Page 116 - The Cell in 40 Topics

The Cell in 40 Topics

So how is it that the veins know when to expand and when to con-
tract? Research has revealed the presence of a chemical messenger: the NO
molecule. It is this molecule, consisting of two atoms, that issues the com-
mand for blood vessels to expand and contract.
Let us now examine the splendid facilities that produce NO in the
depths of your circulatory system.
Under an electron microscope, veins can be seen to have a perfect
structure, in inverse proportion to their size. For example, 10 capillary ves-
sels aligned side by side are no thicker than a human hair. These narrow
vessels’ internal walls are covered by a layer consisting of flat-muscle cells;
expansion and contraction of the veins take place as a result of these mus-
cles’ activities. The muscle cells do not come into direct contact with the
blood, because the endothelial cells form a membranous layer between the
muscle cells and the blood flow.
Like links in a chain, these cells combine together to comprise the en-
dothelial layer. Until the 1980s it was believed that these cells had no other
effect than to facilitate the flow of blood in the veins. The truth emerged
only later, when it was realized that one of the endothelial cells’ responsi-
bilities is to produce the messenger molecule NO (Figure 91).
If we compare the endothelial cell to a factory, you
can think of the NO molecules as its products. The life
span of each NO molecule is about 10 seconds, but NO
is created to transmit the messages it carries during
that brief span—which it does in the
most perfect manner. The messenger
NO molecules secreted by the en-
dothelial cells begin being carried
by the blood in all directions at
high speed. Those that move to-
wards the flat-muscle cells enter
these cells’ membranes. The highly
selective flat-muscle cell membrane

Figure 91.
The endothelial cell produces mol-
ecules of nitric oxide (NO).

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