7.4 The structure of the heart

• The heart is a muscular organ which lies in the thoracic cavity behind the sternum (breastbone).
• The human heart is composed of two separate pumps on the left and right. The left side deals with oxygenated blood from the lungs, whilst the right side deals with deoxygenated blood from the body.
• The atrium is thin-walled and elastic, and it stretches as it collects blood.
• The ventricle has a thick, muscular wall since it has to contract strongly to pump blood across a great distance.
• The heart has two separate pumps for blood in order to maintain a sufficient blood pressure as it travels around the body.
• Blood has to pass through tiny capillaries in the lungs in order to present a large surface area for gas exchange, which drastically reduces the blood pressure to elsewhere in the body. Therefore, mammals have a system where blood is returned to the heart to increase it's pressure before being transported to the rest of the body.
• The right ventricle pumps blood only to the lungs and it has a thinner muscular wall than the left ventricle.
• The left ventricle has a thick muscular wall, enabling it to contract and create enough pressure to pump blood to the rest of the body.
• Both atria contract together and both ventricles contract together to pump the same volume of blood.
• Between each atrium and ventricle are valves that prevent the backflow of blood into the atria when the ventricles contract.
• There are two types of valves : left atrioventricular (bicuspid) valve and right atrioventricular (tricuspid) valve.
• The ventricles pump blood away from the heart and into the arteries.
• The atria receive blood from the veins.
• Vessels connecting the heart and lungs are called pulmonary vessels.
• There are four pulmonary vessels, including the aorta, vena cava, pulmonary artery and pulmonary vein.
• The aorta is connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs.
• The vena cava is connected to the right atrium and brings deoxygenated blood back from the tissues of the body except the lungs.
• The pulmonary artery is connected to the right ventricle and carries deoxygenated blood to the lungs, where its oxygen is replenished and its carbon dioxide is removed.
• The pulmonary artery is different to other arteries because it carries deoxygenated blood instead of oxygenated blood.
• The pulmonary vein is connected to the left atrium and brings oxygenated blood back from the lungs.
• The pulmonary vein is different than other veins because it carries oxygenated blood instead of deoxygenated blood.
• The heart muscle is not supplied by the oxygenated blood passing through it's left side. Instead, it is supplied by the coronary arteries which branch off the aorta shortly after it leaves the heart.
• When the coronary arteries are blocked (e.g. via a blood clot), this can cause myocardial infarction or heart attack. This is because an area of the heart is deprived of blood and therefore oxygen. The muscle cells in this region can no longer aerobically respire, so they die.