Video Transcript
Why is it important for capillaries
to have thin, permeable walls? (A) To increase the rate of blood
flow. (B) To provide capillaries with the
flexibility to move around cells. (C) To prevent the backflow of
blood in the vessels. (D) To allow them to carry blood at
high pressure. Or (E) to allow substances to
diffuse into and out of cells.
There are three main types of blood
vessel in the human body. These are arteries, veins, and
capillaries. You may recall that arteries carry
blood away from the heart and veins carry blood toward and into the heart. In between, the blood passes
through capillaries, which are usually arranged in networks around the cells of a
particular organ or tissue.
As blood flows through the
capillaries, oxygen is delivered to the cells for respiration. And carbon dioxide, a waste product
of respiration, moves from the cells into the capillary so it can be transported to
the lungs and removed from the body. Because there is a steep
concentration gradient for both of these gases, they move into and out of cells by
diffusion. The two properties of capillary
walls described in the question are both important for maximizing the rate of
diffusion.
The fact that the walls are thin
means that the gases only have a short distance to diffuse over. And the fact that they are
permeable means that it is much easier for the gases to diffuse from the capillaries
into the cells and vice versa.
We have therefore determined that
the correct answer is (E). It is important for capillaries to
have thin, permeable walls to allow substances to diffuse into and out of cells.
