CIE AS level biology mcq

Hello, so I think looking at the diameter and wall thickness was the easiest way of identifying them. Please someone correct me if there is a better way of explaining or answering the question

I am assuming you understand why P and Q is an artery and a vein seeing as you got that correct. I can see why you chose B because R has the smallest diameter, much thinner than P and Q and so it could seem like a capillary in comparison.

However, the diameter is measured in mm. Blood capillaries are very very small, small enough so red blood cells are often squeezed against the capillary wall - and if you think about how small red blood cells are, you can't exactly measure one with a ruler. Blood capillaries are about 8-10 microns long - waaaay smaller than 4mm This means it would have to be a vein or artery.

Furthermore, another big giveaway is the composition - capillaries are made of endothelial cells only. They do not contain ANY muscle or collagen at all. Again, they are way to small and they also simply don't need it to carry out their function. A capillary would NOT contain 35% muscle and 25% collagen, however an artery would. A high percentage of muscle is needed for an artery, as seen in R.

I hope this helps - feel free to ask me a question if you don't understand still

Hi, I had a doubt about this question. Could someone explain why the answer is A and not B which I had selected