lymph(redirected from periarteriolar lymph sheath (PALS))
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the fluid that circulates in the lymphatic system of vertebrates and man.
With lack of food, the lymph is a transparent or slightly opalescent fluid. After food is eaten, the lymph becomes white and opaque, and its emulsified fat content increases. The specific gravity of lymph is 1.01–1.016. The reaction of lymph is alkaline (pH = 7.4–9). Its chemical composition is similar to that of blood plasma, although there is less protein (3 percent, as compared to 6.5). Its viscosity is low. Lymph is capable of clotting, but it does so more slowly than does blood. Lymph contains blood cells, among which lymphocytes are especially numerous; there are very few erythrocytes.
There have been several theories of lymph formation, including a filtration theory, a secretory theory, and Ascher’s theory (after the German scientist L. Ascher, who, in 1898, ascribed lymph formation to cells surrounding the lymphatic vessels of the tissues). According to the modern view, lymph formation is ensured by the continual entrance of fluid into the tissues from the blood plasma and into the lymphatic vessels from the tissue spaces. Lymph production is influenced by the permeability of the blood and lymph vessels, the hydrostatic and osmotic pressure of the blood and tissues, the functional state of the organs, the state of neurohumoral regulatory mechanisms, and the physicochemical state and activity of the connective tissue. Movement of lymph in the vessels (lymph circulation) is ensured by the physiological activities of the organs, the contractions of body muscles, and negative venous pressure. Lymph pressure equals 20 mm H2O (200 newtons [N] per m2). Under certain physiological conditions, it may increase to 60 mm (600 N/m2). The human lymphatic system contains a total of 1–2 liters of lymph. The amount of lymph in the body may increase when the hydrostatic pressure in the blood capillaries rises, when there is an increase in metabolic rate, or under pathological conditions.
Poisons and bacterial toxins readily penetrate the lymph. A considerable amount of fat is absorbed from the intestine into the lymph. Inadequate lymph circulation is observed when venous pressure increases and with mechanical pressures (tumors, chronic infections, scars, ligation of lymph vessels); when this happens, lymphostasis leads to an increase in organ volume.
REFERENCESZhdanov, D. A. Obshchaia anatomiia i fiziologiia limfaticheskoi sistemy. [Leningrad] 1952.
Rusznyák, İ., M. Földi, and G. Szabó. Lymphatics and Lymph Circulation, 2nd ed. Oxford, 1967.