Biodynamics of vertebrate circulation. Blood pressure
and blood flow
The pressure that develops within the closed vertebrate
circulatory system is highest at the pump—the heart—and
decreases with distance away from the pump because of
friction within the blood vessels. Because the blood
vessels can change their diameter, blood pressure can
be affected by both the action of the heart and changes
in the size of the peripheral blood vessels. Blood is
a living fluid—it is viscous and contains cells (45
percent ofits volume in human beings)—and yet the effects
of the cells on its flow patterns are small.
Blood enters the atrium by positive pressure from the
venous system or by negative pressuredrawing it in by
suction. Both mechanisms operate in vertebrates. Muscular
movements of the limbs and body, and gravity in land
vertebrates, are forces propelling blood to the heart.
In fishes and amphibians the atrium forces blood into
the ventricle when it contracts. In birds and mammals
the blood arrives at the heart with considerable residual
pressure and passes through the auricles into the ventricles,
apparently without much additional impetus from contraction
of the auricles.
The ventricle is the main pumping chamber, but one
of the features of double circulation is that the two
circuits require different pressure levels. Although
the shorter pulmonary circulation requires less pressure
than the much longer systemic circuit, the two are connected
to each other and must transport the same volume of
fluid per unit time. The right and left ventricles in
birds and mammals function as a volume and a pressure
pump, respectively. The thick muscular wall of the left
ventricle ensures that it develops a higher pressure
during contraction in order to force blood through the
body. It follows that pressures in the aorta and pulmonary
artery may be very different. In human beings aortic
pressure is about six times higher.
Valves throughout the system are crucial to maintain
pressure. They prevent backflow at all levels; for example,
they prevent flow from the arteries back into the heart
as ventricular pressure drops at the end of a contraction
cycle. Valves are important in veins, where the pressure
is lower than in arteries.
Another impetus to blood flow is contraction of the
muscles in the walls of vessels. This also prevents
backflow of arterial blood toward the heart at the end
of each contraction cycle. Input from nerves, sensory
receptors in the vessels themselves, and hormones all
influence blood vessel diameter, but responses differ
according to position in the body and animal species.
Normally, the pressures that develop in a circulatory
system vary widely in different animals. Body size can
be an important factor. The closed circulation systems
of vertebrates generally operate at higher pressures
than the open blood systems of invertebrates; the systems
of birds and mammals operate at the highest pressures
of all.
