sign in sign up
<<
Home , Aortic sac, Aortic sinus, Endocardial tubes, Great vessels, Middle cardiac vein, Primitive atrium

Cardiac anatomy, physiology and development 1

WHY DO WE NEED A left and right atria). The two pumps each serve a different circulation. CARDIOVASCULAR SYSTEM? The right is the pump for the pulmonary circu- lation. It receives from the right , which is then The cardiovascular system serves to provide rapid trans- pumped through the pulmonary into the . Here port of nutrients to the tissues in the body and allow rapid it is oxygenated and gives up carbon dioxide; it then returns removal of waste products. In smaller, less complex organ- via the pulmonary into the left atrium of the , isms than the human body there is no such system because and then enters the left ventricle. their needs can be met by simple diffusion. Evolution of The left ventricle is the pump for the systemic circula- the cardiovascular system provided a means of aiding the tion. Blood is pumped from the left ventricle via the to diffusion process, allowing the development of larger or- the rest of the body. In the tissues of the body, nutrients and ganisms. The cardiovascular system allows nutrients: waste products are exchanged. Blood returns to the right • To diffuse into the system at their source (e.g., oxygen atrium via the superior and inferior vena cavae. from the alveoli). The two circulations operate simultaneously and are ar- • To travel long distances quickly. ranged in series. Unidirectional flow is ensured by valves in • To diffuse into tissues where they are needed (e.g., the heart, pressure differences in the arterial tree and valves oxygen to working muscle). in the veins (Fig. 1.1). This is an active process requiring a pump: the heart. The functions of the cardiovascular system rely on a transport me- CLINICAL NOTE dium: blood. Blood is made up of cells (mainly red and white blood cells) and plasma (water, proteins, electrolytes, etc.). As the heart consists of two separate pumps, failure of an individual pump is possible, e.g., right Functions of the cardiovascular heart failure as a result of severe disease (cor system pulmonale). The main functions of the cardiovascular system are: 1. Rapid transport of nutrients (oxygen, amino acids, glucose, fatty acids, water, etc.). The is made up of , veins, cap- 2. Removal of waste products of metabolism (carbon illaries and lymphatic vessels: dioxide, urea, creatinine, etc.). 1. Arteries transport blood from the heart to the tissues. 3. Hormonal control, by transporting hormones to their 2. Capillaries are where diffusion of nutrients and waste target organs and by secreting its own hormones (e.g., products takes place. atrial natriuretic peptide). 3. Veins return blood from the tissues to the heart. (The 4. Temperature regulation, by controlling heat hepatic portal is an exception. This transports distribution between the body core and the skin. blood from the intestines to the liver.) 5. Reproduction, by producing penis erection and 4. Lymphatic vessels return to the blood any excess water providing nutrition to the foetus via a complex system and nutrients that have diffused out of the capillaries. of placental blood flow. 6. Host defence, by transporting immune cells, antigens HINTS AND TIPS and other mediators (e.g., antibodies). Arteries carry oxygenated blood and veins carry ANATOMY OF THE HEART AND deoxygenated blood. The two exceptions to this rule are the umbilical vessels (supplying the foetus) GREAT VESSELS and pulmonary vessels where this is reversed. Overview of the heart and circulation The volume of blood ejected from one ventricle during The heart consists of two muscular pumps (the left and 1 minute is called the cardiac output. The cardiac output of right ventricles). Each pump has its own reservoir (the each ventricle is equal overall, but there may be occasional 1 Cardiac anatomy, physiology and development

­further subdivided into anterior, middle and posterior parts (Fig. 1.2). The contents of each part are shown in Table 1.1. lungs The structures in the mediastinum are surrounded by loose connective tissue, nerves, blood vessels, and lymph vessels.

Pulmonary It can accommodate movement and volume changes. circulation The heart is in the middle mediastinum, and it has the following relations: heart 1. Superiorly, the great vessels and bronchi. 2. Inferiorly, the diaphragm. 3. Laterally, the pleurae and lungs. 4. Anteriorly, the thymus. 5. Posteriorly, the oesophagus. Systemic circulation The structures visible on a normal chest X-ray are shown in Fig. 1.3.

portal circulation liver gut

manubrium anterior mediastinum rest of body superior mediastinum

oxygenated deoxygenated body of Fig. 1.1 Systemic and pulmonary circulations. sternum

inferior mediastinum beat-by-beat variation. The entire cardiac output of the right ventricle passes through the lungs and into the left side of the heart. The cardiac output of the left ventricle passes into the aorta, and it is distributed to various organs and tissues according to their metabolic requirements or particular­ xiphoid posterior functions (e.g., the kidney receives 20% of cardiac output process diaphragm mediastinum middle so that its excretory function can be maintained). This dis- mediastinum tribution can be changed to meet changes in demand (e.g., Fig. 1.2 Lateral view of the mediastinum. during exercise, the flow to the skeletal muscle is increased considerably). Blood is driven along the vessels by pressure. This pres- sure, which is produced by the ejection of blood from the ventricles, is highest in the aorta (about 120 mmHg above Table 1.1 Contents of the mediastinum atmospheric pressure) and lowest in the great veins (almost Mediastinal atmospheric). It is this pressure difference that moves blood compartment Contents through the arterial tree, through the capillaries, and into Superior Great vessels the veins. Thymus Trachea The mediastinum Oesophagus Anterior Internal thoracic arteries This is the space between the two pleural cavities. It con- Thymus tains all the structures of the chest except the lungs and pleura. The mediastinum extends from the superior tho- Middle Heart and Origins of the great vessels racic aperture to the diaphragm and from the sternum to the vertebrae and is divided into superior and inferior Posterior Descending aorta parts by the plane passing from the sternal angle to the T4/ Oesophagus Sympathetic chain T5 intervertebral disc. The inferior mediastinum is then 2 Anatomy of the heart and great vessels 1

right recurrent left recurrent arch of left common 13 laryngeal nerve laryngeal aorta carotid artery 2 nerve 14 right vagus left vagus nerve nerve brachiocephalic left subclavian trunk artery 1 left phrenic 5 superior nerve vena cava pulmonary 11 12 right phrenic trunk 6 nerve 8 left auricle right pulmonary 3 circumflex artery branch right auricle left coronary 9 right coronary artery 7 artery great cardiac right atrium vein left ventricle anterior cardiac 10 vein left anterior 4 descending atrioventricular artery groove

right marginal right apex Fig. 1.3 Normal posteroanterior (PA) chest X-ray. 1, Arch of artery ventricle aorta/aortic knuckle; 2, clavicle; 3, left atrial appendage; 4, left Fig. 1.4 Sternocostal external view of the heart. dome of diaphragm; 5, left lung; 6, left hilum; 7, left ventricular border; 8, pulmonary trunk; 9, right atrial border; 10, right dome of diaphragm; 11, right lung; 12, right hilum; 13, spine of left common carotid artery arch of aorta vertebrae; 14, trachea. (Courtesy Professor Dame M. Turner- brachiocephalic Warwick, Dr. M. Hodson, Professor B. Corrin and Dr. I. Kerr.) trunk left subclavian artery Pericardium bifurcation of pulmonary azygos vein This is the fibroserous sac that surrounds the heart. It con- trunk sists of two layers, between which there is a small amount pulmonary of . The pericardium is fused with the cen- veins left atrium right atrium tral tendon of the diaphragm at its base, the sternum by the left circumflex sternopericardial ligament anteriorly and with the tunica artery adventitia of the great vessels. left marginal vein coronary sinus left posterior CLINICAL NOTE interventricular inferior vein vena cava When fluid accumulates within the pericardial sac left ventricle this is called a pericardial effusion. If it builds up posterior interventricular artery quickly and begins to affect cardiac function this is Fig. 1.5 Posteroinferior external view of the heart. (Courtesy called cardiac tamponade. (Both are described in of Professor Dame M. Turner-Warwick, Dr. M. Hodson, Chapter 17.) Professor B. Corrin and Dr. I. Kerr.)

3. The sternocostal surface of the heart is formed mainly by the right ventricle. 4. The diaphragmatic surface is formed mainly by the left External structure of the heart ventricle and part of the right ventricle. The heart lies obliquely about two-thirds to the left and 5. The pulmonary surface is mainly formed by the left one-third to the right of the median plane (Figs 1.4–1.6). It ventricle. has the following surfaces: The heart borders of the anterior surface are as follows: 1. The base of the heart is located posteriorly and formed 1. Right: right atrium. mainly by the left atrium. 2. Left: left ventricle and left auricle. 2. The apex of the heart is formed by the left ventricle 3. Inferior: right ventricle mainly and part of left ventricle. and is posterior to the fifth intercostal space. 4. Superior: right and left auricles. 3 Cardiac anatomy, physiology and development

Coronary arteries The are shown in Figs 1.8 and 1.9. The mid-clavicular line left coronary artery arises just distal to the left anterior cusp of the . The right coronary artery arises from manubrium the right anterior aortic sinus just above the right anterior p of sternum cusp of the aortic valve. The coronary arteries are the first a body of branches of the aorta; the heart supplies itself with a blood sternum m supply before any other organ. t

5th intercostal HINTS AND TIPS space Knowledge of the arterial supply to the myocardium is essential in determining which vessel is affected in Fig. 1.6 Surface markings of the heart (a, aortic valve; m, ischaemic heart disease, and allows us to predict the ; p, ; t, ). These sequelae of an event. For example, an inferior infarct are anatomical relations – see Fig. 5.5 for auscultatory areas. caused by disease of the right coronary artery is more prone to bradyarrhythmia as this artery also supplies the sinoatrial (SA) and atrioventricular (AV) nodes. HINTS AND TIPS

When examining the cardiovascular system, it is important to remember that the right ventricle Coronary veins lies anteriorly and faces the sternocostal surface. In certain conditions causing pulmonary The coronary veins drain mainly into the coronary sinus, which drains directly into the right atrium (Figs 1.10 and 1.11). hypertension, the right ventricle is forced to work There are some small veins that drain directly into the heart excessively hard, and this can be felt as a right chambers. Generally, these drain into the right side of the heart. ventricular heave on the precordium. Great vessels ‘Great vessels’ is the term used to denote the large arteries Internal structure of the heart and veins that are directly related to the heart. The great The internal structure of the heart is shown in Fig. 1.7. The arteries include the pulmonary trunk and the aorta (and right atrium contains the orifices of the superior and in- sometimes its three main branches: the brachiocephalic, ferior and coronary sinus. The right ventricle the left common carotid, and the left subclavian). The great is separated from the right atrium by the tricuspid (three veins include the pulmonary veins and the superior and cusps) valve. The right ventricle is separated from its out- inferior venae cavae. The great vessels and their thoracic flow tract (the pulmonary trunk) by the pulmonary valve. branches are illustrated in Figs 1.12–1.14. This has three semilunar valve cusps. The left atrium has the orifices of four pulmonary veins Tissue layers of the heart and in its posterior wall and is separated from the left ventricle pericardium by the mitral (sometimes referred to as bicuspid, i.e., two cusps) valve. The left ventricle is separated from its outflow Figure 1.15 shows the tissue layers of the heart and tract (the aorta) by the aortic valve, which also has three pericardium. semilunar valve cusps. Pericardium The pericardium consists of an outer fibrous pericardial sac, CLINICAL NOTE enclosing the whole heart, and an inner double layer of flat In approximately 1% of the population, the aortic mesothelial cells, called the serous pericardium. The two valve is bicuspid (has only two cusps). This usually layers of the serous pericardium are: goes unnoticed, but puts a person at increased risk 1. The parietal pericardium, which is attached to the of developing aortic stenosis at an earlier age. fibrous sac. 2. The visceral pericardium, which forms part of the epicardium and which covers the heart’s outer surface. 4 Anatomy of the heart and great vessels 1

A right atrium

sinoatrial right superior annulus openings of four node auricle vena cava ovalis pulmonary veins

ascending B left atrium crista aorta terminalis

atrioventricular musculae node pectinati valve of inferior septal cusp inferior vena cava of tricuspid vena cava valve

C right ventricle cusp of pulmonary valve

conus D left ventricle arteriosus cusp of aortic valve supra- cusp of mitral valve ventricular crest chordae tendineae septomarginal wall of left trabecula ventricle interventricular anterior septum papillary muscle trabeculae carnae wall of right ventricle

Fig. 1.7 Internal structure of the four chambers of the heart. (A) Right atrium. (B) Left atrium. (C) Right ventricle. (D) Left ventricle.

5 Cardiac anatomy, physiology and development

branch to pulmonary superior sinoatrial ascending trunk vena cava node aorta left main coronary artery anterior great right atrial left atrium cardiac cardiac branch left atrial branch veins vein circumflex oblique branch coronary vein of left marginal right atrium sinus left atrium branch right anterior small middle coronary interventricular cardiac cardiac artery branch vein vein atrioventricular nodal artery diagonal branches right Fig. 1.10 Anterior view of the heart showing coronary veins. marginal left ventricle apex branch

posterior right interventricular ventricle branch

Fig. 1.8 Anterior surface of the heart showing coronary arteries. The left coronary artery has two terminal branches: the anterior interventricular branch (also called the left great cardiac anterior descending artery, or ‘widow’s artery’) and the vein circumflex branch. The anterior interventricular branch supplies both ventricles and the . left marginal The circumflex branch supplies the left atrium and the vein inferior part of the left ventricle. The right coronary artery coronary supplies the via the right atrial branch. sinus

left posterior ventricular vein left atrium small cardiac vein

middle cardiac vein circumflex Fig. 1.11 Posteroinferior view of the heart showing branch of left coronary veins. coronary artery pulmonary veins posterior ventricular right common branches inferior carotid artery left common vena cava right carotid artery right subclavian left subclavian coronary artery artery artery posterior crux axillary interventricular artery branch atrioventricular nodal artery Fig. 1.9 Posteroinferior surface of the heart showing coronary arteries. The right coronary artery gives off a right marginal branch and a large posterior interventricular branch. Near the apex, the posterior interventricular branch may anastomose with the anterior interventricular branch of the left coronary artery. The right coronary artery mainly brachiocephalic posterior supplies the right atrium, right ventricle, and interventricular trunk intercostal septum. It may also supply part of the left atrium and left ascending descending arteries ventricle. The nodal branch supplies the atrioventricular aorta thoracic aorta node. Fig. 1.12 The thoracic aorta and its branches. 6 Anatomy of the heart and great vessels 1

right left brachiocephalic left vein left superior vena cava right pulmonary left pulmonary veins veins

inferior vena cava

Fig. 1.13 Veins of the .

large blood aorta superior vessel vena cava cavity of heart fibrous bifurcation of pericardium pulmonary pulmonary trunk parietal layer of veins serous pericardium pulmonary veins pericardial cavity visceral pericardium (epicardium) right left atrium myocardium atrium

left ventricle inferior vena cava

Fig. 1.14 Posterior view of the pulmonary vessels. parietal dense irregular CT fibrous pericardium pericardium pericardial cavity The serous pericardium produces approximately 50 mL of visceral pericardium pericardial fluid, which sits in the pericardial cavity formed (epicardium) by the parietal and visceral layers. The primary function myocardium of this fluid is to provide lubrication so that the heart can lamina move within the pericardium during the cardiac cycle. endocardium fibrosa

Heart Fig. 1.15 Tissue layers of the heart and pericardium (CT, The heart itself contains three layers: connective tissue). • Epicardium. • Myocardium. • Endocardium. Epicardium All the muscle layers attach to the fibrocollagenous heart skeleton, which provides a stable base for contraction. The epicardium is a thin layer of connective tissue that con- The atrial myocardium secretes atrial natriuretic peptide tains adipose tissue, nerves and the coronary arteries and veins. (ANP) when stretched, promoting salt and water excre- Myocardium tion. The ventricular myocardium secretes brain natri- The myocardium is the thickest layer of the heart, and uretic peptide (BNP) when stretched, which seems rather it is made up of cardiac muscle cells. The myocardium a misnomer. BNP is sometimes used to monitor left ven- is thickest in the left ventricle and thinnest in the atria. tricular dysfunction in heart failure.

7 Cardiac anatomy, physiology and development

Endocardium The endocardium has three layers: an outermost connec- atrium blood vessels tive tissue layer (which contains nerves, veins and Purkinje in endocardium fibres) a middle layer of connective tissue and an endothe- endothelium lium of flat endothelial cells. fibrous skeleton Heart valves lamina fibrosa The heart valves are avascular (i.e., they have no blood sup- ply) (Fig. 1.16). This is important if bacteria invade the valves because there is little immune reaction and infective endocar- nodule of cusp ditis may result. Their avascular nature also means that they can be replaced with a porcine (pig) or bovine (cow) tissue valve without generating a rejection-like immune response. ventricle chordae tendinae

Cardiac myocytes papillary There are three types of myocytes – work myocytes, nodal muscle cells and conduction fibres: Fig. 1.16 Structure of a . 1. Work myocytes are the main contractile cells. 2. Nodal cells make up the SA node and AV node, and 3. Single, central nucleus. generate cardiac electrical impulses. 4. Branched structure. 3. Conduction (Purkinje) fibres have a greater diameter 5. Attached to neighbouring cells via intercalated than work myocytes (70–80 μm) and allow fast disks at the branch points. These cell junctions conduction of action potentials around the heart. consist of desmosomes (which hold the cells Ultrastructure of the typical cardiomyocyte together via proteoglycan bridges) and gap The typical cardiac myocyte (Fig. 1.17) has the following junctions (which allow electrical features: conductivity). 1. Length of 50–100 μm (shorter than skeletal muscle fibres). 6. Many mitochondria arranged in rows between the 2. Diameter of 10–20 μm. intracellular myofibrils.

opening of transverse intercalated disc tubule

desmosomes

gap junctions sarcolemma cardiac muscle nucleus fibre mitochondrion

Fig. 1.17 Cardiac myocyte arrangement. Myocytes are branched, and they attach to each other through desmosomes to form muscle fibres. Gap junctions enable rapid electrical conductivity between cells. There is an extensive sarcoplasmic reticulum, which is the internal Ca2+ store. The contractile elements within each cell produce characteristic bands and lines. In between each myofibril unit there are rows of mitochondria. Accompanying blood vessels and connective tissue lie alongside each muscle fibre. (Redrawn with permission from Tortora, G.J., Grabowski, S.R., 2000. Principles of anatomy and physiology, ninth ed. John Wiley & Sons, New York.) 8 Anatomy of the heart and great vessels 1

7. T (transverse) tubules organized in diads with tile unit. It is composed of two bands, the A band and the I cisternae of sarcoplasmic reticulum (Fig. 1.18), which band, between two Z lines. enable rapid electrical conduction deep into the cell, 1. The A (anisotropic) band is made up of thick myosin activating the whole contractile apparatus. filaments and some interdigitating actin filaments. 2+ 8. Extensive sarcoplasmic reticulum, which stores Ca 2. The I (isotropic) band is made up of thin actin filaments ions necessary for electrical activity and contraction. that do not overlap with myosin filaments. Troponin and Each myocyte contains many myofibril-like units (simi- tropomyosin are also contained in the thin filaments. lar to the myofibrils of skeletal muscle) (see Fig. 1.18). These 3. The Z line is a dark-staining structure containing α- units are made up of sarcomeres attached end-to-end and actinin protein that provides attachment for the thin collected into a bundle. A sarcomere is the basic contrac- filaments.

A mitochondrion transverse sarcoplasmic sarcolemma (T) tubule thin filament reticulum thick filament

nucleus Z disc M line Z disc

H zone

I band A band I band

sarcomere

B I band A band I band Z lineZ line sarcomere

actin myosin mitochondrion gap junction filaments M line filaments glycogen granules

intercalated sarcoplasmic T tubule disk reticulum sarcolemma lipid droplet Fig. 1.18 Electron micrographic appearance of cardiac muscle. (A) Each myocyte has rows of mitochondria in between myofibril-like units. There is also an extensive sarcoplasmic reticulum and T tubule system. (B) Close-up of a myofibril-like unit shows the following bands: A band, myosin with some actin; I band, actin; Z line, attachment point for actin; M line links myosin fibres. (Reproduced with permission from [A] Williams, P.L. (Ed.), 1989. Gray’s anatomy, thirty-seventh ed. Churchill Livingstone, Edinburgh; [B] Davies, A., Blakeley, A.G.H., Kidd, C., 2001. Human physiology. Churchill Livingstone, Edinburgh.) 9 Cardiac anatomy, physiology and development

DEVELOPMENT OF THE HEART Venous blood initially enters the sinus horns of the from the cardinal veins (a branch of the umbilical AND GREAT VESSELS vein). Within the next few weeks, the whole systemic venous return is shifted to the right sinus horn through the newly The heart develops in the cardiogenic region of the meso- formed superior and inferior venae cavae. The left sinus horn derm from week 3. This region is at the cranial end of the becomes the coronary sinus, which drains the myocardium. embryonic disc. Angioblastic cords (aggregates of endothe- lial cell precursors) develop and here they coalesce to form two lateral . During week 4, these tubes HINTS AND TIPS fuse together to form the primitive heart tube and the heart begins to pump (Fig. 1.19). terms can be understood by From weeks 5 to 8, the primitive heart tube folds and considering what process the term describes. For remodels to form the four-chambered heart. Initially, the example, the is the first septum primitive heart tube develops a series of expansions sepa- to form (primus means first in Latin), and septum rated by shallow sulci (infoldings) (Fig. 1.20). secundum is the second septum to form. The will give rise to parts of both future atria. The will make up most of the left ventricle. The will form the right ventricle. The will form the ascending aorta and In weeks 5–6, the septum primum and the septum se- the pulmonary trunk. cundum grow to separate the right and left atria (Fig.1.21). These septa are incomplete and leave two openings (foram- ina or ostia) that allow blood to move between the atria. The arterial end of heart septum primum grows downwards from the superior poste- rior wall. The foramen (ostium primum) it creates narrows as the septum grows. first aortic arch While the septum primum is growing, the thicker sep- tum secundum also starts to form. This bulbus does not meet the , leaving an opening cordis bulboventricular called the near the floor of the right atrium. sulcus Blood now has to shunt from the right to the left atrium through the two staggered openings in the septum, the ventricle ­foramen ovale and the ostium secundum (Fig. 1.22). At

atrium superior vena cava sinus developing venosus septum secundum septum primum orifice of venous end of heart superior ostium vena cava secundum Fig. 1.19 Primitive heart tube at 21 days. orifice of left atrium inferior vena cava left endocardial orifice of cushion coronary sinus first and second ventricle dorsal right aorta endocardial atrium cushion truncus sectioned arteriosus septum sinus venosus intermedium conus arteriosus inferior vena cava 6th week (40 days) atrioventricular Fig. 1.21 Initial septation of the atria. The septum primum orifice forms at day 33, and eventually leaves a hole (the ostium ventricle secundum). The septum secundum develops later, at day 40, and is deficient at the foramen ovale. (Redrawn with permission from Larsen, W.J., 1997. Human embryology, Fig. 1.20 Primitive heart tube as it folds and expands. second ed. Churchill Livingstone, Edinburgh.) 10 Development of the heart and great vessels 1

superior develop into the great arteries of the neck and thorax, and vena cava septum primum the dorsal aortae develop branches which supply the rest of the body. The paired dorsal aortae connect to the umbilical septum secundum blood arteries, which carry blood to the . flow The umbilical veins carry oxygenated and nutrient-rich ostium blood from the placenta to the foetus. The venous system right secundum (from the foetus, and umbilical veins) drains into atrium left atrium the sinus horns, and subsequently into the venae cavae and right atrium. foramen ovale The shunts a portion of blood from the directly into the inferior vena cava during left gestation. This is vital as it allows oxygenated blood to en- ventricle sectioned ter the right atrium of the heart, to be pumped around the septum muscular intermedium ventricular foetus. septum right The lungs are not functional during gestation, negating ventricle the need for a large . The pulmonary circulation is largely bypassed by two mechanisms. The fo- early 7th week (43 days) ramen ovale enables most of the oxygenated blood in the inferior right atrium to pass into the left atrium and reach the sys- vena cava temic circulation. The develops from the Fig. 1.22 Completed septation of the atria. The septum sixth aortic arch, and connects the pulmonary arteries to primum is deficient superiorly at the ostium secundum. the descending aorta. This allows oxygenated blood not The septum secundum is deficient inferiorly at the foramen ovale. Blood shunts from the right atrium through these shunted through the foramen ovale to enter the systemic two holes in the septa to the left atrium. In this way, blood circulation directly. The duct is kept open during foetal life bypasses the lungs in the foetal circulation. As these two by circulating prostaglandins, and this stimulation may be openings are staggered, fusion of the septum primum continued artificially early in the neonatal period. and secundum will abolish any shunt between the atria. (Redrawn with permission from Larsen, W.J., 1997. Human embryology, second ed. Churchill Livingstone, Edinburgh.) Circulatory adaptations at A series of changes convert the single system of blood flow birth, the two septa are fused together to abolish any fora- around the foetus into dual systems at birth (Figs 1.23 and men between the two atria. 1.24). Blood flow in the umbilical vessels drastically de- During weeks 5–6, the atrioventricular (tricuspid and clines in the first few minutes after birth because of: mitral) valves develop. The heart undergoes some changes • Compression of the cord. that bring the atria and ventricles into their correct posi- • Vasoconstriction in response to cold, mechanical tions and align the outflow tracks with the ventricles. stimuli and circulating foetal catecholamines as a result The inferior part of the bulboventricular sulcus grows of the stress of descending through the birth canal. into the muscular ventricular septum. Growth stops in week At birth, the pulmonary vascular resistance falls rapidly 7 to wait for the left outflow track to develop, leaving an because: interventricular foramen. In weeks 7–8, the truncus arteriosus (the common out- • The thorax of the foetus is compressed on descent, flow tract of the heart) is divided in two by a spiral process emptying the amniotic fluid from the lungs. of central septation, which results in the formation of the • The mechanical effort of ventilation opens the aorta and pulmonary trunk. This septum is called the trun- constricted alveolar vessels. coconal septum. This septum also grows into the ventricles, • Raising PO2 and lowering PCO2 cause vasodilatation of and it forms the membranous ventricular septum, which the pulmonary vessels. joins the muscular ventricular septum. This completes the This produces an increase in the pulmonary blood flow. septation of the ventricles. Swellings develop at the inferior The sudden cessation of umbilical blood flow and the end of the truncus arteriosus, and these give rise to the arte- opening of the pulmonary system cause a change in the rial (pulmonary and aortic) valves. pressure balance in the atria. There is a pressure drop in the right atrium and a pressure rise in the left atrium (caused by Development of the vasculature an increased pulmonary venous return to the left atrium). This changes the pressure gradient across the atrial septum The vasculature develops from the angioblastic cords of and forces the flexible septum primum against the rigid sep- mesoderm. The aortic ends of the primitive heart tube be- tum secundum, closing the foramen ovale. These two septa come the aortic arches and dorsal aortae. The aortic arches fuse together after about 3 months. 11 Cardiac anatomy, physiology and development

pulmonary aorta ductus superior pulmonary ductus arteriosus becomes trunk arteriosus vena cava trunk ligamentum arteriosum superior pulmonary aorta vena cava veins

lung lung lung lung

ductus inferior foramen ovale foramen venosus vena cava becomes fossa ovale gastro gastro- liver ovalis -intestinal intestinal tract tract liver hepatic inferior hepatic portal vein vena cava portal vein aorta umbilical vein becomes umbilical vein ligamentum teres

placenta umbilicus common common iliac artery iliac artery

umbilical umbilical arteries arteries become medial umbilical ligaments Fig. 1.24 Neonatal circulation after birth. Note the closure Fig. 1.23 Foetal circulation in utero. of the foetal shunts (foramen ovale, ductus arteriosus, and ductus venosus) and umbilical vessels. The ductus arteriosus closes 1–8 days after birth. It is thought that as the pulmonary vascular resistance falls, the eration of the ductus results in the formation of the liga- pressure drop in the pulmonary trunk causes blood to flow mentum arteriosum, which attaches the pulmonary trunk from the aorta into the pulmonary trunk through the duc- to the aorta. tus arteriosus. This blood is oxygenated and the increase in The ductus venosus closes soon after birth and becomes PO2 causes the smooth muscle in the wall of the ductus to a remnant known as the ligamentum venosus. The mecha- constrict due to decreased prostaglandin production, ob- nism is unclear, but it is thought to involve prostaglandin structing the flow in the ductus arteriosus. Eventually, the inhibition. The closure is not vital to life as the umbilical intima of the ductus arteriosus thickens – complete oblit- vein no longer carries any blood.

Chapter Summary

• The cardiovascular system is vital to the survival of all other tissues in the human body. • The right ventricle pumps blood to the pulmonary circulation, and the left ventricle pumps blood to the rest of the body. • Knowledge of the coronary arterial anatomy is important when considering which region of the myocardium is affected in ischaemic heart disease, and allows prediction of clinical sequelae. • Appreciation of the development of the foetal circulation and its changes after birth are essential to understanding congenital heart abnormalities and their effects.

12