Page 194 - The Miracle of the Blood and Heart

THE MIRACLE OF THE
BLOOD AND HEART

the cell membrane removes sodium faster than it takes in
potassium, an excess positive charge builds outside the cell.
When it reaches a certain level, the flow suddenly reverses
and sodium ions again begin entering the cells. This sudden
change sets up an electrical current, and the heart cell retreats
by contracting. 92 The process that allows the cells to beat

emerges through these chemical transactions.
The signal for the heartbeat to start depends on one small
knot of cells known as the sinus node, or SA node, in the right
atrium. The electrical signal produced by this node is trans-

Capillary

Intercalated
discs

Sarcolemma
Cardiac muscle cell
Nucleus
a) Diagram
Cardiac muscle
Interca-
lated
discs
Nuclei
Heart
muscle
fiber

b) Photomicrograph

The cardiac muscle possesses a very special structure, in that every one
of the cells comprising it can beat on its own. These cells are just like liv-
Harun ing batteries, using the electrical current that passes over them.
Yahya

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