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Circulatory system

(systema cardiovasculare et

Iymphovasculare)

The role of the circulatory system is to maintain uninter- the heart. Capillaries have the simplest wall structure and
rupted flow of blood around the body, thus permitting are therefore described first.
constant exchange of oxygen and carbon dioxide, uptake of
nutrients and elimination of waste. The circulatory system Cardiovascular system (systema
also serves to transport ions, hormones and enzymes, and cardiovasculare)
contributes to thermoregulation. The blood vascular (car-
diovascular) system is comprised of the heart, as its central Capillaries (vas capillare)
organ, and the blood vessels (arteries, arterioles, capillaries, Capillaries form a finely branched, metabolically active
venules and veins). Together they form a closed circulatory network between the smallest arterioles and the postcap-
system. Connected to the blood vascular system is a system illary venules. These thin-walled tubular vessels (average
of lymph vessels that drain lymph from the tissues. diameter 7–9 μm) permit the passage of blood cells and
The heart can be regarded as a highly differentiated seg- plasma as well as metabolic products into the connective
ment of vessel wall with internal divisions that give rise tissue (interstitium). In addition, capillaries take up blood
to a smaller pulmonary and a larger systemic circulation. cells and interstitial fluid from the loose connective tissue,
Rhythmic contraction of the muscular wall results in cir- conveying these via postcapillary venules into the circulat-
culation of blood. ing blood (Figures 6.1 to 6.5). A capillary consists of:
The vessels travelling away from the heart, the arter-
ies, serve as the conduit for blood and its soluble contents · simple squamous epithelium (endothelium) that
to the peripheral tissues and organs. Arteries divide exten- forms the inner lining of the tubular vessel and
sively into vessels with decreasing diameter. The smaller · a basal lamina (complete or incomplete) externally
arterioles undergo multiple divisions before opening into adjacent to the endothelium; in certain capillaries
dense networks of capillaries, in which gases and meta- this may be absent over segments of the capillary
bolic products are exchanged. Postcapillary segments wall (sinusoids) (Figure 6.2).
converge to form venules (diameter 0.2–1 mm) which
empty into vessels of increasing diameter – the small, Lying adjacent to the outer surface of most capillaries
medium and large veins. Venules and veins return the are perivascular cells called pericytes, that are partially
blood to the heart. enclosed by a basal lamina. These are undifferentiated
For exchange of substances to occur in the capillary mesenchymal cells that retain a significant capacity for
bed, blood must flow at relatively slow speeds and low transformation (e.g. into macrophages, fibroblasts).
pressure. Thus, pressure in capillaries is reduced to 20–35 The structure of the capillary wall is closely related to
mmHg. The veins into which blood passes from the cap- the metabolic activity of the tissue (see also Table 6.1).
illaries constitute a low-pressure system (5–10 mmHg). Passage of substances across the capillary wall can occur
A high-pressure system exists near the heart and in the through transcellular or intercellular pathways. In the
arterial circulation. Mean pressures in the high-pressure transcellular route, substances are transported through the
circulation vary considerably with species (65–140 mmHg). cytoplasm in individual or aggregated pinocytotic vesicles
The structure of the vessel walls varies to accommodate (transcytosis, cytopempsis). This pathway is used for the
the requirements of this dynamic system. In particular, the transfer of water-soluble and larger molecules through
flow of blood is influenced by differences in the degree the endothelium. New vesicles (up to 1000/mm ) are con-
2
of muscle development. The walls of the capillaries are tinuously produced. After passing through the cytoplasm,
devoid of muscle, while muscle tissue forms the bulk of they fuse with the plasmalemma of the endothelial cell and

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