Anatomy and Physiology of the Heart: Key Concepts and Structures

Cardiology

The study of the heart

Heart location

Between the 2nd rib and 5th intercostal space; between the lungs; left of the midline; in the mediastinum; apex pointed toward the left

Heart size and weight

About the size of a closed fist; weighs less than 1 pound

Normal heart rate

Averages about 72 contractions per minute

3 Layers of the Heart (inside to outside)

(1) Endocardium — lines the heart's interior; (2) Myocardium — thickest layer, the heart muscle, allows the heart to pump blood; (3) Epicardium — thin outermost layer that becomes part of the pericardium

Endocardium

Innermost layer of the heart; lines the heart's interior

Myocardium

Thickest layer of the heart; the heart muscle; allows the heart to pump blood; nourished by coronary arteries

Epicardium

Thin outermost layer of the heart; also called visceral pericardium; becomes part of the pericardium

Pericardium

The sac that encloses the heart; has three layers: fibrous pericardium, parietal pericardium, and epicardium (visceral pericardium)

Fibrous pericardium

Outermost layer of the pericardium; anchors the heart to surrounding structures

Parietal pericardium

Middle layer of pericardium; forms the pericardial space

Pericardial space

Space between the parietal pericardium and epicardium; contains 10 to 30 mL of serous fluid to reduce friction

Pericarditis

Inflammation of the pericardial membranes; characterized by pain, a friction rub sound, and secretion of excess serous fluid

Cardiac tamponade

Condition where excess serous fluid in the pericardial space compresses the heart, making it difficult to fill with blood; causes lower blood pressure

Heart as a double pump

Right heart (blue) = pulmonary circuit (deoxygenated blood to lungs); Left heart (red) = systemic circuit (oxygenated blood to body)

Pulmonary circulation

Right side of heart pumps deoxygenated blood to the lungs to pick up oxygen

Systemic circulation

Left side of heart pumps oxygenated blood to all tissues of the body

Great vessels

The large blood vessels attached to the heart: superior and inferior vena cava, pulmonary artery, pulmonary veins, and aorta

Superior and inferior vena cava

Largest veins in the body; return deoxygenated blood from the body to the right atrium

Pulmonary artery

Carries deoxygenated blood from the right ventricle to the lungs (only artery that carries deoxygenated blood)

Pulmonary veins

Carry oxygenated blood from the lungs back to the left atrium (only veins that carry oxygenated blood)

Aorta

Largest artery; carries oxygenated blood from the left ventricle to the body; receives blood from left ventricle

4 Chambers of the Heart

Right atrium, right ventricle, left atrium, left ventricle

Right atrium

Receives low-oxygen (deoxygenated) blood returning from body tissue through the superior and inferior vena cava

Right ventricle

Pumps deoxygenated blood from right atrium to the lungs via the pulmonary artery

Left atrium

Receives high-oxygen (oxygenated) blood from the lungs via pulmonary veins

Left ventricle

Pumps oxygenated blood to the body via the aorta; strongest, thickest, biggest chamber — pumps blood through the aorta

Interventricular septum

Wall that separates the right and left ventricles

Vessels carrying unoxygenated blood

Vena cava and pulmonary artery

Vessels carrying oxygenated blood

Pulmonary veins and aorta

Heart valves — function

Regulate the flow of blood through the heart; prevent backflow; regulate the direction of flow

Atrioventricular (AV) valves

Valves between atria and ventricles; includes tricuspid (right) and bicuspid/mitral (left)

Tricuspid valve

AV valve between the RIGHT atrium and RIGHT ventricle; prevents backflow into the right atrium when the right ventricle contracts; has 3 cusps

Bicuspid valve (Mitral valve)

AV valve between the LEFT atrium and LEFT ventricle; has 2 cusps; also called the mitral valve

Chordae tendineae

Tendon-like cords that attach the AV valve cusps to the ventricular walls; prevent valves from inverting

Semilunar valves

Valves between ventricles and great vessels; includes pulmonic valve (right) and aortic valve (left)

Pulmonic valve

Semilunar valve between the RIGHT ventricle and pulmonary artery; opens when pressure in the right ventricle increases

Aortic valve

Semilunar valve between the LEFT ventricle and the aorta

Heart murmur

Abnormal heart sound caused by blood leaking through a valve (backflow/regurgitation)

Heart sounds — origin

Made by vibrations caused by the CLOSURE of the valves (lubb-dubb sound)

S1 (first heart sound) — 'lubb'

Caused by closure of the AV valves (tricuspid and mitral); best heard over the APEX of the heart

S2 (second heart sound) — 'dubb'

Caused by closure of the semilunar valves (pulmonic and aortic); best heard at the BASE of the heart

Blood flow through the heart (in order)

Body → Superior/Inferior Vena Cava → Right Atrium → Tricuspid Valve → Right Ventricle → Pulmonic Valve → Pulmonary Artery → Lungs (gas exchange) → Pulmonary Veins → Left Atrium → Bicuspid (Mitral) Valve → Left Ventricle → Aortic Valve → Aorta → Body

Coronary arteries

Left and right coronary arteries; supply oxygenated blood to the myocardium (heart muscle itself); drain via coronary veins

Coronary blood flow — exertion

Can increase up to 4-5 times during exertion; if vessels are occluded (blocked), they are already maximally dilated, so the person feels immediate chest pain (angina) on exertion

Coronary blood flow — relaxation

Flow is greatest during myocardial relaxation; when ventricles relax, coronary arteries open up and blood flows in

Coronary anastomoses

Development of additional (collateral) arteries to compensate for a partially occluded coronary artery; the body's natural bypass around an occlusion

Ischemia

Diminished blood flow and oxygen deprivation to the myocardium

Angina

Chest pain caused by ischemia (reduced blood flow to heart muscle)

Myocardial infarction (MI)

Heart attack; myocardial cells die due to prolonged ischemia and leak enzymes into the blood

Cardiac enzyme tests (for MI)

CPK (creatine phosphokinase), AST (aspartate aminotransferase), LDH (lactic dehydrogenase), Troponin; elevated when myocardial cells die and leak

Troponin

Most specific and sensitive cardiac enzyme; leaks into blood when myocardial cells die; primary diagnostic tool for heart attacks

Cardiac conduction system

The electrical system of the heart that initiates and coordinates contractions; SA node → AV node → Bundle of His → Purkinje fibers

SA node (Sinoatrial node)

The heart's natural pacemaker; located in the upper wall of the right atrium; fires action potentials 60-100 times per minute; initiates each heartbeat

AV node (Atrioventricular node)

Receives impulse from SA node; slows the cardiac impulse before sending it to the ventricles; delay allows ventricles to fill with blood from the atria

Bundle of His

Conducts the cardiac impulse from the AV node down to the Purkinje fibers in the ventricles

Purkinje fibers

Conducting fibers that rapidly spread the electrical signal throughout the ventricles, causing them to contract

Conduction pathway (in order)

SA node → atrial conducting fibers → AV node (slows impulse) → Bundle of His → right and left bundle branches → Purkinje fibers → ventricles contract

Automaticity

The ability of cardiac pacemaker cells to generate an electrical signal without assistance from the CNS (nerves)

Rhythmicity

The regular, consistent firing of a cardiac impulse by the SA node; gives the heart its steady rhythm

Dysrhythmia (Dysrhythmic)

When the heart rhythm is disturbed/irregular

Normal SA node firing rate

60 to 100 action potentials (beats) per minute