heart and thorax 2026

External Intercostals

• Location: Between ribs, superficial intercostal layer

• Layer/Compartment: Superficial layer of three intercostal muscle layers

• Position: Superficial to internal intercostals; fibers run obliquely downward and anteriorly (hands in pockets direction)

• Origin: Inferior border of rib above

• Insertion: Superior border of rib below

• Action: Elevate ribs (inspiration/inhalation); expand thoracic cavity

Internal Intercostals

• Location: Between ribs, middle intercostal layer

• Layer/Compartment: Middle layer of three intercostal muscle layers

• Position: Deep to external intercostals, superficial to innermost intercostals; fibers run obliquely downward and posteriorly (perpendicular to external intercostals)

• Origin: Superior border and costal groove of rib below

• Insertion: Inferior border of rib above

• Action: Depress ribs (forced expiration); decrease thoracic cavity volume

Innermost Intercostals

• Location: Between ribs, deepest intercostal layer

• Layer/Compartment: Deepest layer of three intercostal muscle layers

• Position: Deep to internal intercostals; fibers run same direction as internal intercostals

• Origin: Superior border of rib below

• Insertion: Inferior border of rib above

• Action: Depress ribs (forced expiration); assist internal intercostals

• Note: Separated from internal intercostals by intercostal neurovascular bundle

Transversus Thoracis

• Location: Internal (deep) surface of anterior thoracic wall

• Layer/Compartment: Deep to sternum and costal cartilages

• Position: Posterior surface of sternum and costal cartilages; continuous with transversus abdominis

• Origin: Posterior surface of xiphoid process and body of sternum

• Insertion: Internal surfaces of costal cartilages 2-6

• Action: Weakly depresses ribs (assists forced expiration)

True ribs (1-7): Attach directly to sternum via own costal cartilage

False ribs (8-10): Attach indirectly to sternum (costal cartilages join cartilage of rib 7)

Floating ribs (11-12): No anterior attachment to sternum

Head (of Rib)

• Location: Posterior medial end of rib

• Function: Articulates with vertebral bodies (superior and inferior costal facets)

• Articulations: Usually articulates with two adjacent vertebral bodies and intervertebral disc

Neck (of Rib)

• Location: Short section between head and tubercle

• Function: Connects head to tubercle; narrow region

Tubercle (of Rib)

• Location: Posterior aspect of rib, at junction of neck and body

• Function: Articulates with transverse process of corresponding vertebra (transverse costal facet)

• Parts: Articular part (smooth facet) and non-articular part (roughened, for ligament attachment)

Body (Shaft of Rib)

• Location: Main curved portion of rib

• Function: Forms thoracic cage wall; protects thoracic organs

• Features: Costal groove on inferior internal surface (houses intercostal neurovascular bundle); angle of rib (where rib curves anteriorly)

Facets (of Rib)

• Location: On head and tubercle of rib

• Function: Articular surfaces for vertebral articulations

• Head facets: Superior and inferior facets articulate with vertebral bodies

• Tubercle facet: Articulates with transverse process

Manubrium

• Location: Superior portion of sternum

• Function: Articulates with clavicles (sternoclavicular joints) and first two ribs

• Features: Jugular notch (superior), clavicular notches (lateral), facets for ribs 1-2

• Landmark: Sternal angle (manubriosternal joint with body, at rib 2 level - T4-T5 vertebral level)

Jugular Notch (Suprasternal Notch)

• Location: Superior border of manubrium, midline

• Function: Palpable landmark; U-shaped depression

• Clinical: Palpable at base of neck

Body (of Sternum)

• Location: Middle and largest portion of sternum

• Function: Articulates with ribs 2-7 via costal cartilages

• Features: Facets for costal cartilages along lateral borders

Xiphoid Process

• Location: Inferior tip of sternum

• Function: Attachment point for diaphragm and abdominal muscles

• Features: Variable shape (pointed, bifid, or perforated); ossifies late in life

• Clinical: Landmark for CPR hand placement (avoid pressing directly on xiphoid)

Ascending Aorta

• Location: Begins at left ventricle, ascends in middle mediastinum

• Course: Ascends 5cm from aortic valve to aortic arch

• Region Supplied: Heart via coronary arteries (right and left coronary arteries branch from ascending aorta immediately above aortic valve)

• Contained within: Pericardial sac

Aortic Arch

• Location: Superior mediastinum, arches over left main bronchus

• Course: Curves posteriorly and to left, becomes descending aorta at T4-T5

• Branches (anterior to posterior): Brachiocephalic artery, left common carotid artery, left subclavian artery

• Region Supplied: Head, neck, upper limbs via branches

Brachiocephalic Artery

• Location: First branch of aortic arch (right side only)

• Course: Ascends right, divides into right common carotid and right subclavian

• Region Supplied: Right side of head, neck, right upper limb

Subclavian Arteries

• Location: Base of neck, arches over first rib

• Right: Branches from brachiocephalic artery

• Left: Branches directly from aortic arch (third branch)

• Region Supplied: Upper limb, brain (via vertebral artery)

Descending Thoracic Aorta

• Location: Posterior mediastinum, left of midline

• Course: Descends from T4-T5 to T12, passes through diaphragm (becomes abdominal aorta)

• Branches: Posterior intercostal arteries (pairs 3-11), bronchial arteries, esophageal arteries, pericardial branches

• Region Supplied: Thoracic wall, esophagus, bronchi, pericardium

Posterior Intercostal Arteries

• Location: Run along inferior border of ribs in costal grooves

• Origin: Pairs 1-2 from supreme intercostal artery (from subclavian); Pairs 3-11 from descending thoracic aorta

• Course: Run with intercostal vein (superior) and intercostal nerve (inferior) in costal groove - "VAN" arrangement

• Region Supplied: Intercostal muscles, thoracic wall, spinal cord (via spinal branches)

Subclavian Veins

• Location: Anterior to subclavian artery, anterior to anterior scalene muscle

• Course: Continuation of axillary vein; joins internal jugular vein to form brachiocephalic vein

• Region Drained: Upper limb

Brachiocephalic Veins

• Location: Superior mediastinum

• Course: Left and right brachiocephalic veins (formed by junction of subclavian and internal jugular veins) unite to form superior vena cava

• Right brachiocephalic: Short and vertical

• Left brachiocephalic: Longer, crosses midline anterior to aortic arch branches

• Region Drained: Head, neck, upper limbs

Superior Vena Cava

• Location: Superior and middle mediastinum, right side

• Course: Formed by union of left and right brachiocephalic veins; descends and enters right atrium

• Region Drained: All structures above diaphragm (head, neck, upper limbs, thoracic wall)

• Tributaries: Azygos vein (enters posteriorly)

Inferior Vena Cava

• Location: Passes through diaphragm at T8 level; brief course in middle mediastinum

• Course: Ascends through abdomen, pierces diaphragm, immediately enters right atrium

• Region Drained: All structures below diaphragm (abdomen, pelvis, lower limbs)

Azygos Vein

• Location: Posterior mediastinum, right of vertebral column

• Course: Ascends right side of vertebral column; arches over right main bronchus to enter SVC

• Tributaries: Right posterior intercostal veins, hemiazygos vein, accessory hemiazygos vein, esophageal veins, bronchial veins

• Region Drained: Thoracic wall, mediastinal structures

• Function: Provides collateral circulation between SVC and IVC systems

Hemiazygos Vein

• Location: Posterior mediastinum, left of vertebral column (lower half)

• Course: Ascends left side (T9-T12 level); crosses midline at T8-T9 to drain into azygos vein

• Tributaries: Lower left posterior intercostal veins (9-11), left subcostal vein

• Region Drained: Lower left thoracic wall

Accessory Hemiazygos Vein

• Location: Posterior mediastinum, left of vertebral column (upper half)

• Course: Descends left side (T5-T8 level); crosses midline to drain into azygos vein

• Tributaries: Upper left posterior intercostal veins (4-8), left bronchial veins

• Region Drained: Upper left thoracic wall

Phrenic Nerve

• Location: Anterior mediastinum and middle mediastinum

• Origin: C3, C4, C5 nerve roots ("C3, 4, 5 keeps the diaphragm alive")

• Course: Descends anterior to anterior scalene muscle in neck; enters thorax; descends along lateral pericardium (between pericardium and mediastinal pleura); passes through diaphragm

• Function: Motor innervation to diaphragm (primary muscle of respiration); sensory to central diaphragm, pericardium, mediastinal pleura

Vagus Nerve (CN X)

• Location: Posterior mediastinum

• Origin: Brainstem (medulla oblongata)

• Course: Descends through neck in carotid sheath; enters thorax; right vagus passes posterior to root of right lung and along esophagus; left vagus passes anterior to aortic arch and along esophagus

• Branches in thorax: Recurrent laryngeal nerves, cardiac branches, pulmonary branches, esophageal branches

• Function: Parasympathetic innervation to heart (slows heart rate), lungs (bronchoconstriction), esophagus (peristalsis)

Intercostal Nerves

• Location: Between ribs, in costal grooves

• Origin: Ventral rami of thoracic spinal nerves T1-T11 (T12 is subcostal nerve)

• Course: Run in costal groove with posterior intercostal artery and vein - "VAN" arrangement (Vein, Artery, Nerve from superior to inferior)

• Function: Motor to intercostal muscles; sensory to thoracic and abdominal wall skin (dermatomal distribution)

Primary Bronchi

• Location: Middle mediastinum

• Course: Right and left primary bronchi branch from trachea at carina (T4-T5)

• Right primary bronchus: Wider, shorter, more vertical (foreign objects more likely to enter right side)

• Left primary bronchus: Narrower, longer, more horizontal (passes under aortic arch)

• Function: Conduct air from trachea to lungs; each bronchus enters lung

Apex (heart)

• Location: Inferior, left, anterior tip of heart

• Position: Points downward, anteriorly, and to the left; typically at 5th intercostal space, left midclavicular line

• Formed by: Left ventricle

• Function: Where apical impulse (heartbeat) can be palpated

• Clinical: "Point of maximal impulse" (PMI)

Base

• Location: Posterior, superior surface of heart

• Position: Opposite the apex; directed posteriorly and superiorly

• Formed by: Mainly left atrium, with some contribution from right atrium

• Function: Where great vessels enter and exit heart

• Note: Base is NOT the inferior surface (common misconception)

Pericardium

• Location: Surrounds heart and roots of great vessels

• Function: Protects heart, prevents overdistension, anchors heart in mediastinum, reduces friction

• Layers: Fibrous pericardium (outer tough layer), serous pericardium (inner, divided into parietal and visceral layers)

Parietal Pericardium

• Location: Inner layer of fibrous pericardium

• Position: Lines inner surface of fibrous pericardium

• Function: Secretes pericardial fluid into pericardial cavity

• Continuous with: Visceral pericardium (epicardium) at reflection points around great vessels

Visceral Pericardium (Epicardium)

• Location: Outer layer of heart wall, adheres directly to myocardium

• Position: Innermost layer of serous pericardium

• Function: Secretes pericardial fluid; outer layer of heart wall

• Continuous with: Parietal pericardium at great vessel roots

Pericardial Cavity: Potential space between parietal and visceral pericardium containing small amount (~15-50mL) of pericardial fluid for lubrication

Myocardium

• Location: Middle layer of heart wall

• Position: Between epicardium (outer) and endocardium (inner)

• Structure: Cardiac muscle tissue; thickest in left ventricle, thinner in atria

• Function: Contracts to pump blood; generates force for circulation

Right Atrial Auricle (Right Atrial Appendage)

• Location: Small pouch-like extension of right atrium

• Position: Projects anteriorly from right atrium; wraps around aortic root

• Function: Increases atrial volume; contains pectinate muscles

Left Atrial Auricle (Left Atrial Appendage)

• Location: Small pouch-like extension of left atrium

• Position: Projects anteriorly from left atrium; lateral to pulmonary trunk

• Function: Increases atrial volume; contains pectinate muscles

• Clinical: Common site of thrombus formation in atrial fibrillation

Anterior Interventricular Sulcus

• Location: Groove on anterior surface of heart between right and left ventricles

• Position: Runs from coronary sulcus (atrioventricular groove) toward apex

• Function: External landmark indicating position of interventricular septum

• Contains: Left anterior descending artery (LAD), great cardiac vein, fat

Posterior Interventricular Sulcus

• Location: Groove on posterior (diaphragmatic) surface of heart between ventricles

• Position: Runs from coronary sulcus toward apex on posterior surface

• Function: External landmark indicating position of interventricular septum

• Contains: Posterior descending artery (PDA), middle cardiac vein, fat

Interventricular Septum

• Location: Wall separating left and right ventricles

• Position: Runs between ventricles from base to apex

• Parts: Muscular portion (thick, inferior 2/3), membranous portion (thin, superior 1/3)

• Function: Prevents mixing of oxygenated and deoxygenated blood

• Contains: AV node and bundle of His in membranous portion (part of conduction system)

Trabeculae Carneae

• Location: Inner surface of ventricles

• Position: Irregular muscular ridges projecting into ventricular chambers

• Function: Increase surface area; prevent suction effect during contraction; assist in ventricular contraction

• Note: More prominent in ventricles than in atria

Papillary Muscles

• Location: Project from ventricular walls (anterior, posterior, septal)

• Position: Arise from ventricular myocardium; project into ventricular cavities

• Function: Anchor chordae tendineae; contract during systole to prevent valve prolapse

• Location specifics: Right ventricle has 3 (anterior, posterior, septal); left ventricle has 2 (anterior, posterior)

Pectinate Muscles

• Location: Interior of atria (especially atrial auricles)

• Position: Parallel muscular ridges in atrial walls

• Function: Increase atrial contractility; increase surface area

• Extent: Separated from smooth posterior atrial wall by crista terminalis (right atrium)

Crista Terminalis

• Location: Right atrium

• Position: Vertical muscular ridge on internal surface; separates rough anterior portion (pectinate muscles) from smooth posterior portion

• Function: Landmark; embryological junction; contains sinoatrial (SA) node at superior end

Conus Arteriosus (Infundibulum)

• Location: Superior portion of right ventricle

• Position: Smooth-walled outflow tract leading to pulmonary valve and pulmonary trunk

• Function: Funnel-shaped region directing blood from right ventricle to pulmonary trunk

• Note: Smooth-walled (no trabeculae) unlike rest of right ventricle

Mitral Valve (Bicuspid Valve)

• Location: Between left atrium and left ventricle

• Structure: Two cusps/leaflets (anterior and posterior)

• Function: Prevents backflow of blood from left ventricle to left atrium during systole

• Attachments: Chordae tendineae connect cusps to papillary muscles

Tricuspid Valve

• Location: Between right atrium and right ventricle

• Structure: Three cusps/leaflets (anterior, posterior, septal)

• Function: Prevents backflow of blood from right ventricle to right atrium during systole

• Attachments: Chordae tendineae connect cusps to papillary muscles

Chordae Tendineae

• Location: Attach valve cusps to papillary muscles

• Structure: Fibrous cords ("heart strings")

• Function: Prevent valve leaflets from prolapsing (everting) into atria during ventricular contraction

• Action: Papillary muscles contract → tension on chordae → hold valve leaflets in place

Valve Flap (Leaflet)

• Location: Mobile portions of valves

• Structure: Thin fibrous tissue covered by endocardium

• Function: Opens to allow blood flow, closes to prevent backflow

Valve Cusps

• Location: Individual leaflets of valves

• Function: Mitral has 2 cusps; tricuspid has 3 cusps; semilunar valves have 3 cusps each

Pulmonary Valve

• Location: Between right ventricle (conus arteriosus) and pulmonary trunk

• Structure: Three semilunar cusps (left, right, anterior)

• Function: Prevents backflow from pulmonary trunk to right ventricle during diastole

• Action: Opens during ventricular systole; closes during ventricular diastole

• Note: No chordae tendineae or papillary muscles (unlike AV valves)

Aortic Valve

• Location: Between left ventricle and ascending aorta

• Structure: Three semilunar cusps (left, right, posterior/non-coronary)

• Function: Prevents backflow from aorta to left ventricle during diastole

• Action: Opens during ventricular systole; closes during ventricular diastole

• Special feature: Coronary artery ostia (openings) located immediately above left and right cusps (coronary arteries fill during diastole)

Ostia to Coronary Arteries

• Location: In aortic sinuses (dilations in ascending aorta immediately above aortic valve cusps)

• Position: Left coronary ostium above left aortic cusp; right coronary ostium above right aortic cusp

• Function: Openings where coronary arteries originate from ascending aorta

• Filling: Coronary arteries fill during diastole (when aortic valve is closed and aortic pressure is higher)

Right Coronary Artery (RCA)

• Location: Originates from right aortic sinus

• Course: Descends in right atrioventricular (coronary) sulcus; wraps around to posterior surface

• Branches: Right marginal artery, posterior descending artery (PDA) in 70% of people (right-dominant), SA nodal artery (60%), AV nodal artery

• Supplies: Right atrium, right ventricle, inferior wall of left ventricle, SA node (usually), AV node (usually)

Left Coronary Artery (LCA)

• Location: Originates from left aortic sinus

• Course: Short main stem (1-2cm); bifurcates into LAD and circumflex

• Branches: Left anterior descending (LAD), left circumflex

• Supplies: Most of left ventricle, interventricular septum, left atrium

Left Circumflex Artery

• Location: Branch of left coronary artery

• Course: Continues in left atrioventricular sulcus; wraps around to posterior surface

• Branches: Left marginal artery, posterior descending artery (in 10% - left dominant)

• Supplies: Left atrium, lateral and posterior walls of left ventricle, SA node (40%)

Left Anterior Descending Artery (LAD / Anterior Interventricular Artery)

• Location: Branch of left coronary artery

• Course: Descends in anterior interventricular sulcus toward apex

• Branches: Diagonal branches, septal perforating branches

• Supplies: Anterior wall of left ventricle, most of interventricular septum, apex

• Clinical: "Widowmaker" — most critical coronary artery; occlusion causes massive anterior MI

Posterior Descending Artery (PDA / Posterior Interventricular Artery)

• Location: Usually branch of RCA (70% right-dominant), sometimes from circumflex (10% left-dominant), or both (20% co-dominant)

• Course: Descends in posterior interventricular sulcus toward apex

• Supplies: Posterior interventricular septum, inferior walls of both ventricles

Coronary Sinus

• Location: Posterior atrioventricular sulcus (coronary sulcus) on posterior surface

• Course: Runs in coronary sulcus; drains into right atrium (between IVC and tricuspid valve)

• Tributaries: Great cardiac vein, middle cardiac vein, small cardiac vein, posterior vein of left ventricle, oblique vein of left atrium

• Function: Main venous drainage of heart (collects ~75% of cardiac venous blood)

Great Cardiac Vein

• Location: Accompanies LAD in anterior interventricular sulcus

• Course: Ascends in anterior interventricular sulcus; turns left in coronary sulcus; continues as coronary sinus

• Drains: Anterior left ventricle, interventricular septum, left atrium

Small Cardiac Vein

• Location: Accompanies right marginal artery

• Course: Runs along right margin of heart; drains into coronary sinus

• Drains: Right atrium, right ventricle

Middle Cardiac Vein

• Location: Accompanies posterior descending artery in posterior interventricular sulcus

• Course: Ascends in posterior interventricular sulcus toward base; drains into coronary sinus

• Drains: Posterior walls of ventricles, posterior interventricular septum

Describe and trace the path/sequence of the circulatory loops from left ventricle back to left atrium