Overview
Of The Cardiovascular System.
The cardiovascular system
is composed of the heart, blood vessels and blood. In simple terms, its main
functions are:
1 Distribution
of O2 and nutrients (e.g. Glucose, amino acids) to all body tissues
2 Transportation
of CO2 and metabolic waste products (e.g. Urea) from the tissues to
the lungs and excretory organs
3 Distribution
of water, electrolytes and hormones throughout the body
4 Contributing
to the infrastructure of the immune system
5 Thermoregulation.
Blood is composed of plasma, an aqueous solution containing
electrolytes, proteins and other molecules, in which cells are
suspended. The cells comprise 40–45% of blood volume and are mainly erythrocytes,
but also white blood cells and platelets. Blood volume is about
5.5 L in an ‘average’ 70-kg man.
Figure 1 illustrates the ‘plumbing’ of the cardiovascular system. Blood
is driven through the cardiovascular system by the heart, a muscular pump divided into left and
right sides. Each side contains two chambers, an atrium and a ventricle,
composed mainly of cardiac muscle cells. The thin-walled atria serve to fill or
‘prime’ the thick-walled ventricles,
which when full constrict forcefully, creating a pressure head that drives the
blood out into the body. Blood enters and leaves each chamber of the heart
through separate one-way valves, which open and close reciprocally (i.e. one
closes before the other opens) to ensure that flow is unidirectional.
Consider the flow of blood, starting with its exit from the left
ventricle.
When the ventricles contract, the left ventricular internal pressure
rises from 0 to 120 mmHg (atmospheric pressure = 0). As the pressure rises, the
aortic valve opens and blood is expelled into the aorta, the first and
largest artery of the systemic circulation. This period of ventricular
contraction is termed systole. The maximal pressure during systole is
called the systolic pressure, and it serves both to drive blood through
the aorta and to distend the aorta, which is quite elastic. The aortic valve
then closes, and the left ventricle relaxes so that it can be refilled with
blood from the left atrium via the mitral valve. The period of relaxation is
called diastole. During diastole aortic blood flow and pressure diminish
but do not fall to zero, because elastic recoil of the aorta continues
to exert a diastolic pressure on the blood, which gradually falls to a
minimum level of about 80 mmHg. The difference between systolic and diastolic
pressures is termed the pulse pressure. Mean arterial blood pressure (MABP) is pressure
averaged over the entire cardiac cycle. Because the heart spends approximately
60% of the cardiac cycle in diastole, the MABP is approximately equal to the
diastolic pressure + one-third of the pulse pressure, rather than to the
arithmetic average of the systolic and diastolic pressures.
The blood flows from the aorta into the major arteries, each of
which supplies blood to an organ or body region. These arteries divide and
subdivide into smaller muscular arteries, which eventually give rise to
the arterioles – arteries with diameters of <100 µm. Blood enters the
arterioles at a mean pressure of about 60– 70 mmHg.
The walls of the arteries and arterioles have circumferentially arranged
layers of smooth muscle cells. The lumen of the entire vascular system
is lined by a monolayer of endothelial cells. These cells secrete
vasoactive substances and serve as a barrier, restricting and controlling the
movement of fluid, molecules and cells into and out of the vasculature.
The arterioles lead to the smallest vessels, the capillaries,
which form a dense network within all body tissues. The capillary wall is a
layer of overlapping endothelial cells, with no smooth muscle cells. The
pressure in the capillaries ranges from about 25 mmHg on the arterial side to
15 mmHg at the venous end. The capillaries converge into small venules,
which also have thin walls of mainly endothelial cells. The venules merge into
larger venules, with an increasing content of smooth muscle cells as they
widen. These then converge to become veins, which progressively join to
give rise to the superior and inferior venae cavae, through which
blood returns to the right side of the heart. Veins have a larger diameter than
arteries, and thus offer relatively little resistance to flow. The small pressure gradient between venules
(15 mmHg) and the venae cavae (0 mmHg) is therefore sufficient to drive blood
back to the heart.
Blood from the venae cavae enters the right atrium, and then the right
ventricle through the tricuspid valve. Contraction of the right
ventricle, simultaneous with that of the left ventricle, forces blood through
the pulmonary valve into the pulmonary artery, which progressively subdivides
to form the arteries, arterioles and capillaries of the pulmonary
circulation. The pulmonary circulation is shorter and has a much lower
pressure than the systemic circulation, with systolic and diastolic pressures
of about 25 and 10 mmHg, respectively. The pulmonary capillary network within
the lungs surrounds the alveoli of the lungs, allowing exchange of CO2
for O2. Oxygenated blood enters pulmonary venules and veins, and
then the left atrium, which pumps it into the left ventricle for the
next systemic cycle.
The output of the right ventricle is slightly lower than that of the left
ventricle. This is because 1-2% of the systemic blood flow never reaches the
right atrium, but is shunted to the left side of the heart via the bronchial
circulation (Figure 1) and a small fraction of coronary blood flow drains into
the thebesian veins (see Chapter 2).
Blood vessel functions
Each vessel type has important functions in addition to being a conduit
for blood.
The branching system of elastic and muscular arteries progressively
reduces the pulsations in blood pressure and flow imposed by the intermittent
ventricular contractions.
The smallest arteries and arterioles have a crucial role in regulating
the amount of blood flowing to the tissues by dilating or constricting. This
function is regulated by the sympathetic nervous system, and factors generated
locally in tissues. These vessels are referred to as resistance arteries,
because their constriction resists the flow of blood.
Capillaries and small venules are the exchange vessels. Through
their walls, gases, fluids and molecules are transferred between blood and
tissues. White blood cells can also pass through the venule walls to fight
infection in the tissues.
Venules can constrict to offer resistance to the blood flow, and the
ratio of arteriolar and venular resistance exerts an important influence on the
movement of fluid between capillaries and tissues, thereby affecting blood
volume.
The veins are thin walled and very distensible, and therefore
contain about 70% of all blood in the cardiovascular system. The arteries
contain just 17% of total blood volume. Veins and venules thus serve as volume
reservoirs, which can shift blood from the peripheral circulation into the
heart and arteries by constricting. In doing so, they can help to increase the cardiac
output (volume of blood pumped by the heart per unit time), and they are
also able to maintain the blood pressure and tissue perfusion in essential
organs if haemorrhage (blood loss) occurs.
