effective osmoles) that exert osmotic pressure
Complete this table.
An effective osmole must be […] in fluid
Freely dissolved
(i.e. can’t be sequestered in organelles inside cell, won’t exert pressure)
Fluid exchange across capillaries
- Through what pathway is water exchanged from the capillaries into the interstitium?
- What is the driving force(s)?
- What is the net filtration pressure?
- What does it mean when this value is (+)?
- What does it mean when this value is (-)?
B
Thinking of the net filtration pressure equation (below), what are some potential causes of interstitial edema?
