Humans are comprised of four main tissue types. These tissue types include nervous tissue, epithelial tissue, connective tissue, and muscular tissue. Epithelial tissue covers the surface of the body and forms the lining of most internal cavities. Functions of epithelial tissue include protection, secretions, absorption, and filtration. Our skin is the largest organ in our bodies and is made up of epithelial tissue. Skin protects our body from dirt, bacteria, and other microbes that may be harmful to us. Epithelial cells come in different shapes and sizes depending upon where they are found in the body. These cells can be thin, flat, cube shaped, or elongated. There are three types of muscular tissue. The types of muscular tissue include: skeletal muscle, cardiac muscle, and smooth muscle tissue. Skeletal muscle is voluntary and its function is to move our bodies. A signal is sent from the brain for the muscle to contract. Cardiac muscle is found in the heart and it is involuntary. Involuntary means that you cannot consciously control the pumping of your heart. Smooth muscle is found in the walls of internal organs and blood vessels. Smooth muscle is also involuntary. The function of muscular tissue is movement.