Page 22 - The Cell in 40 Topics

The Cell in 40 Topics

The nerves departing from these receptors are connected to the pitu-
itary gland, just like a cable connection. Under conditions of normal blood
pressure, these receptors are constantly stimulated to send a continual elec-
trical current to the pituitary gland by means of these nerves (Figure 11). So
long as the pituitary gland continues to receive these signals, it will prevent
the secretion of the hormone vasopressin. The ever-ready members of the
security system never go into action, as long as they receive "all clear" mes-
sages from the center where the system is established. 1
So when does this security firm, the pituitary gland, go into action? In
the event of any serious bleeding, the body loses a lot of blood, and the vol-

ume of blood—and hence, the blood pressure—in the veins is reduced, pre-
senting a grave danger for the patient.
The moment that blood pressure falls, the receptors in the blood ves-
sels send a signal to the pituitary gland, and the heart is interrupted. This
causes the pituitary gland to sound the alarm and to secrete the hormone
vasopressin (Figure 12). When the pituitary gland realizes that the signal
has been interrupted, it immediately displays what seems to be an excep-
tionally conscious piece of behavior—amazing, since this apparently con-
scious behavior is entirely performed by tiny molecules composed of
combinations of particular atoms.
The hormone vasopressin causes the muscles around the blood ves-
sels to contract immediately, which causes blood pressure to rise. This sys-
tem is actually highly complex, consisting of a number of inter-related
components, suggests a great many questions worthy of reflection.
How do the hypothalamus cells that generate the hormone vaso-
pressin know the structure of the muscle cells in the blood vessels located
so far away from them?
How have they learned that these vessels must be contracted in order

for blood pressure to rise?
How can these cells produce the exact chemical formula that enables
this contraction to take place?
How did such a flawless alarm system along the nerves connecting
the heart and the pituitary gland come into being?

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