What is Cardiology?
Caridology is the medical study of the structure, function, and disorders of the heart. It is a branch of medicine that involves the diagnosis and treatment of heart diseases.

Introduction on the Cardiovascular System - Website to help you understand the Cardiovascular System

How does Cardiology apply to Occupational Therapy?
The cardiovascular system applies to occupational therapy especially in any situation or setting. We always need to be aware of their heart conditions so we know when to stop if problems arise. For an example, if we are focusing on strength and endurance and we notice that they are short of breath, we need to know how to handle the situation. Symptoms of increased heart rate and shortness of breath include redness in the face, sweating, high pulse, chest pain, and light headedness. If this happens during an occupational therapy treatment session, we would stop the activity, comfort the patient and call 911 if neccesary. It is important to be aware of all signs and symptoms regarding patients with a heart condition. If a serious situation was to arise make sure you document it and further discuss issues with your advisor and other practitioners if neccesary.

The Cardiovascular System
The cardiovascular system is made up of the heart and the blood vessels. The heart is in charge of pumping the blood and the blood vessels are in charge of channeling and delivering the blood throughout the rest of the body. Arteries are thick-walled tubes with a circular covering of elastic fibers, which contain a filling of muscle that absorbs the pressure wave of a heartnbeat and slows the blood down. This pressure is called your pulse. Arteries are used to carry the blood that is filled with nutrients away from the heart to other parts of the body to supply them with "good" blood. The arteries divide into smaller "tubes" called arterioles and they divide into smaller "tubes" called capillaries. The capillaries are the smallest of all blood vessels.
In every tissue of every organ, blood delivers what the cell needs to stay alive and takes away the waste products that the cells don't need. Capillaries join together and form small veins, which then flow into larger main veins. The main veins deliver deoxugenated blood back to the heart. Veins have thin walls because the blood has lost the pressure which forced it out of the heart. So the blood which flows through the veins on its way to the lungs oozes along slowly and its on its way to be reoxygenated.
Back at the heart, the veins enter a vessel called the pulmonary arteries. It flows along the pulmonary arteries to the lungs to collect oxygen, then back to heart's left side to begin its journey around the body again.

Cardiovascular System - Arteries and Veins Animation - Website and Video on the Circulatory System

This picture shows you what was explained above. You can see how the oxygenated blood comes from the heart and goes to the organs in the body (kidneys in this case). And then the blood goes back to the lungs to get reoxygenated.

Sources: http://www.innerbody.com/anim/card.html