. Principles of modern biology. Biology. Responses of Higher Animals: The Effectors - 441 into ducts, and usually the active secretory cells are localized in bulbous chambers, called acini, at the blind ends of the finer ducts. Raw materials used by the gland cells in forming their special secretion are brought to the gland by the blood, which circulates through the capillaries, in close association with each acinus (Fig. 24-9). Certain glands, such as the sweat and tear glands, synthesize no special substances, but merely extract the components of their juices from the blood. Such glands expe
. Principles of modern biology. Biology. Responses of Higher Animals: The Effectors - 441 into ducts, and usually the active secretory cells are localized in bulbous chambers, called acini, at the blind ends of the finer ducts. Raw materials used by the gland cells in forming their special secretion are brought to the gland by the blood, which circulates through the capillaries, in close association with each acinus (Fig. 24-9). Certain glands, such as the sweat and tear glands, synthesize no special substances, but merely extract the components of their juices from the blood. Such glands expend energy in selecting and concentrating their products, since tears and sweat, compared to blood, contain more salts and scarcely any organic substances, except for traces of urea. But most glands also perform the synthesis of special- ized secretory products, for example, the enzymes of the digestive glands and the hor- mones of the endocrine glands. Some glands, especially in the endocrine system, appear to secrete continuously, al- though this activity is subject to change by excitations from the nervous system, or by the action of hormones. But many glands discharge their products only in response to periodic stimulation, in which case each ex- citation is accompanied by a well-defined action potential. Precisely how a gland cell discharges its secretion into its duct is not well understood. Secretory granules (p. 31), when present,. RESTING •* J- 4> EXCITED Fig. 24-10. How secretion may occur. An increase of osmotic solute, together with increased permeability (or complete breakdown) of the membrane on the duct side of the cell, causes a flow of water from the lymph space through the cell, carrying its secretion out into the duct. may disintegrate just prior to their discharge (Fig. 24-10), or such granules may be swept bodily into the duct through the end of the cell bordering on the lumen. Probably the membrane at this end of the cell momentarily disintegrates, eit
Size: 1400px × 1784px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
