Hematology is the study of blood: what it is made of, how it works, and the illnesses that change it. A hematology lab looks at red cells (which carry oxygen), white cells (which fight infection), and the sticky parts of blood that help a cut stop bleeding. This is the American spelling. In British English the same field is haematology, with an extra a after the h. You will see hematology in US hospitals and journals, and haematology in UK writing and in many older textbooks. Though actually, they are not two different sciences; only the letters change. The person who does this work is a hematologist. Everyday English often just says blood tests or blood doctor; hematology is the name of the field itself.