Very confused about blood pressure, resistance and cardiac output :(

[WanderingGuitarist]

Please help me relate resistance, blood flow velocity, and cardiac output.

Also, I have a question about why the answer is A.

Wouldn't lowering the diameter cause an increase in pressure? This would cause an increase in the hydrostatic pressure due to it equaling Pressure of arteries - pressure of veins?

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[aldol16]

Wouldn't lowering the diameter cause an increase in pressure? This would cause an increase in the hydrostatic pressure due to it equaling Pressure of arteries - pressure of veins?

Are you sure the answer is A? What you said above is consistent with the answer being B. Reducing diameter increases linear blood velocity because the volume flow rate must be constant. Your body is (essentially) a closed circuit and you're not losing any blood, so the flow rate of blood through any one point in the circle must equal that of another point in the circuit. So linear velocity increases. If velocity increases, then static pressure decreases according to Bernoulli's equation. That's just a conservation of energy.

 

[WanderingGuitarist]

Nope. The answer is B. I am so sorry for the confusion.

 

[victorias]

Decrease diameter = increase resistance = increase Pressure = Decrease flow rate (less blood flows through)

A. no change in the concentration of the blood so it wouldn't favor movement into/out of the capillaries any differently
B. Hydrostatic pressure will decrease as the flow rate has decreased and less blood is passing through the capillaries
C. Capillary and venous resistance can't be equalized by changing diameter, they two have different surface areas and we don't know based on the question stem if the other variables are assumed to be constant between capillaries and veins for the purpose of this question
D. Proteins and particle don't flow in/out of the capillaries, only water flows due to different hydrostatic and oncotic pressures

 

[aldol16]

Decrease diameter = increase resistance = increase Pressure = Decrease flow rate (less blood flows through)

For an incompressible fluid, decreasing the diameter of the tube will increase linear velocity because volume flow rate must remain constant (you can't lose blood out of a closed circuit - similar to Kirchhoff's laws). Increase in linear velocity actually decreases pressure due to Bernoulli's equation. That's why B is correct.

A. no change in the concentration of the blood so it wouldn't favor movement into/out of the capillaries any differently
B. Hydrostatic pressure will decrease as the flow rate has decreased and less blood is passing through the capillaries
C. Capillary and venous resistance can't be equalized by changing diameter, they two have different surface areas and we don't know based on the question stem if the other variables are assumed to be constant between capillaries and veins for the purpose of this question
D. Proteins and particle don't flow in/out of the capillaries, only water flows due to different hydrostatic and oncotic pressures

A. You're forgetting that osmotic pressure is only one factor affecting movement of fluid. Pressure differentials also affect fluid movement. But in this case, since pressure is lowered, the differential would actually favor movement into the vessel, not the interstitial space.

B. Hydrostatic pressure and linear flow rate change in opposite directions according to Bernoulli's equation so they can't both decrease.

D. While proteins generally don't flow into/out of capillaries, particles/solutes definitely do. That's the whole point of having capillaries - to deliver nutrients to the tissues of the body. Small molecules diffuse through capillary walls while larger molecules travel within vesicles.

 

[maydaymalone]

I have always been a bit confused about hydrostatic pressure and osmotic pressure. Are they opposite? Is osmotic pressure the pressure exerting fluid out of the vessel and is hydrostatic pressure the pressure pushing fluid back in?

 

[aldol16]

I have always been a bit confused about hydrostatic pressure and osmotic pressure. Are they opposite? Is osmotic pressure the pressure exerting fluid out of the vessel and is hydrostatic pressure the pressure pushing fluid back in?

Great question! Unfortunately, it opens up quite a few topics that are often unclear to students. So to directly answer your question, they don't have to necessarily be the opposite. The hydrostatic pressure is simply the pressure of the fluid and is just like pressure in any other fluid you know, including atmospheric pressure. Osmotic pressure is slightly different. Osmotic pressure isn't really a "pressure at all." It's defined as the pressure that must be applied to keep water from moving across a semipermeable membrane when there are unequal concentrations of solutes between the two sides. So what it really measures is the entropic drive for water to travel from somewhere it is concentrated (in dilute solutions) to where it is not (in concentrated solutions). This tendency can go in both directions. So for instance, say you have a semipermeable membrane in a beaker bisecting it. On the left, you have some solute dissolved and on the right you don't. Let's say that the pressure on the right is greater than the pressure on the left. So hydrostatics would predict that water goes from right to left. But on the left, water is more dilute (there are solutes dissolved that occupy space that could have been occupied by water). So water will want to move to the left because of entropy. Which factor wins out depends on which drive is greater. But you could also have pressure greater on the left. In that case, both factors would drive movement of water to the right. Your body has many homeostatic mechanisms that keep these factors in balance so that just the right amount of fluid gets retained.

So the second thing that most students get confused about is the sign of osmotic pressure. Positive osmotic pressure means that you have to apply a certain pressure to keep water from moving across the membrane into the solution of interest. This means that it's actually measuring a "sucking" force from the solution that draws the water in as opposed to a pressure that pushes water in. Now, this sign convention is only a convention so there's no need to sweat the details. One could just as easily have reversed the signs (which would be more intuitive since then the side water is more "concentrated" on would have the greater pressure) and have gotten the same result. But the sign convention is just one thing to keep in mind for the MCAT, because you'll often see osmotic pressure defined as a "sucking" force that draws water in as opposed to a pushing force.

 

[maydaymalone]

@aldol16 thank you for the explanation!