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C HAPTER 2 / Systemic and Pulmonary Circulation and Oxygen Delivery 43
Artery Vein
Elastic tissue
Tunica interna
Vein
V V
Tunica media Vein
Tunica externa
Blood flow
Arteriole Venule
Capillary
Endothelium
■ Figure 2-1 Schematic drawing of the major structural characteristics of the principal segments of blood
vessels. The relative amounts of elastic tissue and fibrous tissues are largest in the aorta and least in small
branches of the arterial tree. Small vessels have more prominent smooth muscle in the media. Capillaries con-
sist only of endothelial cells. The walls of the veins are much like the arterial walls, but are thinner in relation
to their caliber. (From iDAMS 3746-08-17.)
Capillaries Continuous endothelium is found in skin; skeletal, smooth, and
Capillaries branch from terminal arteriolar segments. The capil- cardiac muscle; and the lungs. There are several mechanisms by
lary wall consists of endothelial cells and basal lamina; there is no which substances pass through continuous endothelium. Water
tunica media or adventitia. Capillary diameter is 4 to 8 mm, and solutes pass through intercellular junctions (40 to 1 Å) driven
P
P
which is just large enough to allow the deformable red blood cells predominantly by a pressure gradient ( P) driving fluid out of the
to pass through. 14,16 Not all exchange vessels in an area are si- vessels. This outward flow is partly counterbalanced by forces
multaneously open. During periods of increased metabolism, cap- drawing water back into the vessels. Lipid-soluble substances
illary recruitment increases the number of open and perfused ex- (CO 2 , O 2 ) pass directly through the cell by diffusion; cytoplasmic
change vessels, thereby decreasing the distances for diffusion vesicles transport solutes and water back and forth through the
between exchange blood vessels and cells, as well as increasing the endothelium; and vesicles intermittently fuse to create channels in
total surface area for exchange between the capillaries and cells. 13 the cell. The junctions between the cells are responsible for the
In microvascular beds located in the ears, fingers, and toes in high permeability of the membrane to “ultrafiltrate,” or protein-
humans and many other mammals, there are arteriovenous vascu- free fluid, and for the rapid diffusion of small ions. The continu-
lar channels that bypass the exchange vessels and allow blood to ous endothelium is relatively impermeable to plasma proteins and
flow directly from arterioles to venules. 13 These arteriovenous large molecules.
anastomoses, which are richly innervated by the sympathetic Fenestrated vascular endothelium is located in the gastrointesti-
nervous system, are important in local temperature control in nal mucosa, glands, renal glomerular capillaries, and peritubular
these areas and even of the whole body in some conditions. 17 capillaries. The endothelium has openings (fenestrae) that expose
the basement membrane (renal glomerular capillaries) or are cov-
Exchange Vessel Endothelium ered by a thin diaphragm (gastrointestinal mucosa, renal per-
The endothelium of exchange vessels in various organs contains at itubular capillaries). The fenestrated endothelium has a higher
least four different structures that determine the rate of filtration permeability to water and small solute molecules than continuous
and bulk transport of water and solutes and the exchange of larger endothelium, whereas its permeability to plasma proteins is low,
molecules (Fig. 2-2). The structure of the membrane (continuous, similar to continuous endothelium. 13
fenestrated, discontinuous and tight junction) varies depending Discontinuous endothelium is located in the hepatic cells, bone
on the location of the vascular bed. 18 All four types of endothe- marrow, and splenic sinusoids. Discontinuous endothelium con-
lium have a continuous basement membrane, with the exception tains gaps in the endothelium and basement membrane and is
of the discontinuous endothelium. permeable to proteins and other large molecules.