Science Factoid

Each red blood cell contains about 200 million hemoglobin molecules. If this much hemoglobin were suspended within the plasma rather than enclosed within the cells, blood would be so thick the heart would have difficulty pumping it.

(Mader, S.S. Human Biology; Fifth Edition; WCB/McGraw-Hill: Boston, MA, 1998, p. 109.)

It's amazing what you can learn in high school biology.