Cardiovascular System Basics

The cardiovascular system moves blood through a closed circuit: the right heart sends deoxygenated blood to the lungs, and the left heart sends oxygenated blood through the aorta to systemic tissues. Venous return fills the atria and ventricles; ventricular contraction then propels blood into the pulmonary artery or aorta. The coronary arteries supply the myocardium itself, so impaired coronary flow can weaken the pump that depends on that oxygen supply. Cardiac output is calculated as heart rate multiplied by stroke volume. Stroke volume reflects preload, afterload, and contractility. Preload is ventricular filling before contraction; afterload is the resistance the ventricle must overcome; contractility is the strength of myocardial contraction. A very rapid rhythm can lower output even when the heart rate appears “strong” because diastolic filling time becomes too short. Severe vasoconstriction raises afterload and makes ejection more difficult. A slow rate, poor contractility, inadequate filling, or excessive resistance can therefore reduce tissue perfusion. Blood pressure is one part of the perfusion picture, not the whole picture. The body may temporarily preserve blood pressure through tachycardia and vasoconstriction while cerebral,...