Figure 626

Glands are composed of epithelial cells. Exocrine-gland secretions enter ducts, whereas hormones or other substances secreted by endocrine (ductless) glands diffuse into the blood.

terial diffuses into the blood, which carries it throughout the body. The endocrine glands secrete a major class of chemical messengers, the hormones, and in practical usage the term "endocrine gland" has come to be synonymous with "hormone-secreting gland." However, it should be noted that there are "ductless glands" that secrete nonhormonal, organic substances into the blood. For example, the liver secretes glucose, amino acids, fats, and proteins into the blood. The substances secreted by such nonendocrine glands serve as nutrients for other cells or perform special functions in the blood, but they do not act as messengers and therefore are not hormones.

The substances secreted by glands fall into two chemical categories: (1) organic materials that are, for the most part, synthesized by the gland cells, and (2) salts and water, which are moved from one extracellular compartment to another across the glandular epithelium. Ultimately this salt and water come from the blood supplying the tissue.

Organic molecules are secreted by gland cells via all the pathways already described: diffusion in the case of lipid-soluble materials, mediated transport for some polar materials, and exocytosis for very large molecules such as proteins. Salts are actively transported across glandular epithelia, producing changes in the extracellular osmolarity, which in turn causes the osmotic flow of water. In exocrine glands, as the secreted fluid passes along the ducts connecting the gland to the luminal surface, the composition of the fluid originally secreted may be altered as a result of absorption or secretion by the duct cells (Figure 6-26). Often the composition of the secreted material at the end of the duct varies with the rate of secretion, reflecting the amount of time the fluid remains in the duct, where it can be modified.

Most glands undergo a low, basal rate of secretion, which can be greatly augmented in response to an appropriate signal, usually a nerve impulse, hormone, or a locally generated chemical messenger. The mechanism of the increased secretion is again one of altering some portion of the secretory pathway. This may involve (1) increasing the rate at which a secreted organic substance is synthesized by activating the appropriate enzymes, (2) providing the calcium signal for exocy-tosis of already synthesized material, or (3) altering the pumping rates of transporters or opening ion channels.

The volume of fluid secreted by an exocrine gland can be increased by increasing the sodium pump activity or by controlling the opening of sodium channels in the plasma membrane. The increased movement of sodium across the epithelium increases the sodium concentration in the lumen, which in turn increases the flow of water by osmosis. The greater the solute transfer, the greater the water flow.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Movement of Molecules Across Cell Membranes CHAPTER SIX


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