Fig. 2.1
Components of the cardiovascular system; systemic circulation is shown in white, pulmonary circulation in grey
2.1.2 Components of the Cardiovascular System
Heart
The structure of the heart is illustrated in Fig. 2.2. The heart has two main phases; a contraction phase when blood is ejected from the left and right ventricles, and a relaxation phase when the chambers fill with blood returning via the venous system. The heart valves prevent backflow and operate in a passive manner associated with pressure differences. During ejection, the aortic and pulmonary valves are open and the tricuspid and mitral valves closed. During filling, the aortic and pulmonary valves are closed and the other two valves are open. The left ventricle ejects blood into the relatively high-pressure systemic system, hence has a much thicker wall then the right ventricle, which ejects blood into the low-pressure pulmonary system.
Fig. 2.2
Principal components of the heart. Reproduced from Wikipedia according to the GNU Free Documentation License; diagram authored by Wapcaplet and Yaddah. https://commons.wikimedia.org/wiki/File:Diagram_of_the_human_heart_(cropped).svg
Composition of vessels
The vessels are composed of three layers, as shown in Fig. 2.3. These layers are:
Adventitia. The outermost layer, primarily consisting of collagen fibres layered in a spiral fashion.
Media. A layer consisting of smooth muscle, elastin sheets (layered circumferentially) and collagen fibres.
Intima. The innermost layer consisting of a single layer of endothelial cells. These line the lumen, and hence are in contact with flowing blood. There is also a basement membrane immediately beneath the endothelium.
Fig. 2.3
Principal components of vessel wall. Note that vessel curvature is not to scale. Images reproduced from ‘Structure and function of blood vessels’ Openstar; under a creative commons licence http://creativecommons.org/licenses/by/3.0/legalcode. Download for free at http://cnx.org/contents/58db2cce-b3d9-4904-9049-80a6cd89264b@4
From a mechanical perspective, the two main constituents of the vessel wall are elastin and collagen. These are considered further in Chap. 4. Elastin is highly deformable with a low Young’s modulus whilst collagen has a nonlinear behaviour with high values of Young’s modulus and a high breaking strength. The ratio of elastin to collagen is the principal determinant of the overall elastic behaviour of a vessel. If the ratio is high, the vessel is elastic and deforms under increasing pressure. If the ratio is low, the vessel does not deform much under pressure.
Types of vessel
The vessels shown in Fig. 2.3 can be grouped and are described in this section. The microscopic structures of the different types of vessel are closely linked to the vessel’s specific function.
Arteries (diameter 1–30 mm). Arteries carry blood away from the heart. Systemic arteries must withstand relatively high pressures and so have thick walls consisting of the three basic layers described earlier. Arteries are subcategorised, on the basis of their wall composition, into elastic and muscular arteries. Elastic arteries such as the aorta and its major branches are low resistance vessels and have a high elastin/collagen ratio. The high elastin content results in high distensibility. This allows them to accommodate the volume of blood ejected from the heart and also enables the storage of energy. More distal arteries, such those supplying the organs and those in the leg and arms, are muscular in nature. Muscular arteries have a thicker medial layer which has less elastin and more smooth muscle than that of elastic arteries. These vessels are also known as distributive arteries.
Arterioles (diameter 10–100 μm). Arterioles have all three layers (adventitia, media and intima) but of much reduced thickness compared to arteries. Proportionally, the thickness of the media is large consisting mainly of smooth muscle cells. These enable the lumen size to be controlled over a wide range. Constriction and dilatation of the arterioles controls the flow to capillaries. Diameter ranges from 100 μm to around 10 μm for the smallest (terminal) arterioles.
Capillaries (diameter 4–40 μm). These have a very thin wall consisting of only endothelium and basement membrane. There are three types of capillary. The most common are the continuous capillaries; these are found in skin and muscle. Whilst the endothelial cells of these capillaries are closely coupled by tight junctions, small gaps are present which control the passage of fluids and small molecules. Fenestrated capillaries have pores (fenestrations), which give greater permeability to fluids and allow certain small molecules to pass through. They are found in the intestine and kidney. Sinusoidal capillaries are the least common and have large gaps allowing greater volume of materials to pass through. These are found in the liver, spleen and endocrine glands, for example. The diameter of continuous and fenestrated capillaries is in the range 4–10 μm, however sinusoidal capillaries can have much larger diameters of up to 40 μm. Capillaries are around one mm in length.
Venules (diameter 10–200 μm). These have all three layers but are much thinner than arterioles with an almost absent medial layer.
Veins (diameter 1–25 mm). Veins return the blood to the heart. The venous system has a much lower pressure than the arterial system and consequently the wall thickness of veins is much less than that of arteries. Intermediate sized veins contain valves, which prevent backflow of blood. Larger veins including the vena cava do not. The adventitial layer is thicker than the medial layer and the elastin/collagen ratio is small compared to arteries. This makes veins relatively stiff when fully distended, but when veins are under low or negative pressure they may collapse.
2.1.3 Functions of the Cardiovascular System
The cardiovascular system has several functions; transport of molecules, defence and healing, thermoregulation and maintenance of fluid balance between different tissues in the body.
Transport of molecules
The cardiovascular system transports molecules from one vascular bed to another. Entry and exit of molecules into the cardiovascular system occurs through the walls of the capillaries. For example, oxygen is transported from the pulmonary vascular bed to vascular beds all over the body where oxygen is needed for metabolism. Carbon dioxide is a waste product of metabolism and is made in tissues all over the body. Carbon dioxide is transported from vascular beds to the lungs, where it is discharged into the alveoli. Glucose, amino acids, vitamins and minerals are discharged into the blood from the vascular beds of the gastrointestinal tract. As far as transport of molecules is concerned, the function of the rest of the cardiovascular system is to provide passage of molecules from one capillary bed to another.
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