Editing Anatomy and Physiology (section)

== Cardiovascular System ==

=== Structure and Blood Flow ===

The heart has four chambers. Deoxygenated blood returns through the superior and inferior vena cava to the '''right atrium''', passes the '''tricuspid valve''' into the '''right ventricle''', exits through the '''pulmonic valve''' to the pulmonary arteries and lungs. Oxygenated blood returns by the pulmonary veins to the '''left atrium''', passes the '''mitral valve''' into the '''left ventricle''', and is ejected through the '''aortic valve''' into the aorta.

The left ventricle has the thickest wall because it pumps against systemic vascular resistance.

The coronary arteries fill during '''diastole''', which is why tachycardia, by shortening diastole, worsens myocardial ischemia.

=== Conduction ===

The '''sinoatrial (SA) node''' is the normal pacemaker at 60 to 100 beats per minute. The impulse travels to the '''atrioventricular (AV) node''' (40 to 60 beats per minute intrinsic rate), where it is briefly delayed to allow atrial filling of the ventricles, then through the bundle of His, the bundle branches, and the Purkinje fibers (20 to 40 beats per minute).

On the electrocardiogram: the '''P wave''' is atrial depolarization, the '''QRS complex''' is ventricular depolarization, and the '''T wave''' is ventricular repolarization.

=== Cardiac Output ===

Cardiac output equals '''heart rate multiplied by stroke volume''', normally about 4 to 8 liters per minute in an adult. Stroke volume depends on:
* '''Preload''': the volume stretching the ventricle at end diastole. Increased by fluid overload, decreased by hemorrhage, dehydration, and venous dilators.
* '''Afterload''': the resistance the ventricle pumps against. Increased by hypertension, vasoconstriction, and aortic stenosis.
* '''Contractility''': the force of contraction, improved by positive inotropes and reduced by ischemia, acidosis, and hypoxia.

The '''Frank-Starling law''' states that greater stretch produces stronger contraction, up to a point beyond which the failing ventricle cannot keep up.

Blood pressure is a product of cardiac output and systemic vascular resistance. '''Mean arterial pressure''' of roughly 65 mmHg or more is generally needed to perfuse the vital organs.

=== Vessels ===

Arteries carry blood away from the heart and are thick and elastic. Arterioles are the main site of resistance. Capillaries are one cell thick and are the only site of exchange. Veins are thin walled, hold about two thirds of blood volume, and rely on valves and skeletal muscle pumping for return.

'''Starling forces''' at the capillary determine edema: hydrostatic pressure pushes fluid out and oncotic pressure from plasma proteins pulls it back. Low albumin (liver failure, malnutrition, nephrotic syndrome) lowers oncotic pressure and causes edema despite a normal or low blood volume. Normal serum albumin is about 3.5 to 5.0 g/dL.

=== Baroreceptors and the Renin-Angiotensin-Aldosterone System ===

Baroreceptors in the carotid sinus and aortic arch detect pressure changes within seconds and adjust sympathetic and parasympathetic output. Over hours to days, low renal perfusion triggers renin release, which produces angiotensin I, converted by angiotensin-converting enzyme to angiotensin II, a potent vasoconstrictor that also stimulates aldosterone release and sodium and water retention.

'''Nursing application:'''
* This system explains the mechanism of angiotensin-converting enzyme inhibitors and angiotensin receptor blockers, and explains why they can cause hyperkalemia.
* Baroreceptor sensitivity declines with age, which is why orthostatic hypotension and falls are common in older adults. Teach patients to change position slowly and dangle their legs before standing.
* '''Left-sided heart failure''' backs blood into the pulmonary circulation: dyspnea, orthopnea, paroxysmal nocturnal dyspnea, crackles, cough, and pink frothy sputum. '''Right-sided heart failure''' backs blood into systemic veins: jugular venous distention, peripheral edema, hepatomegaly, ascites, and weight gain. Daily weight is the most reliable indicator of fluid status; about 1 kilogram equals about 1 liter of fluid.