KS3: Look at prepared blood smears through a microscope. Get the students to make a red blood cell from plasticine so they can see the thick rim and the thin centre. Work out the advantage of this shape over a flat disc or a sphere.
KS4: Try to repeat Harvey's experiments on the human forearm (details in most KS4 textbooks). See www.bbc.co.ukgcsebitesize for straightforward information. Look at the way haemophilia and sickle-cell anaemia are inherited; most textbooks will have sample family trees and problems.
Questions on these topics will be in GCSE papers.
KS5: Look at the molecular structure of haemoglobin, the way it carries oxygen and also how it is changed in the sickle-cell condition. See www.bio.ph.ic.ac.ukmolbiomolshb for 3D images of haemoglobin. Use the information in standard texts to show how the distribution of sickle-cell anaemia has changed during the 20th century; consider the link between this disease and resistance to malaria, and how this is can be used to show natural selection in action. The Sickle Cell Society is a source of lots of information: www.sicklecellsociety.org
Explain the way the antigens and antibodies work in blood grouping and get students to work out why some people are "universal donors" and others "universal recipients". Also look at the link between this reaction, the immune response and organ transplantation. For background information, see www.bts.org.ukhistory.htm