Confused:Pressure in the Capillaries, Net Movement, Hydrostatic/Osmotic Pressure

[fizzlin]

---

Members don't see this ad.

So, I've been trying to understand this concept, and I still am having a very difficult time. Went through a few different books/read through threads on this topic, but I'm having a brain block.

I read through this thread:
http://forums.studentdoctor.net/showthread.php?t=186717


Since it seemed helpful, but instead, left me more confused.
I like the diagram EK provides on p. 142, but I don't understand what's going on.


Why is the osmotic pressure lower on the arterial side in comparison to the hydrostatic side? How did the osmotic pressure increase on the venule side? Can someone explain this for the capillaries?

Pressure is high to low? So, for osmotic pressure, there must be more solutes in the capillary rather than the interstitial space for the arterial side, while the opposite on the other side? But then, hydrostatic pressure draws into the equation, and high hydrostatic means greater concentration of fluid in the capillary than interstitial in arterial side??? Basically, I'm just extremely, extremely confused, and if anyone could logically explain the terms and process to me--then that would be awesome.


Also, why is there a low blood pressure in the capillaries, venules, small veins, large veins, vena cavae progressively? I think I'm missing some large fundamental concept somewhere.

 

[fizzlin]

Anyone? 😕

 

[wjs010]

Doesn't hydrostatic pressure mean more fluid would be on the outside of the capillaries? Osmotic pressure draws water in due to {solute} and hydrostatic pushes it out , I'm sorry ill have to review this section . It's been a few weeks since I studied this topic lol

 

[Romz]

So, I've been trying to understand this concept, and I still am having a very difficult time. Went through a few different books/read through threads on this topic, but I'm having a brain block.

I read through this thread:
http://forums.studentdoctor.net/showthread.php?t=186717


Since it seemed helpful, but instead, left me more confused.
I like the diagram EK provides on p. 142, but I don't understand what's going on.


Why is the osmotic pressure lower on the arterial side in comparison to the hydrostatic side? How did the osmotic pressure increase on the venule side? Can someone explain this for the capillaries?

Pressure is high to low? So, for osmotic pressure, there must be more solutes in the capillary rather than the interstitial space for the arterial side, while the opposite on the other side? But then, hydrostatic pressure draws into the equation, and high hydrostatic means greater concentration of fluid in the capillary than interstitial in arterial side??? Basically, I'm just extremely, extremely confused, and if anyone could logically explain the terms and process to me--then that would be awesome.


Also, why is there a low blood pressure in the capillaries, venules, small veins, large veins, vena cavae progressively? I think I'm missing some large fundamental concept somewhere.

Hydrostatic pressure is due to the elasticity of the arteries. Logically makes sense as veins do not have this same elasticity. Remember, the heart is pumping blood towards the entire body and must exert force against a high pressure gradient. This elasticty of the arteries sort of squeezes the blood during rebound, what we know as hydrostatic pressure.
How do we maintain pressure in the venules? we know that they have the capacity to stretch yet are not elastic. We have osmotic pressure at the venule end of the capillary bed to help keep the pressure "high' and the blood flowing. The veins absorb fluid from their surroundings to keep this pressure gradient and, with the help of valves, transport the blood back to the heart.

In terms of your question, the hydrostatic pressure sort of squeezed fluid out during arterial flow and the venule side of the capillary bed just kind of takes it back up because it needs it. Thats why we say the venous circulation has a higher volume of fluid than arterial circulation, because veins can reuptake lost fluid from arterial circulation in order to maintain the needed pressure.

I hope this helps.

 

[Mopitt16]

At the arterial side of the capillaries, hydrostatic pressure is relatively high (though the arterioles decrease the hydrostatic pressure significantly due to high resistance) and thus fluid moves through the capillary walls into the interstitial space.

As you move through the capillary bed, you lose fluid to the interstitial space. Osmotic pressure is proportional to the molarity of the fluid (which is moles of solute per volume). So as you lose fluid to the interstitial space, you lose fluid volume, increase the molarity of the blood and increase the osmotic pressure of the blood. This causes water in the interstitial fluid to move back down its concentration gradient into the capillaries due to the relative solute concentrations (higher in the capillaries).

So on the ARTERIAL side of the capillaries, the hydrostatic pressure dominates the osmotic pressure, but on the VENOUS side of the capillaries, the increased osmotic pressure dominates the hydrostatic pressure (decreased due to the capillaries inherent resistance to flow, like a circuit); this causes fluid to move into the intersitial space on the arterial side with nutrients and back into the capillaries without nutrients on the venous side.

Lots of information, but hopefully this cleared things up.

 

[fizzlin]

Thank you!!! Yes, the explanations definitely help. I shouldn't be tripping up on any hydrostatic/osmotic pressure questions now.... or at least hopefully.


The key thing is that for the capillaries, osmotic pressure is constant, and hydrostatic pressure changes!