Heart Anatomy and Structure

• The heart resides in the pericardial cavity within the mediastinum of the thorax.
• A folded membrane surrounds the heart.
• The outer part of the membrane forms the pericardial sac.
• The inner part of the membrane is the epicardium, the heart's outer layer.
• Deep to the epicardium is the myocardium, the heart's middle layer, made of cardiac muscle.
• The endocardium is the heart's inner lining, continuous with the endothelium of blood vessels.
• Blood flows in the cardiovascular system because of pressure gradients.
• The heart's contraction generates high pressure, pushing blood into vessels.
• The heart is in the thoracic cavity, between the lungs in the mediastinum, about the size of a fist
• The top of the heart is broad (the base), tapering to the apex.

Internal Structures

• Each ventricle is separated from the atrium by an atrioventricular (AV) valve.
• The right atrium empties into the right ventricle through the tricuspid valve (right AV valve).
• The tricuspid valve has 3 leaflets (cusps) anchored by chordae tendineae.
• Chordae tendineae are fibrous bands connecting valve cusps to papillary muscles, preventing backflow during ventricular contraction.
• Ventricles empty into large arteries through semilunar (SL) valves.
• Semilunar valves have 3 flaps of tissue.
• Right ventricle contraction flattens the flaps of the pulmonic SL valve against the pulmonary trunk, ejecting blood.
• When contraction stops, blood moves towards the ventricle, pulling the flaps closed.
• The pulmonary trunk divides into pulmonary arteries, carrying deoxygenated blood to the lungs for gas exchange.
• In the lungs, carbon dioxide diffuses into alveoli, and oxygen diffuses into pulmonary capillaries.
• Pulmonary veins return oxygen-rich blood to the left atrium.
• The pulmonic circuit involves blood flow from the right heart to the lungs and back to the left heart.
• Blood from the left atrium flows through the mitral valve (bicuspid valve, left AV valve) into the left ventricle.
• The mitral valve structure is like the tricuspid, but has 2 cusps anchored by chordae tendineae.
• Left ventricle contraction forces blood through the aortic SL valve into the aorta.
• The aortic SL valve's characteristics mirror the right SL valve.
• The aorta delivers blood to systemic tissues for oxygen delivery and carbon dioxide removal.
• Deoxygenated blood returns to the right atrium via the superior vena cava (from the upper body) and inferior vena cava (from the lower body).
• The systemic circuit includes vessels from the aorta to systemic tissues and veins returning to the heart.
• Atria contract first, moving blood into ventricles, then ventricles contract, moving blood into great vessels.
• Both sides of the heart move the same amount of blood.
• The pulmonic circuit is shorter than the systemic circuit.
• Longer vessels increase resistance and pressure, so the left side of the heart has to generate greater force due to extra thickness of the left ventricular wall.
• The pressure in the aorta is greater than the pulmonic trunk, so the aortic wall is thicker.

Coronary Circulation

• Blood flowing through heart chambers doesn't nourish cardiac tissue.
• Coronary circulation supplies nutrients and removes wastes.
• Coronary arteries branch from the aorta, delivering oxygen/nutrient-rich blood.
• Coronary veins return blood to the coronary sinus, which empties into the right atrium.
• Anterior cardiac veins bypass the coronary sinus and empty directly into the right atrium.

Cardiac Cycle

• The cardiac cycle includes contraction (systole) and relaxation (diastole) phases for the heart chambers during one heartbeat.
• Both sides of the heart go through the same phases at the same time due to the cardiac conduction system.
• Depolarization signals calcium release and influx for muscle contraction, like skeletal muscle.
• Cardiac, like skeletal, muscle requires calcium to bind troponin and move tropomyosin for cross bridge formation and contraction.
• Blood flows from high to low pressure areas.
• The entire cardiac cycle consists of the changes that occur during the relaxation (diastole) and contraction (systole) of the heart.
• Changes in the cardiac cycle correlate with electrical changes shown on an electrocardiogram (ECG or EKG).
• The ventricular filling phase occurs between ECG waves and continues into the first part of each ECG wave.
• Valve positions and pressure differences dictate blood flow direction during each phase.

Ventricular Filling

• Ventricular filling starts when AV valves open between atria and ventricles, usually around the end of the T wave on the ECG.
• AV valves open because atrial pressure exceeds ventricular pressure.
• SL valves are closed before this phase and remain closed throughout.
• Blood fills ventricles.
• Atria simultaneously receive blood from the systemic (right atrium) and pulmonic (left atrium) circuits.

Atrial systole

• Atrial systole/Ventricular diastole, in the latter part of the ventricular filling phase, atrial depolarization occurs (P wave), and the atria contract.
• Atrial systole forces blood into ventricles, which are in diastole (relaxed).
• At the end of ventricular filling, ventricles are filled with blood.
• Atria begin to relax.

Ventricular Systole

• Isovolumetric Contraction, ventricular systole begins after ventricular depolarization (~2/3 of the QRS complex).
• As ventricles contract, pressure rises, pushing blood toward AV valves, causing them to shut.
• AV valve closure creates the first heart sound, S1 (lub).
• SL valves remain closed.
• Both AV and SL valves are closed, and ventricles continue contracting, increasing pressure.
• The isovolumetric phase continues until ventricular pressure exceeds outflow vessel pressure (pulmonic trunk and aorta).
• The SL valves open when ventricular pressure exceeds that in outflow vessels.

Ventricular Ejection

• Ventricular ejection occurs when blood flows from ventricles into the pulmonic trunk and aorta as SL valves open.
• Atria are relaxed (diastole) and filling.
• Ventricles contract (systole), forcing blood into outflow tracts.
• This starts around 2/3 of the QRS wave and continues to 2/3 of the T wave.

Ventricular Diastole

• Ventricles begin to repolarize (T wave).
• Ventricular relaxation follows soon after (2/3 of the T wave).
• As ventricles relax, pressure decreases.
• Pressure in outflow vessels (pulmonic trunk and aorta) becomes greater than in ventricles.
• Blood flows back toward ventricles down the pressure gradient.
• SL valves snap shut, creating the second heart sound, S2 (dup).

Isovolumetric Relaxation

• Closing of the SL valves starts the isovolumetric relaxation phase.
• Both AV and SL valves are closed, and ventricles are in diastole (relaxation).
• The isovolumetric phase is brief because the atria continue to build up pressure while relaxed and filling.
• When atrial pressure exceeds ventricular pressure, AV valves open, and ventricular filling begins.

Heart Sounds

• The cardiac cycle is a set of atrial and ventricular contractions.
• The events are accompanied by pressure changes, valve movements, and blood flow.
• These events produce vibrations, creating the sounds associated with the heartbeat.
• A normal human heartbeat has two well-defined sounds per cycle.
• Heart sounds are easily heard using a stethoscope.
• The first sound (S1) primarily results from AV valve closure.
• The second heart sound (S2) mainly develops from semilunar valve closure.
• Heart sounds provide information about valve condition and murmurs.
• A heart murmur is turbulence caused by abnormal blood flow.
• Murmurs can occur during ventricular systole due to AV valve problems or between heart sounds as blood flows through narrowed vessels (aorta or pulmonic trunk).