[box type=”download”] Appreciation of how cardiovascular components are suited to their primary functions: Veins as thin-walled capacitance and return vessels Arteries as elastic pulsation–dampening distribution vessels Capillaries and venules as sites of exchange and return Normal adult values for total blood volume, cardiac output and stroke volume Mean arterial pressure [MAP] as (diastolic + one-third pulse) pressure and rationale for this Values of mean blood pressure before and after capillary beds and of the CVP [/box]
The vascular system consists of arteries and arterioles, thin-walled capillaries that allow the diffusion of gases and metabolites, and venules and veins.
Varying amounts of smooth muscle are contained within the vessel walls.
Capillaries contain no smooth muscle.
The inner surface of all blood vessels is lined with a thin monolayer of endothelial cells.
Large arteries are elastic and partially damp out oscillations in pressure; stiff arteries (age, atherosclerosis) result in larger oscillations.
Small arteries contain relatively more muscle and are responsible for controlling tissue blood flow.
Veins have a larger diameter than equivalent arteries, and provide less resistance.
They have thin, distensible walls and contain ∼70% of the total blood volume.
Large veins are known as capacitance vessels and act as a blood volume reservoir.
Large veins in the limbs contain one-way valves, and muscle activity acts as a pump assisting the return of blood to the heart.
The heart has an intrinsic pacemaker and requires no nervous input to beat normally, although modulated by the autonomic nervous system.
The volume of blood pumped per minute (cardiac output) is ∼5 L at rest in humans, although this can increase to above 20 L during exercise.
The volume ejected per beat (stroke volume) is ∼70 mL at rest.
Contraction of the heart is called systole; the period between each systole, when the heart refills with blood, is called diastole.
The systemic circulation
During systole, the pressure in the left ventricle increases to ∼120 mmHg, and blood is ejected into the aorta.
Systolic pressure is the maximum arterial pressure during systole (∼110 mmHg).
During diastole, arterial blood flow is partly maintained by elastic recoil of the walls of large arteries.
The minimum pressure reached before the next systole is the diastolic pressure (∼80 mmHg).
The difference between the systolic and diastolic pressures is the pulse pressure.
The mean arterial pressure, (MAP) cannot be calculated by averaging these pressures, because for ∼60% of the time the heart is in diastole.
So, MAP is estimated as the diastolic pressure plus one-third of the pulse pressure, e.g. 80 + 1/3(110 − 80) ≈ 90 mmHg.
The major arteries divide repeatedly – the smallest (<100 μm) are called arterioles which regulate tissue blood flow – (resistance vessels).
The mean blood pressure at the start of the arterioles is ∼65 mmHg.
The arterioles divide into capillaries in the tissues, and these rejoin into venules, the smallest veins.
Capillaries and small venules provide the exchange surface between blood and tissues, contain no smooth muscle and are called exchange vessels; some gas exchange also occurs across the walls of small arterioles.
The pressure on the arterial side of capillaries is ∼25 mmHg and, on the venous side, ∼15 mmHg.
Venules converge into veins and finally the vena cava and to the right atrium.
The pressure in the vena cava at the level of the heart is called the central venous pressure (CVP), and is close to 0 mmHg.
The pulmonary circulation
The right atrium helps to fill the right ventricle, which pumps blood into the pulmonary artery and lungs.
The pulmonary circulation is shorter than the systemic, and has a lower resistance to flow.
Less pressure is therefore required to drive blood through the lungs; the pulmonary artery pressure is ∼20/15 mmHg.
Gas exchange occurs in capillaries surrounding the alveoli (small air sacs) of the lungs.
These rejoin to form pulmonary venules and veins, and oxygenated blood is returned through the pulmonary vein to the left atrium, and thence to the left ventricle.
The metabolic requirements of the lungs are met by the separate bronchial circulation, the venous outflow (de-oxygenated blood) of which returns to the left side of the heart.