Walls of the Heart
The walls of the heart consist of layers of muscle, wound several times
around the atria and the ventricles in a complicated arrangement. The
layers of muscle around the ventricles are thicker than those around the
atria. The heart muscle pumps blood by its cyclic contractions and
relaxation. The contracted phase of the heart is called systole, and the
relaxed phase is called diastole. Each contraction begins in the walls of
the atria, and this squeezes the blood from the atria into the ventricles;
a moment later, the walls of the ventricle contract. Squeezing the blood
out of the heart into the ventricles; a moment later, the walls of the
ventricle contract, squeezing the blood out of the heart into the arteries.
Since the walls of both atria contract jointly, and the walls of both
ventricles also contract jointly, the right and the left pumps in the heart
always operate in unison. The layers of muscle around the left ventricle
are much thicker than those around the right ventricle. The left
ventricle generates the highest pressures and does most of the
mechanical work.
Under conditions of rest, the heart typically goes through 70 cycles of
contraction and relaxation per-minute, that is, 70 heartbeats per minute.
Each contraction lasts about 0.3s, and each relaxation about 0.5s.
However, under conditions of heavy exercise or stress, the heart rate
may be as high as 180 heartbeats per minute. The rate of flow of blood
through each side of the heart is 5.5 liters per minute at rest. Trained
athletes attain a rate of flow of up to 35 liters per minute, during heavy
exercise. The total volume of blood -
in the human body is 5 to 6 liters, and therefore a rate of flow of 5.5
liters per minute implies that, on average, a parcel of blood takes
just about a minute to travel around the complete cardiovascular
circuit. Since the blood flows on a closed circuit, without any loss of
blood, the rate of flow must be the same everywhere along the circuit.
Thus, if 5.5 liters per minute flow through the cavel venis, then the
same amount must flow through the systemic capillaries. However, the
speed of flow is different at different points of the circuit, as required
by the equation of continuity. The mean speed is about 0.2 m/s in the
aorta, but it is much lower in the capillaries, only about 0.3 mm/s.
The pressure of the blood in the arteries fluctuates during each stroke
of the pump. At the base of the aorta, the pressure during systole
reaches 120 mm-Hg. and during diastole it falls to about 80 mm-Hg.
The mean pressure is about 100 mm-Hg Table- 2. The pressure in the
veins fluctuates much less; in the vena cava, near the heart, the
pressure is nearly steady and nearly zeros. Thus, the mean pressure
difference across the systemic segment of the circulatory system is
about 100mm-Hg. A pressure difference is required to balance the
viscous resistance of blood and to maintain a more or less steady rate
of flow. Blood has a viscosity of 2.1 × 10-3s. N/m2, about three times
that of water. According to Poiseuille’s equation, the pressure drop
along a tube, such as an artery or a vein, is directly proportional to the
viscosity, the length, and the rate of flow, and inversely proportional to
the fourth power of the radius of the tube.
Most of the pressure drop actually occurs in the small arteries, or
arterioles, that connect to the capillaries. In the capillaries themselves,
the pressure drop is not so large, even though these are the smallest
vessels in the circulatory system, with a radius of only 3× 10-6m. The
arterioles have a larger radius than the capillaries, but they also have a
larger length and a higher rate of flow, and because of this they
contribute a larger pressure drop than the capillaries.
