can someone clarify: osmotic pressure, hydrostatic pressure etc.

[eng_dude786]

I'm still not fully grasping the concept of osmotic pressure, and hydrostatic pressure.

 

In a PS question, it said that pressure in the feet would be highest because of the formula: P=Po +density*g*height,

 

but in the BS it says that blood pressure increases near the heart and decreases to its lowest as it moves away from the heart.

 

can someone plzzzzzzzzzzzz clarify. I feel like jumping off a cliff, because I can NOT seem to understand the loop of henle without this basic concentration difference in solute and osmolality. Someone help!

[Steve-0]

First of all, osmotic pressure has nothing to do with the pressure in either of the examples dealing with blood vessels that you described. Osmotic pressure results from a solute imbalance between two solutions separated by a membrane that is impermeable to the solute. The solvent will move to the side with more solute and the water level will therefore be higher on this side. The pressure difference between the two solutions is the osmotic pressure.

 

As for blood pressure at different heights in the body, pressure is highest in the feet because there is a greater volume of blood resting on top of it here than in any other body part (when upright). This is like any other physics problem involving pressure of a solution. The pressure is proportional to the depth by P= density x g x depth. In effect, your feet are the "deepest" part of your body.

 

Finally, because volume flow rate is constant for ideal fluids, you would expect the pressure to be proportional to the surface area of the vessels of the body (by the continuity equation- Q=vA). Blood is not an ideal fluid however, and therefore the pressure is greatest directly after the pump (the heart) and steadily decreases so that it is lowest in the vessels that feed back into the heart (the vena cavae).

 

Not sure if this is what you were looking for but it's a start at least.

[The Law]

I'm still not fully grasping the concept of osmotic pressure, and hydrostatic pressure.

 

In a PS question, it said that pressure in the feet would be highest because of the formula: P=Po +density*g*height,

 

but in the BS it says that blood pressure increases near the heart and decreases to its lowest as it moves away from the heart.

 

can someone plzzzzzzzzzzzz clarify. I feel like jumping off a cliff, because I can NOT seem to understand the loop of henle without this basic concentration difference in solute and osmolality. Someone help!

 

Hey eng_dude,

 

 

There are two issues here. First is fluid exchange in capilliaries, second is how the nephrons are working.

 

Issue #1:

I think it's best to think of it this way, hydrostatic pressure pushes fluids out of the capillaries. It is greatest at the start of the capilliaries, and weakest at the end of the capillaries. This pressure depends on the power of blood driving from the heart. If you just think of it logically (I sure am not smart enough to quantify it ), the further you move from the heart, the more resistance the fluid will get, and so the weaker the pressure is going to be. This means, when you are near the start of the capillary (the arteriole end), fluid is being driven out of the capillaries. Now, it's important to remember that blood is very concentrated in solute molecules. Albumin plays a critical role in this regard. Water will rush into wherever solute is concentrated. The blood is very concentrated, so it rushes into the capillary at the furthest side (veinous side) from the start of the capillary. You may be wondering, if blood is so concentrated, why isn`t this always happening...

 

This is where the concepts of hydrostatic pressure and osmolarity take place.

 

At the start: HYDROSTATIC PRESSURE (from the heart pumping) is HIGH, OSMOLARITY (water`s need to go to where solutes are more concentrated) is also still high (favouring entry into blood since blood is concentrated).... The reason that water leaves though is because the hydrostatic pressure is still high enough to push water out.

 

Near the end of the capillary: HYDROSTATIC PRESSURE from the heart is DECREASED greatly, but OSMOLARITY is still HIGH! Now, water can rush in since there`s the force pushing it out (they hydrostatic pressure) is decreased.

 

BOTTOM LINE: Hydrostatic pressure PUSHES FLUID OUT. Osmolarity pushes it back into capillary before it returns to the vein.

 

 

Issue 2:

 

http://www.sumanasinc.com/webcontent/animations/content/kidney.html

 

Watch this animation, it does a tremendous job at explaining things. The key point is, hydrostatic pressure pushes fluids out of the glomerelus into the bowman`s capsule. The concentration of filtrate is around that of plasma, but the kidney is still able to end up making the filtrate much more concentrated that the plasma in the end! This means that depsite concentrations being similar, it can still retain water.

 

How does this work... It works by the process described in the video. The ascending limb releases ions (and only ions) into the interstitial fluid. This concentrates the fluid in the interstitial fluid. Since the descending limb is only permeable to water, as fluid goes down the nephron, the filtrate becomes more and more concentrated since water is leaving the nephron due to osmosis.

 

Now, remember, ascending limb is releasing ions into fluid so this is once again making the filtrate unconcentrated. However, the distal convuluted tubule also releases water returning the fluid to its concentrated state. This effect is enhanced by ADH, which causes the distal convuluted tubule to become even more permeable to water.

 

I hope this helps, let me know if you need me to clarify - this stuff can get confusing because of the number of processes involved, but really it`s not that bad once you work it out.

[eng_dude786]

thanks law!