What is the most important organ? I am confident that the majority of you will say it is heart. Heart works as your body’s “blood pump”. To be more specific, the heart is a muscular organ located at the left of the middle of the chest between the lungs that is part of the circulatory system (one of your body systems that is responsible for carrying blood, oxygen, nutrients, hormones throughout your body). Heart consists of 2 sides; the right side of your heart receives blood from the body and pumps it to the lungs, and the left side of your heart receives blood from the lungs and pumps it to the body. Heart is mainly responsible for maintaining blood pressure, controlling heart beat rate, circulation of blood, and transportation of nutrients throughout all parts of your body.
Structure of the Heart
Now, let’s dive into how your heart is actually structured and the functions of each part. Heart can be divided into 5 main parts: heart walls, heart chambers, blood vessels, heart valves, and heart conduction system.
Heart Walls
Heart wall is self-explanatory. As its name indicates, the heart wall separates inner parts of the heart with other parts of the body. Heart wall consists of 3 distinct layers: epicardium, myocardium, and endocardium.
Epicardium, also known as visceral pericardium, is the outermost layer that is composed of connective tissue, which gives support and structure, and adipose tissue, which is considered as body fat. Epicardium protects inner heart layers (myocardium and endocardium) and helps production of pericardial fluid, which is a fluid located in the pericardial cavity between serous pericardium and epicardium that reduces friction within pericardial membranes allowing heart to beat easily. Epicardium also contains coronary blood vessels, which supplies blood to the heart wall.
The middle layer is called Myocardium. Myocardium is composed of cardiac muscle fibers and fibroblast (cells found in connective tissue). It is the thickest layer of the heart wall, while its thickness varies in different parts of the heart. Myocardial muscle fibers (atrioventricular bundle and Purkinje fibers) carry electrical impulses that will carry out the conduction system.
Endocardium, the very innermost layer of the heart wall, is composed of smooth muscles and elastic fibers. It also covers heart valves, which will be discussed later. If your endocardium is infected by bacteria or other microbes, this is called endocarditis and can be very fatal.
Image source: https://www.thoughtco.com/the-heart-wall-4022792
Heart Chambers
Your heart is divided into left and right sides by a wall called “septum”, and each side has two chambers: atrium and ventricle.
Here is how 4 heart chambers (left and atrium and left and right ventricles) work together. First of all, deoxygenated blood that has been circulating your body will move to the right atrium through venous circulation (venae cavae). Deoxygenated blood will move to the right ventricle, which is located right under the right atrium, then it will move to the pulmonary artery for pulmonary circulation (lungs). Next, oxygenated blood in the lungs will move to the left atrium through pulmonary veins, to left ventricles, and to your body for systemic circulation. This cycle will continue.
Image source: https://medlineplus.gov/ency/imagepages/19612.htm
Heart Blood Vessels
There are many blood vessels that are connected to your heart that help pumping blood throughout your body. Three very well-known blood vessels are veins, arteries, and capillaries. I believe many of you are familiar with their mechanisms: arteries carry oxygenated blood away from the heart to body tissues, veins bring deoxygenated blood back to your heart, and capillaries connect the arteries and the veins together to allow movement of oxygen, nutrients, and waste products. Yet, there are more specific blood vessels present that are responsible for blood flow and oxygenation.
Image source: https://www.britannica.com/science/blood-vessel
First of all, there is a blood vessel called venae cavae. This blood vessel is responsible for transportation of deoxygenated blood from the systemic circulation to the right atrium. There are two types; superior vena cava is responsible for receiving blood from head and upper part of the body and inferior vena cava is responsible for receiving blood from lower part of the body.
There are more specific blood vessels that are responsible for blood transportation. Pulmonary arteries transport deoxygenated blood from right ventricles to lungs for them to be oxygenated. After that, oxygenated blood will move from the lungs to the left atrium through pulmonary veins. Then, aorta, which is the longest artery of the body, will pump oxygenated blood to the left ventricles and to other parts of the body. Coronary arteries will carry oxygenated blood to heart, and coronary veins will “dump” deoxygenated blood from heart into the right atrium.
Heart Valves
Heart valves are “one-way” valves that allow blood to flow between chambers and prevent backflow of blood in the opposite direction. There are 4 valves present in your heart: tricuspid valve and mitral valve, which are located between atria and ventricles, and aortic valve and pulmonic valve which are located between the ventricles and blood vessels.
Tricuspid valve has 3 “leaflets”, and they separate the right atrium from the right ventricle. When it opens, it allows blood flow from the right atrium to the right ventricle, and prevents “backflow” of blood into the opposite direction. Unlike other 3 valves, the mitral valve has 2 leaflets, and they separate the left atrium from the left ventricle. Just like the tricuspid valve, it allows blood flow from the left atrium to the left ventricle, and prevents backflow of blood into the opposite direction.
The other 2 heart valves have almost exactly the same mechanism as the previous two. Pulmonic valve separates the right ventricle from pulmonary artery, allows blood flow from the right ventricle to the lungs to be oxygenated, and prevents backflow of blood from pulmonary arteries to the right ventricle. Similarly, the aortic valve separates left ventricles from the aorta, allows blood flow from the left ventricle to the aorta, and prevents blood backflow from the aorta to the left ventricle.
Conduction System
Conduction system is a mechanism that controls your heartbeat by a network of electrical signals, signaling cells, and nerves and muscle tissues. The cycle begins when a node called sinoatrial node creates an electrical impulse. The electrical signal will first travel to the atrium, which makes the atrium contract and empty the blood in it. Then the signal travels to the atrioventricular node, and there, it delays the electrical signal until the atrium fully empties the blood in it. Then, in order, the impulse will travel to the atrioventricular bundle (bundle of His) and Purkinje fiber, which makes the ventricles contract. These contractions will cause the heart to beat and the delays of signals using nodes will control the heart beat rate.
The Conduction system is very important as it helps keep the heart rate very steady at a set state. This means that it controls how blood flows throughout your body and transportation of oxygen and nutrients. The electrical signals will speed up when you need more oxygen (e.g. when you are exercising) so that blood can transport oxygen faster, or they will slow down when you do not need that much oxygen (e.g. when you are taking a break).
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