Evidence-based physiotherapy and rehabilitation resources
Editorial & review policy
Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · The trunk
A muscular pump the size of a fist, slung in a tough bag in the middle of the chest. Its wiring and its blood supply are laid out so logically that you can predict what a blockage will damage. This chapter works from the outside in.
The chest holds two pleural sacs, one per lung. Between them runs a corridor from the thoracic inlet, the ring of bone at the root of the neck, down to the diaphragm: the mediastinum. Everything in the chest that is not lung or pleura sits in it.
One plane divides it, through the sternal angle in front and the disc between the fourth and fifth thoracic vertebrae behind. Above is the superior mediastinum; below, the inferior mediastinum, which the pericardium divides into three.
Figure 1 · The mediastinum, divided
| Division | Where it is | What sits there |
|---|---|---|
| Superior | Superior thoracic aperture, the thoracic inlet, down to the sternal angle plane | Aortic arch and branches, brachiocephalic veins, upper superior vena cava, trachea, oesophagus, thoracic duct, vagus and phrenic nerves, thymus |
| Anterior | Sternum to pericardium | Fat, lymph nodes, the last of the thymus. The narrowest part |
| Middle | The pericardium and its contents | Heart, roots of the great vessels, phrenic nerves, main bronchi |
| Posterior | Pericardium to vertebrae five to twelve | Descending aorta, oesophagus, thoracic duct, azygos and hemiazygos veins |
Why the sternal angle is the best landmark in the chest
It is the ridge where the manubrium meets the body of the sternum. Find it and you have found:
The heart hangs inside a double-walled bag. Almost every pericardial problem follows from its three layers and the one space between two of them.
| Layer | What it is | Why it matters |
|---|---|---|
| Fibrous pericardium | A tough outer sac, fused below to the central tendon of the diaphragm and blended above with the great vessels | Anchors the heart. It will not stretch at short notice |
| Parietal serous layer | A smooth membrane lining that sac | One half of the sliding surface |
| Pericardial cavity | The gap between the serous layers: a potential space holding a film of fluid | The heart moves inside it almost without friction |
| Visceral serous layer | The same membrane reflected onto the heart, the epicardium | The other half. Coronary vessels run beneath it in fat |
Picture a fist pushed into a partly inflated balloon. The fist is the heart, the layer on your knuckles is the visceral layer, the outer wall the parietal layer, and the trapped air the cavity.
Where the great vessels enter and leave, the visceral layer turns back on itself to become the parietal layer, and the turn leaves two blind recesses. The transverse sinus runs behind the aorta and pulmonary trunk, so a surgeon can pass a finger or a tape around both together. The oblique sinus is a cul-de-sac behind the left atrium.
Watch that word. It is doing three other jobs in this chapter: the aortic sinuses are bulges in an artery wall, the coronary sinus is a vein, and the sinuatrial node is muscle.
Figure 2 · The heart in the middle mediastinum, with the pericardium opened
Illustration to be added
Two panels from the front. Panel one: chest wall cut away, showing the heart inside an intact fibrous pericardium, lungs drawn back either side, the sac fused below to the central tendon of the diaphragm, and the phrenic nerve on its lateral surface. Panel two: a window cut in the fibrous sac with the flaps folded out, showing the parietal serous layer lining the flaps, the visceral layer on the heart, the thin cavity between them marked by a bracket, and the aorta, pulmonary trunk and superior vena cava leaving through the roof where the serous layers turn back. Pericardium pale grey-blue, muscle brick, bone ivory, nerves gold, artery red, vein blue, navy outlines on white, no text.
Two consequences you should be able to explain
The outer layers are supplied by the phrenic nerve, from the third to fifth cervical nerves. Its third and fourth cervical fibres share segments with the supraclavicular nerves, which supply the skin over the shoulder tip — so pericardial pain is felt there. And fluid in the cavity cannot push the fibrous sac outwards, so pressure rises instead and the thin-walled right chambers stop filling.
The heart sits behind the sternum, not on the left. Two thirds of its mass lies left of the midline and the apex points that way, which is where the impression comes from. It is tilted, so the right ventricle lies nearest the front.
The next table is worth the trouble because a chest film shows only an outline. Knowing which chamber makes each edge is how an enlarged left ventricle or a stretched right atrium is spotted on a plain film.
| Surface or border | Formed mainly by |
|---|---|
| Anterior surface, against the sternum | The right ventricle |
| Inferior surface, on the diaphragm | The left ventricle, with part of the right |
| Base, facing backwards | The left atrium, in front of the oesophagus |
| Apex | The left ventricle |
| Right border on a chest film | The right atrium, superior vena cava above |
| Left border on a chest film | The left ventricle, with the left auricle — the small ear-shaped pouch of the left atrium — and the pulmonary trunk above, and the aortic arch above those |
| Inferior border on a chest film | Mostly the right ventricle |
The apex beat is the one part you can find with a fingertip: fifth left intercostal space, just medial to the midclavicular line, about nine centimetres from the midline.
Two pumps sit side by side in one organ. The right sends blood a short way to the lungs; the left drives it round the whole body. They beat together but do not work equally hard, and the walls show it.
| Chamber | Receives from | Sends to | Worth noticing |
|---|---|---|---|
| Right atrium | Both venae cavae and the coronary sinus | Right ventricle | Thin walled. The fossa ovalis marks the opening that bypassed the lungs before birth |
| Right ventricle | Right atrium | Pulmonary trunk | Forms most of the front. The moderator band crosses it carrying conducting tissue |
| Left atrium | Four pulmonary veins | Left ventricle | Lies at the back, against the oesophagus |
| Left ventricle | Left atrium | Aorta | The thickest wall, two to three times the right. It forms the apex |
Figure 3 · The chambers and valves of the heart
Illustration to be added
Three panels. Panel one: the heart opened from the front with the front wall of both ventricles removed, showing the three tricuspid cusps with chordae running to the papillary muscles of the right ventricle, the two mitral cusps with larger papillary muscles, and the septum between; draw the left ventricular wall roughly two to three times the thickness of the right. Panel two: the right atrium opened, showing the openings of both venae cavae and the coronary sinus, the pectinate muscles ridging the inside of the right auricle, the ear-shaped pouch of the atrium, and the fossa ovalis. Panel three: the four valve rings from above with the atria removed, pulmonary in front and left, aortic just behind it, mitral behind and left, tricuspid behind and right, set in the fibrous ring, with the coronary openings in the aortic sinuses. Muscle brick, cusps pale ivory, fibrous ring navy, cavities white, no text.
Four valves, one job each: stop blood going backwards.
| Valve | Between | Cusps | Prevents |
|---|---|---|---|
| Tricuspid | Right atrium and right ventricle | Three | Backflow into the atrium as the ventricle contracts |
| Mitral, or bicuspid | Left atrium and left ventricle | Two | Backflow into the atrium as the ventricle contracts |
| Pulmonary | Right ventricle and pulmonary trunk | Three, half-moon | Backflow into the ventricle as it relaxes |
| Aortic | Left ventricle and aorta | Three, half-moon | Backflow into the ventricle as it relaxes |
The two kinds are held shut differently. Atrioventricular cusps are tethered by fine cords, the chordae tendineae, running to papillary muscles in the ventricular wall. Those muscles contract with the ventricle and hold the cords tight, so the cusps cannot flip back. The half-moon cusps need none of that: blood falling back fills their pockets and forces them shut.
All four rings sit in a band of dense fibrous tissue, which also insulates the atria electrically from the ventricles.
Figure 4 · Where a valve lies, and where it is heard
Now the point that confuses everybody. A valve is not heard where it lies. All four are crowded behind the sternum, and sound travels the way the blood is going. So you listen downstream, where the vessel beyond it comes nearest the chest wall.
| Valve | Lies behind | Best heard at |
|---|---|---|
| Pulmonary | Medial end of the left third costal cartilage | Left second space, at the sternal edge |
| Aortic | The sternum at the third space, just left of the midline | Right second space, at the sternal edge |
| Mitral | Left half of the sternum at the fourth costal cartilage | The apex, fifth left space, midclavicular line |
| Tricuspid | The sternum at the fourth space, just right of the midline | Lower left sternal edge, fourth or fifth space |
Cardiac muscle does not wait to be told; isolated heart muscle beats on its own. The conducting system adds not the beat but its order, so the atria empty before the ventricles squeeze, and the ventricles squeeze from the bottom up.
It is not nerve, either, but modified cardiac muscle. Nerves change the rate the heart chooses; they do not carry the impulse.
Figure 5 · The conducting system, step by step
Two steps repay a second look. The delay at the atrioventricular node stops the ventricles contracting on top of the atria. The atrioventricular bundle is the only muscular bridge across the insulating ring in a normal heart, so interrupting it separates the two halves of the heart.
A few people are born with an extra strand of muscle crossing that ring elsewhere. It gives the impulse a second, abnormal route to the ventricles, which is why the hedge "in a normal heart" belongs in that sentence.
A chamber full of blood cannot feed its own wall. Two arteries leave the aorta almost at once, from bulges above the aortic valve called the aortic sinuses. Learn each branch together with its territory.
| Artery | Course | Supplies |
|---|---|---|
| Right coronary | Right aortic sinus, along the groove between right atrium and ventricle, then to the back | Right atrium, most of the right ventricle, and in most people both nodes |
| Right marginal branch | Along the inferior, or acute, margin towards the apex, which it usually does not reach | Wall of the right ventricle |
| Posterior interventricular branch | Down the posterior interventricular groove, between the two ventricles. From the right coronary in most people | Inferior wall of both ventricles, back third of the septum |
| Left coronary | Left aortic sinus. A short stem dividing at once into the two branches below | Most of the left side of the heart |
| Anterior interventricular branch, or left anterior descending | Down the anterior interventricular groove, all the way to the apex | Front wall of the left ventricle, front two thirds of the septum, part of the right |
| Circumflex branch | Left in the atrioventricular groove, then to the back | Left atrium and the side wall of the left ventricle |
Dominance means simply this: which of the two coronary arteries gives the posterior interventricular branch. In most people it is the right coronary, which is why a right coronary blockage damages the inferior wall of the left ventricle too.
The left side of the heart feeds itself while it is resting
When the left ventricle contracts — systole, the squeeze — its thick wall clamps the vessels running through it and flow almost stops. Most left coronary filling therefore happens in diastole, the resting phase between beats. Diastole shortens far more than systole as the rate rises, and that is the anatomical basis of effort-related chest pain.
The right ventricle pumps against far lower pressure, so its wall never squeezes hard enough to shut its own vessels. Right coronary flow carries on through systole and is fairly even across the whole cycle.
Venous return follows the arteries back. Most collects into the coronary sinus, a short wide vein lying in the back part of the atrioventricular groove, between the left atrium and the left ventricle. It opens into the right atrium beside the inferior caval opening.
It receives the great cardiac vein from the anterior interventricular groove, the middle from the posterior interventricular groove and the small from the right border.
Figure 6 · The coronary arteries and the veins of the heart
Illustration to be added
Three panels. Panel one: the front of the heart, with the right coronary artery leaving the right aortic sinus, running in the atrioventricular groove and giving the right marginal branch; and the left coronary leaving the left aortic sinus behind the pulmonary trunk, dividing into the anterior interventricular branch down the anterior interventricular groove to the apex and the circumflex turning left. Panel two: the back and diaphragmatic surface, with the right coronary giving the posterior interventricular branch down the posterior interventricular groove, and the coronary sinus lying separately in the back part of the atrioventricular groove between left atrium and left ventricle, opening into the right atrium and receiving the great, middle and small cardiac veins. Panel three: a short-axis section of the ventricles seen from below, divided into three shaded territories -- anterior interventricular, the front wall of the left ventricle and the front two thirds of the septum, in pale red; circumflex, the side wall, in pale gold; right coronary, the inferior wall, the back third of the septum and the whole right ventricular free wall, in pale blue -- with the two interventricular grooves marked as the boundary points between territories. Arteries red, veins blue, muscle brick, navy outlines on white, no text.
The traffic runs in the direction that surprises students most. Pulmonary arteries carry blood low in oxygen; pulmonary veins carry it rich. A vessel is named for direction of travel, never for contents.
The venae cavae empty into the right atrium, and four pulmonary veins into the left. The pulmonary trunk leaves the right ventricle and divides right under the aortic arch, which is why the left pulmonary artery ends up tethered to the arch by the cord described below.
The ascending aorta gives the two coronary arteries and nothing else. That is why the heart wall takes the highest-pressure blood in the body, and takes it before any other organ. The aorta then becomes the arch, which gives the brachiocephalic trunk, the left common carotid and the left subclavian, in that order — so the left subclavian is the last branch before the aorta turns down behind the heart.
The ligamentum arteriosum runs from the pulmonary trunk to the underside of the arch, the remnant of a vessel that bypassed the unused lungs before birth. The left recurrent laryngeal nerve hooks under the arch beside it, which is why a chest problem can change the voice.
One word first, because it runs through the rest of this section. When a branch is blocked the muscle it fed dies, and that is an infarction. The wall that dies is simply the wall that branch supplied.
| Problem | Anatomy behind it | What you find |
|---|---|---|
| Blocked anterior interventricular branch | Front wall of the left ventricle and front of the septum | The largest territory of any branch, so the greatest loss of muscle |
| Blocked right coronary artery | Inferior wall and, in most people, both nodes | Inferior damage, often with a slow rate or heart block |
| Blocked circumflex branch | Side wall of the left ventricle | Lateral damage, the least common of the three |
| Complete heart block | The atrioventricular bundle is the only normal route across the insulating ring | Atria and ventricles beat independently and far apart in rate |
| A leaking mitral valve after infarction | The cusps are held by chordae from papillary muscles | If a papillary muscle fails, the cusp is unheld and blood escapes backwards |
| Cardiac tamponade | The fibrous sac does not stretch at short notice | Pressure rises, the right chambers cannot fill, output falls |
| Pericarditis | The outer layers are supplied by the phrenic nerve | Sharp pain behind the sternum, sometimes referred to the shoulder tip or the ridge of the trapezius, easier sitting forward |
| Cardiac pain felt elsewhere | Cardiac sensation enters the cord at upper thoracic levels | Pain reported in the chest wall and inner arm, not at the heart |
That last row deserves an explanation, because patients ask about it. The heart cannot tell the brain where it is. Its sensory fibres enter the first few thoracic segments, which also carry sensation from the front of the chest and the inner arm, so the brain reports it there. Other fibres run in the vagus nerve, and their connections explain pain in the neck and jaw.
Why a physiotherapist needs this chapter
You will meet these patients again in cardiac rehabilitation. Which artery was involved tells you which wall of muscle was lost. Because the right coronary artery usually supplies both nodes, a blockage there can slow the heart or block conduction as well as killing the inferior wall. And left coronary filling in diastole explains why symptoms appear at one heart rate and not below it.
It lies centrally, behind the sternum. Two thirds of its bulk is left of the midline and the apex points that way. That is all the "left" there is to it.
The chamber against the sternum is the right ventricle. The left forms the apex, the left border and the diaphragmatic surface.
They are crowded together behind the sternum. You listen downstream, because that is the way the sound travels.
It does not. The trunk leaves the heart with blood low in oxygen; the pulmonary veins return it rich.
The contracting left ventricular wall squeezes its own vessels shut, so most of its filling happens between beats. That is why a fast rate provokes symptoms in a narrowed artery. Do not stretch the rule to the whole heart: the right ventricle squeezes far more gently, and right coronary flow continues through systole.
It is modified cardiac muscle. Nerves alter the rate the heart chooses; they do not carry the impulse.
Tap a question to see the answer and the reasoning.
Answer: (B) It also marks the second costal cartilage, both ends of the aortic arch, and the tracheal bifurcation.
Answer: (D) It lies between the pericardium and vertebrae five to twelve.
Answer: (B) Fluid cannot push the tough sac outwards, so pressure rises and the right chambers stop filling.
Answer: (C) It carries fibres from the third to fifth cervical nerves. Its third and fourth cervical fibres share segments with the supraclavicular nerves, which supply the skin over the shoulder tip.
Answer: (B) The heart is tilted, so the right ventricle lies closest to the sternum.
Answer: (B) About nine centimetres from the midline. Further out means an enlarged or displaced heart.
Answer: (B) All four are crowded behind the sternum. The mitral valve itself lies behind the left half of the sternum at the fourth costal cartilage. Listening downstream separates them.
Answer: (B) Without the pause the ventricles would squeeze while the atria were still filling them.
Answer: (B) It is the largest territory of any branch, so a blockage costs the most muscle.
Answer: (B) It lies in the back part of the atrioventricular groove and opens beside the inferior caval opening.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| Clinically Oriented Anatomy Moore, Dalley and Agur |
The clearest account of the mediastinum and pericardium. |
| Gray's Anatomy for Students Drake, Vogl and Mitchell |
Schematic drawings of the chambers, valves and coronary territories. |
| Clinical Anatomy by Regions Snell |
Surface anatomy of the heart and the valve areas. |
| Last's Anatomy: Regional and Applied Sinnatamby |
Precise on the great vessels and mediastinal relations. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents