Cardiology

Cardiac Anatomy

Introduction

The heart is a four chamber muscular pump forcing blood around the pulmonary and systemic circulations. The direction of flow between the atria and ventricles and the ventricles and aortic and pulmonary arteries is controlled by valves which allow only one way flow.

The heart has four chambers and is about the size of a clenched fist. It is enveloped in a layer of fibrous pericardium within the pericardial sac. It is suspended by the major vessels. It lies between the lungs and behind the sternum. Inferiorly the surface is formed by the right ventrle and left ventricle and part of the right atrium posteriorly. The inferior surface is in contact with the diaphragm. Posteriorly lies the base of the heart formed by the left atrium in close contact with the the descending aorta and oesophagus. The heart is a conical shaped structure with both an apex and base lying in the middle of the mediastinum. The apex of the cone points down and out and its anatomical surface marking is the mid clavicular line in the 5th Intercostal space.

The heart is made up of four chambers. The two atria acts as storage vessels for blood returning to the heart. The two ventricles act as pumps. On the left side the left atrium receives oxygenated blood from the lungs from the four pulmonary veins. The left atrium and left ventricle are separated by the semilunar mitral valve. The mitral valve has anterior and posterior leaflets. The left ventricle pumps blood into the systemic circulation and has a thick muscular wall

On the right side the right atrium receives deoxygenated blood from the systemic circulation from the inferior and superior vena cavae. The right atrium and ventricle separated by tricuspid valve which has three leaflets. Blood is ejected with systole across the pulmonary valve into the pulmonary artery. Right sided pressures are low and the RV is thin walled compared with the left.

The right border of the heart is formed almost entirely by the right atrium. The left border of the heart is formed almost entirely by the left ventricle with the left atrial appendage superiorly. The base or posterior surface is formed almost entirely by the left atrium which is closely opposed to the oesophagus (important for transoesophageal echo). Inferior or diaphragmatic surface of the heart is made up by the right and left atrium

Right atrium has a thin muscular wall. The superior and inferior vena cava empty into the right atrium. Posteriorly lies the fossa ovalis with a surrounding limbus. Muscular ridges lie between caval openings called the crista terminalis. Right atrium also contains the opening of the coronary sinus.

Right Ventricle Thick walled adjacent to the RA and forms anterior and inferior surfaces of the heart. The LV septum bulges into it. The walls are lines with muscles bands. The RA and RV are in communication through the tricuspid valve. This has 3 cusps. leaflets are held by two papillary muscles. Blood is ejected via the pulmonary valve. This is formed of 3 semilunar leaflets - two anterior and one posterior.

Left atrium thin walled to store blood pre diastole. Lies behind the RA and forms base of heart facing posteriorly. The left atrial appendage lies superiorly and is a source of clot. Blood arrives from four pulmonary veins with 2 on each side. In close contact posteriorly with oesophagus, descending aorta and left bronchus.

Left Ventricle the main pumping chamber. Forms the apex with the tip pointing anteriorly, to the left and slightly inferiorly. It forms left heart border and part of anterior and inferior surfaces. Thickened muscular walls. The Mitral valve with its tow leaflets lies posteriorly and the aortic outflow tract is superior. The mitral valve leaflets are held by papillary muscles. The anterior cusp is the larger and lies between the orifices of aortic and mitral valves. The aortic valves has 3 leaflets - anterior and two posterior (AAPP).

Heart right anatomy

Coronary Arteries

Note: The first organ supplied with blood by the heart is the heart itself. The blood vessels of the heart are shown below. Coronary flow is discussed within the physiology section.

Ascending Aorta

The ostia for the coronary arteries lie just above the aortic valve leaflets in a space called the aortic sinus. The right coronary comes off first and is the most proximal to the valve and then the left.

Left coronary artery arises from the left posterior aortic sinus and is closely related to the left coronary cusp to form the

  • Left main stem which branches almost immediately into the
  • Left anterior descending artery (LAD) which travels between right and left ventricles towards the apex.
    • The LAD gives off the
      • Diagonal branches (D1)
      • Diagonal branches (D2)
      • Septal branches
  • Circumflex artery (CX) which lies in the left AV groove between the left atrium and left ventricle and supplies the vessels of the lateral wall of the left ventricle.
    • Posterior descending artery (PDA). (10% of patients have a left dominant circulation in which the Cx also supplies)
    • Marginal branch (M1)
    • Marginal branch (M2)

Right coronary artery is the first branch of the aorta and arises from the anterior aortic sinus and runs in the AV grove between right atrium and ventricle. It gives of the

  • Acute marginal branch runs along the margin of the right ventricle above the diaphragm.
  • Sinus node branch in 60% (otherwise supplied by the Cx)
  • Atrioventricular node branch
  • Posterior descending artery (RCA dominant) in over 65% which supplies the inferior wall of the left ventricle and inferior part of the septum

Applied anatomy: Coronary Angiography

Knowledge of coronary anatomy is needed for interpreting coronary angiograms and the general management of 'coronary artery disease' whih is nearly always related to atherosclerosis and plaque formation and rupture. Angiograms are done to show evidence of stenosis or blockage in coronary vessels which can be correlated with cardiac symptoms or evidence of myocardial dysfunction. The oblique angles of the angiogram mean that it does not look like the appearance of the vessels looking at the front of the heart. So be aware of these views.

Here is an excellent web site with images of a left coronary angiogram Left coronary angiogram

Here is the same site with images of a right coronary angiogram Right coronary angiogram

Structure of Blood vessels

One cannot have a circulation with just the pump as you also need the pipes. There are several different types. All act as conduits of blood. One of the things to note is that the pipes are not inert structures. Even the very thin endothelium the forms the inner layer of the vessel which contributes a tiny amount to the diameter of a vessel has hugely important roles in terms of blood flow, coagulation, infection all of which is dealt with under physiology. Although the pipes get narrower as flow proceeds there are more of them so overall resistance gets smaller. Most resistance to flow is controlled dynamically by neurohumoural factors within the arterioles.  

Basic structure

  • Tunica intima: innermost layer. Covered by squamous endothelium with some connective tissue and smooth muscle. forms the inner 'non stick' surface of the vessel. It is an active producer of various vasoactive substances.
  • Tunica media: middle layer with elastic lamina and smooth muscle cells and collagen fibres.
  • Tunica adventitia:is connective tissue (dense or loose) that forms the outer boundary of the vessel
  • Vasa vasorum are small blood vessels that supply the tunica media and tunica adventitia

Large muscular arteries e.g. Aorta, Iliacs

  • Tunica intima:internal elastic lamina
  • Tunica media:with elastic lamina and smooth muscle cells and collagen fibres
  • Tunica adventitia: connective tissue (dense or loose) that forms the outer boundary of the vessel

Small muscular arteries e.g. Carotids, Coronary

  • Tunica intima: internal elastic lamina
  • Tunica media: internal elastic lamina forms a well defined layer with smooth muscle cells and collagen.
  • Tunica adventitia:external surface

Arterioles e.g. Penetrating vessels in brain

  • Tunica intima: endothelium still rests on a internal elastic lamina
  • Tunica media: 1-3 concentric layers of smooth muscle cells. difficult to identify an external elastic lamina
  • Tunica adventitia: hard to define from surrounding connective tissue.
  • Arterioles receive both sympathetic and parasympathetic innervation.

Capillaries

  • Tunica intima: single endothelial cell
  • Tunica media:not present other than occasional pericyte
  • Tunica adventitia:none
  • Diameter: 4-15 micrometres

Venules

  • Tunica intima: very thin
  • Tunica media: may be surrounded by pericytes
  • Tunica adventitia:difficult to detect

Veins

  • Tunica intima: very thin
  • Tunica media: may be bands of smooth muscle which can alter tone. Collagen and elastin in larger veins.
  • Tunica adventitia: can be poorly defined
  • Unidirectional valves