The cardiovascular or circulatory system is responsible for the movement of blood throughout the body, with the purpose of delivering oxygen and nutrients to tissues, disposing waste products from the tissues plus carrying hormones as cellular signals.

It consists of a pump (the heart) and an extensive network of channels (the arteries, capillaries and veins). The arteries carry blood away from the heart; the capillaries allow for exchange of material between the blood and the tissues; the veins return the blood to the heart. In addition to blood vessels, a second network of channels, the lymphatic vessels, will also be studied in this exercise. These vessels carry lymph (intercellular fluid) which is filtered through lymph nodes and eventually returned back to the blood.

Learning Objectives:

• Understand that the circulatory system comprises the heart, blood vessels, and lymphatic vessels.
• Understand that the walls of the heart are composed of three complex layers, endocardium, myocardium, and epicardium, which are all different histologically.
• Know the histological features of arteries and veins as found within three layers: tunica intima, tunica media, and tunica adventitia.
• Understand that both arteries and veins are classified primarily according to the cellular and the extracellular components of the adventitia and media and also according to size.
• Understand that capillaries are described as continuous, fenestrated, or discontinuous depending on their structure and function, that capillary beds are the sites for the formation of lymph, and that arteriovenous anastomoses (AV shunts) enable blood to bypass capillaries.
• Know the histological organization of lymphatic capillaries and larger lymphatic vessels, their relationship to lymph nodes, and how they differ from blood vessels.

Let's first look at the heart.