GS-1 Biological Sciences Question 19

[gth19453667]

following the sequence provided by the answer doesnt the substance return to the heart before it reaches teh lungs? why is lungs a better answer choice than heart?

[jellywing_2058]

Yes the active agent does reach the heart before it does the lungs, however what is important here is which capillaries are reached first. The blood does not go through capillaries of the heart, but through the ventricle. The first capillaries it reaches are the ones in the lungs as shown in the explanation.

[calena7178147]

You are right in assuming the brachiocephalic goes to the heart first and then the lungs. However the lungs are very rich is capillary beds, and the substance rapidly diffuses through them.

Consider this: would deoxygenated blood (ie blood from a vein in the arm) be heading straight for a capillary bed in muscle (ie in the heart)? That would make no sense since the heart muscle requires oxygenated blood. So the blood goes to the capillary beds in the lungs, gets oxygenated, returns to the left heart then gets pumped into systemic circulation including little “heart arteries” (coronaries) that supply oxygenated blood to the capillaries of the heart muscle.

[chronicida2879]

One might ask, why wouldn't the compound affect the heart anyway? After all, it's muscle that's pretty much bathing in blood.

The answer is, the heart is covered (on the inside) by endocardium that is similar to normal vascular endothelium, which is less permeable than capillaries.

It is still possible, however, that the compound is able to penetrate. The question stem does not rule it out.

[mcat_premed3832]

It is not clear to me if you are suggesting that the compound in question may somehow diffuse through the endocardium and then diffuse through the myocardium. Of course the heart is not a static container but, if it were, the process would take a tremendous amount of time.

Besides the fact that one would have to question the purpose of having said in the question stem that the agent "completely diffuses through capillary beds," consider that in one heart beat, most of the blood has already moved from the heart to the lung.

[khayes37216]

As a customer, I would ask that you be very careful when formulating questions and finding support for your suggested answers.

I am a critical care nurse (yes, I am taking the MCAT, but that's another story) and have always been taught that when you are giving meds to someone in cardiac arrest or a malignant cardiac rhythm has taken hold, you must push the meds into the LEFT BRACHIOCEPHALIC VEIN, so that they may FIRST affect the heart. This is common physiology and common sense. Indeed, clinically you do see the cardiac effects first and then the pulmonary effects. i see where you're coming from with the fact that the lungs have a more extensive capillary bed, but please do be careful with your accuracy.

[mcat_premed3832]

I would be very interested in a reference that specifically explains how a medication arriving in the heart could diffuse through the heart wall faster than a few heart beats and going through the coronaries and then the capillary bed of the heart (which is the primary way that the heart cells receive nutrients). Certainly, direct diffusion cannot compete with the speed of the effect in the pulmonary capillaries.

On the other hand, there is a different logic for the left over the right brachiocephalic that respects the internal "plumbing" of the cardiovascular system: the left vein is physically closer to the heart and will result in a faster cardiac effect but going through the coronary arteries (predominantly).

So I agree with your statement "you must push the meds into the LEFT BRACHIOCEPHALIC VEIN, so that they may FIRST affect the heart"; but this is fully consistent with this problem since the push is a cardiac medication. But, the cardiac medication will mostly pass through pulmonary circulation before having its cardiac effect.

Of course, this discussion is for interest since this is beyond the knowledge required for the MCAT. The basic internal plumbing would be required knowledge.