Heart Anatomy

Anatomy & Physiology II: The Cardiovascular System - The Heart

External Anatomy of the Heart

• Position: Located in the thoracic mediastinum between the two pleural cavities.

• Base: Upper end of the heart where major vessels are connected.

• Apex: Inferior pointed tip of the heart.

Heart Structures: Arteries and Veins

• Right Coronary Artery (RCA)

  • Main branches include:

  • Right marginal branch of RCA

  • Posterior interventricular branch of RCA

• Left Coronary Artery (LCA)

  • Main branches include:

  • Anterior interventricular branch of LCA

  • Circumflex branch of LCA

• Veins

  • Great cardiac vein

  • Middle cardiac vein

  • Small cardiac vein

Pericardial Space

• Defined as the space between the parietal pericardium and the visceral pericardium (epicardium).

• Contains pericardial fluid, which provides lubrication for the heart during movement.

Heart Wall

• Consists of three layers:

  • Epicardium:

  • Also known as the visceral pericardium.

  • Outermost layer composed of simple squamous epithelium.

  • Myocardium:

  • Middle layer consisting of cardiac muscle cells (cardiocytes) arranged in concentric layers.

  • Endocardium:

  • Innermost layer lined with simple squamous epithelium, covering heart chambers and valves.

Heart Chambers

1. Right Atrium (RA)

   • Receives deoxygenated blood from systemic circulation via:

     • Superior vena cava

     • Inferior vena cava

     • Coronary sinus

   • Contains the fossa ovalis and the SA node.

   • Sends blood to the right ventricle (RV).

2. Right Ventricle (RV)

   • Receives deoxygenated blood from RA.

   • Pumps blood to the pulmonary circuit through the pulmonary trunk.

3. Left Atrium (LA)

   • Receives oxygenated blood from lungs via 2 left and 2 right pulmonary veins.

   • Sends blood to the left ventricle (LV).

4. Left Ventricle (LV)

   • Largest and thickest chamber of the heart.

   • Receives oxygenated blood from LA.

   • Pumps blood to systemic circulation through the aorta.

Important Structural Features

• Interatrial Septum:

  • Separates the right atrium and left atrium.

• Interventricular Septum:

  • Separates the right ventricle and left ventricle.

Heart Valves

1. Right AV Valve (Tricuspid Valve)

   • Located between the RA and RV.

2. Left AV Valve (Bicuspid or Mitral Valve)

   • Located between the LA and LV.

   • Blood pressure closes both AV valves during ventricular contraction.

   • Connected to papillary muscles by chordae tendineae.

3. Pulmonary Semilunar Valve

   • Positioned between RV and pulmonary trunk.

   • Prevents backflow from pulmonary trunk into RV.

4. Aortic Semilunar Valve

   • Located between LV and aorta.

   • Prevents backflow from aorta into LV.

Coronary Circulation

• Heart requires oxygen (O2) and nutrients to function.

• 5% of blood pumped by the heart (approximately 250 mL/min) goes to the coronary circulation.

• Right and Left Coronary Arteries: originate at the base of the ascending aorta in a region called the aortic sinus.

• Main Branches of the Right Coronary Artery:

  • Marginal artery

  • Posterior interventricular artery

  • Supplies blood to RA and SA node.

• Main Branches of the Left Coronary Artery:

  • Anterior interventricular artery (supplies blood to both ventricles and anterior 2/3 of IV septum).

  • Circumflex artery (supplies LA and posterior wall of LV).

• Note: Numerous anastomoses provide alternative routes for blood flow (collateral circulation).

Cardiac Veins

• Great Cardiac Vein: parallels the anterior interventricular artery.

• Middle Cardiac Vein: parallels the posterior interventricular artery.

• Small Cardiac Vein: parallels the marginal artery.

• All coronary veins drain into the coronary sinus, collecting 90% of coronary blood to send to the RA.

Blood Flow Pathway Through the Heart

1. Blood enters the RA from superior and inferior venae cavae.

2. Blood flows through the right AV (tricuspid) valve into the RV.

3. Contraction of the RV opens the pulmonary valve, directing blood into the pulmonary trunk.

4. Blood is distributed by right and left pulmonary arteries to the lungs, unloading CO2 and loading O2.

5. Oxygenated blood returns from the lungs via pulmonary veins to the LA.

6. Blood in the LA flows through the left AV (bicuspid) valve into the LV.

7. Contraction of the LV opens the aortic valve, sending blood into the ascending aorta.

8. Blood is distributed to the body, where it delivers O2 and collects CO2.

Heart Sounds

• Valves open and close in response to pressure changes:

  • 1st Heart Sound (S1): "lubb" occurs with closure of AV valves.

  • 2nd Heart Sound (S2): "dupp" occurs with closure of semilunar valves.

Clinical Application: Heart Murmurs

• Causes:

  • Abnormal sounds produced by blood flow through valves that are either:

  • “Too leaky” (Insufficiency).

  • “Too closed” (Stenosis).

Cardiac Microscopic Anatomy

• Types of Cardiac Muscle Cells:

  • Conducting Cells:

  • Also known as autorhythmic cells.

  • Control and coordinate heartbeats through automaticity.

  • Form the heart's electrical conduction system.

  • Contractile Cells (Cardiocytes):

  • Produce contractions to propel blood.

  • Constitute the myocardium and are small and branched.

Intercalated Discs

• Cardiocytes are connected by intercalated discs.

• Composition:

  • Gap Junctions: Allow rapid propagation of electrical signals, resulting in myocardium working as a syncytium.

  • Desmosomes: Provide structural strength by creating strong connections between cardiocytes.

Cardiac Conducting System

• Sequence of electrical signal transmission:

  1. SA Node (Sinoatrial Node): Pacemaker of the heart, establishes heart rate (sinus rhythm).

  2. Internodal Pathways: Cause contraction of atria.

  3. AV Node (Atrioventricular Node).

  4. AV Bundle (Bundle of His): Splits to form left and right bundle branches.

  5. Purkinje Fibers: Cause contraction of ventricles.

Heart Rate Regulation

• Normal Resting Heart Rate: 60-100 beats per minute (bpm).

  • Tachycardia: Heart rate >100 bpm.

  • Bradycardia: Heart rate <60 bpm.

  • Newborns: 100-160 bpm.

Autonomic Nerve Supply to the Heart

• Autonomic signals modulate but do not initiate heartbeat:

  • Chronotropic Effect: Affects heart rhythm.

  • Inotropic Effect: Affects force of contraction.

Clinical Application: Myocardial Infarction (Heart Attack)

• Definition: The most common cause of death in the US.

• Cause: Coronary artery disease, developing an atherosclerotic plaque.

  • Plaque may weaken and rupture, forming a thrombus that blocks blood flow, causing coronary ischemia.

  • Leads to death of cardiac muscle cells due to lack of oxygen.

• Consequences of Lack of O2:

  • Damage to electrical cells leading to arrhythmias (e.g., ventricular fibrillation, cardiac arrest).

  • Death of myocardial cells leading to left/right ventricular failure.

Diagnosis of Myocardial Infarction

• Imaging techniques like digital subtraction angiography (DSA) scan used to detect arterial blockage.

Treatment Options for Myocardial Infarction

• Balloon Angioplasty:

  • Involves inserting a catheter with a collapsed balloon, which is then dilated to widen the artery.

• Stent Placement:

  • A stent remains in the coronary artery to keep it open.

• Coronary Artery Bypass Graft (CABG):

  • Creating new pathways for blood flow using grafts from other arteries/veins.

  • May involve single, double, triple, or quadruple bypasses.

Congestive Heart Failure (CHF)

• Definition: Results from either ventricle's inability to eject blood effectively.

• Common causes: myocardial infarction, chronic hypertension, valvular insufficiency, congenital defects.

Effects of Ventricular Failure

1. Right Ventricular Failure:

   • If RV pumps less than LV, blood backs up into systemic circulation:

     • Symptoms include jugular vein distention, hepatomegaly, ascites, peripheral edema.

2. Left Ventricular Failure:

   • If LV pumps less than RV, blood backs up into pulmonary circulation:

     • Symptoms include shortness of breath, cyanosis, fatigue, pulmonary edema.