Mechanical Properties of the Heart
Adapted from Dr. Marc Dickstein's Lecture Notes

This document describes some of the basics of heart physiology, which may help one understand the purpose of the heart simulator.

The Circulatory Loop

The Circulatory Loop is comprised of two main fluid pumps and a network of vascular tubes. The loop can be divided into the pulmonary system which contains the right ventricle, the pulmonary arteries, the pulmonary capillaries, and pulmonary veins, and the systemic system which contains the left ventricle, the systemic arteries, the systemic capillaries, and the systemic veins. Each pump provides blood with energy to circulate through its respective vascular network. While these pumps are pulsatile (i.e. blood is delivered into the circulatory system intermittently with each heart beat), the flow of blood in the vasculature becomes more and more steady as it approaches capillary networks; in essence, the large blood vessels serve as a type of shock absorber.

If all of the blood followed this simple pathway, then it should be clear that the amount of blood ejected by the Right Ventricle (RV) must equal the amount of blood ejected by the Left Ventricle (LV). Consider that if the cardiac output of the RV exceeded that of the LV, it would mean that more blood was being introduced into the pulmonary vasculature than was being removed. (It also would mean that more was being removed from the systemic circulation than was being introduced.) This type of discrepancy, no matter how small, cannot be sustained. Therefore, many factors come into play that ensure appropriate "matching" of right and left ventricular outputs.

The Cardiac Cycle

The state of the ventricle alternates between relaxed (termed diastole), and active contraction (termed systole). When relaxed, the ventricle is more suited to receive blood from the venous sytem. When actively contracting, the ventricle ejects a portion of its contents into the arterial system. The cycling of theses phases in the LV is concurrent with cycling in the RV. This cycling is governed by the conduction system.

Systole may be subdivided into two phases:

1. Isovolumic contraction, during which the ventricular chamber is developing sufficient pressure to exceed the pressure in the artery and thereby open the interposing valve (pulmonary or aortic), and
2. Ejection, during which time blood is being actively expelled from the ventricular chamber.

Diastole may be subdivied into four phases:

1. Isolvolumic relaxation, during which the chamber pressure is rapidly declining until it falls below atrial pressure and thereby opens the interposing valve (tricuspid or mitral),
2. passive filling, during which blood moves from the atrium to the ventricle thus dissipating any pressure gradient,
3. diastasis, during which there is little movement of blood as the pressures in the atrium and ventricle matched, and
4. atrial contraction, during which the atrium undergoes active contraction and develops increased pressure thus re-establishing a pressure gradient between the atrium and ventricle and causing a second burst of ventricular filling.

	These images are taken from a trans-esophageal echocardiography exam. The Left ventricle chamber is outlined in red. Note the proximity of the mitral valve to the aortic valve.The following frames will demostrate the phases of the cardiac cycle.
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