I recommend you check out this histology site from Boston University. It is one of the best I have seen; easy to navigate, quick to respond, very complete and excellent micrographs.

This page from Guidant has well organized information about the basics of the heart and blood vessels, very relevant to what you need to know. The animation of blood flow through the heart and lungs is excellent, particularly if you use the "Step Through" feature.

This page by Alexander Imholz at Prince George's Community College has an extensive list of histology links as well as other A&P and Biology resources.

This is an excellent tutorial on heart anatomy and the cardiac cycle.

This site from the University of Houston is one of the most comprehensive I have seen. It includes some excellent tutorials as well as some great links to other sites. It includes pages for Human A&P, Biology, Microbiology, Biochemistry, Genetics, and more.