Introduction

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Part 4

Description of cardiovascular system
The cardiovascular system consists of the heart, a muscular hydrodynamic pump, and its conduit vessels, the arteries and veins. Through this hydraulic circuit flows blood carrying oxygen and nutrients to all parts of the body, and transporting accumulated toxic metabolites to the kidneys, liver and lungs for elimination. The heart in fact is two pumps, the right and left heart, which are connected in series and which independently circulate blood to the lungs or to the rest of the body, respectively. Each of these pumping systems, right and left, consists of a small, thin walled atrium which collects blood and forces it into a larger thick walled ventricle which generates enough pressure to move the blood to the lungs or the rest of the body. Since the heart muscle alternately contracts to expel blood and relaxes to take in blood, the pressure in arterial conduits varies in a pulsatile fashion.

Reflecting this, blood pressure in the body (the left heart or systemic circulation) is measured as the peak systolic pressure and the trough diastolic pressure, typically 120 and 80 mm of mercury, respectively. The heart maintains these perfusion pressures by pumping 2 to 3 ounces of blood per contraction, approximately 72 times per minute. Thus, the heart pumps over 500,000 gallons of blood per year, and will beat over two and a half billion times in a lifetime.

Ventricular contraction
Unidirectional flow of blood through the heart is maintained by flexible valves at the entrance and exit points of both ventricles. As the valve leaflets snap closed they produce an audible sound which can be heard with a stethoscope, or even an ear pressed against the chest wall. The cardiac entrance valves (mitral on the left and tricuspid on the right) close at the beginning of ventricular contraction to prevent blood from being expelled backward into the venous system and produce the first heart sound. Likewise, the cardiac exit valves (aortic on the left and pulmonary on the right) are open during ventricular contraction to allow blood to flow into the arteries, but snap shut as the ventricles relax to prevent reverse flow of arterial blood into the heart, and produce the second heart sound.

Cardiovascular irregularities

Closure of the cardiac valves thus produces the "lub-dub....lub-dub...." sounds one hears when listening to the heart. Under normal circumstances blood flow through the heart, valves and vessels is smooth and silent because there is no turbulence. However, when the cardiac valves malfunction, either by leaking (a condition termed regurgitation) or by not fully opening (termed stenosis), turbulent blood flowing across the diseased valve produces a rushing sound or "heart murmur" by which a Cardiologist can frequently diagnose a particular valvular condition. Valvular disease may be congenital, such as being born with misshapen leaflets, or acquired such as rheumatic heart disease or bacterial and fungal valvular infections, termed infective endocarditis. Although the heart muscle can adapt to the extra work demands imposed by valvular regurgitation or stenosis by increasing the amount of heart muscle (cardiac hypertrophy), over many years this compensatory process is ultimately overwhelmed and the heart abnormally dilates and fails. Therefore, severe valvular disease generally requires surgical replacement with a mechanical or biological prosthesis to prevent the development of heart failure.

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