Biodynamics of vertebrate circulation. Control of heartbeat
and circulation
Many factors, such as temperature, oxygen supply, or
nervous excitement, affect heartbeat and circulation.
Blood circulation is controlled mainly via nerve connections,
sensory receptors, and hormones. These act primarily
by varying the heart's pulse rate, amplitude, or stroke
volume and by altering the degree of dilation or constriction
of the peripheral blood vessels (i.e., those blood vessels
near the surface of the body).
Temperature has a direct effect on heart rate, and
one of the ways in which mammals regulatetheir internal
temperature is by controlling peripheral blood circulation.
Mammals are endothermic (warm-blooded) vertebrates;
their internal temperature is kept within narrow limits
by using heat generated by the body's own metabolic
processes. Lizards are ectothermic (cold-blooded); they
obtain heat from the external environment by, for example,
basking in the sun. The effects of oxygen concentration
on the heart and blood vessels is rapid. Oxygen deficiency
in the cardiac tissue causes dilation of the coronary
capillaries, thereby increasing blood flow and oxygen
supply.
Most effects on the circulation are indirect and complex.
All vertebrate hearts receive inputfrom nerves; for
example, stimulation of a branch of the vagus nerve
causes the release of acetylcholine at the nerve endings,
which depresses the heart rate. Other nerve endings
release norepinephrine, which increases the heart rate.
Less directly, nervous stimulation brought about by
stress causes the release of the hormones epinephrine
and norepinephrine into the bloodstream. These substances
not only make the heart beat faster and with a greater
amplitude, but they also divert blood to the muscles
by constricting the vessels in theskin and gut. This
prepares the animal physiologically for physical exertion.
Numerous other chemicals, such as nicotine, affect heart
rate directly or indirectly.
Two other factors are important in the context of circulatory
regulation—the concentrations of inorganic ions and
sensory receptors in blood vessel walls. Sodium, potassium,
and calcium ions are always involved in changes of electrical
potential across cell membranes. A change in their concentrations,
therefore, influences heartbeat profoundly. External
calcium concentration can, for example, determine the
conductance of sodium across the cardiac cell membranes.
Sensory receptors in the walls of blood vessels register
blood pressure. They arefound in the aorta, carotid
arteries, pulmonary artery, capillaries in the adrenal
gland, and the tissues of the heart itself. Impulses
from the receptors travel to the medulla of the brain,
fromwhere messages are sent via motor nerves to the
heart and blood vessels. Regulation is thus achieved
according to the body's needs.
