Anatomy of the Heart 1.pdf

Cardiovascular System The Heart Chambers and the Double Circulation Welcome to this lecture which is entitled The Heart Chambers and the Double Circulation.  Cardiovascular System The Heart Chambers and the Double Circulation...

Cardiovascular System The Heart Chambers and the Double Circulation Welcome to this lecture which is entitled The Heart Chambers and the Double Circulation.  Cardiovascular System The Heart Chambers and the Double Circulation Part 1: The Double Circulation The lecture is divided into 3 parts, and in this first part, we’ll consider the double circulation.  Learning Outcomes After this lecture you should be able to: ▪ Give an account of the double circulation in the adult ▪ Describe the principal features of each of the chambers of the heart ▪ State how the circulation in the fetus differs from that of the adult, and describe the changes that take place at birth ▪ List the problems that may occur in persistent fetal structures after birth Here are the learning outcomes for the entire lecture.  Learning Outcomes After this lecture you should be able to: ▪ Give an account of the double circulation in the adult ▪ Describe the principal features of each of the chambers of the heart ▪ State how the circulation in the fetus differs from that of the adult, and describe the changes that take place at birth ▪ List the problems that may occur in persistent fetal structures after birth However, the learning outcome for this part is straight-forward. After the lecture you should be able to Give an account of the double circulation in the adult The remaining outcomes will be dealt with in subsequent sections.  The Systemic Circulation The heart is divided into a right side and a left side. The left side pumps blood to the various systems of the body, but not to the lungs. The main artery to leave the left side of the heart is the aorta, and it carries oxygenated (that is oxygen rich) blood. It delivers that blood to the capillary beds of the major systems and regions, that can utilise that oxygen. It is blood that is also rich in nutrients. The veins take the deoxygenated blood back to the heart. This circulation is known as the systemic circulation.  The Pulmonary Circulation The pulmonary circulation is from the right side of the heart. It sends oxygen depleted blood to the lungs. The artery that does that is called the pulmonary trunk. In the lungs, gas exchange takes place, and the veins of the lungs take oxygen rich blood back to the left side of the heart, ready to become part of the systemic circulation once again.  Cardiovascular System The Heart Chambers and the Double Circulation Part 2: The Heart Chambers This is part 2 of the lecture on the heart, and in this section, we’ll open up each chamber to see what lies inside.  Learning Outcomes After this lecture you should be able to: ▪ Give an account of the double circulation in the adult ▪ Describe the principal features of each of the chambers of the heart ▪ State how the circulation in the fetus differs from that of the adult, and describe the changes that take place at birth ▪ List the problems that may occur in persistent fetal structures after birth The learning outcome for this part is that afterwards you should be able to: Describe the principal features of each of the chambers of the heart The remaining outcomes will be discussed in Part 3.  Right Atrium Right auricle Musculi Sinus pectinati venarum Fossa Crista ovalis terminalis Opening of AV-valve Coronary sinus IVC The right atrium is full of interesting structures! Firstly, it has an earlobe or auricle that lies in front of the 1st part of the aorta. When one opens up the atrium, the anterior wall can be seen to be composed of muscular ridges called musculi pectinati. These arise from a vertical ridge which runs from the superior vena cava to the inferior vena cava. This is the crista terminalis. In its upper end there is an important part of the electrical conducting system of the heart, the sinu-atrial node. The term used for the muscular wall here is interesting because ‘pecten’ in Latin means ‘comb’, and the appearance of the ridges running perpendicular to the crista, is strikingly similar to a comb. The wall behind the crista terminalis is smooth and is known as the sinus venarum. The difference between the anterior and posterior parts of the atrium indicates different embryological origins of these parts of the wall, the details of which can be picked up in an embryology textbook if you are interested. On the septal wall of the sinus venarum there is a depression, the fossa ovalis. This is a remnant of the foramen ovale of the fetus. This was a hole which closed at birth. We’ll discuss that in the next part of the lecture. Do not confuse the terms ‘fossa’ (pit) with ‘foramen’ (hole). It begins as a foramen and becomes a fossa. There are three openings into the right atrium; the SVC, IVC and the coronary sinus. The latter is the main vein draining the heart itself. The IVC and coronary sinus have rudimentary valves, but these are non-functioning in the adult. The exit route out of the right atrium is via the right atrioventricular valve which is known as the tricuspid valve, since it has three cusps.  Cardiac Ultrasound There are different ways to obtain ultrasound images of the heart, but all of these different views give good appreciation of the ventricles. Let’s look at the parasternal short axis in particular.  Parasternal Short Axis Ultrasound RV LV This axis shows the shape of the ventricles in cross-section. Notice how the right ventricle is crescent shaped, whilst the left ventricle is round. The wall of the left ventricle is about three times the thickness as the one on the right. Indeed the roundness and thickness of the left ventricle bulges towards the right ventricle, forcing it to adopt a crescent shape. The wall of the left ventricle is 3 times the size of the right in cross- section. This reflects the fact that the right side (pulmonary) is a low- pressure system, whilst the left (systemic) is a high-pressure system.  The Right Ventricle The inlet to the right ventricle is guarded by the tricuspid valve. The outlet is located near the superior part of the chamber, on the superior (or anterior) wall. This outlet is to the pulmonary trunk, and is guarded by the pulmonary semilunar valve. The interior walls of the ventricles are highly muscular. The ridges formed by the myocardium are the trabeculae carneae, which literally translates as “meaty ridges”! These help to create a turbulent flow of blood within the chambers.  Right Ventricle1) Septal papillary muscle chordae tendineae In this enlarged image we can see that there are also muscles inside the chamber attached to the wall. These are the papillary muscles. There are anterior and posterior in both ventricles, plus a variable number of additional septal papillae in the right ventricle. Attached to apices of the papillae are fibrous cords. These are chordae tendineae. In turn these cords attach to the margins of the atrioventricular valve cusps. These papillary muscles and chordae act to prevent the cusps of the valve from blowing upwards into the atria during ventricular contraction. There is a bundle passing from the interventricular septum to the anterior papillary muscle, known as the moderator band (or septomarginal bundle). This contains part of the conducting system of the heart that we will discuss in the next lecture.  Right Ventricle2 The region of the septal wall close to the outflow tracts of the heart is smooth, and this reflects the membranous portion of the fetal heart septum. In the adult this is called the conus arteriosus or infundibulum. The word infundibulum means a funnel, and this funnel serves to direct the blood out of the right ventricle via the pulmonary valve.  Ventricular Septal Defect (VSD) The infundibulum is an area that can often fail to develop properly, and give rise to a hole in the heart. This is a ventricular septal defect, or VSD. Ventricular septal defects can also occur in the muscular portion of the septum, but are less common. Atrial septal defects (ASD’s) can also occur and these are fairly common, with approximately 20% of the population having a small patent foramen ovale called a probe patency. Most of those individuals are unaware of this, as it does not permit shunting of blood normally. More serious conditions include large deficiencies in the septum and hence a large patent foramen ovale. Sometimes a complete absence of the inter-atrial septum is seen. This is called common atrium syndrome.  Tetralogy of Fallot A persistent interventricular foramen is usually associated with a mal- formation of the aorta and pulmonary trunk. One such condition is known as tetralogy of Fallot, and occurs when the developing outflow tracts divide unequally. This condition is characterised by a narrow pulmonary trunk (called a pulmonary infundibular stenosis), a defect in the interventricular septum, an “over-riding” aorta and a hypertrophied right ventricle.  Left Atrium Left auricle Pulmonary veins Left atrium The left atrium receives the pulmonary veins, carrying oxygenated blood from the lungs. It is entirely smooth-walled internally, except for the auricle. This reflects the invasion of the endothelium of the pulmonary veins during development. The auricle is the only remnant of the original atrial chamber in the embryo.  Left Heart R. Pulmonary veins Left Mitral valve atrium Chordae tendineae Papillary muscles The valve between the left atrium and the left ventricle is the mitral valve. This is a bicuspid valve. That is it only has two cusps. The anatomical features of the left ventricle are similar to those of the right, except the left ventricle is larger and has thicker walls. There is no moderator band in the left ventricle. There are usually only two papillary muscles, and it lacks septal papillae that are a feature of the right ventricle.  The Left Ventricle The smooth-walled portion of the interventricular septum leads up to the aortic semilunar valve, and hence the outlet of the chamber is to the aorta.  Cardiovascular System The Heart Chambers and the Double Circulation Part 3: The Fetal Circulation This is the final part of the lecture on the Heart Chambers and the Double Circulation. In this section, we will consider the circulation of the fetus.  Learning Outcomes After this lecture you should be able to: ▪ Give an account of the double circulation in the adult ▪ Describe the principal features of each of the chambers of the heart ▪ State how the circulation in the fetus differs from that of the adult, and describe the changes that take place at birth ▪ List the problems that may occur in persistent fetal structures after birth The learning outcomes for this part of the lecture are that afterwards you should be able to: State how the circulation in the fetus differs from that of the adult, and describe the changes that take place at birth List the problems that may occur in persistent fetal structures after birth  The Double Circulation The heart is a muscular pump with four chambers. The pump however, serves two systems:- the pulmonary and the systemic. The pulmonary system is on the embryological right side of the heart, and the systemic system is on the embryological left side. The right atrium receives deoxygenated blood from the venae cavae, which have returned blood from the body. Blood then passes to the right ventricle via the right atrioventricular valve and then to the pulmonary trunk. The deoxygenated blood is then oxygenated within the lungs. Blood from the lungs travels via the pulmonary veins and enters the left atrium. It then passes to the left ventricle via the left atrioventricular valve before being ejected via the aorta. The aorta transports the oxygen-rich blood via its branches to the rest of the body.  Cardiac Output What is perhaps surprising, is that although the right-heart and left-heart have very different volumes of tissue to supply, the amount of blood ejected to both is exactly the same. This has to be the case, as the input to the systemic circulation (i.e. blood entering the left atrium) is the same as the output from the pulmonary circulation (blood ejected from the right ventricle). It is hence no surprise that the two heart systems are pulled together as one.  Fetal Circulation Before birth, there is no need for a pulmonary circulation, as the lungs are not yet functional. There is no gas there to exchange until the first breath is taken. The pulmonary blood vessels are closed for business, and as a result there is a high pressure on the pulmonary side. This is a bit like congestion on the motorway. One needs a way to avoid the traffic jam. There are two ways in which blood bypasses the pulmonary system in the fetus.  Fetal Circulation2 Ductus arteriosus Foramen ovale Firstly, there is a hole in the atrial septum. This is the foramen ovale. Secondly, there is a vessel which connects the pulmonary trunk to the aorta. This is the ductus arteriosus. Because the pressure within the pulmonary system is high, and the systemic system has a relatively lower pressure, the blood will move from the region of high pressure to one of low pressure.  Fetal Circulation3 Blood arriving in the right atrium from the superior vena cava is deoxygenated, much like that in the adult. However, the blood from the inferior vena cava is partially oxygenated. This is because the oxygen has been absorbed across the placenta via the mother. I’ve represented this in purple – the result of mixing red and blue together. There is a rudimentary valve of the inferior vena cava that shunts blood towards the foramen ovale. It will also be encouraged to head towards the left atrium by the pressure gradient. 80% of the blood will enter the left atrium via this route. The rest will travel via the ductus arteriosus. Blood is oxygenated in the placenta. At birth, the lung beds open suddenly as the first breath is taken. The pulmonary system pressure drops, and this triggers a closure of both the foramen ovale and ductus arteriosus. The foramen ovale is then known as the fossa ovalis (since it is no longer a hole), and the ductus arteriosus shrivels and fibroses to form the ligamentum arteriosum.  Patent Fetal Circulation in Newborn Patent Foramen Ovale Patent Ductus Arteriosus Occasionally, the foramen ovale fails to close. This is “patent foramen ovale” if large, or “probe patency” if small. When this occurs, the pressure gradient is such that the systemic system is at a higher pressure than the pulmonary system, and blood flows from left to right. Thus oxygenated blood re-enters the pulmonary system instead of being distributed to the body tissues that need it. Probe patency occurs in approximately 20% of the population, without this causing a pathological state. However, patent foramen ovale is serious. A patent ductus arteriosus is equally disastrous, since the high pressures in the aorta will force blood back to the pulmonary arteries, and thence back to the lungs.  Respiratory System The Heart Chambers and the Double Circulation And with that short section on the fetal circulation concluded, that ends this lecture on the Heart Chambers and the Double Circulation. In the next lecture, we will look at the three most significant aspects of the heart, including the conducting system, the coronary circulation and the heart valves. I look forward to you joining me there.