Heart

Cor

Also known as: cor, cardia

The heart is a hollow, four-chambered muscular pump about the size of a clenched fist. It lies inside the pericardium in the middle mediastinum and drives blood through the pulmonary and systemic circulations.

Show on body map Cardiac Conduction System & ECG Heart — Internal Anatomy Respiratory Tract

Overview

Dimensions≈12 × 8 × 6 cm

Mass≈250–350 g

Apex beatLeft 5th intercostal space, midclavicular line

Beats per day≈100 000

Cardiothoracic ratio<0.5 on PA chest radiograph

Position and orientation. The heart lies obliquely in the middle mediastinum, with about two-thirds of its mass to the left of the median plane. Its base is formed mainly by the left atrium and faces posteriorly toward the T5–T8 vertebrae, separated from them by the oesophagus and descending aorta. Its apex is formed by the left ventricle. It points anteriorly, inferiorly and to the left, and lies behind the left 5th intercostal space about 9 cm from the midline. The heart is rotated so that the right ventricle forms most of the front (sternocostal) surface and the left atrium is the most posterior chamber.

Surface landmarks. The coronary (atrioventricular) sulcus encircles the heart between atria and ventricles and carries the right coronary artery, the circumflex artery and the coronary sinus. The anterior and posterior interventricular sulci mark the septum and carry the LAD and posterior interventricular arteries. The point where the coronary and posterior interventricular sulci meet is called the crux of the heart.

Size. The adult heart measures about 12 × 8 × 6 cm. It weighs about 250–300 g in women and 300–350 g in men. It beats about 100 000 times a day and pumps roughly 7 000 L.

Location: Middle mediastinum, enclosed in the pericardium. It lies behind the sternum and the 2nd–6th costal cartilages, rests on the central tendon of the diaphragm and sits between the two lungs and pleural sacs.

Parts & subdivisions

Apex
Formed by the left ventricle. It produces the palpable apex beat in the left 5th intercostal space, midclavicular line.
Base (posterior surface)
Formed mainly by the left atrium, with a small contribution from the right atrium. It receives the pulmonary veins and lies in front of the oesophagus.
Sternocostal (anterior) surface
Formed mainly by the right ventricle, with the right atrium on the right and a strip of left ventricle on the left.
Diaphragmatic (inferior) surface
Formed mainly by the left ventricle and part of the right ventricle, which are separated by the posterior interventricular sulcus. It rests on the central tendon of the diaphragm.
Pulmonary surfaces
The right pulmonary surface is formed by the right atrium. The left is formed by the left ventricle, which makes the cardiac impression in the left lung.
Borders (chest radiograph)
The right border is the right atrium, between the SVC and IVC. The inferior border is mostly the right ventricle. The left border is the left ventricle and left auricle. The superior border is formed by the atria, auricles and great vessels.
Right atrium
Receives systemic venous blood.
Right ventricle
Pumps blood to the lungs.
Left atrium
Receives pulmonary venous blood.
Left ventricle
Pumps blood to the body.

Anatomy

Innervation

The heart is supplied through the cardiac plexus.

Sympathetic. Fibres from T1–T4/T5 travel in the cervical and thoracic cardiac nerves. Acting through β1 receptors, they increase heart rate, contractility and AV conduction.

Parasympathetic. The vagus nerves act through M2 receptors to slow the SA node (mainly the right vagus) and AV conduction (mainly the left vagus).

Visceral pain afferents. These travel with the sympathetic fibres to T1–T4, so ischaemic pain is referred to the chest, left arm and jaw.

Arterial supply

Right and left coronary arteries, which arise from the aortic sinuses just above the aortic valve. Left ventricular perfusion occurs mainly in diastole. Resting coronary flow is ≈225–250 mL/min (4–5% of cardiac output), and the heart already extracts ≈70–75% of the delivered oxygen at rest.

Venous drainage

Most venous blood returns through the cardiac veins into the coronary sinus and then the right atrium. The anterior cardiac veins open directly into the right atrium, and the smallest cardiac veins (venae cordis minimae) open into all four chambers.

Lymphatic drainage

Subendocardial and myocardial lymphatics drain to a subepicardial plexus. From there, vessels follow the coronary arteries to the tracheobronchial and brachiocephalic (anterior mediastinal) lymph nodes.

Relations

Anteriorly: the sternum, costal cartilages and anterior margins of the lungs. The cardiac notch of the left lung leaves part of the pericardium uncovered. Posteriorly: the oesophagus, descending aorta, azygos vein and the T5–T8 vertebrae. Inferiorly: the diaphragm. Laterally: the mediastinal pleura and lungs, with the phrenic nerves and pericardiacophrenic vessels between pericardium and pleura.

Development

The heart forms from a primitive heart tube derived from splanchnic mesoderm (the cardiogenic field). It starts beating around day 22, loops to the right and is septated into four chambers between weeks 4 and 7.

Function

Pumps deoxygenated blood to the lungs (right heart) and oxygenated blood to the body (left heart).
Generates its own rhythm through the sinoatrial node and conduction system (automaticity).
Creates arterial pressure, the driving force for blood flow and glomerular filtration.
Secretes natriuretic peptides (ANP from the atria, BNP from the ventricles), which promote sodium excretion and vasodilation.

Clinical significance

Apex beat and cardiomegaly

A displaced, diffuse apex beat suggests left ventricular dilatation. A heaving, sustained apex beat suggests pressure overload, as in aortic stenosis or hypertension. On a posteroanterior chest radiograph, a cardiothoracic ratio above 0.5 suggests cardiomegaly.

Referred cardiac pain

Cardiac pain afferents enter cord segments T1–T4. These segments also receive input from the chest wall and the medial arm (T1–T2 dermatomes). As a result, myocardial ischaemia is felt as central chest pressure that radiates to the left arm, neck or jaw.

Dextrocardia

In dextrocardia the heart lies in the right hemithorax. When it is part of complete situs inversus it is usually functionally normal. Kartagener syndrome (primary ciliary dyskinesia) is a recognised association.

Test yourself

Which papillary muscle is most likely to rupture after an inferior myocardial infarction?

Anterolateral papillary muscle of the LV
Posteromedial papillary muscle of the LV
Anterior papillary muscle of the RV
Septal papillary muscle of the RV

Answer: Posteromedial papillary muscle of the LV

The posteromedial papillary muscle usually has a single blood supply from the posterior interventricular artery. The anterolateral muscle has a dual supply from the LAD and circumflex. Rupture causes acute severe mitral regurgitation.

Fixed (non-respiratory) splitting of the second heart sound is characteristic of:

Right bundle branch block
Atrial septal defect
Left bundle branch block
Severe aortic stenosis

Answer: Atrial septal defect

In an ASD, the left-to-right shunt overloads the right ventricle with volume, and the shunt cancels out the normal respiratory swings in right heart filling. P2 is therefore delayed equally in inspiration and expiration. RBBB causes wide splitting that still varies with breathing. LBBB and severe AS cause reversed (paradoxical) splitting.

Using the Fick principle: O₂ consumption is 250 mL/min, arterial O₂ content is 200 mL/L and mixed venous O₂ content is 150 mL/L. What is the cardiac output?

2.5 L/min
5 L/min
7.5 L/min
12.5 L/min

Answer: 5 L/min

CO = VO₂ ÷ (CaO₂ − CvO₂) = 250 ÷ (200 − 150) = 250 ÷ 50 = 5 L/min.