Heart, mediastinum and great vessels

The mediastinum is the space between the two pleural sacs, bounded by the sternum in front, the thoracic vertebrae behind and the thoracic inlet above. A plane from the sternal angle to the lower border of T4 separates the superior mediastinum from the inferior mediastinum, which is further divided into anterior, middle and posterior parts. The middle mediastinum contains the heart, the pericardium, the roots of the great vessels and the phrenic nerves. The pericardium is a fibrous sac fused below to the central tendon of the diaphragm and above to the adventitia of the great vessels. Its inner serous layer is a closed sac: the parietal layer lines the fibrous pericardium and the visceral layer (epicardium) covers the heart, with the pericardial cavity between them. Two recesses matter clinically: the transverse sinus behind the aorta and pulmonary trunk, where a surgeon can pass a clamp, and the oblique sinus behind the left atrium. The fibrous pericardium is supplied by the phrenic nerves, which is why pericardial pain can be referred to the shoulder.

The heart has a base facing back and to the right, an apex in the left fifth intercostal space in the midclavicular line, and sternocostal, diaphragmatic and left pulmonary surfaces. The right atrium receives the venae cavae and the coronary sinus; its smooth posterior part and rough anterior part meet at the crista terminalis. The right ventricle has trabeculae carneae, three papillary muscles and the moderator band, which carries the right bundle branch. The left atrium receives the four pulmonary veins, and the left ventricle, with a wall about three times thicker than the right, has two papillary muscles. The mitral and tricuspid valves close when ventricular pressure rises; their cusps are held by chordae tendineae attached to papillary muscles, which contract first so the cusps do not evert. The aortic and pulmonary valves are semilunar with three cusps and no chordae; the aortic sinuses above the cusps give rise to the coronary arteries. The sinoatrial node lies in the upper crista terminalis near the superior vena cava, the atrioventricular node in the interatrial septum near the coronary sinus opening, and the bundle of His runs through the membranous septum before dividing into the bundle branches and Purkinje network.

Cardiac muscle cells are striated, branched, usually have one central nucleus, and are joined end to end by intercalated discs. The discs carry desmosomes and fasciae adherentes that resist the pull of contraction and gap junctions that let the action potential spread from cell to cell, so the atria and the ventricles each behave as a syncytium. Cardiac cells are rich in mitochondria and have T tubules at the Z line. The heart wall has three layers: endocardium (endothelium on connective tissue, continuous with the vessel lining and carrying the Purkinje fibres in its subendocardial layer), myocardium and epicardium (the visceral pericardium, mesothelium over fat and the coronary vessels). Every blood vessel has a tunica intima, media and adventitia. Elastic arteries such as the aorta have many elastic laminae in the media and smooth the pulse; muscular arteries have a thick smooth-muscle media between internal and external elastic laminae and regulate distribution; arterioles have one to three muscle layers and set peripheral resistance. Veins have a thin media, a relatively thick adventitia and valves in the limbs. Capillaries are endothelium on a basal lamina and are continuous (muscle, brain), fenestrated (kidney, gut, endocrine glands) or discontinuous sinusoids (liver, spleen, bone marrow).

The right coronary artery leaves the anterior aortic sinus, runs in the right atrioventricular groove and gives the sinoatrial nodal branch in about 60 percent of people, the right marginal branch and, in most hearts, the posterior interventricular (posterior descending) artery, which also supplies the atrioventricular node. The left coronary artery leaves the left posterior aortic sinus as a short main stem and divides into the anterior interventricular (left anterior descending) artery, which supplies the anterior two thirds of the interventricular septum and most of the left ventricle, and the circumflex artery, which runs in the left atrioventricular groove to the left marginal branch. The artery that gives the posterior interventricular branch defines dominance; right dominance is the commonest pattern. Anastomoses between the branches exist but are small, so the coronaries are functional end arteries and sudden occlusion infarcts the territory. Most venous blood drains through the great cardiac vein (with the anterior interventricular artery), the middle cardiac vein (with the posterior interventricular artery) and the small cardiac vein into the coronary sinus, which opens into the right atrium between the inferior vena cava and the tricuspid valve; the anterior cardiac veins and the venae cordis minimae open directly into the chambers.

The ascending aorta gives only the coronary arteries. The arch gives the brachiocephalic trunk (right common carotid and right subclavian), the left common carotid and the left subclavian; the common carotids divide at the upper border of the thyroid cartilage into internal (brain, orbit) and external (face, neck) carotids, and the subclavians continue as the axillary, brachial, radial and ulnar arteries of the upper limb. The thoracic aorta gives posterior intercostal, bronchial and oesophageal branches; the abdominal aorta gives the coeliac trunk, superior and inferior mesenteric arteries, renal, gonadal and lumbar arteries and ends at L4 as the common iliacs. The internal iliac supplies the pelvis and gluteal region, and the external iliac becomes the femoral, popliteal, anterior and posterior tibial arteries of the lower limb. The superior vena cava, formed by the two brachiocephalic veins behind the first right costal cartilage, drains the head, neck, upper limbs and, through the azygos vein, the thoracic wall. The inferior vena cava forms at L5 from the common iliacs and receives lumbar, renal, right gonadal, right suprarenal and hepatic veins. Blood from the gut, spleen and pancreas goes first to the liver through the portal vein, formed behind the neck of the pancreas by the splenic and superior mesenteric veins. Portocaval anastomoses at the lower oesophagus, the rectum, the umbilicus and the retroperitoneum open up when portal pressure rises, producing oesophageal varices, haemorrhoids and caput medusae.