Blood is a connective tissue that functions to transport oxygen, nutrients, waste, and more throughout the body. It is composed of formed elements such as red blood cells, white blood cells, and platelets suspended in plasma. Red blood cells carry oxygen and carbon dioxide, white blood cells protect the body from infection, and platelets help the blood to clot. Blood is essential for life as it transports these critical substances, regulates pH and temperature, and protects the body from pathogens.