<P> The body consists of many different types of tissue, defined as cells that act with a specialised function . The study of tissues is called histology and often occurs with a microscope . The body consists of four main types of tissues--lining cells (epithelia), connective tissue, nervous tissue and muscle tissue . </P> <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>

Branches of anatomy that study the body at different levels of detail