Cardiology is the branch of medicine pertaining to the heart and the vascular system of the human body.  It’s a subspecialty of Internal Medicine.

Structure

In the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left (although sometimes it is on the right, see dextrocardia), underneath the breastbone.  The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts).  The left lung is smaller than the right lung because the heart occupies more of the left hemithorax.  The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs.  The pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective tissue; and a double membrane structure containing a serous fluid to reduce friction during heart contractions (the serous pericardium).  The mediastinum, a subdivision of the thoracic cavity, is the name of the heart cavity.

The apex is the blunt point situated in an inferior (pointing down and left) direction.  A stethoscope can be placed directly over the apex so that the beats can be counted.  It is located posterior to the 5th intercostal space in the left mid-clavicular line.  In normal adults, the mass of the heart is 250-350 g (9-12 oz), or about three quarters the size of a clenched fist, but extremely diseased hearts can be up to 1000 g (2 lb) in mass due to hypertrophy.  It consists of four chambers, the two upper atria (singular: atrium ) and the two lower ventricles.  On the left is a picture of a fresh human heart which was removed from a 64-year-old male.

Functioning

The function of the right side of the heart (see right heart) is to collect de-oxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange).  This happens through a passive process called diffusion.  The left side (see left heart) collects oxygenated blood from the lungs into the left atrium.  From the left atrium the blood moves to the left ventricle which pumps it out to the body.  On both sides, the lower ventricles are thicker and stronger than the upper atria.  The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.

Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle.  Here it is pumped out the pulmonary semilunar valve and travels through the pulmonary artery to the lungs.  From there, blood flows back through the pulmonary vein to the left atrium.  It then travels through the bicuspid valve to the left ventricle, from where it is pumped through the aortic semilunar valve to the aorta.  The aorta forks, and the blood is divided between major arteries which supply the upper and lower body.  The blood travels in the arteries to the smaller arterioles, then finally to the tiny capillaries which feed each cell.  The (relatively) deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.

The heart is effectively a syncytium, a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges.  This relates to electrical stimulation of one cell spreading to neighboring cells.  Its pumping is so strong, that if the aorta, would be cut off, then the blood would squirt to a level of 4 meters.

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