Question 13

[stoneprod7112]

It would seem to make since that lactic acid or low ph would means muscle use so therefore why would the heart not pump harder when this is signaled to help eradicate the o2 debt caused by muscle use?

[stoneprod7112]

I still don't understand this situation and take the MCAT in a couple of weeks if anyone could help answer the above question

[quadalpha]

[mcat_premed3832]

OK, first we will just discuss the reasoning expected from the MCAT's point of view. Then we can talk about physiology.

The last 2 paragraphs of the passage suggest that lactic acid leads to decrease contractility and the question suggests that increasing intracellular Ca++ leads to increased contractility (i.e. sympathetic stimulation). Since these processes are in opposition, it would be reasonable to assume that decreasing pH would not be helpful to increasing intracellular Ca++.

Now physiology: I think you might be confounding a different issue which is the question of unloading oxygen (ie. shifting of the oxygen dissociation curve). As you know, a hard working muscle is hot (increased metabolism/temperature), acid (lactic acid), hypercarbic (high CO2 means lower pH also) and high phosphates (esp. 2,3 DPG in RBCs). All these features of a hard working muscle leads to a shift to the right of the O2 curve - meaning increased unloading of oxygen (BIO 7.5.1).

None of the above is to be confused with the fact that a hard working muscle does not want to continue working hard. You can postulate the evolutionary benefit to prevent injury (myocardial infarction) or cardiac arrest, that if the heart muscle is working so hard that acid is building up then it must cool down. To explain the processes involved is beyond the introductory level knowledge required for the MCAT.

[quadalpha]

That makes sense.

On another note, what exactly is the mechanism for increasing output? The passage seems to first suggest increased ventricular volume, as if the muscles were stretched to hold more blood; then the passage suggests increases in frequency of heart beat ("In other words ... higher the rate of ventricular contraction"); and finally, the answer to question 12 seems to suggest a "stronger" beat due to formation of more crossbridges.

[mcat_premed3832]

You deduced well. The relevant equation is:

Cardiac Output (Q) = SV × HR

where SV is stroke volume and HR is heart rate. Muscle strength can affect either SV or HR. All of the above are not required knowledge for your exam.