Arteriole, Capillary, Venule

[MedPR]

So there's a net loss of fluid between an artery and vein connected by a capillary network. However, blood is supposed to be isotonic in all vessels. How is this possible?

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[chiddler]

I'm not sure if this is sheer coincidence or if the mcat qotd inspired you to write this, but today's question answers just that:

"Most of the fluid returns at the venule end because blood pressure:


D. Decreases and large solutes stay the same."

 

[aSagacious]

The Starling equation demonstrates that the capillary and ISF hydrostatic and oncotic pressures determine the net flux of fluid across the capillary membranes.

Where:
Jv = net flux of fluid
Kf = some constant
σ = reflection coefficient
Pc = capillary hydrostatic pressure
Pi = interstitial fluid hydrostatic pressure
πc = capillary oncotic pressure
πi = interstitial fluid oncotic pressure

In a physiologic setting, the following are generally true:
Pc > Pi
πc > πi

...which results in a net filtration into the interstitial space. This would lead to a buildup of fluid in the extra-cellular space but lymph channels (with a near-zero hydrostatic pressure) allow for the clearance of any extra fluid.

 

[MedPR]

Thanks for the input, but neither of your answers are explaining (or I'm missing how they are explaining) why arterial blood and venous blood are isotonic.

 

[MedPR]

I'm not sure if this is sheer coincidence or if the mcat qotd inspired you to write this, but today's question answers just that:

"Most of the fluid returns at the venule end because blood pressure:


D. Decreases and large solutes stay the same."

Coincidence. We discussed it in my A&P2 lecture today, but I didn't get a chance to ask. I remember seeing in EK Bio that about 10% fluid is lost to the interstitium, which made me think that the venous blood just after the capillary was hypertonic to the arterial blood just before the capillary. This is what my teacher said as well, but then he said that eventually (further away from the capillary bed), blood is isotonic.

 

[aSagacious]

Sorry, I didn't really address your question in my previous answer.

At the arterial end the blood is rich in nutrients. It proceeds to dump these off (often by non-passive processes - ie active transport, etc) into the interstitial space. On the venous side of the capillary bed, the cells have been continually secreting waste products, which are then reabsorbed into the capillary. This approximately balances out, resulting in a net isosmotic filtration of fluid.

 

[Class1P]

As blood flows through capillaries the hydrostatic pressure pushes water out into the interstitial tissue. As the blood goes along and loses water that pressure is reduced due to that loss. Now that they hydrostatic pressure is low and the blood is slowly becoming not isotonic due to loss of fluid it starts to pick up fluid in the venous system due to osmotic pressure since there are a large number of solutes left in the blood.