arterial pressure

[BodybldgDoc]

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At the arterial end of a capillary bed, why does the osmotic pressure end up being lower than its hydrostatic pressure?

 

[DailyDrivenTJ]

Hydrostatic pressure is just so strong at the arteriole end (as opposed to venous end) being closer to the heart. Once it passes the capillary beds, it loses most of the hydrostatic pressure.

Osmotic pressure stays almost constant through the capillary, it is just the hydrostatic pressure dropping across the capillary bed and this relative pressure difference between the osmotic and the hydrostatic pressure helps with the movement of the fluid in and out of the capillary.

 

[PennPB]

At the arterial end of a capillary bed, why does the osmotic pressure end up being lower than its hydrostatic pressure?

Because the oncotic/colloid osmotic pressure is established by an increase in protein concentration as fluid is filtered through the capillary network. Capillary hydrostratic pressure is greater at the arterial end because fluid tends to leave at arterial end and is reclaimed at the venous end. Think of it in terms of permeability along the vessels, and you'll be good to go.

 

[jdmsamurai]

here is a better way to think of it....

on the aterial side its nothing but smooth muscles...and as the left ventricle is pumping blood its causing alot of pressure...and since the smooth muscle acts like a rubber...it swells up and has a "recoil effect" keeping the pressure high, while on the venous side is not covered with smooth muscles...(most of the blood volume is on the venous side..btw)...when the blood gets to the capillaries it reaches "leaky cells" and "porous cells"...so the pressure causes hydrostatic pressure pushing outward (many holes in your rubber under high pressure), but we also know that pressure falls off with distance...so as its traveling down the capillaries its hydrostatic pressure falls (due to distance and leaky holes).....

now onto colloid osmostic pressure. well, in our blood we have plasma which is made up of many ions...one of the big ones is albumin which is a protein. we learn that in osmosis water tends to travel to the side where the concentration is highest...well as the hydrostatic pressure is pushing blood and fluids out (along with some ions)...the capillaries gets really concentration with proteins...so guess what? waters starts going back in which we called colloid osmostic pressure which is highest at the venous end....

and fun fact...
edema: build up of liquid...
ever wonder why kids in poor countries have "swollen belly sydrome?"...well they dont eat so their body can not make (anabolize/assimilate) any proteins...so as the hydrostatic pressure is pushing fluid out...there is no proteins (colloid osmotic pressure) pushing fluid back in....

hope that helps 🙂

 

[BodybldgDoc]

why is there smooth muscle only on the arterial side and none on the venous side? In that case what kind of muscle is there on the venous side?

 

[jdmsamurai]

there is smooth muscle but not that much until you hit the veins...
so it goes like this

artery > arteroiles > capillaries > venules > vein
(alot of SM > few SM > none > none > few SM)
SM = smooth muscle

the arteries need the smooth muscle more for the pressure gradient and recoil effect....

 

[BodybldgDoc]

great thanks. your explanation made a lot of sense.

 

[jdmsamurai]

sure anytime...im always trying to answer your questions 🙂