transport

As lining and covering tissues, epithelia are involved in absorptive, secretory, and excretory functions. There are some fundamental differences between these processes. Secretion involves genetic control by a gene on a chromosome. The endoplasmic reticulum(ER) and Golgi apparatus(GA) of the cell build and package the secretory product released across the apical surface. Absorption and excretion involve movement of nutrients or waste products across cells between the blood and interstitial fluids surrounding cells. Note, all three can involve transport across the apical surface. Absorption and excretion, however, can involve transport across the basal surface with diffusion to or from capillaries in connective tissues beneath epithelia.