The heart is located directly on top of the diaphragm behind the sternum. It is positioned in the middle mediastinum, between the left and right lungs. It is roughly cone-shaped with a broad base and a blunt apex. It lies obliquely, the apex pointing forwards and to the left, close to the 5th intercostal space and the base pointing backwards and to the right. Between the base and the apex the heart measures approximately 12 cm; it is approximately 9 cm across its widest diameter and 6 cm from front to back. The heart weighs approximately 300g in the male and 250g in the female.
The heart is anchored to the diaphragm, to the back of the sternum and to the great vessels by the pericardium (see below for further information).
NB The heart in our 3D model is larger than the average, as this individual suffered from heart disease.
The heart is a myocardial muscular pump consisting of four chambers, two auricles, four valves and a muscular septum all enclosed within a fluid filled sac, the pericardium.
The heart wall is made up of three layers, endocardium, myocardium and epicardium. The endocardium is the smooth thin membrane that lines the inner surface of the heart chambers. The myocardium is the heart muscle itself, and varies in thickness depending on its location, being thin in the atria and thick in the ventricles. The epicardium is a thin outer membrane of the heart wall and is also described as the inner most layer of the serous pericardium known as the visceral pericardium.
The pericardium is three layers of fibrous connective tissue that keeps the heart in place, limits its motion, prevents it from over expanding and reduces the friction as it beats between it and its surrounding structures.
The serous pericardium is a closed sac composed of two thin membranous layers; the visceral and parietal layers. The visceral layer lies directly on the outer surface of the heart wall and the parietal layer lies directly on the deep surface of the fibrous pericardium. Between the visceral and parietal layer is a thin cavity, the pericardial cavity, filled with a viscous pericardial fluid. The pericardial cavity and fluid allows the layers of the pericardium to slide over each other as the heart beats, reducing any friction.
The fibrous pericardium is a thick fibrous sac that encloses the heart and serous pericardium and anchors it within the chest cavity. It does not directly attach to the heart itself but instead attaches to the great vessels of the heart, the diaphragm and sternum, as well as the parietal layer of the serous pericardium beneath. It functions to limit the motion of the heart and because of its fibrous nature it also resists stretch and prevents the heart from over expanding.
The heart is divided into left and right sides by the muscular interventricular septum which is located between the base and the apex of the heart. It runs obliquely through the heart, separating the right and left atria and the right and left ventricles. Its position is marked on the surface of the heart by the anterior and posterior interventricular grooves.
The atria are the two upper chambers of the heart and are positioned near its base. The auricles are little flap like appendages of the atria. The right atrium receives de-oxygenated blood from the entire body via the superior and inferior vena cava. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. The atrial walls are thin as they only have to squeeze blood past the interventricular valves into their corresponding left or right ventricles.
The ventricles are the two lower chambers of the heart and are positioned near its apex. Their walls are much thicker than those of the atria, reflecting their function. The right ventricle receives de-oxygenated blood from the right atrium and pushes it into the pulmonary trunk to the lungs. The left ventricle walls are especially thick because it receives oxygenated blood from the left atrium and has to push it into the aorta and around the entire body.
For blood pressure and pulse see "Circulation" Chapter.
The valves of the heart guard the entrance to the right and left ventricles, the aorta and the pulmonary trunk. All of the valves prevent the blood from flowing in the wrong direction, by making sure that the blood does not flow back into the chamber it has just come from.
The atrio-ventricular valves are located between the atria and the ventricles and prevent the blood in the ventricles from flowing back into the atria. The right atrio-ventricular valve is known as the tricuspid valve because it has 3 leaflets, tri- meaning 3. The left atrio-ventricular valve is known as the bicuspid valve because it has 2 leaflets, bi- meaning 2. When the atrio-ventricular valves close they create a lub sound; the first heart sound. The pulmonary valve and the aortic valves guard the entrance to the pulmonary trunk and the aorta respectively and prevent blood from flowing back into the ventricles. Each valve has 3 cresentric shaped cusps. When the ventricles are contracting the cusps of the valve are pushed flat against the vessel wall, therefore keeping the valve open. When the ventricles stop contracting the blood immediately tries to flow back in the opposite direction (back into the ventricles). This returning blood flows into the valve cusps, opening them out and closing the valve, blocking the flow of blood back into the ventricles. When the pulmonary and aortic valves close they create a dub sound; the second heart sound.
The heart muscle (myocardium) itself must be constantly supplied with nutrients in the form of oxygenated blood and drained of waste products in the form of de-oxygenated blood. This is carried out by the coronary vessels and their healthy circulation is therefore essential for the heart to function. Left and right coronary arteries arise from the walls of the ascending aorta just above the cusps of the aortic valve.
Blood is forced into the coronary arteries as the ventricles stop contracting (ventricular diastole). The blood immediately tries to flow back in the opposite direction (back into the left ventricle). This returning blood flows into the aortic valve cusps, opening them out and closing the aortic valve, but allowing the blood to flow into the openings to the coronary arteries. The left and right coronary arteries and their main branches follow the atrioventricular and interventricular grooves on the surface of the heart and are often embedded in pericardial fat. The branches of the left and right coronary arteries communicate with each other by joining (anastamosing) together around the heart to form continuous loops. This is very important if one of the vessels becomes blocked as it provides an alternative route for blood to get to the myocardium.
Coronary veins accompany the coronary arteries and merge to become the coronary sinus which empties into the right atrium. Numerous small veins pierce the heart walls to directly drain into the heart chambers.
At rest the heart beats 60 - 80 times a minute and functions to pump de-oxygenated blood to the lungs and oxygenated blood around the body. The cardiac cycle is the sequence of events that occurs for the heart to beat. Diastole is when the ventricles and atria are relaxed and allows blood to flow from the atria into the ventricles via the open atrio-ventricular valves. The atria then contract, squeezing the remaining blood into the ventricles. The increased pressure inside the ventricles causes the atrio-ventricular valves to close, preventing the back flow of blood. The ventricles then contract (ventricular systole), and the further increase in pressure opens the semilunar valves allowing the blood to flow into the pulmonary trunks and aorta.
The heart has its own conduction system to transmit electrical impulses, so it can beat independently of nervous control. The myocardium of the heart wall is made of specialised muscle which spontaneously depolarizes to cause contraction. The autonomic nerves that travel to the heart, serve only to control the rate and intensity of the heart beat. If the heart had no parasympathetic and sympathetic input it would beat about 100 beats a minute.
Each heart beat is initiated at the sino-atrial node (SAN). The SAN is located in the superior aspect of the right atrium close to the entry of the superior vena cava. It rhythmically creates an electrical signal (action potential) which spreads throughout the myocardium of the atria, making them contract. Internodal fibres in the atria conduct the impulse to the atrio-ventricular node (AVN), which is found in the intermuscular septum at the junction between the atria and ventricles. Here the signal is paused momentarily to allow the atria to complete their contraction before the ventricles contract. The impulse then travels along the atrio-ventricular bundles of HIS located in the intermuscular septum between the ventricles. Emerging from the atrio-ventricular bundles are Purkinje fibres, which relay the impulse at six times the speed of normal myocardium to the ventricular walls, causing them to contract simultaneously.
The nerves to the heart are part of the autonomic nervous system (parasympathetic and sympathetic). The vagus nerve carries parasympathetic nerve fibres to the heart which decrease the heart rate and causes its contractions to be less powerful. The cardiac nerves carry sympathetic nerve fibres to the heart which increase the heart rate and causes its contractions to be more powerful.
The vagus nerve is the tenth (X) cranial nerve and its fibres originate in the medulla oblongata, which is the cardiac centre of the brain and coordinates the innervation of the heart. From here the vagus nerve travels down the neck to the thorax where it contributes to the cardiac plexus at the base of the heart. The cardiac nerves originate from the lower cervical and upper thoracic spinal cord and their fibres travel to the cervical ganglia in the neck via the sympathetic trunks. The cervical ganglia give off cardiac nerves which travel to the base of the heart and with the vagus nerve make up the cardiac plexus.
'Congenital heart disease' refers to conditions which are present at birth;
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