SECRETION

Secretion is a process generally brought about by an organ called a gland. A gland whether simple or complex in structure may be looked upon as a tube whose walls are composed of highly specialized epithelial cells, gland cells. The tube is closed at the end. In many glands the other end of the lumen opens up, either directly or by means of a special duct, onto a free surface, such as the skin, the interior of the mouth, etc. The materials produced by the gland are poured onto this free surface, for which reason the secretion of this type of gland is spoken of as an external secretion. The gland is surrounded by a dense network of capillaries.

The distinct process takes place in a gland: the gland cell serves as a transfer agency or it acts as a manufacturing plant or both. In the first instance, certain materials, water and NaCl are taken out of the cell, passed into the duct and secreted on a free surface. All types of glands transfer water in this manner.

Other glands take certain materials out of the blood stream and chemically transform them into new compounds, they then being poured into the duct. The activity of the gland is normally accompanied by a great dilation of its blood vessels. Without this increased flow of blood a gland cannot function for any appreciable length of time. Nevertheless in many instances its blood flow in itself is not the direct cause of secretion, for by administering certain drugs it is possible to stop the secretion completely although the flow of blood continues.

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