The cardiovascular system
T
he
cardiovascular system is divided for descriptive purposes into two
main parts.
The circulatory system, consisting of the heart, which acts as a pump, and the blood vessels through which the blood circulates
The lymphatic system, consisting of lymph nodes and lymph vessels, through which colourless lymph flows (see Ch. 6).
The two systems communicate with one another and are intimately associated.
The heart pumps blood into two anatomically separate systems of blood vessels (Fig. 5.1).
the pulmonary circulation
the systemic circulation.
T
he
right side of the heart pumps blood to the lungs (the pulmonary
circulation) where gas exchange occurs; i.e. C02
leaves the blood and enters the lungs, and 02
leaves
the lungs and enters the blood. The left side of the heart pumps
blood into the systemic circulation, which supplies the rest of the
body. Here, tissue wastes are passed into the blood for excretion,
and body cells extract nutrients and 02.
The circulatory system ensures a continuous flow of blood to all body cells, and its function is subject to continual physiological adjustments in order to maintain
an adequate blood supply. Should the supply of oxygen and nutrients to body cells become inadequate, tissue damage occurs and cell death may follow.
Blood vessels
Arteries and arterioles
T
hese
are the blood vessels that transport blood away from the heart. They
vary considerably in size and their walls consist of three layers of
tissue (Fig. 5.3):
tunica adventitia or outer layer of fibrous tissue
tunica media or middle layer of smooth muscle and elastic tissue
tunica intima or inner lining of squamous epithelium called endothelium.
The
amount of muscular and elastic tissue varies in the arteries
depending upon their size and function. In the large arteries,
sometimes called elastic arteries, the tunica media consists of more
elastic tissue and less smooth muscle. This allows the vessel wall to
stretch, absorbing the pressure w
ave
generated by the heart as it beats. These proportions gradually
change as the arteries branch many times and become smaller until in
the arterioles
(the
smallest arteries) the tunica media consists almost entirely of
smooth muscle. This enables their diameter to be precisely
controlled, which regulates the pressure within them. Systemic blood
pressure is mainly determined by the resistance these tiny arteries
offer to blood flow, and for this reason they are called resistance
vessels.
Arteries have thicker walls than veins and this enables them to withstand the high pressure of arterial blood.
Anastomoses and end-arteries
Anastomoses are arteries that form a link between main arteries supplying an area, e.g. the arterial supply to the palms of the hand (p. 100) and soles of the feet, the brain, the joints and, to a limited extent, the heart muscle. If one artery supplying the area is occluded, anastomotic arteries provide a collateral circulation. This is most likely to provide an adequate blood supply when the occlusion occurs gradually, giving the anastomotic arteries time to dilate.
End-arteries are the arteries with no anastomoses or those beyond the most distal anastomosis, e.g. the branches from the circulus arteriosus (circle of Willis) in the brain or the central artery to the retina of the eye. When an end-artery is occluded the tissues it supplies die because there is no alternative blood supply.
Veins and venules
Veins are blood vessels that return blood at low pressure to the heart. The walls of the veins are thinner than those of arteries but have the same three layers of tissue
(Fig. 5.3). They are thinner because there is less muscle and elastic tissue in the tunica media, because veins carry blood at a lower pressure than arteries. When cut, the veins collapse while the thicker-walled arteries remain open.
When an artery is cut blood spurts at high pressure while a slower, steady flow of blood escapes from a vein.
Some veins possess valves, which prevent backflow of blood, ensuring that it flows towards the heart (Fig. 5.4). They are formed by a fold of tunica intima and strengthened by connective tissue. The cusps are semilunar in shape with the concavity towards the heart. Valves are abundant in the veins of the limbs, especially the lower limbs where blood must travel a considerable distance against gravity when the individual is standing. They are absent in very small and very large veins in the thorax and abdomen. Valves are assisted in maintaining one-way flow by skeletal muscles surrounding the veins (p. 88).
The smallest veins are called venules.
Veins are called capacitance vessels because they are distensible, and therefore have the capacity to hold a large proportion of the body's blood. At any one time, about two-thirds of the body's blood is in the venous system. This allows the vascular system to absorb (to an extent) sudden changes in blood volume, such as in haemorrhage; the veins can recoil, helping to prevent a sudden fall in blood pressure.