This fluid portion of the blood is mostly water. Plasma also contains ions, glucose, lipids, amino acids, vitamins, hormones, and dissolved gases. In mammals, red blood cells are biconcave disks that transport oxygen. Red blood cells contain hemoglobin, an iron-containing protein that binds with oxygen. They form in the red bone marrow from stem cells.
Phagocytic cells remove the oldest cells from the bloodstream.
Leukocytes are derived from stem cells in the bone marrow. There are five types of white blood cells. Eosinophils and basophils have lesser roles in the system. Substances released from platelets initiate blood clotting. The human heart is divided into right and left halves.