The heart in its most basic terms can be described as a pump, it's function being to rythmatically pump blood around the body. It is a cone-shaped, hollow, muscular organ that is positioned in the thoracic cavity within the mediastinum. It is tilted slightly to the left. The heart is made up of three layers of tissue: pericardium, myocardium and endocardium. Pericardium being the outer covering of the heart, myocardium being specialised cardiac muscle found only within the heart and endocardium being the lining of the myocardium and the heart valves.
Systemic and Pulmonary Circulation.
The human cardiovascular system has double circulation, the pulmonary circulation and the systemic circulation. The right side of the heart or the pulmonary circulation pumps de-oxygenated blood to the lungs where carbon dioxide is excreted and oxygen absorbed, once this exchange of gasses has taken place oxygenated blood is pumped to the right side of the heart. The left side of the heart or systemic circulation pumps the oxygenated blood within the heart to the body where another exchange of gasses takes place, involving every cell within the body, and then takes the now de-oxygenated blood back to the heart.
Blood Flow Through the Heart.
The superior and inferior vena cava deposit de-oxygenated blood from the body into the right atrium. This then passes through the tricuspid valve into the right ventricle. It is then pumped up through the pulmonary valve (which clasps shut to prevent the back flow of blood into the right ventricle) into the pulmonary artery, which after leaving the heart branches into two arteries, the right and left pulmonary arteries. It is these that carry the blood to the lungs to deposit carbon dioxide (or waste) and absorb oxygen. Newly oxygenated blood is returned to the heart via two pairs of veins (one pair from each lung), the left pulmonary veins and the right pulmonary veins. These veins empty their contents into the left artrium, which then passes through the bicuspid valve into the left ventricle. From the left ventricle blood is pumped up into the aorta through the aortic valve, which as with the pulmonary valve, clasps shut to prevent a back flow of blood. The aorta is the first artery of the general circulation, from here blood is pumped around the body.
It is the coronary arteries that supply the heart with arterial blood. They receive around 5% of the oxygenated blood pumped from the heart. The left and right coronary arteries are the first branches off the aorta. They lay around the surface of the heart and their individual branches penetrate deep within the myocardium. Both the left and right coronary arteries each divide into two branches. The left coronary artery divides into the circumflex branch and the anterior interventricular branch. The circumflex branch supplies the left atrium and the posterior left ventricular wall. The anterior interventricular branch supplies the interventricular septum and the anterior wall of both ventricles. The right coronary artery divides into the marginal branch and the posterior interventricular branch. The marginal branch supplies the lateral part of the heart, including the right atrium whilst the posterior interventricular branch supplies the posterior ventricular wall.
The cardiac cycle is a series of events from one heartbeat to the next. There are two main phases in the cardiac cycle, contraction and relaxation. Contraction is called systole and relaxation is called diastole. Each cardiac cycle consists of:
•Atrial Systole - The cardiac cycle starts with the superior and inferior vena cava depositing deoxygenated blood into the right atrium. This happens at the same time as the four pulmonary veins deposit oxygenated blood into the left atrium. Blood start to flow through the atrioventricular valves into the ventricles. The sino-atrial node (SAN) transmits an electrical signal that spreads out over the myocardium of the atria causing them to contract. This empties the atria and completes ventricular filling.
•Atrial Diastole - Once ventricular filling is completed, relaxation of the atria or atrial diastole occurs, which lasts until the cardiac cycle is completed.
•Ventricular Systole - This begins once ventricular filling is complete and the atria relax. The atrio-ventricular node (AVN) picks up the signal sent out by the SAN then conducts impulses down through the bundle of His and on through the Purkinje fibres. This results in a contraction of the ventricles, pumping blood up into the pulmonary arteries and the aorta.
•Ventricular Diastole - The ventricles relax. The stage is sometimes called 'complete diastole' as the atria are also still relaxed. At this stage the atria begin to fill with blood in preparation for the next cycle and the myocardium recovers untill the next contraction.