The heart is located in the space between the lungs known as the thoracic cavity in the mediastium. It lies slightly to the left and is approximately the size of the fist.
The interior of the heart is divided into four chambers; the two upper chambers are referred to as the atria, and the lower are the ventricles. A wall known as the septum divides the two sides of the heart. There are four valves that control the way blood flows in and out of the heart chambers. The tricuspid valve, also known as the right atrioventricular, and the pulmonary valve are located on the right side of the heart. The mitral valve, also known as the left atrioventricular, and the aortic valve are situated on the left.
Three layers of tissue make up the heart wall - the pericardium is the outer layer, the myocardium lies in the middle and the endocardium makes up the inside. The atrial walls are much thinner than those of the ventricles, and as we shall see, this is because the atria simply push the blood through the valves into the ventricles, but it is the task of the ventricles to send the blood out to the entire body, and it is the left ventricle that contains the thickest heart wall.
The function of the heart is to pump the blood around the body in order to deliver properties such as nutrients and oxygen. It does this by pumping oxygenated blood out to the body via the aorta; this is the largest blood vessel in the body. The blood being transported to the arms, the neck, and the head, travels from the heart by way of the aortic arch. Blood travelling to the lower body flows out from the descending aorta. Nutrients, oxygen and various hormones are delivered throughout the body and then the blood returns to the heart deoxygenated.
The blood returning from the upper body comes into the heart from the large vein known as the superior vena cava, and blood coming back from the rest of the body returns via the inferior vena cava. From here the blood enters into the right atrium which contracts and the tricuspid (right atrioventricular) valve opens, the blood then flows through into the right ventricle which contracts sending the blood into the pulmonary valve and then out into the pulmonary artery. The blood is then transported to the lungs where the oxygen is renewed.
Oxygenated blood returns to the heart from the lungs via the pulmonary veins. The blood enters the left atrium which contracts and propels the blood through the mitral (left atrioventricular) valve into the left ventricle which then contracts and pushes the blood through the aortic valve and out into the aorta.
Blood circulation between the heart and lungs is referred to as pulmonary circulation. That which involves the heart and the rest of the body is known as systemic circulation. However, the two systems work in conjunction to each other as deoxygenated blood enters into the right atrium at the same time as oxygenated blood comes into the left atrium, and with every heartbeat the two atria contract, which is known as atrial systole, sending the blood to the ventricles which then also contract, and this is termed as ventricular systole. Following ventricular systole there is a pause and the atria and ventricles relax, this is known as complete cardiac diastole, and it lasts for just under half a second.
The contractions depend upon the heart's electrical conduction. The electrical impulses within the heart originate in the sinoatrial node, which is sometimes referred to as the heart's natural pacemaker. This is located in the wall of the right atrium near the opening of the superior vena cava. The electrical impulses then stimulate atrial systole by travelling through the atria and on to the atrioventricular node which is situated in the atrial septum (between the two atria) and near to the mitral (left atrioventriclar) valve. The atrioventricular node becomes stimulated and sends the impulses along through a bundle of conducting fibres in the ventricular septum; this is known as the atrioventricular bundle, sometimes referred to as the bundle of His, and from here the impulses spread out into the ventricles triggering ventricular systole.
This is how the heart beats and pumps blood around the body. In a healthy body this procedure takes place on average of around sixty to eighty times per minute during regular activities. This figure may be lower when resting and will increase during strenuous exercise.