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the minute blood vessels that run through all human and animal tissues and form a network between the arterioles transporting blood to the tissues and the venules carrying blood from the tissues. Gases and other substances are exchanged between the blood and adjacent tissues through the capillary walls.
The capillaries were first described by the Italian naturalist M. Malpighi (1661) as the missing link (whose existence had been predicted by W. Harvey) between the venules and the arterioles. Capillaries usually vary in diameter from 2.5 to 30 μm. Wide capillaries are also called sinusoids.
Acapillary wall consists of three layers: an inner, or endothelial, layer; a middle layer, or basement membrane; and an outer layer, or tunica adventitia. The endothelial layer consists of flat polygonal cells that change according to their condition. The endothelial cells are characterized by the presence in the cytoplasm of a large number of micropinocytotic vesicles, 300–1, 500 Å in diameter, which move between the edge of the cell facing the lumen and the edge facing the tissues. They transport portions of the substances needed to bring about an exchange between blood and tissues. Between the endothelial cells are slit-like spaces, 100–150 Å across, and two types of intercellular compounds, with and without areas of obliteration. The basement membrane, 200–1, 500 A wide, is made up both of a cellular component and of a noncellular component that consists of intertwined fibrils embedded in a homogeneous substance rich in mucopolysaccharides. The cellular component (pericytes, or Rouget cells) is completely enveloped by the noncellular component. The adventitial tunic consists of fibroblasts, histiocytes, and other cellular and fibrous structures, and the interstitial substance of connective tissue. The layer passes into the connective tissue that surrounds the capillaries and forms the pericapil-lary zone.
The ultrastructure of the wall of an arterial capillary differs from that of a venous capillary in the size of the lumen (the arterial and venous lumens are generally about 8 μam and 7–12 μam, respectively) and in the orientation of the nuclei of the endothelial cells (in an arterial capillary the long axis of the nucleus runs along the course of the capillary; in a venous capillary, it runs perpendicularly). The endothelial layer is smooth and thick in an arterial capillary but attenuated, with numerous cytoplasmic processes, in a venous capillary. In an arterial capillary the swelling of the nuclei and cytoplasm of the endothelial cells usually results in closure of the lumen, but in the cells of a venous capillary the same process only constricts the lumen. The permeability of the capillary wall is due mainly to the permeability of the endothelium. The noncellular component of the basement membrane also plays a part in the permeability of the capillary wall.
Some investigators believe that the pericyte is a contractile cell capable, like a muscular cell, of actively changing the size of the capillary lumen. Others maintain that the pericyte is a special cell involved in the motor innervation of the capillary: in response to a nerve impulse transmitted from the central nervous system through the pericyte to the endothelial cells, the latter instantly accumulate fluid (swell) or release it (shrink), causing the size of the capillary lumen to change. The ultrastructure of the capillary wall varies from organ to organ. For example, in muscular organs the capillaries have a wide endothelial layer and a narrow basement layer. In kidney capillaries the basement layer is wide but the endothelial cells are attenuated and in places have membrane-covered openings, or fenestrae. Both the endothelial layer and the basement layer are thin in the lungs. The capillaries of bone marrow have no basement layer at all, and the capillaries of the liver and spleen have pores. Capillaries are classified according to the ultrastructure of the endothelial and basement layers in the various organs.
One of the principal biological properties of the capillary wall is its reactivity: the capacity for prompt and adequate change in the activity of all components of the wall in response to environmental influences. Changes in the reactivity of the capillary wall may underlie the pathogenesis of several diseases.
Lymph capillaries, unlike capillaries of the blood circulatory system, have only an endothelial layer, which lies against the surrounding connective tissue and is attached to it by collagen fibrils with special filaments. Lymph capillaries run through almost all human and animal organs and tissues except the brain, splenic parenchyma, lymph nodes, cartilage, sclera, crystalline lens, and certain other tissues. The shape and outlines of the lymphatic network vary with the structure and function of the organ and with the properties of the connective tissue in which the capillaries are found. The lymph capillaries perform the function of drainage, helping colloidal solutions of protein substances not reaching the blood-bearing capillaries to flow out of the tissues and removing foreign particles and bacteria from the body. The wall of a lymph capillary is permeable to small and large molecules passing both through the endothelial cells, aided by the micropinocytotic vesicles, and through the intercellular spaces, which are wider than in the blood-bearing capillaries and are not closed by areas of obliteration. Lymph from the intercellular spaces collects in the lymph capillaries, which merge to form lymphatic vessels.
REFERENCESZhdanov, D. A. Obshchaia anatomiia i fiziologiia limfaticheskoi sistemy. Moscow, 1952.
Shakhlamov, V. A. Kapilliary. Moscow, 1971.
Krogh, A. Anatomiia i fiziologiia kapilliarov. Moscow, 1927. (Translatedfrom German.)
V. A. SHAKHLAMOV