The heart is a vital organ which is the size of a fist and weighs 250-300 grams. It sits in the middle of the chest and is surrounded on both sides by the lungs and lies behind the wind pipe also known as the trachea. The food pipe or esophagus runs behind the heart. Therefore, the heart is actually in the middle of the chest ie below the breast bone and not in the left chest as most people think.
The heart is also an organ and thus it needs to be supplied by blood. Blood flowing through the chambers of the heart do not supply the heart muscle with much oxygen and nutrients. The heart depends on the blood vessels called coronary arteries for its energy supply. The coronary arteries start from the base of the aorta which is connect to the left lower chamber called the left ventricle (LV) and they lie on the outer surface of the heart.
There are two main coronary arteries, the left coronary artery and the right coronary artery. Blood flows in these coronary arteries mainly during the relaxation phase of the heart contraction cycle (diastole). The left coronary artery arises from the left side of the aorta and starts off as a large main artery (LM) which then splits into 2 smaller arteries. They are the left anterior descending coronary artery (LAD) and the left circumflex coronary artery (LCX). The left anterior descending artery runs down the front of the heart and supplies blood mainly to the left ventricle (LV) and is one of the most important blood vessels of the heart.
The left circumflex coronary artery curves to the left wall of the heart and supplies blood to the left wall of the LV. The right coronary artery (RCA) arises from the right side of the aorta and runs along the right side of the heart and supplies the right ventricle (RV) and to the back of the heart and supplies the back wall of the LV. Any obstruction of blood flow in the coronary arteries will deprive the heart of blood, oxygen and nutrients.
The heart is made up of two upper chambers called the atrium and two lower chambers called the ventricles. The upper and lower chambers are divided into the right and left sides by a septum. The upper septum is called the inter-atrial septum and the lower septum is called the inter-ventricular septum. The blood vessels connected to the atria are the vena cava on the right side and the pulmonary veins on the left side. The blood vessels connected to the ventricles are the pulmonary artery on the right side and the aorta on the left side. There are valves that regulate the direction of blood flow through the heart.
The valve that sits between the right atria (RA) and ventricle (RV) is called the tricuspid valve and correspondingly, the valve that sits between the left atria (LA) and the left ventricle (LV) is called the mitral valve. There are also valves that sit at the entrance of the blood vessels that connect to the ventricles and they are named after those blood vessels, for example the valve that sits at the entrance of the pulmonary artery is called the pulmonary valve and the valve that sits at the entrance of the aorta is called the aortic valve.
Blood flows only in one direction forwards through the heart and the valves prevent the blood from flowing backwards. Deoxygenated blood from the body and the head flow towards the heart and enter through the vena cava. It then flows through the tricuspid valve into the right ventricle (RV). The blood is then pumped out of the RV into the lungs through the pulmonary artery.
In the lungs, the blood is re-oxygenated by gas exchange from the air we breathe in. The blood is then directed back to the heart through the pulmonary veins into the left atrium (LA). As a typical circulatory cycle in the body, after the blood flows into the left ventricle (LV) through the mitral value, it is being pumped back to the body and the head via the aorta.