Anatomy of the Heart and the Great Vessels

Heart overview

Closed system of tubes - carry blood to and from capillary beds

2 separate circulations

1. Pulmonary circulation

2. Systemic circulation

Heart location and orientation

Located in the thoracic cavity, within the mediastinum

Borders of the mediastinum:

• Pleural lining of the lungs

• Diaphragm

• Thoracic vertebrae

• Sternum

The mediastinum is divided into a superior mediastinum and inferior mediastinum by a horizontal plane trhough the sternal angle - the heart is located in the inferior mediastinum

• The ‘apex’ of the heart projects anteriorly towards the ribs

• The ‘base’ of the heart projects posteriorly towards the vertebrae

Pericardium and layers

Pericardium: a fibroserous sac that surrounds thee heart and the roots of the great vessels

• Fibrous pericardium: blends with the fibres of the diaphragm, holding the heart in place, prevents overdistension/enlargement of the heart

• Serous pericardium: consists of 2 layers and cavoty containing fluid, layers glide over each other

  • 2 layers (parietal and visceral)

Pericarditis - causes, symptoms and treatment

Inflammation of the pericardium

Causes:

• Infection, autoimmune disorder, trauma

Symptoms:

• Pain, shortness of breath, increased and irregular heartbeat

• Pericardium is innervated by the phenic nerve (C3 - C5)

  • Referred pain to skin of shoulder and chest

Treatment:

• NSAIDs, corticosteroids, antibiotics

• Pericardiocentesis (removal of excess fluid)

Pericardial Effusion - what is it and treatment

Build up of excess fluid in the pericardial cavity

Can lead to cardiac tamponade (compression of the heart) reducing force of contractions and potentially leading to cardiac arrest

Treatment:

• Non-severe: NSAIDs, antibiotics

• Severe: pericardiocentesis (removal of excess fluid by needle)

What are the 3 layers of the heart wall?

Endocardium

Myocardium

Epicardium

What are the 2 types of septums?

Interatrial septum

Interventricular septum

Purpose of heart valves and 4 types of heart valves

Prevent regurgitation of blood back

Heart valve disease involves a defect in a heart valve

• Valve narrowing (stenosis)

• Incomplete valve closure (regurgitation)

Heart valve replacement surgery replaces a faulty heart valve with a new functional heart valve - untreated heart valve disease may lead to blood clots, stroke or heart failure

4 heart valves:

1. Tricuspid valve - between right atrium and right ventricle

2. Pulmonary valve - between right ventricle and pulmonary trunk

3. Bicuspid (mitral) valve - between the left atrium and left ventricle

4. Aortic valve - between the left ventricle and aorta

How many cusps does the tricuspid and bicuspid (mitral) valve have?

Tricuspid valve - 3 cusps

Bicuspid (miral) - 2 cusps

Chordae tendineae

Papillary muscle

Semilunar valves

Aortic and pulmonary valve

The cusps fill with blood forcing them together, causing closure

2 cusps in the aortic valve contain openings for the coronary arteries

Mitral valve stenosis (narrowing) - clinical signs, diagnosis and treatment

Clinical signs

• High left atrial pressure

• Pulmonary hypertension

Diagnosis

• Chest X-ray; echocardiogram

Treatment

• Diuretics (congestion)

• Beta blockers/ Ca²⁺ channel blockers (heart rate)

• Warfarin (anticoagulant)

• Valvuloplasty

Myocardial infarction

Result of blockage of the coronary arteries

Commonly known as a heart attack

• Major cause of death due to coronary artery and thrombosis

• Causes necrosis in the the wall of the heart

Coronary arteries

Left and right coronary arteries

• Don’t overlap their supply or anastomose

• Blockage = no blood supply

• Cardiac muscle ischamia

• Myocardial infection and necrosis

Coronary veins

Cardiac veins drain into the coronary sinus which drains into the right atrium

What are the main anatomical features of the right atrium?

• Receives deoxygenated blood

• Has relatively thin walls because it only pumps blood into the right ventricle

Contains openings of:

• Superior vena cava

• Inferior vena cava

• Coronary sinus

Communicates with the right ventricle through the tricuspid valve

What is the function of the right atrium?

1. Receives deoxygenated blood from the systemic circulation

2. Receives blood from the SVC, IVC and coronary sinus

3. Passes blood into the right ventricle

4. Contracts during atrial systole to assist ventricular filling

What are the main features of the right ventricle?

• Receives blood from the right atrium through the tricuspid valve

• Pumps blood into the pulmonary trunk

• Has a thinner wall than the left ventricle

• Contains papillary muscles and chordae tendineae which prevent reflux into the right atrium

• Contains the moderator band which carries the right bundle branch of the atrioventricular bundle

What is the main anatomical feature of the left atrium?

• Receives oxygenated blood from the lungs

• Receives blood through the four pulmonary veins

• Has a relatively smooth internal surface

• Contains pectinate muscles mainly within the left atrial appendage

• Communicates with the left ventricle through the mitral valve

What is the function of the left atrium?

1. Receives oxygenated blood from the pulmonary veins

2. Stores and transfers this blood to the left ventricle

3. Contracts during atrial systole to assist filling of the left ventricle

What are the main features of the left ventricle?

• Receives oxygenated blood from the left atrium through the mitral valve

• Pumps blood into the aorta

• Has the thickest muscular wall of all four chambers

• Contains trabeculae carneae

• Contains larger papillary muscles and chordae tendineae due to increased pressure

• Generates the high pressure required for systemic circulation

Why is the left ventricular wall thicker than the right ventricular wall?

• The left ventricle must pump blood around the entire systemic circulation

• This creates much greater resistance than pumping blood to the nearby lungs

• Therefore, the left ventricle must generate a much higher pressure

• Its thicker myocardium allows it to produce this pressure

Great vessels of the heart - anterior view diagram

Great vessels of the heart - posterior view diagram

How do the AV valves prevent reflux?

During ventricular systole:

1. Ventricular pressure rises

2. AV valves close

3. Papillary muscles contract

4. Chordae tendineae become tense

5. The valve cusps are prevented from inverting into the atria

6. Blood is therefore directed into the pulmonary trunk/aorta

How do semilunar valves prevent reflux?

During ventricular systole:

• Ventricular pressure becomes greater than arterial pressure

• Semilunar valves open

• Blood is ejected into the pulmonary trunk/aorta

During diastole:

• Arterial pressure becomes greater than ventricular pressure

• Blood attempts to flow backwards

• The cusps fill with blood and close

Electrical conduction system of the heart - ECG

Cardiac conduction system process

1. Impulse originates from the sinoatrial (SA) node which is located in the right atrium - acts as the heart’s natural pacemaker

2. The impulse spreads across the right and left atrium causing atrial depolarisation and contraction

3. Impulse reaches the atrioventricular (AV) node located in the lower part of the right atrium near the interatrial septum

4. AV node introduces conduction delay allowing time for ventricles to fill with blood before contracting

5. Impulse then passed into the bundle of His

6. Bundle of His carries electrical excitation from the AV node through the fibrous skeleton of the heart into the interventricular septum

7. Purkinje fibres distribute electrical excitation throughout the ventricular myocardium allowing rapid, coordinated ventricular depolarisation and contraction

What is the cardiac plexus formed from?

• Parasympathetic fibres from vagus nerve

• Sympathetic fibres from sympathetic trunk

Neuronal control of the heart - parasympathetic and sympathetic stimulation

Parasympathetic stimulation:

• Decreases heart rate

• Reduces force of contraction

• Constricts coronary arteries

Sympathetic stimulation:

• Increases heart rate

• Increases force of contraction

Referred pain

• Visceral afferent innervation of the heart corresponds to T1 – T4 spinal cord segments

• Skin dermatomes T1 – T4 map to skin of the chest and inner-side of the arm on left side of body

• When cardiac cells die during myocardial infarction this results in referred pain in these dermatome

Arrhythmia - what is it and treatment

Irregular heartbeat - atrial fibrillation

• ‘Episodes’ or ‘burts’ of irregular and rapid beating of atria

• Irregular blood flow can lead to clot formation

Treatment:

• Rate control medications (beta blockers)

• Anticoagulants (warfarin)

• Pacemaker implantation