Your body cells use oxygen you breath to get energy from the foods you eat. The process is called "Cellular Respiration." This process or function consists of the cell using oxygen to break down sugars in the body. When the cell uses the oxygen to break down sugars, carbon-dioxide is produced and energy is released, thus producing the energy your body needs.
"Hemoglobin" is a protein with Red Blood cells (also referred to as RBC's) that are responsible for the transport of oxygen. The Cardiovascular System consists of the heart and vessels and may also be referred to as the vascular system or the circulatory system. The heart functions as a muscular pump that pushes blood through the vessels to all areas of the body and the vessels function to carry it back to the heart as well. Hence the name "circulatory."
They have to have a higher density than water to have the ability to stay on the sea floor. Otherwise they would just float to the surface of the water
