Capillaries and sinusoids

The smallest arterioles break up into a number of minute vessels called capillaries. Capillary walls consist of a single layer of endothelial cells sitting on a very thin basement membrane, through which water and other small-molecule substances can pass. Blood cells and large-

molecule substances such as plasma proteins do not normally pass through capillary walls. The capillaries form a vast network of tiny vessels that link the smallest arterioles to the smallest venules. Their diameter is approximately that of an erythrocyte (7 jam). The capillary bed is the site of exchange of substances between the blood and the tissue fluid, which bathes the body cells.

Entry to capillary beds is guarded by rings of smooth muscle {precapillary sphincters) that direct blood flow. Hypoxia (low levels of oxygen in the tissues), or high levels of tissue wastes, indicating high levels of activity, dilate the sphincters and increase blood flow through the affected beds.

S inusoids are wider than capillaries and have extremely thin walls separating blood from the neighbouring cells. In some there are distinct spaces between the endothelial cells. Among the endothelial cells there may be many phagocytic macrophages, e.g. Kupffer cells in the liver. Sinusoids are found in bone marrow, endocrine glands, spleen and liver. Because of their larger lumen, the blood pressure in sinusoids is lower than in capillaries, and blood flow is slower.

Blood supply

The outer layers of tissue of thick-walled blood vessels receive their blood supply via a network of blood vessels called the vasa vasorum. Vessels with thin walls and the endothelium of the others receive oxygen and nutrients by diffusion from the blood passing through them.

Heart

The heart is a roughly cone-shaped hollow muscular organ. It is about 10 cm long and is about the size of the owner's fist. It weighs about 225 g in women and is heavier in men (about 310 g).

Position

T he heart lies in the thoracic cavity (Fig. 5.9) in the mediastinum (the space between the lungs). It lies obliquely, a little more to the left than the right, and presents a base above, and an apex below. The apex is about 9 cm to the left of the midline at the level of the 5th intercostal space, i.e. a little below the nipple and slightly nearer the midline. The base extends to the level of the 2nd rib.

Organs associated with the heart (Fig. 5.10)

Inferiorly - the apex rests on the central tendon of

the diaphragm Superiorly - the great blood vessels, i.e. the aorta,

superior vena cava, pulmonary artery

and pulmonary veins

Structure

The heart is composed of three layers of tissue (Fig. 5.11): pericardium, myocardium and endocardium.

Pericardium

The pericardium is made up of two sacs. The outer sac consists of fibrous tissue and the inner of a continuous double layer of serous membrane.

The outer fibrous sac is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its inelastic, fibrous nature prevents overdistension of the heart.

T he outer layer of the serous membrane, the -parietal pericardium, lines the fibrous sac. The inner layer, the visceral pericardium, or epicardium, which is continuous with the parietal pericardium, is adherent to the heart muscle. A similar arrangement of a double membrane forming a closed space is seen also with the pleura, the membrane enclosing the lungs (see Fig. 10.15, p. 247).

The serous membrane consists of flattened epithelial cells. It secretes serous fluid into the space between the visceral and parietal layers, which allows smooth movement between them when the heart beats. The space between the parietal and visceral pericardium is only a potential space. In health the two layers are in close association, with only the thin film of serous fluid between them.