query: How many red blood cells are in the human body?
passage: Red blood cell
Adult humans have roughly 20–30 trillion red blood cells at any given time, constituting approximately 70% of all cells by number.[18] Women have about 4–5 million red blood cells per microliter (cubic millimeter) of blood and men about 5–6 million; people living at high altitudes with low oxygen tension will have more. Red blood cells are thus much more common than the other blood particles: there are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets per microliter.
passage: Blood cell
In 1658 Dutch naturalist Jan Swammerdam was the first person to observe red blood cells under a microscope, and in 1695, microscopist Antoni van Leeuwenhoek, also Dutch, was the first to draw an illustration of "red corpuscles", as they were called. No further blood cells were discovered until 1842 when French physician Alfred Donné discovered platelets. The following year leukocytes were first observed by Gabriel Andral, a French professor of medicine, and William Addison, a British physician, simultaneously. Both men believed that both red and white cells were altered in disease. With these discoveries, hematology, a new field of medicine, was established. Even though agents for staining tissues and cells were available, almost no advances were made in knowledge about the morphology of blood cells until 1879, when Paul Ehrlich published his technique for staining blood films and his method for differential blood cell counting