hydrostatic pressure

[joonkimdds]

---

Members do not see this ad.

According to Kaplan
"At the arteriole end of the capillary bed, the hydrostatic pressure is
approximately 36 mm Hg while the opposing osmotic pressure is approximately 25 mm Hg. The larger
hydrostatic pressure forces fluid out of the capillaries. At the venule end of the capillary bed, the osmotic
pressure across the wall is greater than the hydrostatic pressure, which has dropped to 15 mm Hg. This
difference tends to draw fluid into the capillaries (osmosis). Hence, most of the fluid is forced out of the
capillaries at the arteriole end, and is reabsorbed by the capillaries at the venule end."


it says fluid is forced out of the capillaries at the arteriole end, and is reabsorbed by the capillaries at the venule end.

I think it should say fluid is forced out of the arteriole end to capillaries
and forced out of the capillaries to venule end. because fluid flows from high to low pressure.

 

[gdunk]

I think it should say fluid is forced out of the arteriole end to capillaries
and forced out of the capillaries to venule end. because fluid flows from high to low pressure.

But fluid is forced out of the arteriole end from the capillaries, not to them.

And at the venule end, fluid is forced into the capillaries, not out of them.

 

[doc3232]

Arteriole to arteriole-end capillaries to surrounding cells to veule-end capillaries to venioles.

Firs is by hydrostatic pressure. Blood returns to capillaries by osmotic/osnotic pressure.

 

[joonkimdds]

oh...right right right....
by the way, what really is the osmotic pressure here?
is everything inside the blood vein called hydrostatic pressure and everything outside the blood vein called osmotic pressure?

 

[doc3232]

oh...right right right....
by the way, what really is the osmotic pressure here?
is everything inside the blood vein called hydrostatic pressure and everything outside the blood vein called osmotic pressure?

So some of the blood goes into the tissues. But large proteins can't. These large proteins cause osmotic pressure, forcing the blood back into the capillaries.

 

[jay47]

oh...right right right....
by the way, what really is the osmotic pressure here?
is everything inside the blood vein called hydrostatic pressure and everything outside the blood vein called osmotic pressure?

Okay, think of it this way:

Hydrostatic Pressure: this is always facing out of the vessel, be it veinule or arteriole end. This is caused by the pressure the fluid exerts on the vessel from the heart pumping blood. Just like how a hose gets stiff when you force water into it, the same concept applies, except this hose is permeable to the water.

Osmotic Pressure: this is caused by proteins (like Doc said), solutes, and other dissolved particles in the blood. It is always into the vessel, artery or vein. Solutions with more dissolved particles will want to "pull" water into their solutions.

So, what you have is a balance between outward and inward forces in all capillaries and veins. The hydrostatic pressure is greater near the arteriole end (the one closest to the artery, heart, etc..), therefore fluid flows out (because this is the outward pressure). Once some of this pressure is relieved because fluid flows out, osmotic pressure is actually greater. Then fluid flows back in; this is closer to the venule end.

However, not all of the fluid that left at the arteriole end do to hydrostatic pressure comes back in. Some of it stays in the interstitial space, and eventually ends up draining into the lymphatic system.

Hope this helps, Jay.👍