BIOC63H3 Chapter Notes -Cardiac Muscle Cell, Vascular Smooth Muscle, Aortic Body
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Overview of the Cardiovascular System (Figure 1)
The cardiovascular system is composed of the heart, blood vessels and blood. In simple
terms, its main functions are:
1. Distribution of 02 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.
Figure 1 illustrates the „plumbing‟ of the cardiovascular system.
Blood is driven through the cardiovascular system by 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 served 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 systematic 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 differences between systolic and diastolic pressure is termed the
pulse pressure. Mean aortic pressure is pressure averaged over the entire cardiac
cycle. Because the heart spends approximately 60% of the cardiac cycle in diastole, the
mean aortic pressure is approximately equal to the diastolic pressure + one-third of the
pulse pressure, rather than to the arithmetic average of the systolic and diastolic
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 um.
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
The arterioles lead to the smallest vessels, the capillaries, which from 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 many 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 10mmHg,
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.
Blood vessel functions
Each vessel 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
The smallest arteries and arterioles play 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 contriction 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 bloodflow, 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 are able
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 hemorrhage (blood loss) occurs.