Page 12 - 新思维科学学生用书8 样章

1.2 Gas exchange

Air sacs

The photograph shows a tiny part of the lungs,
seen through a powerful microscope. You can
see the lungs are mostly holes. These holes

are called air sacs. Another name for them
is alveoli.

There are also lots of very tiny blood vessels
in the lungs, wrapped around the air sacs.
You cannot see them in the photograph, but

they are shown in the diagram below. These
blood vessels are capillaries.

Part of the lungs, viewed through a powerful microscope

The structure of an air sac

This diagram shows one of the air sacs in the lungs. The air sac has a wall made of one layer of cells.

These cells are very thin.
air moving in air moving out

blood flowing
blood flowing towards the
from the heart heart

di usion
air inside sac of oxygen
thin wall of air sac di usion of
thin wall of carbon dioxide
blood capillary
red blood cell
blood plasma

An air sac in the lungs

You can see that there is a blood capillary around the outside of the alveolus. The capillary is pressed
tightly against the alveolus. The wall of the capillary is also made of a single layer of very thin cells.

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