Anatomy of the Heart
The female heart weighs about 8 ounces (approximately 2 ounces less than a man’s) and is about the size of a clenched fist. Although it lies in the center of the chest, the bottom of the heart tilts toward the front left side of the chest so it appears to be on the left. The heart is the only muscle in the body that works continuously without needing to rest. It pumps more than 8 pints of blood throughout the female body every minute.
The heart is made up of four chambers. The left atrium and the right atrium on top mainly collect the blood, and the left and right ventricles on the bottom pump the blood. Because the ventricles act as the main pumps for the heart, they are thicker and more muscular than the atria. The left and right sides of the heart are divided by a muscular wall called the septum. The right side of the heart receives oxygen-depleted blood and pumps it to the lungs via the pulmonary artery. The left ventricle pumps oxygen-rich blood out to the body via the aorta; the left ventricle is the main pumping chamber and has thick muscular walls.
Four valves regulate the flow of blood between the heart’s chambers:
- The tricuspid valve regulates the flow of blood from the right atrium to the right ventricle.
- The bicuspid mitral valve regulates the flow of blood between the left atrium and the left ventricle. It is called the mitral valve because it resembles a bishop’s pointy hat, or a miter.
- The pulmonary valve regulates blood flow between the right ventricle and the pulmonary artery (leading to the lungs).
- The aortic valve regulates blood flow from the left ventricle to the aorta.
The valves are pushed open by the force of the blood flow; once the pressure drops, they slam shut like trap doors ensuring the blood flows in one direction only. The lub-dub noise associated with the heartbeat is the sound of the valves opening and shutting.
The Heart & Circulation – The Heartbeat
At rest, the heart beats 60 to 70 times per minute, beating faster during exertion such as exercise. The heartbeat is controlled by an electrical conduction system that sends electrical charges to the heart muscle causing it to expand and contract. The electrical signal originates in the sinus node, a group of nerve cells located in the right atrium that acts as a natural pacemaker. When the sinus node emits an impulse, it passes through the atria, stimulating them to contract and open their respective valves, forcing the blood to flow down into the ventricles. The electrical pulse then passes through the atrioventricular node (or AV node), a kind of junction box located between the atria and the ventricles. A fraction of a second later, the electrical impulse reaches a group of fibers called the bundle of His, which split into the right and left bundle branches in the right and left ventricles. As the impulse moves along these fibers, it triggers the muscle walls of the ventricles to contract, pumping blood into the circulation.
The contraction period is known as systole and the relaxation period is called diastole; together they make up the cardiac cycle.
The Heart & Circulation – The Circulatory System
The circulatory system is the network of blood vessels that delivers oxygen-rich blood throughout the body and transports oxygen-poor blood from the body’s tissues back to the lungs
The circulatory system is comprised of two pathways, or loops:
- The first is the pulmonary circulation. On this loop, blood leaves the heart to collect oxygen from the lungs (what we breathe in) and deposit carbon dioxide (what we breathe out).
- On the second loop, the systemic circulation, arteries carry the oxygen-rich blood throughout the body, delivering oxygen to the tissues and collecting waste and carbon dioxide. The now–oxygen-depleted blood returns to the heart via the veins.
The blood vessels involved include:
- Arteries. These carry oxygen-rich blood (which is red) away from the heart to the rest of the body with the exception of the pulmonary artery, which carries oxygen-depleted blood from the heart to the lungs. Blood coming from the heart flows at a high pressure, so arteries have thicker walls than veins. The aorta is the main artery, and it carries oxygen-rich blood from the heart to the rest of the body.
- Veins. These carry oxygen-poor blood (which is blueish) to the heart from the rest of the body with the exception of the pulmonary veins, which carry oxygen-rich blood to the heart from the lungs. The main veins are the superior vena cava, which brings oxygen-depleted blood to the heart from the upper section of the body (the head and arms), and the inferior vena cava brings oxygen-depleted blood to the heart from the lower sections of the body (the trunk and legs)
The Heart & Circulation – The Blood Vessels of the Heart
As the hardest working muscle in the body, the heart needs its own circulation system to ensure a constant supply of oxygen. These blood vessels, called the coronary arteries, run along the heart’s surface and supply the muscle with oxygen. The two main coronary arteries are the left and right coronary arteries, which branch out from the aorta. A heart attack occurs when one or more of these coronary arteries are blocked.
The left coronary artery begins as the left main coronary artery, which is only about one-half inch long before it divides into two branches: the left anterior descending artery carries blood down the front of the heart to the ventricles, and the left circumflex coronary artery supplies the side and some of the back of the heart. The right coronary artery provides blood mainly to the right and under side of the heart. Blockages of the left main and left anterior descending arteries are usually more life-threatening than blockages of the left circumflex or the right coronary artery.