osmotic pressure vs. hydrostatic pressure

[yumi43]

the more i read about this, the more confused i get. can someone please explain why osmotic pressure is less than hydrostatic pressure at the arteriole end of the capillary bed, and why osmotic pressure is higher than hydrostatic pressure at the venule end? thank you.

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[whawha]

the more i read about this, the more confused i get. can someone please explain why osmotic pressure is less than hydrostatic pressure at the arteriole end of the capillary bed, and why osmotic pressure is higher than hydrostatic pressure at the venule end? thank you.

As blood moves from the arteriole end to the venule end, there is an exchange of stuff between blood vessles and interstitial fluid.

So, before it enters the capillary bed, the osmotic pressure is lower than hydrostatic pressure (not much solute is present). After it has gone through the capillary bed, it has absorbed some materials which increased the omsmotic pressure of the blood. (more solute is present at the end of the capillary bed)

This is how I interpret this, but there may be a better explanation... hope this helps.

 

[r8 dds]

http://forums.studentdoctor.net/showthread.php?t=534556

 

[nze82]

the more i read about this, the more confused i get. can someone please explain why osmotic pressure is less than hydrostatic pressure at the arteriole end of the capillary bed, and why osmotic pressure is higher than hydrostatic pressure at the venule end? thank you.

Hydrostatic Pressure: is the pressure that the fluid exerts on the walls of its container. In human body, the hydrostatic pressure refers to the pressure that the blood exerts on the walls of the arteries and veins.

Osmotic Pressure: is the pressure required to prevent the flow of water across a semipermeable membrane via osmosis.

At the arterial end, the hydrostatic pressure is greater than the osmotic pressure. Why?
At the arterial end, you have a larger volume of blood flowing through the vessel.
At the arterial end, fluid leaks out of the capillaries. Why?
At this end, the hydrostatic pressure is greater than osmotic pressure. Therefore, fluid is forced out of the capillaries and into the interstitial fluid.

At the venous end, the hydrostatic pressure drops, but the osmotic pressure remains the same. Why?
At the venous end, the volume of blood is smaller compared to that of the arterial end (remember that some of the fluid has leaked out at the arterial end). Therefore, the hydrostatic pressure drops. However, the concentration of solutes within the blood is the same at both ends. Therefore, the osmotic pressure remains the same.
At the venous end, fluid leaks into the capillaries. Why?
At this end, the hydrostatic pressure falls below the osmotic pressure, causing fluid to enter the capillaries.

Video:
http://highered.mcgraw-hill.com/sit...exchange_across_the_walls_of_capillaries.html