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The Heart

From Mediwikis

Basic Anatomy and Four Chambers

The heart is one of the key organs in the body, if not the key organ of the body. The heart acts as the pump that drives all of the blood around the body and drives the blood to and from the lungs to be oxygenated. To go back to basics, the heart is made up of a special type of muscle fibre that acts as a syncitium (works as a team) to enable coordinated contraction. The heart also contains specialised nerve fibres and tracts with electrical nodes that act to instruct the heart muscle on how and when to beat to create a very specialised and specific system.

The heart is made up of four chambers, two atria and two ventricles, these are known as the left atria, left ventricle, right atria and right ventricle. The left and right sides of the heart are separated by a septum (a wall of heart muscle). The four chambers are spaces between within the heart where the blood is pumped.

The right side of the heart is where the blood enters from the body and is pumped to the lungs, the walls of the right side of the heart are much thinner and smaller than the walls on the left side of the heart. This difference in wall thickness is due to the left side of the heart having to pump the blood that returns from the lungs to the rest of the body, this means that much more force is needed and so the wall has to be much thicker. This difference in wall thickness enables the left side of the heart to be identified when compared to the right side of the heart when seen in isolation from the rest of the cadaver.

The heart has its own blood supply known as the coronary circulation, a series of vessels that run in and over the surface of the heart to supply the heart muscle with the oxygen and nutrients it needs.

The vessels that run in and out of the heart are also key in the anatomy of the heart itself and when seen in situ these great vessels almost dwarf the heart itself. These will be discussed in more detail later in this page.

The coronary sulcus is a groove which circles the heart and seperates the atria from the ventricles. The anterior and posterior interventricular sulci separate the two ventricles

Four Chambers Function

The four chambers and their characteristics are key to the function of the heart. The heart has to be thought of in 2 halves, a left and a right. The right side deals with blood coming from the body and going to the lungs to be oxygenated and the left deals with newly oxygenated blood coming from the lungs and going to the rest of the body. Each half of the heart has an atria and a ventricle.

The atria are smaller than the ventricles and have much thinner walls, this is because they only have to pump the blood that they recieve down into the ventricles:

  • The right atrium receives blood via the SVC and IVC (superior and inferior vena cava), the superior vena cava enters into the top of the right atrium and the inferior vena cava enters through the bottom. Next to the entrance of the inferior vena cava into the right atrium there is a small opening through which the coronary sinus empties. The coronary sinus is the conduit for deoxygenated blood from the coronary circulation (the vessels supplying the heart muscle itself) back into the main circulation to be re-oxygenated. When looking at the heart in situ the right atrium lies over part of the anterior surface of the heart on the right side, (the rest of the anterior wall of the heart is made up of the right ventricle, with the left half of the heart being located round the back).
  • The left atrium receives blood from the lungs via the four pulmonary veins, there are two pulmonary veins entering on the left side of the left atrium and two pulmonary veins entering on the right side of the left atrium. The left atrium makes up some of the posterior surface of the heart. Just below the border of the left atrium on the surface of the heart is the coronary sinus.

The ventricles are larger than the atria and have much thicker walls, especially the left ventricle which has a much thicker muscle wall than the other chambers of the heart. The walls of the ventricles are thicker than those of the atria as they have to pump the blood out into the body, the walls of the left ventricle are the thickest as they have to pump blood to the extremities and the rest of the body, (not just the lungs like the right ventricle):

  • The right ventricle receives blood from the right atrium via the tricuspid valve (for details on the heart valves, see below) and pumps blood up and out of the heart to the lungs via the pulmonary valve into the pulmonary trunk.
  • The left ventricle receives blood from the left atrium via the mitral valve and pumps blood up and out of the heart via the aortic valve into the aorta to the rest of the body. The aortic valve acts as the gateway into the coronary circulation for oxygenated blood, (for details see below).

Further Anatomy of the Chambers

The right atrium is divided into two sections by the crista terminalis, a ridge of muscle on the inside which can be located from the outside due to the shallow, vertical groove called the sulcus terminalis cordis. Also draining into the right atrium are the smallest cardiac veins, whose openings are scattered throughout the atrium.

The right ventricle is dotted with irregular structures called trabeculae carneae. Some of these are papillary muscles which attach chordae tendineae which connect to the tricuspid valve. The papillary muscles are named after the section of the tricuspid valve they connect to though sometimes there will be no septal papillary muscle. A single specialised trabeculum called the septomarginal trabeucla or moderator band is responsible for carrying some of the cardiac conduction.

The left ventricle has got finer trabeculae carneae but much larger papillary muscles than the right ventricle. It has two papillary muscles (as it has a bicuspid valve) the anterioir and posterior.

Anatomy of the Heart Valves

There are four main valves in the heart, they are the tricuspid, bicuspid, pulmonary and aortic valves. The tricuspid and bicuspid valves lie between the atria and ventricles, the tricuspid between the right atrium and right ventricle and the bicuspid between the left atria and left ventricle, they are the atrio-ventricular valves. The pulmonary and aortic valves lie between the ventricles and the great vessels that leave the heart. The pulmonary artery between the right ventricle and pulmonary trunk (leading to the pulmonary arteries) and the aortic artery between the left ventricle and the aorta.

The anatomy of the individual valves is important in understanding the function of the heart, what can go wrong and what can be done to fix problems within, and due to the heart. Dealing with each valves at a time, running from where venous blood enters the heart from the body:

  • Tricuspid Valve
The tricuspid valve is the atrioventricular valve between the right atrium and right ventricle. It has three fibrous cusps (as indicated in its name - tri-cuspid) and is open when the atria contract to force blood into the ventricles and is closed when the ventricles contract to force blood up and out of the heart. The outer ring of the cusps is attached to the fibrous skeleton of the heart and the free edges of the cusps meet together to form a tight valve to prevent backflow of blood.
To stop the valve cusps being blown back up into the atria by the pressure when the ventricles contract, the valve cusps are attached to the ventricular heart muscle with some strands of fibrous tissue known as chordae tendinae which are then attached to the ventricular heart muscle with papillary muscle. Papillary muscle can be seen within the ventricles as small hillocks of muscle.
  • Bicuspid Valve
The bicuspid valve is the atrioventricular valve between the left atrium and the left ventricle. It has valve cusps like the tricuspid valve but instead of three cusps the bicuspid only has two (as indicated by the name - bi-cuspid). It works in exactly the same as the tricuspid valve and has the same layout of chordae tendinae and papillary muscle. The only difference between the tricuspid and bicuspid (other than being on the other side of the heart) is the number of cusps the valves possess.
  • Pulmonary valve
The pulmonary valve is located between the right ventricle and the pulmonary trunk that leads to the pulmonary arteries that carry deoxygenated blood to the lungs to be reoxygenated. The pulmonary valve does not have the same anatomy as the bicuspid or tricuspid but instead works using a semilunar cusp system. The semilunar cusp system is based on three pouches of fibrous material that hang from the vessel down towards the heart. When the ventricles pump blood up and out of the heart and gravity tries to bring this blood back down into the heart at the end of the ventricular contraction the falling blood fills the cusps which then interlock together to prevent blood re-entering the heart. In affect they "catch" the falling blood and in doing so cause a barrier to prevent further blood falling and re-entering the heart. This concept can be difficult to visualise, drawing diagrams to display each step may help. Also not that the aortic valve is similar in structure to this but has two sinuses next to the left and right valve which are where the coronary arteries branch from. The Pulmonary valve has a left and right cusp like the aortic valve however the aortic valve has an anterior cusp while the pulmonary valve has a posterior cusp.

Clinical Note It is important to note the importance of valvular disease as it can lead to changes in the function of the heart, ventricular hypertrophy and cardiac failure. The two main types of valvular disease are incompetance (aka insufficiency) and stenosis. An incompetance is where the heart valve doesn't close sufficiently causing a back flow of blood in the wrong direction (this leads to the presence of murmurs on auscultation of the heart). A stenosis of a heart valve is the opposite of an incompetance, basically the heart valves do not open properly so less blood can fit through the valve. The different types of heart disease do not usually occur in isolation and both will be present to differing degrees, (for further details on valvular heart disease see other pages on this wiki).

In terms of clinical pathology it should also be noted that during a myocardial infarction the papillary muscle or valve cuspes themselves can be damaged leading to the collapse or complete failure of heart valves.

Surface Markings

One of the skills needed in examining the cardiovascular system in a patient is the ability to locate the areas on the chest wall where the heart valves can be heard effectively. The order in which the heart valves are auscultated can be remembered by using the acronym PATMe, (the left and right refered to in the description is the patient's left and right). Remember that the sternal angle (level with rib 2) can be used to count down to other intercostal space levels:

  • P ulmonary valve = on the left edge of the sternum in the 2nd intercostal space (just below the sternal angle)
  • A ortic valve = on the right edge of the sternum in the 2nd intercostal space (just below the sternal angle)
  • T ricuspid valve = on the left edge of the sternum in the fifth intercostal space
  • M itral valve = just under the left nipple in the fifth intercostal space
  • e = (just inserted to make PATM easier to remember!)

Note: for further information on the sternal angle, its location and importance see the Respiratory Anatomy page.

Fibrous Skeleton of the Heart

The fibrous skeleton of the heart is a very interesting feature that leads to an electrical isolation between the atria and the ventricles on both sides of the heart. This electrical isolation leads to a more effective control of when the fibres of heart muscle contracts.

The fibrous skeleton is made up of dense tissue rings around each of the four valves in the heart that cause a plane to be formed between the atria and the ventricles, this region of fibrous skeleton is known as the annulus fibrosis.

The fibrous skeleton has other functions alongside electrically isolating the atria and ventricles, these include:

  1. helping to keep the valves in the correct shape for effective function
  2. acting as a point of attachment into the body of the heart for the valve cusps

Histology of the Heart

There are several different layers of the heart. The Endocardium is an outerlayer fused to the visceral serous pericardium. The myocardium is the muscle and the epicardium is the innermost layer.

The pace maker cells are located high in the right atrium. They are non contractile but can spontaneously depolarise to start an action potential which will spread regeneratively throughout the heart (more on this later.

Heart muscle is similar to skeletal muscle in several respects. It is striated and composed of myofibrils made up of sarcomeres. There are several key differences though:

  • The T tubules are short and broad and do not form triads with two fused sections of sarcoplasmic reticulum. This is because the reticulum in cardiac muscle does not have the terminal cisternae which make up the triads. They instead make up diads as they associated with one sarcoplasmic reticulum.
  • The T tubules also run along the Z line not the ends of the thick filaments, they are less extensive but have the same job in that they allow for electromechanical transduction (conversion of electrical energy from ion imbalance to mechanical movement).
  • Cardiac muscle relies nearly entirely on aerobic respiration therefore had large amounts of myoglobin for storing oxygen and large amounts of mitochondria.
  • Cardiac muscle contains intercalated disks. These are points where two adjacent cells join. They are held together by desmosomes, adherens junctions and gap junctions. The plates allow for the muscle to be stable in its three dimensional structure and the gap junctions allow the rapid transfer of ions necessary for contraction of the whole sheet of cardiac muscle. Also the myfibrils on each side of the disc are anchored together meaning that all the cells can contract at once with maximum efficiency.
  • There's also ion transfer between the cells meaning that they function as a single unit (syncytium). This is primarily due to cell branching, which skeletal muscle does not have.

There are also several functional differences between cardiac and skeletal muscle:

  • Cardiac muscle does not need neural stimulation to contract, this property is know as automaticity.
  • Innervation by autonomic nerves alters the pace set by the pacemaker cells but does not directly influence contraction.
  • Tetanic contraction (rapid successive contractions keeping the muscle tense) is not possible in cardiac muscles as they have a longer refractory period than skeletal muscles .
  • It has little glycogen and gains little ATP from glycolysis so when denied oxygenated blood the part of the heart affected quickly becomes damaged.
  • Branches of the muscle fibres are connected by adherens junctions, these consist of two sets of proteins. The first are catenins who bind to the ends of actin filaments and to cadherins which bind to eachother extracellularly.

Mechanism of Contraction

In the membranes of tubules there are L-type calcium ion channels also known as dihydropyridine receptors. In skeletal muscle these open and then force calcium channels in the SR (ryanodine receptors) to open causing the sarcoplasm to flood with calcium. In cardiac muscle it is slightly different as the calcium influx from the L-type channels is a trigger flux which starts calcium induced calcium release which causes contraction. The important thing about the difference is that in skeletal muscle the maximum amount of contraction is always produced by the myoctes (meaning stronger or weaker contraction is achieved by activating multiple myocytes) whereas in cardiac muscle the ANS regulates the size of trigger influx and therefore the strength of contraction. This makes sense as the heart cannot control how many myocytes will contract.

Resting Potential

  • During a resting potential sodium potassium ATPase pump moves sodium out and potassium in (3:2) ratio.
  • Potassium channels are open so potassium leaks out due to concentration gradient but is somewhat retained due to the electrochemical gradient as the cell is more negative than the ECF (-90mV for cardiomyoctes)
  • Therefore the diffusion gradient of potassium ions maintains resting potential.
  • Sodium and potassium channels are closed during resting potential

Action potential

Phase 0

  • Action potential in an adjacent cardiomyocte or pacemaker cell makes the membrane potential roise above the usual-90mV
  • The sodium ion channels snap open at -70mV, this is the threshold potential.
  • Influx of sodium down both a concentration and electrochemical gradient raises membrane potential to slightly above 0mV. This is the overshoot.
  • The sodium channels are time dependant so then close.L-type calcium channels open at -40mV and stay open for longer causing an influx of calcium

Phase 1

  • Some potassium ion channels open and potassium moves out of the cell lowering the membrane potential to 0mV

Phase 2

  • The inward flow of calcium is opposed to the outward flow of potassium caused by the potassium ion channels (which were stimulated to open by the original depolarisation but open on a delay).
  • The membrane potential slowly drops (repolarisation)

Phase 3

  • The calcium ion channels now start to close (again they are time limited).
  • Potassium efflux now vastly outstrips calcium influx so membrane potential begins to drop.
  • Sodium and potassium ions are returned to their pre action potential environment by a variety of channels and the calcium is moved outside the cell
  • The sodium channels won't be reactivated until the membrane potential reaches -50mV or lower, due to the long period of plateau of action potential due to the calcium channels this gives cardiomyocytes a much longer refractory period than skeletal myocytes.


Overview of Mechanism and Hormonal Influences

  • Many cells are linked to acetylcholine receptors which are linked to inhibitory G proteins which reduce adenylate cyclase activity and increase potassium currents producing less powerful contractions.
  • L-type calcium ion channels can be phosphorylated to increase their chance of opening and increase the size of the trigger flux
  • PKA also can phosphorylate the pumps and pump inhibitors involved in returning the calcium ion concentration to normal after contraction. This has two effects in that the as the pumps run faster the heart muscles relax faster and provide a longer time for the ventricles to fill with blood. Also as SERCA (sarcoplasmic reticulum calcium ATPase) is moving Ca2+ back into the SR it's stimulation allows for the myoctye to store more calcium ions in the SR and therefore have a larger concentration release during an action potential so have a more powerful contraction.

Cellular Level of the Contraction

Calcium influx into cells reaches the T tubules during phase 2 of contraction. The sarcoplasmic reticulum has calcium channels of its own, known as ryanodine receptors. These will open when the calcium moves into the cell as a result of the action potential. This is calcium induced calcium release or CICR and is the mechanism whereby an action potential and its associated contraction propagates throughout the heart.

The rise in the intracellular level of calcium ions causes the calcium to bind to the Tn-C subunit of troponin, this inhibits the Tn-I subunit (check microanatomy of the CV system for more information on contractile protein structure) as a conformation change occurs shifting the troponin complex off of the myosin binding sites on the actin. The Myosin head has ATP attached. ATP is hydrolysed to ADP (by an ATPase on myosin) which allows it to bind to the actin via forming cross bridges.

The interaction between myosin and actin causes the myosin head to swivel and pull the actin filament, making the muscle contract.

As the Calcium ion channels deactivate in the cell membrane there is no longer a trigger influx. This means that the ryanodine receptors stop being stimulated so the Sarcoplasmic reticulum stops releasing calcium ions. Equally SERCA pumps calcium ions back into the sarcoplasmic reticulum. As calcium concentration falls it beings to dissociate from Tn-C and so the inhibitory function of Tn-I is restored.

Pacemaker Cellular Function

Pacemaker cells have HCN channels in them. HCN channels are opened by the phase 3 of an action potential. They cause the membrane to slowly become more depolarised and cause the cell to undergo a spontaneous action potential. Rising cAMP levels cause more channels to open and therefore more action potentials thereby raising heart rate. This gives the autonomic nervous system the ability to control heart rate as it can influence levels of cAMP with hormones like adrenaline.

The sympathetic nervous system will increase the heart rate by releasing adrenaline which will bind with g coupled protein receptors linked with adenylyl cyclase, which produces more cAMP. Parasympathetic nervous system will be connected to Adenylyl cyclase but by an inhibitory g protein coupled receptor. The PSNS releases acetylcholine not adrenaline.

The SAN has an intrinsic rate of about 100bpm but is usually limited by the PSNS. The AVN has a much lower intrinsic rate (40bpm) but is usually never produces action potentials at its intrinsic rate due to it receiving them at a faster rate from the SAN (overdrive suppression). The Purkinje fibres have an even lower intrinsic rate. An increase in heart rate is called positive chronotrophy while a decrease is negative chronotrophy. If you have it Lippincott's illustrated physiology has a diagram on page 196 showing the neural influences on heart rate and the pathways each of them use which acts as a very good summary.

Pericardial Sac

The pericardial sac is a fibrous layer of pericardium that covers the heart in the mediastinum, very much like how the pleura covers the lungs. The pericardial sac is located approximately in the midline of the body in the thoracic cavity and ends with fusion into the diaphragm and superiorly fusing to the sheaths (adventitia) of the great vessels. The tough outer layer, the Fibrous Pericardium, fuses with the diaphragm at a feature on the diaphragm known as the central tendon. Because of this fixing of the heart via the pericardium onto the diaphragm the heart moves on inspiration and expiration with the diaphragm. The diaphragm and heart move upwards and forwards on inspiration and downwards and backwards on expiration. The Fibrous pericardium is also attached to the sternum via sternopericardial ligaments.

Within the tough fibrous pericardium is the serous pericardium. This, like the pleura of the lungs, has a visceral and parietal layer with the visceral being next to the heart itself. There is a very small amount of fluid between the two layers, the space known as the pericardial cavity.

Clinical Note There are many conditions that effect the pericardium, inflammation of the pericardium due to infection (known as pericarditis) causes the layers of the pericardium to become more stuck together instead of running freely over each other. This 'sticking' leads to a feature on auscultation known as a "rub". This is often mistaken for a heart attack as the patient complains of chest pain however in the case of pericarditis the pain may be relieved by sitting forward and an ECG will differentiate if from a myocardial infarction.

Another condition associated with the pericardium is cardiac tamponade where the pericardial cavity becomes filled with fluid (for example blood) and this restricts the heart's movement by reducing the space in which it can beat. Compressing the heart leads to a reduction in cardiac function and ability. A pericardial effusion is a build up of liquid in between the two layers of the serous pericardium which can lead to cardiac tamponade.

Finally there is a condition called constrictive pericarditis. This is an abnormal thickening of the pericardial sac which can constrict the heart.

Blood vessels

Artery supply is delivered by the internal thoracic arteries (pericardiophrenic branch) and the pericardial veins drain into the internal thoracic veins.

Pericardial Sinuses

The pericardial sinuses are areas where the serous (visceral) layer forms small pouches, these are blind ending sinuses. They are formed as the serous parietal layer of the pericardium is continuous with the visceral layer of serous pericardium around the roots of the vessels leaving the heart. There are two main sinuses associated with the pericardium:

  1. Transverse Sinus - located between the pulmonary artery and vein as they leave the heart
  2. Oblique Sinus - runs round the back of the heart between the SVC (superior vena cava) and IVC (inferior vena cava)

Conducting System within the Heart

The conducting system of the heart can be imagined to be like a series of pathways and junctions that tell the heart muscle what to do and when to contract. It is made up of nodes, bundles and fibres.

First lets think about the nodes, these emit and control the pace of the heart. There are two nodes, the sinoatrial node (SAN) and the atrioventricular node (AVN):

  • The sinoatrial node is the pacemaker of the heart, it decides when the heart is going to beat and sets the pace of the heart rate. The SAN responds to various factors that it uses to know when to cause the heart to beat faster/slower so the blood supply to the rest of the body is at its most efficient all the time. The SAN is located near where the SVC meets with the right atrium. When the SAN fires a wave of electrical impulse travels down through the heart muscle causing the atria to contract together forcing the blood in them down into the ventricles. This wave of electrical impulse also activates the AVN.
  • The atrioventricular node responds to activation from the SAN and sends a wave of impulse down through the atrioventricular bundle.

The atrioventricular bundle is a collection of specialised cells that carry the wave of electrical impulse down to the base of the ventricles through the septum of the heart to the Purkinje fibres. These Purkinje fibres then spread out through the walls of the ventricles and the wave of electrical impulse passing through them causes the ventricles to contract from the bottom upwards. It is important that the ventricles contract from the bottom upwards so that the blood in them is forced up and out of the heart.

To summarise the wave of electrical impulse runs from the SAN out over the atria from the top down to the AVN which passes it down to the base of the ventricles where it is released to run over the ventricles from the bottom upwards.

Borders of the Heart

The borders of the heart are important to understand because of the implications to clinical practise, it is good to know where the heart is for many reasons, for example:

  1. When percussing the chest the heart should be avoided
  2. When auscultating the chest
  3. When interpreting clinical signs or results of imaging investigations

etc.

The right border of the heart is made up of the vena cava (superior and inferior) and the right atrium, it lies just to the right of the sternum. The left border of the heart is made up by the aortic arch (at the top) then the pulmonary trunk (further down) and the left ventricle (at the bottom) and can be found to the left of the sternum with the end of the left ventricle being in line with the mid-clavicular line just under the left nipple.

Surfaces of the Heart

  • The base (posterior) surface is made up of the left atrium, a small portion of the right atrium and some of the Vena Cava and pulmonary veins.
  • The anterior surface is made up mainly of the right ventricle with some of the left ventricle on the left and some of the right atrium on the right.
  • The diaphragmatic surface is made up of the left ventricle and a small portion of the right ventricle.
  • The left pulmonary surface consists of the left ventricle and the left atrium
  • The right pulmonary surface consists of the right atrium.

The Coronary Circulation

The heart muscle itself is supplied by its own circulation, the coronary circulation. This is based around a network of vessels that can be seen running over the surface of the heart and also dive down to run within the heart muscle and fat that coats the heart. The coronary arteries run in special grooves in the heart's surface known as sulci.

The coronary circulation originates from two special openings in the aortic valve where oxygenated blood enters the two main coronary arteries, the left and right coronary arteries, (see above under valves). The coronary circulation works as every other circulation with arteries, capillary beds and veins. The coronary veins carrying venous deoxygenated blood drains into the coronary sinus that goes on to drain into the right atrium to be re-circulated to the lungs.

The RCA (right coronary artery) runs down in the AV sulci (between the atria and ventricles on the surface of the heart). It has 3 major branches

  • The sinu-atrial nodal branch supplies the SAN
  • Further down the right marginal branch splits off just before the right margin of the heart to supply the right atrium
  • In a right dominant heart the posterior interventricular branch arises from the RCA (in the less common left dominant it arises from the left anterior descending branch)

The RCA supplies the SAN and the AVN (AV node) and the LAD (left anterior descending branch of LCA - left coronary artery) coronary artery supplies the interventricular septum (the area of heart muscle between the left and right ventricle).

The LCA has two major branches, splitting between the left auricle (atrium) and the pulmonary trunk:

  • The circumflex branch which moves posteriorly
  • The descending or anterior interventricular branch which runs anteriorly supplying the left ventricle.

If the RCA supplies the AV node (AVN) and the back of the heart the heart is seen as "Right dominant" but if the circumflex artery supplies the AV node (AVN) and the back of the heart, the heart is seen as "left dominant". If asked to label branches which you cannot remember derive the name by the direction is it going and the next largest vessel it is attached to (e.g. diagonal branch of anterior interventricular branch).

The veins all drain to the coronary sinus. The great cardiac vein starts at the apex of the heart on the anterior side and travels up and round, under the right auricle where it joins with the posterior cardiac vein and opens out into the coronary sinus. Following the sinus it splits on the right to the middle cardiac vein which travels down to the apex, opposite to the great cardiac vein and the small cardiac vein, which reaches to the anterior side

Clinical Note The coronary circulation is very suceptible to atheroma formation and cholesterol deposition. This can lead to disasterous consequence if one of the main vessels supplying the heart becomes blocked. Blockage would lead to a lack of blood supply to areas of heart muscle, causing myocardial infarction. Partial blockage of the coronary vessels leads to angina where the heart muscle is receiving some, but not adequate, blood supply.

Drawing of the heart showing coronary arteries and major blood vessels

Superior Mediastinum and Thoracic Inlet (Vessels)

The subclavian arteries provide blood supply to the arms (one each) and the common carotid artery (CCA) provides blood supply to the face and neck.

Note That each subclavian artery (runs under the clavicle on either side) gives a branch known as the internal thoracic (aka mammary) artery that runs just lateral to either edge of the sternum. This internal thoracic artery has two terminal branches:

  1. Superior Epigastric - runs down to the abdominal wall
  2. Musculophrenic - runs down the costal margin, through the diaphragm to the final intercostal space

Surface Markings

The superior mediastinum can be visualised as running from the sternal angle (described in detail in the anatomy of the respiratory system) to the body of the fourth thoracic vertebrae (T4). The inferior mediastinum runs from the body of the fourth thoracic vertebrae (T4) to the lower edge of the eighth thoracic vertebrae (T8).

The apex beat is important to locate as if it is displaced it can indicate some clinical problems such as an enlarged left ventricle. The apex beat is the area on the surface of the skin where the heart's beat can be felt to have the most force, it can be palpated in the fifth intercostal space, 9 cm from the midline (i.e. just under the left nipple).

Nerves

Phrenic Nerves

There are 2 phrenic nerves, one for either side of the body, they origanate from cervical vertebrae 3, 4 and 5 and they innervate the diaphragm with motor and sensory fibres, (remember C3, 4, 5 keeps the diaphragm alive). The right phrenic passes behind the superior vena cava and then down outside the pericardium. The left phrenic nerve passes posteriorly to the right lung root and continues down outside the pericardium. The phrenic nerves innervate the fibrous pericardium.

Clinical Note When the diaphragm is inflammed there can be pain referred to the shoulder tips due to the same spinal levels supplying more than one area in the body. So someone presenting with shoulder tip pain may have a problem with their diaphragm.

Vagus Nerves

The vagus nerves are cranial nerves (CN X) and therefore arise from within the cranial cavity. The Vagus nerves run via the mediastinum as part of their path to the abdominal cavity. The vagus nerves deal with and control parasympathetic functions, (remember the parasympathetic system is the "rest and digest" system). They innervate the serous parietal pericardium and a branch innervates the SAN, providing the ability to slow or speed up rate of contraction.

The right and left vagus nerves take different routes through the thoracic cavity and mediastinum:

  • The Right Vagus Nerve
This nerve runs between the brachiocephalic trunk (which is the first artery out of the arch of the aorta) and the right brachiocephalic vein (which joins with the left brachiocephalic vein to form the SVC - superior vena cava). one of the defining points where the right vagus nerve may be located is as it crosses over the top of the right bronchus back towards the oesophagus, as it makes this movement the azygos vein crosses over the right vagus nerve.
  • The Left Vagus Nerve
The left vagus nerve runs between the left common carotid and the left subclavian arteries to get into the superior mediastinum, from here it runs down near the aorta to disapear under the diaphragm. There is an important branch of the left vagus nerve that should be noted at this point, this is the left recurrent laryngeal nerve. The left recurrent laryngeal nerve branches off just below the arch of the aorta and crosses over the anterior surface of the aorta to ascend back up to the larynx (as suggested by the name).