# Mammal size and capillary wall thickness

How does capillary wall thickness vary with the size of the animal?

Background/context of question:

Of the following, Figure 1 is most consistent with the fact that, in general, smaller mammals have:

A] thicker capillary walls than do larger mammals.

B] a more rapid metabolic rate than do larger mammals.

C] smaller surface area/volume ratios than do larger mammals.

D] capillaries of more variable cross-section area than do larger mammal

My working:

Of the following, Figure 1 is most consistent with the fact that, in general, smaller mammals have:

A] thicker capillary walls than do larger mammals.

B] (correct answer) a more rapid metabolic rate than do larger mammals. This is correct because a smaller animal has a higher surface area to volume ratio and hence radiates/loses heat quicker, and so must metabolise food quicker to keep their temperature up.

C] smaller surface area/volume ratios than do larger mammals. This is wrong as a smaller object has a higher surface area per volume

D] capillaries of more variable cross-section area than do larger mammal

My issue: I am trying to work out why A and D are wrong.

I can't find anything on the Internet about how capillary wall thickness (Answer choice A) or capillary cross sectionn (Answer choice D) vary with animal size.

If I was going to have a guess based on the stimulus:

For Answer choice A : From the graph, we know smaller animals have less oxygen partial pressure (as they lie to the right) and less HB saturation (as their curves lie to the bottom). If capillary walls were thicker, I would think this means that the oxygen partial pressure would be GREATER. I.e. if I think of a really thick walled balloon, it would be more difficult to blow up and increase in volume (and hence, have a higher pressure) than a thin walled balloon. However, as I know C is the correct answer, A (and my reasoning above) must both be wrong.

For Answer choice D: I don't really know how variation in cross section would have an effect. Maybe by providing a bottle-neck and increasing pressure in that way?

• I believe your problem lies in understanding the oxygen dissociation curve. The higher the need for oxygen (e.g. because of metabolic rate or physical activity), the greater the shift to the right. So generally, increasingly smaller animals, having increasingly higher metabolic rates, have curves that shift progressively to the right. Capillary wall thickness is fairly constant in mammals: about one cell thick. Commented Oct 26, 2016 at 3:12
• Thanks @anongoodnurse ! That explains why I couldn't find anything about the capillary wall thickness and animal size relationship ; I assumed as A wasn't the answer that the opposite i.e. that LARGER mammals have thicker capillary walls was true (rather than them being equal). Commented Oct 26, 2016 at 3:35