Cells come in different types and are organized into tissues to perform specific body functions. Tissues are composed of groups of similar cells working together. The document then describes different types of epithelial tissues that line and cover organs and structures throughout the body, including simple squamous, simple cuboidal, simple columnar, pseudostratified columnar, stratified squamous, stratified cuboidal, stratified columnar, transitional epithelia and glandular epithelia. It also notes that blood is composed of red and white cells suspended in plasma, and its function is to transport substances around the body.