The heart is a muscular organ just slightly bigger than a person’s loosely clenched fist. It is located in the thorax - more specifically, between the two lungs, in a space called the mediastinum. The heart is covered by a tough membrane called the pericardium, that separates the heart from the other structures in the mediastinum. This position allows the heart to do its job, which is to pump oxygen-rich blood to the entire body, and to send poorly oxygenated blood to the lungs, where gas exchange takes place. The heart is made up of four chambers, and as a whole can be functionally divided into the right heart, made up of the right atrium and the right ventricle, and a left heart, made up of the left atrium and left ventricle. And both atria have pouch-like protrusions called auricles, which can increase their capacity when needed.

Ok, now, poorly oxygenated blood from our bodies and tissues enters the right atrium through the superior vena cava and inferior vena cava. From there, blood passes into the right ventricle, which pumps into the pulmonary trunk, on a voyage towards the lungs and is considered part of the pulmonary circulation. On the other hand, after gas exchange takes place in the lungs, oxygenated blood returns from the lungs through the four pulmonary veins, which drain into the left atrium. Then, oxygenated blood goes in the left ventricle and from there, it’s pumped into the aorta so that it reaches the whole body and is considered part of the systemic circulation.

So, looking at it in three dimensions, the heart looks like an upside down, tipped-over pyramid with four sides, a base that’s mostly posterior and an apex, or tip, that points anteriorly and slightly to the left.

On an anterior or posterior view in two dimensions, the heart is shaped like a trapezoid, so it has a superior and inferior as well as a right and left border. It is important to understand what comprises the borders of the heart because the heart is rotated to the left on its longitudinal axis within the mediastinum, so the apex is directed more postero-laterally.