STRUCTURES AND FUNCTIONS
The heart is a four-chambered organ that lies in the mediastinal space in the thorax.
The heart is divided by the septum, forming the right and left atrium and the right and left
Valves separate the chambers of the heart:
o Mitral valve separates the left atrium and the left ventricle.
o Aortic valve separates the left ventricle and the aorta.
o Tricuspid valve separates the right atrium and the right ventricle.
o Pulmonic valve separates the right ventricle and the pulmonary artery.
The heart is:
o Composed of three layers: endocardium, myocardium, and epicardium.
o Surrounded by a fibroserous sac called the pericardium.
The right side of the heart receives blood from the body (via the vena cava) and pumps it
to the lungs where it is oxygenated. Blood returns to the left side of the heart (via the
pulmonary arteries) and is pumped to the body via the aorta.
The coronary circulation provides blood to the myocardium. The right and left coronary
arteries are the first branches of the aorta.
The conduction system consists of specialized cells that create and transport electrical
impulses. These electrical impulses initiate depolarization (contraction) of the
myocardium and ultimately a cardiac contraction.
Each electrical impulse starts at the SA node (located in the right atrium), travels to the
AV node (located at the atrioventricular junction), through the bundle of His, down the
right and left bundle branches (located in the ventricular septum), terminating in the
The electrical activity of the heart is recorded on the electrocardiogram (ECG).
Systole, contraction of the myocardium, results in ejection of blood from the ventricles.
Relaxation of the myocardium, or diastole, allows for filling of the ventricles.
Cardiac output (CO) is the amount of blood pumped by each ventricle in 1 minute. It is
calculated by multiplying the amount of blood ejected from the ventricle with each
heartbeat, the stroke volume (SV), by the heart rate (HR) per minute: CO = SV HR. Factors affecting SV are preload, afterload, and contractility. Preload is the volume of
blood in the ventricles at the end of diastole, and afterload represents the peripheral
resistance against which the left ventricle must pump.
Cardiac reserve refers to the heart’s ability to alter the CO in response to an increase in
demand (e.g., exercise, hypovolemia).
Stimulation of the sympathetic nervous system increases HR, speed of conduction
through the AV node, and force of atrial and ventricular contractions, whereas
stimulation of the parasympathetic nervous system decreases HR.
Baroreceptors, located in the aortic arch