Two pumps in one organ
The human heart is a muscular organ about the size of a fist, lying slightly left of center in the chest between the lungs. Functionally it behaves like two pumps joined side by side, separated by a muscular wall called the septum. The right side receives blood that has returned from the body and sends it to the lungs. The left side receives oxygen-rich blood from the lungs and sends it out to the rest of the body.
The two sides pump in step but handle different loads. The left side drives blood through a much larger network of vessels, so its walls are thicker.
The four chambers
Each side has an upper and a lower chamber.
- Right atrium: Receives oxygen-poor blood returning from the body through two large veins, the superior and inferior venae cavae.
- Right ventricle: Pumps that blood through the pulmonary artery toward the lungs.
- Left atrium: Receives oxygen-rich blood from the lungs through the pulmonary veins.
- Left ventricle: The most powerful chamber, which pumps blood into the aorta, the body’s largest artery.
The atria are thin-walled collecting chambers that mostly deliver blood to the ventricles below. The ventricles are the main pumping chambers.
The valves: one-way doors
Four valves keep blood moving forward and prevent backflow.
- The tricuspid valve lies between the right atrium and right ventricle.
- The pulmonary valve lies between the right ventricle and the pulmonary artery.
- The mitral valve (also called the bicuspid valve) lies between the left atrium and left ventricle.
- The aortic valve lies between the left ventricle and the aorta.
The valves between atria and ventricles are anchored by thin cords attached to small muscles in the ventricle walls, which stop the flaps from flipping backward under pressure. Valves open and shut passively in response to pressure differences on either side. The familiar sounds heard through a stethoscope are generally attributed to valves closing.
The path of a single drop of blood
Following blood around the full loop shows how the two circuits connect:
- Oxygen-poor blood enters the right atrium and flows into the right ventricle.
- The right ventricle sends it through the pulmonary valve into the pulmonary artery and on to the lungs.
- In the lungs, blood picks up oxygen and gives up carbon dioxide.
- Oxygen-rich blood returns via the pulmonary veins to the left atrium and passes into the left ventricle.
- The left ventricle pumps it through the aortic valve into the aorta, which branches to supply the body.
- After delivering oxygen to tissues, blood returns through veins to the right atrium.
The lung loop is called pulmonary circulation, and the loop to the rest of the body is systemic circulation. The vessels that make up those loops are covered in the discussion of blood vessels.
The wall of the heart
The heart wall has three layers: a thin outer covering (epicardium), a thick middle layer of cardiac muscle (myocardium) and a smooth inner lining (endocardium). The whole organ sits in a protective sac called the pericardium, which contains a small amount of lubricating fluid.
Cardiac muscle is distinct from skeletal muscle. Its cells are linked by specialized connections so that electrical activity spreads quickly from cell to cell and the muscle contracts as a coordinated unit. Cardiac muscle is also highly resistant to fatigue, which is essential for an organ that works continuously for a lifetime.
The heart’s own blood supply
Blood passing through the chambers does not nourish the thick muscle wall. Instead, the heart has its own vessels, the coronary arteries, which branch from the base of the aorta and wrap around the surface. They supply the muscle, and cardiac muscle depends heavily on a steady flow. When a coronary artery becomes narrowed or blocked, part of the muscle may be deprived of oxygen, which is the basis of many heart problems. Factors such as cholesterol levels and blood pressure are widely studied as contributors to arterial disease.
A brief word on control
The heart beats without conscious effort. A small cluster of specialized cells in the right atrium acts as the natural pacemaker, and nerve signals and hormones can speed up or slow the rate to match the body’s needs. The details of that electrical sequence are covered elsewhere under heartbeat.
Common structural problems, in general terms
Some people are born with differences in heart structure, such as holes in the septum or malformed valves. Valves can also narrow (stenosis) or leak (regurgitation) later in life. Heart muscle may weaken or thicken in response to long-term strain. These are broad descriptions, and the causes and implications vary widely.
This article is a general overview of anatomy and is not medical advice. Anyone with concerns about heart health should talk to a qualified clinician.