Re: heart

Does the heart have it's own blood supply?

Absolutely! The muscles of the heart need blood just like any other muscles in the body. Oxygen and nutrients can only diffuse a short distance away from a blood supply, so using the blood being pumped by the heart as a source would require the walls of the heart to be very thin, i.e. just a few cells thick. Thin walls are fine if you're the size of a flea, but to pump blood around a larger body, the heart needs much more muscle mass, so much thicker walls. This can only be accomplished by giving the walls of the heart their own capillary beds to supply them with oxygen and nutrients.

The capillary beds in the walls of the heart are supplied by the Coronary Vessels. The coronary arteries arise from the base of the aorta as it exits the heart and fan out under the visceral pericardium (the outer skin of the heart), sending smaller arterioles into the muscle to supply oxygenated blood to the capillaries. The capillaries drain into the coronary veins which join to form the coronary sinus which carries the used blood directly back to the right atrium, separating the coronary circulation from the systemic and pulmonary circulations. The coronary arteries can be visualized by injecting them with dye while monitoring the heart with X-rays. This procedure is called an angiogram, and is used to look for thickening and clots in the coronary arteries which could lead to a heart attack (also called a "coronary" because it involves loss of part of the coronary circulation).

If so, how can this be?

The separation of the coronary and systemic circulations occurs in the embryo during development. Early in the development of the heart, a cluster of cells lying dorsal to (behind) the heart migrate out across the surface of the heart to form a thin membrane called the epicardium (epi- upon cardium the heart). The epicardium then divides into two layers: an outer "skin" (the visceral pericardium) and an inner network of blood vessels. The epicardial vessels connect to the rest of the circulation at the closest points, which also happen to be the major vessels from the heart: the (ventral) aorta and the common cardinal veins. For various the epicardial vessels empty almost exclusively into the left common cardinal vein. As the embryo continues to develop, the body uses the right common cardinal vein more than the left, until finally the right common cardinal in responsible for the systemic circulation and the left common cardinal is responsible for only the coronary circulation. At this point, the right common cardinal vein is called the superior vena cava, and the left common cardinal vein is called the coronary sinus, with its own access to the right atrium of the heart. So the coronary circulation is separate from the systemic and pulmonary circulations because it uses a branch of the embryonic circulation that becomes specific for the heart.