<P> Cells that lie on surfaces exposed to the outside world or gastrointestinal tract (epithelia) or internal cavities (endothelium) come in numerous shapes and forms--from single layers of flat cells, to cells with small beating hair - like cilia in the lungs, to column - like cells that line the stomach . Endothelial cells are cells that line internal cavities including blood vessels and glands . Lining cells regulate what can and can't pass through them, protect internal structures, and function as sensory surfaces . </P> <P> Organs, structured collections of cells with a specific function, sit within the body . Examples include the heart, lungs and liver . Many organs reside within cavities within the body . These cavities include the abdomen and pleura . </P> <P> The circulatory system comprises the heart and blood vessels (arteries, veins and capillaries). The heart propels the circulation of the blood, which serves as a "transportation system" to transfer oxygen, fuel, nutrients, waste products, immune cells and signalling molecules (i.e., hormones) from one part of the body to another . The blood consists of fluid that carries cells in the circulation, including some that move from tissue to blood vessels and back, as well as the spleen and bone marrow . </P> <P> The digestive system consists of the mouth including the tongue and teeth, esophagus, stomach, (gastrointestinal tract, small and large intestines, and rectum), as well as the liver, pancreas, gallbladder, and salivary glands . It converts food into small, nutritional, non-toxic molecules for distribution and absorption into the body . </P>

Images of different parts of the human body