Cells may have slender extensions of the cell membrane to form cilia or the smaller extensions called microvilli. The microscopic microvilli effectively increase the surface area of the cell and are useful for absorption and secretion functions. A dramatic example is the human small intestine. The tissue has small fingerlike extensions called villi which are collections of cells, and those cells have many microvilli to even further increase the available surface area for the digestion process. According to Audesirk & Audesirk, this can give an effective surface area of about 250 square meters for absorption.