Light micrograph of a section through the wall of a gallbladder. The interior surface (top) of the gallbladder is a mucous membrane with many folds lined with simple columnar epithelium. Beneath the mucous membrane is connective tissue (blue) and a muscular layer of smooth muscle (red, across centre) arranged in bundles. Below the muscle is an artery (left) and a vein (centre). The gallbladder stores and concentrates bile produced by the liver and secretes it into the small intestine where it helps the body to digest fat. Magnification: x100 when printed at 15cm tall.