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CARDIOVASCULAR SYSTEM. It is formed of 4 chambers; 2 atria and 2 ventricles, which are separated by atrio-ventricular (coronary) groove.
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It is formed of 4 chambers; 2 atria and 2 ventricles, which are separated by atrio-ventricular (coronary) groove. The right and left atria are separated longitudinally by the interatrial septum. The right and left ventricles are separated longitudinally by the interventricular septum. It is a muscular hollow organ that pumps the blood to various parts of the body. It is located in the mediastinum (the cavity between the lungs). It is surrounded by the pericardium. THE HEART
Pericardium • It is the covering of the heart. It is formed of 2 types: • Fibrous pericardium: outer tough fibrous conical sac which surrounds the serous pericardium. The apex (above) surrounds the pulmonary trunk, ascending aorta and superior vena cava. The base (below) rests on the central tendon of diaphragm. It protects the heart and prevents its over distension. • Serous pericardium: a closed serous sac which is formed of: • Visceral layer: adherent to the heart. • Parietal layer: lines the fibrous pericardium. • The cavity between these 2 layers is called pericardial cavity which contains a thin film of serous fluid to provide a slippery surface for the movements of the heart. NB: Pericardial effusionis accumulation of excessive amount of fluid in the pericardial cavity leading to compression of the heart. It is drained by a needle inserted in the 5th or 6th left intercostal space near the sternum
The heart is a cone-shaped. It has apex, base, 2 surfaces and 4 borders. Apex: It is directed downwards, forwards and to the left. It is formed by the left ventricle only. It lies opposite the left 5th intercostal space (3 ½ inches from midline). Base: It is directed upwards, backwards and to the right. It is formed by the 2 atria (mainly the left). It is lies in front of middle 4 thoracic vertebrae External features of the heart Anterior view
Sterno-costal surface: It is the anterior surface. It is formed of atrial and ventricular parts, separated by the atrioventricular groove. Atrial part: formed mainly by the right atrium. Ventricular part: formed by right ventricle (right ⅔) and left ventricle (left ⅓), which are separated by the anterior interventricular groove. Diaphragmatic surface: It is the posterior surface, which rests on the diaphragm. It is formed of right ventricle (right ⅓) and left ventricle (left ⅔), which are separated by the posterior interventricular groove. 4 borders: Upper border: formed by the 2 atria (mainly the left). Lower border: formed by the right ventricle and apical part of left ventricle. Right border: formed by the right atrium. Left border: formed by the left ventricle and left auricle. Posterior view
Chambers and flow of blood in the heart • Right atrium: • It is the right upper chamber of the heart which receives venous blood (deoxygenated) from whole body (through the superior vena cava, inferior vena cava, and the coronary sinus). During the interval when the ventricles are not contracting, blood passes down through the right atrio-ventricular orifice, guarded by the right atrio-ventricular (tricuspid) valve into the right ventricle. • Right ventricle: • It is the right lower chamber of the heart which receives blood from the right atrium and pumps it to the lungs through the pulmonary trunk, which divides into two pulmonary arteries, which lead to the left and right lungs, respectively. The following events occur in the right ventricle: • When the right ventricle contracts, the tricuspid valve close and prevent blood from moving back into the right atrium. • When the right ventricle relaxes, the initial backflow of blood in the pulmonary artery closes the pulmonary (semilunar) valve and prevents the return of blood to the right ventricle.
Left atrium: • It is the left upper chamber of the heart which receives oxygenated blood from the lungs through 4 pulmonary veins (2 from each lung). The left atrium, like the right atrium, is a holding chamber for blood. When the left ventricle relaxes, blood passes through the left atrio-ventricular orifice, guarded by the left atrio-ventricular (mitral) valve into the left ventricle. • Left ventricle: • It is the left lower chamber of the heart which receives blood from the left atrium and pumps it through the aorta into the whole body. The following events occur in the left ventricle, simultaneously and analogously with those of the right ventricle: • When the left ventricle contracts, the mitral valve close and prevent blood from moving back into the left atrium. • When the left ventricle relaxes, the initial backflow of blood in the aorta closes the aortic (semilunar) valve and prevents the return of blood to the left ventricle.
Flow of blood in the heart
Blood supply of heart Arterial supply: • Right coronary artery: • It arises from the beginning of the ascending aorta. • It passes in the coronary groove then curves around the right side of the heart till the posterior interventricular groove where it ends by anastomosis with the circumflex branch of left coronary artery. • It gives the following branches: • Right marginal artery: runs along the lower border of the heart. • Posterior interventricular artery: passes in the posterior interventricular groove. • Left coronary artery: • It arises from the beginning of the ascending aorta. • It passes between the pulmonary trunk and left auricle for a short distance then divides into anterior interventricular artery and circumflex arteries. • It gives the following branches: • Anterior interventricular artery: passes in anterior interventricular groove. • Circumflex coronary artery: rotates around left side of the heart at the coronary groove where it ends by anastomosis with right coronary artery.
Venous drainage: • Coronary sinus: a wide short venous channel which collects the majority of venous drainage of the heart. It is lodged in the posterior part of the coronary groove (between the left atrium and left ventricle) and opens in the right atrium. • Tributaries: • Great cardiac vein • Middle cardiac vein • Small cardiac vein • Oblique vein of left atrium • Posterior vein of left ventricle • Anterior cardiac veins: 3-4 small veins draining anterior surface of right ventricle and ends in right atrium. • Venae cordis minimi: minute veins which open directly in all chambers of the heart especially on the right side.
Conductive system of the heart The conductive system of the heart is concerned with the initiation and propagation of the cardiac impulse. It consists of specialized cardiac muscle fibers. It includes the following components: • Sino-atrial node (SAN): • It present at the junction of SVC and right atrium. • It is the pacemaker, in which the impulse is initiated. • Atrio-ventricular node (AVN): • It is present in the lower part of the interatrial septum. • It receives the impulse from SAN by either: • Spreading through the ordinary atrial myocardium. • Spreading through 3 interatrial tracts (anterior, middle and posterior). • Atrio-ventricular bundle (AVB): • It starts from the AVN and descends in the interventricular septum till the apex of the heart and then divides into 2 bundle branches; right and left branches. • It transmits the impulse from the AVN to the wall of the ventricles.
Blood circulations The circulation of the blood within the cardiovascular system can be distinguished into 3 types of circulations, which are communicating together. These are: • Pulmonary circulation: the deoxygenated blood travels from the right ventricle to the lungs through the pulmonary trunk. Within the lungs, gaseous exchange takes place and the oxygenated blood returns from the lungs to the left ventricle through 4 pulmonary veins. Right ventricle pulmonary arteries lungs pulmonary veins left atrium • Systemic circulation: the oxygenated blood travels from the left ventricle via the aorta which gives branches to the various areas of the body. Gas, nutrient, and waste exchange occur across the capillary walls. The venous (deoxygenated) blood is collected to reach to the right atrium via 3 veins; SVC, IVC and coronary sinus. Left ventricle aorta arteries arterioles capillaries venules veins right atrium • Portal circulation: is the circulation which starts by capillaries and ends in capillaries (or sinusoids) without entering the systemic or pulmonary circulation. The most common type is the hepatic portal circulation where the venous blood from the capillaries of the gastrointestinal tract is collected into veins that join to form the portal vein. The later enters the liver where it breaks into the liver sinusoids. The sinusoids are drained by 2 hepatic veins which open in the IVC. Veins from gastrointestinal tract portal vein liver 2 hepatic veins IVC
Arteries • They are elastic vessels, which carry blood away from the heart. • They are branching so that; a big artery gives medium-sized arteries, which in turn give small-sized arteries, arterioles, small arterioles and finally arterial capillaries. • They may communicate together, forming "arterial anastomosis", through which the blood can find an alternative channels if the main pathway is obstructed. • Anastomosis is rich in the regions where movements can interfere with continuous constant circulation e.g. around knee and elbow. • The arteries which have no communications with the neighbouring arteries are called "end arteries" e.g. coronary arteries. • Some arteries may have tortuous course, especially those which are subjected to stretch e.g. lingual, uterine and facial arteries. • There are 3 kinds of arteries (according to size and function): • Elastic arteries are the largest arteries e.g. aorta and other nearby branches. They contain a large amount of elastic tissue, which enables them to expand as blood enters their lumen from the contracting heart. • Muscular arteries are medium-sized arteries e.g. arteries of the limbs. They contain abundant smooth muscle fibers, which allow them to regulate blood flow by vasoconstriction or vasodilatation. • Arterioles are small arteries. Most arterioles contain considerable smooth muscles. The smallest arterioles consist of endothelium surrounded by a single layer of smooth muscle. Arterioles regulate the flow of blood into capillaries by vasoconstriction and vasodilatation.
Connections between arteries and veins • Capillaries: • Are microscopic blood vessels with extremely thin walls. They are lined with single layer of endothelium. Capillaries penetrate most body tissues forming network called capillary beds. The thin walls of the capillaries allow the diffusion of O2 and nutrients out of the capillaries, while allowing CO2 and wastes into the capillaries. • Sinusoids: • Are similar to capillaries in that they are thin-walled blood vessels, but they have irregular and wider spaces than capillaries. They are seen in many sites e.g. liver, spleen, bone marrow and suprarenal gland. The cells lining the sinusoids include phagocytic cells. • Arterio-venous anastomosis (shunts): • Are direct connections between small arterioles and small venules without the intervention of the capillaries. They are numerous in the in the skin of lips, nose, tips of the fingers and toes, intestinal mucosa and in the cavernous tissues of the sex organs. Their walls are surrounded by sphincters which open and close controlling the blood supply to the involved organs.
