A cell is the smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane. Microscopic organisms typically consist of a single cell. An animal cell becomes rounded before cell division. Plant cells do not change shape before the cell division. The muscle cells include skeletal muscles, cardiac muscles, and smooth muscles. Skeletal muscles make up 40% of your body total mass forms biceps and other muscles. Nerve cells are the primary cells in the nervous system. They are responsible for relaying electrical messages to cells and tissues in other organ systems. Blood is a specialized body fluid. It has four main components: plasma, red blood cells, white blood cells, and platelets. They transport oxygen and nutrients to lungs and tissues in the body. The relationship between organs and what the organs do are for example the lung takes in oxygen and blood transfers it through out the body. The digestive system and the walls of the small intestine absorb the digested nutrients into the bloodstream. The blood delivers the nutrients to the rest of the body. The circulatory system consists of the heart and the blood vessels running through the entire body. The arteries carry blood away from the heart; the veins carry it back to the heart. When we breathe, we inhale oxygen and exhale carbon dioxide. Respiration is achievied through the nose and mouth.