BS Kaplan question on blood pressure and oxygen delivered to tissues

[nfg05]

Which of the following factors will NOT influence the amount of oxygen delivered to body tissues?

A. surface area of red blood cells
B. blood pressure
C. surface area of alveolio
D. rate of respiration

Kaplan says B, but I'm not exactly convinced. BP obviously changes when you are exercising and the heart is pumping faster to circulate more blood no? Now I always get a little confused on this but assuming the cross sectional area of the blood vessels is the same at rest and during exercise (not sure if this is actually a valid assumption), the blood flowing at a higher velocity would decrease blood pressure right?

In any case, the blood pressure seems to me to have an effect on the amount of oxygen delivered to the tissues, so what reasoning are they using to say it doesn't?

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[AsuKa]

during exercise, sympathetic nervous system kicks in

epi dilates the vasculature of the skeletal muscles, brain, and heart, and constricts skin, gi tract, kidney

so this would i believe, lead to a change in the cross sectional area of blood vessels

 

[MeRLin443]

The answer is B. ADH and Aldosterone both increase water reabsorption in the blood (the first directly the second indirectly). This leads to an increase in plasma volume and therefore an increase in blood volume and blood pressure. But because there are no more erythrocytes only more plasma, the amount of oxygen to the tissue doesn't increase.

 

[seraph524]

Which of the following factors will NOT influence the amount of oxygen delivered to body tissues?

A. surface area of red blood cells
B. blood pressure
C. surface area of alveolio
D. rate of respiration

Kaplan says B, but I'm not exactly convinced. BP obviously changes when you are exercising and the heart is pumping faster to circulate more blood no? Now I always get a little confused on this but assuming the cross sectional area of the blood vessels is the same at rest and during exercise (not sure if this is actually a valid assumption), the blood flowing at a higher velocity would decrease blood pressure right?

In any case, the blood pressure seems to me to have an effect on the amount of oxygen delivered to the tissues, so what reasoning are they using to say it doesn't?

Let's go through these choices one by one...

A. an increase in the surface area of red blood cells means that they are able to hold more oxygen - thereby delivering more oxygen to tissues

B. an increase in blood pressure will only increase rate of blood flow, not the actual pressure gradient between capillaries and the tissue. now, you can argue that increase in flow would deliver more oxygen to the tissues, however, if the blood cells are moving too fast to actually allow diffusion to take place, then it doesnt help

C. increase in surface area of alveoli will certainly allow more oxygen to enter the blood in the lungs

D. increase in rate of respiration will increase partial pressure of oxygen in the lungs, decreasing partial pressure of carbon dioxide, allowing for a greater gradient in the lungs, more oxygen will diffuse into the blood.


So, in the end, there really isnt a clear cut choice, however, the best one appears to be B. And honestly, Kaplan isnt really known for writing good questions...real MCAT questions are written by leaders in their respective fields, so expect no ambiguities on the real thing.

 

[Kaustikos]

So, in the end, there really isnt a clear cut choice, however, the best one appears to be B. And honestly, Kaplan isnt really known for writing good questions...real MCAT questions are written by leaders in their respective fields, so expect no ambiguities on the real thing.

For the most part.

 

[seraph524]

Good point. I remember a bio question on my MCAT that I was sure there were no right answers. O_O