Cardiovascular system

9 Cardiovascular system




ANATOMY



HEART



Development of the heart


The heart begins to develop towards the end of the third week of gestation as a pair of endothelial tubes which fuse to become the primitive heart tube. This develops within the pericardial cavity from which it is suspended from the dorsal wall by a dorsal mesocardium.


The primitive heart tube develops grooves which divide it into five regions: the sinus venosus, atrium, ventricle, bulbus cordis and truncus arteriosus (Fig. 9.1). The arterial and venous ends of the tube are surrounded by a layer of visceral pericardium. The primitive heart tube then elongates within the pericardial cavity, with the bulbus cordis and ventricle growing more rapidly than the attachments at either end, so that the heart first takes a U-shape and later an S-shape. At the same time it rotates slightly anticlockwise and twists so that the right ventricle lies anteriorly and the left atrium and ventricle posteriorly (Fig. 9.1). Despite this, and an increase in the number of vessels entering and leaving, they still continue to be enclosed together in this single tube of pericardium.



As the tube develops, the sinus venosus becomes incorporated into the atrium and the bulbus cordis into the ventricle. Endocardial cushions develop between the primitive atrium and ventricle. An interventricular septum develops from the apex up towards the endocardial cushions.


The division of the atrium is slightly more complicated. A structure called the septum primum grows down to fuse with the endocardial cushions, but leaves a hole in the upper part which is termed the foramen ovale. A second incomplete membrane develops known as the septum secundum. This is just to the right of the septum primum and foramen ovale. Thus a valve-like structure develops which allows blood to go from the right to the left side of the heart in the fetus (Fig. 9.2). At birth, when there is an increased blood flow through the lungs and a rise in the left atrial pressure, the septum primum is pushed across to close the foramen ovale. Usually the septa fuse, obliterating the foramen ovale and leaving a small residual dimple (the fossa ovalis). The sinus venosus joins the atria, becoming the two venae cavae on the right and the four pulmonary veins on the left (Fig. 9.1).





Fetal circulation


Before birth the circulation (Fig. 9.2) obviously differs from that in the adult because oxygen and food must be obtained from maternal blood instead of from the lungs and the digestive organs. Oxygenated blood from the placenta travels along the umbilical vein, where virtually all of it bypasses the liver in the ductus venosus joining the inferior vena cava (IVC) and then travelling on to the right atrium. Most of the blood then passes straight through the foramen ovale into the left atrium so that oxygenated blood can go into the aorta. The remainder goes through the right ventricle with the returning systemic venous blood into the pulmonary trunk. In the fetus the unexpanded lungs present a high resistance to pulmonary flow, so that blood in the main pulmonary trunk would tend to pass down the low resistance ductus arteriosus into the aorta. Thus the best-oxygenated blood travels up to the brain, leaving the less well-oxygenated blood to supply the rest of the body. The blood is returned to the placenta via the umbilical arteries, which are branches of the internal iliac artery. At birth when the baby starts to breathe, there is a rise in the left atrial pressure, causing the septum primum to be pushed against the septum secundum and thus to close the foramen ovale. The blood flow through the pulmonary arteries increases and becomes poorly oxygenated, as it is now receiving the systemic venous blood.


The pulmonary vascular resistance is also abruptly lowered as the lungs inflate, and the ductus arteriosus becomes obliterated over the next few hours or days. This occurs by a prostaglandin-dependent mechanism which causes the muscular component of the ductal wall to contract when exposed to higher levels of oxygen at the time of birth. Closure of the ductus arteriosus is less likely to occur in very premature babies or those with perinatal asphyxia. Ligation of the umbilical cord causes thrombosis and obliteration of the umbilical arteries, vein and ductus venosus. The thrombosed umbilical vein becomes the ligamentum teres in the free edge of the falciform ligament.



Congenital abnormalities of the heart and great vessels


Given the complex nature of the development of the heart, it is hardly surprising that there are a number of congenital abnormalities, which may be classified as follows.




Left to right shunts (late cyanosis)









Anatomy of the heart



Surfaces and borders


The heart (Fig. 9.4) is a muscular organ which pumps the blood around the arterial system. It consists of four chambers: right and left atria and right and left ventricles. When viewed from the front it has three surfaces and three borders. The anterior surface consists almost entirely of the right atrium and right ventricle with a narrow strip of left ventricle on the left border and the auricle of the left atrium just appearing over the top of this. It lies just behind the sternum and costal cartilages. The posterior surface consists of the left ventricle and left atrium with the four pulmonary veins entering it, and the right edge is visible. The inferior or diaphragmatic surface consists of the right atrium with the IVC entering it and the lower part of the ventricles.



The three borders are the right, the inferior and the left. The right is made up entirely of the right atrium with the SVC and IVC. This extends from the third to the sixth right costal cartilage approximately 3 cm from the midline. The inferior border consists of the right ventricle and the apex of the left ventricle. It extends from approximately 3 cm to the right of the midline at the level of the sixth costal cartilage to the apex which is in the fifth left interspace in the mid-clavicular line (approximately 6 cm from the midline). The left border extends from the apex up to the second left interspace approximately 3 cm from the midline (Fig. 9.5). The outline of the heart can be seen clearly on a chest x-ray (Fig. 9.6). The apex of the heart is the lowest and most lateral point on the chest wall at which the cardiac impulse can be felt. As the heart is in contact with the diaphragm, it moves with each respiration. However, the anterior fibres of the diaphragm are short, so that the central tendon on which the heart rests moves relatively less.





Chambers of the heart


The heart (Fig. 9.7) consists of a right side which pumps blood through the lungs and the left side which pumps it through the systemic circulation. The atria collect blood from the veins and pump it into the ventricles during ventricular relaxation (diastole). When the ventricles are full they contract (systole), the valves between the atria and ventricles close, and the ventricles discharge their contained blood into the appropriate great vessel.









Conducting system


Although cardiac muscle is similar to skeletal muscle in many ways, it does have certain differences. Cardiac muscle cells tend to be shorter and are frequently Y-shaped and are linked at each end to other muscle cells. At the sites of attachment there is an intercalated disc which, as well as anchoring the membranes of the cells, permits the spread of electrical activity. Cardiac muscle cells are able to contract both spontaneously and rhythmically, and indeed isolated cells in culture contract regularly. As all the cells are in contact with each other and can all contract spontaneously, those with the fastest rate of contraction will drive the others. These are situated in the wall of the right atrium at the upper end of the crista terminalis (Fig. 9.9) and are termed the sinoatrial node (SA node or ‘pacemaker of the heart’). From there the cardiac impulse spreads through the atrial muscles to reach the atrioventricular node, which lines the atrial septum close to the opening of the coronary sinus. From there the atrioventricular bundle (of His) passes through a channel in the fibrous skeleton of the heart to the membranous part of the interventricular septum, where it divides into a right and left bundle branch. The left bundle is larger than the right and divides into an anterior and posterior fascicle. These run underneath the endocardium to activate all parts of the ventricular musculature in such a way that the papillary muscles contract first and then the wall and septum in rapid sequence from the apex towards the outflow track, with both ventricles contracting together. The atrioventricular bundle is normally the only pathway through which impulses can reach the ventricles.




Blood supply to the heart


The arterial supply (Fig. 9.4) is of great clinical importance, as coronary occlusion is the chief cause of mortality in the western world. The right and left coronary arteries arise from the anterior and the left aortic sinuses, respectively, just above the aortic valve, and the main branches lie in the interventricular and the atrioventricular grooves.




Left coronary artery

Arising from the left aortic sinus the left coronary artery (the left main stem) varies from 4–10 mm in length and is the most important artery in the human body, in that occlusion will invariably lead to rapid demise! If stenosis of this artery is diagnosed, urgent operation is required to bypass it. It continues passing to the left behind the pulmonary trunk, reaching the atrioventricular groove. It is initially under cover of the left auricle, where it divides into two branches of equal size: the anterior interventricular (left anterior descending) and the circumflex artery. The circumflex artery continues around the left surface of the heart in the atrioventricular groove to anastomose with the terminal branches of the right coronary artery. The left anterior descending (also known as ‘the widow maker’!) runs down to the apex of the heart in the anterior interventricular groove, supplying the walls of the ventricles down the interventricular septum. It gives off the diagonal branch and goes on to anastomose with the posterior interventricular artery. However the natural anastomosis is poor and unless there has been a gradual stenosis giving time for collaterals to develop, sudden occlusion of a mild stenosis from plaque rupture (see p. 271), is almost invariably fatal, hence it’s nickname.


There are some reasonably common variations. Firstly, the left coronary and circumflex artery may be larger and longer than usual and give off the posterior interventricular artery before anastomosing with the right coronary, which is smaller than usual. This occurs in approximately 10% of the population and is known as ‘left dominance’. Another 10% have ‘codominant’ coronary circulation with equal contribution to the posterior interventricular branch. In approximately one-third of individuals the left main stem may divide into three rather than two branches. The third branch, the intermediate, lies between the left anterior descending and the circumflex and may be of large calibre and supply the lateral wall of the left ventricle.


The blood supply to the conducting system is of clinical importance. In just under 60% of the population the SA node is supplied by the right coronary artery, while in just under 40% it is supplied by the circumflex (dual supply in 3%). The AV node is supplied by the right coronary artery in 90% and circumflex in 10%.





Pericardium





Clinical features





Cardiac surgery








AORTA AND GREAT VESSELS









Descending thoracic aorta


This is the continuation of the arch and starts opposite the lower border of the 4th thoracic vertebra and slightly to the left of it. It ends in the midline at the lower border of the 12th thoracic vertebra, where it passes behind the median arcuate ligament of the diaphragm.



Branches


These can be classified into three groups.








Branches




Relations


To the right from above downwards are the right crus of the diaphragm, the cisterna chyli and the commencement of the azygos vein. From the level of the superior mesenteric artery downwards, the IVC is closely applied to the right side of the aorta, although it gradually becomes more posterior at the lower end so that the iliac veins lie behind the iliac arteries.


To the left is the left crus of the diaphragm, the fourth part of the duodenum, the duodenojejunal flexure and the left sympathetic trunk.


Posteriorly are the upper four lumbar vertebrae.


Anteriorly at the level of the coeliac trunk, the lesser sac of peritoneum separates the aorta from the lesser omentum and liver. Below that, the left renal vein crosses the abdominal aorta immediately below the origin of the superior mesenteric artery. This is at the level of the neck of the vast majority of abdominal aortic aneurysms. It is usually possible to get a clamp on just below the renal vein, but occasionally the aneurysm extends high up, stretching the renal vein like a ribbon across it. Because the left renal vein has tributaries from the left adrenal and from the left ovarian or testicular, the left renal vein can be divided providing it is sufficiently far to the right not to impair the entrance of these vessels, which can then act as venous collaterals. The inferior mesenteric vein also runs quite close to the aorta at this level. In an elective aneurysm this is not a problem, but when there is a large haematoma following a leak, it is possible to damage it if one is not aware of its presence. Also the third part of the duodenum may be adherent to an aneurysm, which may be a particular problem if it is an inflammatory aneurysm. When the anastomosis between a graft and aorta has been done, it is important to have some tissue between it and the duodenum (usually the wall of the aneurysm sac is used). If this is not done there is a small risk of a fistula developing between the anastomosis and the duodenum (aortoduodenal fistula) which is an uncommon but serious cause of haematemesis and melaena. The pancreas lies anterior to the aorta with the third part of the duodenum below. Below this lie the parietal peritoneum and peritoneal cavity with the line of attachment of the mesentery to the small bowel.


It should be noted that in a slim person the aorta and IVC are remarkably close to the anterior abdominal wall. The lumbar vertebrae have a large body, spinal canal and spinous process. These vessels are thus at risk, for example, when inserting a needle to obtain a pneumoperitoneum. It is also worth noting that the bifurcation of the aorta is approximately at the level of the umbilicus, so that aneurysms of the abdominal aorta are normally above this level (although they may, of course, involve the common iliacs).




Right subclavian artery


This arises from the bifurcation of the brachiocephalic artery and courses to the outer border of the first rib where it becomes the axillary artery (Fig. 9.12). It arches laterally over the apex of the lung to reach the superior surface of the first rib, where it lies in a groove just behind the insertion of the scalenus anterior. It is divided into three parts by the scalenus anterior muscle. The first part is medial to it and gives off three branches.








The second part of the subclavian artery lies deep to the scalenus anterior muscle. This gives off the costocervical trunk which supplies the deep structures of the neck, and also the superior intercostal artery which gives off the first and second posterior intercostal arteries.


The third part is lateral to the scalenus anterior and normally has no branches.








The great systemic veins of the thorax


The SVC which carries blood into the right atrium is formed from the union of the right and left brachiocephalic veins (Fig. 9.10). These receive blood from the head and neck and upper limbs as well as from the upper half of the body wall of the trunk.






BLOOD SUPPLY TO THE TRUNK







Iliac arteries


See Fig. 9.11.







Inferior vena cava


From its origin at the level of the 5th lumbar vertebra to the right of the midline and behind the right common iliac artery, the IVC ascends vertically through the abdomen, piercing the central tendon of the diaphragm to the right of the midline to empty into the right atrium (Fig. 9.11). It is larger than the aorta, and as it ascends, is related anteriorly to the small intestine, the third part of the duodenum, the head of the pancreas with the common bile duct and then the first part of the duodenum. It lies in a deep groove in the liver before piercing the diaphragm. It receives the right and left hepatic veins from the liver. Sometimes these fuse to give one trunk going into the vena cava, but occasionally the central hepatic vein opens separately. In partial liver resections or in operations for transplantation, it is obviously important to know the precise anatomy prior to surgery. See Chapter 17.




Blood supply of the head and neck


The brachiocephalic artery and the left common carotid artery in the chest have already been described (pp. 235–237). Each common carotid artery enters the neck (Fig. 9.16), from behind the sternoclavicular joint, and thereafter on both sides they have a similar course and relationships. They ascend in the carotid fascial sheath with the internal jugular vein lying laterally and the vagus nerve between and somewhat behind them. The cervical sympathetic chain ascends immediately posterior to the carotid sheath, while the sternocleidomastoid muscle is superficial to it. The carotid sheath is crossed superficially by the omohyoid muscle. At the level of the upper border of the thyroid cartilage the common carotid artery bifurcates into the internal and external carotid artery. There are no other branches of the common carotid.




Internal carotid artery


This commences at the bifurcation of the common carotid artery, and at its origin is dilated into the carotid sinus which acts as a baroreceptor. In the bifurcation is the carotid body, a chemoreceptor. Both are supplied by the ninth cranial nerve. At first the internal carotid lies lateral and slightly more superficial to the external, but it rapidly passes medial and posterior to it, as it ascends to the base of the skull between the side wall of the pharynx and the internal jugular vein. The upper part of the internal carotid artery and the internal jugular vein are closely related to the last four cranial nerves (Fig. 9.17). The internal carotid is separated from the external in the upper part by the styloid process, the stylopharyngeus muscle, and the glossopharyngeal nerve and pharyngeal branch of the vagus.



At the base of the skull the internal carotid enters the petrous temporal bone in the carotid canal, and subsequently gives off the ophthalmic artery, the anterior and middle cerebral arteries and the posterior communicating artery. There are no branches of the internal carotid in the neck.


It should be noted that atheromatous emboli may arise from stenoses at the origin of the internal carotid. When they do so, they may cause transient attacks of blindness (amaurosis fugax) on the same side if emboli travel to the ophthalmic artery. However, if they go to the cerebral cortex, they will cause transient ischaemic attacks (sensory or motor) on the opposite side of the body due to the decussation of the nerve pathways.





BLOOD SUPPLY OF THE UPPER LIMB (Fig. 9.19)



Axillary artery


The axillary artery is the continuation of the subclavian artery, extending from the outer border of the first rib to the lower border of teres major. It is divided into three parts by the pectoralis minor muscle. Surgical division of this muscle displays the axillary artery, which may be helpful in operations such as axillo-femoral bypass. The muscle should be divided as close as possible to its insertion into the coracoid process as the blood supply comes from below, thus reducing bleeding from the cut muscle and avoiding leaving necrotic muscle made necrotic by ischaemia. It is enclosed in the axillary sheath along with the axillary vein and the components of the brachial plexus. The vein lies medial to the artery, and the cords of the brachial plexus are arranged around the artery. The pectoralis major covers it apart from its distal end. It conveniently has one branch on the first part, two from the second and three from the third. These are:






There is a rich arterial anastomosis around the scapula, which may be an important collateral channel in cases of obstruction of the distal subclavian artery.






VENOUS DRAINAGE OF THE UPPER LIMBS (Fig. 9.20)



Superficial veins


The veins in the digit drain into a dorsal venous arch on the back of the hand. Two veins are formed from the dorsal and venous arch: the cephalic and the basilic.






Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Cardiovascular system
Premium Wordpress Themes by UFO Themes
%d bloggers like this: