Cardiovascular (Heart) System

The heart is a muscular, hollow, cone-shaped organ that pumps blood continuously throughout life, and it is one of the hardest-working muscles in the human body. It is roughly the size of a clenched fist, lies in the thoracic cavity between the two lungs, and sits slightly to the left of the centre of the chest, just behind the sternum (breastbone). During development it arises from the mesoderm germ layer of the embryo, and once formed it never rests, contracting and relaxing rhythmically to keep blood moving and blood pressure steady.

• It is about the size of a clenched fist.
• It is located slightly to the left of the centre of the chest.
• It lies between the lungs in the thoracic cavity, slightly towards the left of the sternum (breastbone).
• The heart continuously pumps blood throughout the body and is one of the most important organs responsible for sustaining life.
• The heart is derived from the embryonic mesoderm germ layer.
• It functions throughout a person's lifespan and is one of the most robust, hardest-working muscles in the body.
• Besides humans, most other animals also possess a heart — even invertebrates such as grasshoppers have a heart-like pumping organ, though it does not work the same way a human heart does.
• It is made up of specialised cells that generate their own electrical impulses, which set and coordinate its rhythmic beating.

Because the heart works ceaselessly and must protect itself from friction as it beats, it is enclosed and cushioned by a special covering, which leads us to its wall.

The heart's central job is to keep blood moving so that oxygen and nutrients reach every cell while wastes are carried away; in doing so it also keeps blood pressure steady.

• It pumps deoxygenated blood to the lungs for oxygenation (pulmonary side).
• It maintains blood circulation and steady blood pressure in the body.
• It drives the transport of oxygen, nutrients, gases, hormones and wastes to and from every tissue, delivering oxygen and glucose and carrying away carbon dioxide.

The heart is wrapped in a tough, double-walled sac called the pericardium, which both encases the organ and anchors it in place; between its outer (parietal) layer and its inner (serous) layer runs a thin film of pericardial fluid that lubricates the heart and lets it beat without friction against the lungs and diaphragm. The wall of the heart itself is then built up of three distinct layers, and knowing them in order — outside to inside — is a common exam point.

• Parietal pericardium — the outer layer.
• Serous pericardium — the inner layer.
• Pericardial fluid — runs between the two layers and lubricates the heart during contractions and movements of the lungs and diaphragm.
• The heart's outer wall consists of three layers.

The middle layer, the myocardium, is the thickest and is made of cardiac muscle found nowhere else in the body; it is this muscle that contracts to do the pumping, which is best understood once we look inside at the chambers.

Inside, the human heart is divided into 4 chambers — two upper atria and two lower ventricles — so that it works as a double pump, the right side and the left side acting side by side but never mixing their blood. The thin-walled atria receive blood from the veins and pass it to the ventricles, while the larger, more muscular ventricles pump blood out into the arteries.

• The four chambers are the right atrium, right ventricle, left atrium and left ventricle.
• The atria are smaller chambers that receive blood from the veins and pump it into the ventricles.
• The ventricles are larger, more muscular chambers that pump blood out into the arteries.
• The right atrium and right ventricle together form the "right heart."
• The left atrium and left ventricle together form the "left heart."
• A wall of muscle called the septum separates the two sides of the heart.
• Valves prevent backflow, keeping blood flowing one way through the heart.

• Great veins entering the heart: superior vena cava and inferior vena cava.
• Great vessels of the lungs: pulmonary artery and pulmonary veins (right and left).
• The four chambers: right atrium, left atrium, right ventricle, left ventricle.
• The four valves: pulmonary valve, tricuspid valve, mitral valve, aortic valve.
• The muscular septum dividing the two sides.

The crucial point that the source diagram gets wrong, and that exams test, is the side that carries which blood: the right heart handles deoxygenated blood on its way to the lungs, while the left heart handles oxygenated blood on its way to the body.

To keep that blood moving one way only, each chamber is guarded by a valve.

Valves are flaps of tissue that snap shut to prevent backflow, keeping blood travelling in a single direction through the heart; there are four of them, sitting between the chambers and at the exits to the great arteries. The two atrioventricular (AV) valves lie between the atria and the ventricles, while the two semilunar valves guard the openings into the pulmonary artery and the aorta. The single most-asked fact is which AV valve sits on which side, so it is worth memorising the table exactly.

• The snapping shut of these valves produces the familiar "lubb–dubb" heart sounds.

Having the chambers and valves in place, the next question is what makes them contract in the right order.

The heartbeat is not triggered by the brain but is generated by the heart itself, which contains specialised cells that produce their own electrical impulses; this self-starting property is called myogenic stimulation. The impulse begins in the sinoatrial (SA) node, a small patch of cells in the wall of the right atrium.

• It produces the electrical pulses that drive heart contractions and sets the rhythm of the heartbeat.
• The SA node is known as the natural pacemaker of the heart.

Because the SA node fires on its own, the heart can keep beating its rhythm even when nerve signals are removed, and when this natural pacemaker fails, doctors fit an artificial pacemaker; with the wiring understood, we can now follow the blood itself.

In humans the blood passes through the heart twice for every full trip around the body, a pattern called double circulation, with the heart acting as a double pump that drives two circuits.

• Pulmonary circulation carries deoxygenated blood from the heart to the lungs and brings oxygenated blood back to the heart.
• Systemic circulation pumps oxygenated blood from the heart to every organ and tissue, and deoxygenated blood returns to the heart.
• The heart is itself a muscle needing a constant oxygen supply, met by a third type — coronary circulation.
• Coronary circulation supplies oxygenated blood to the heart muscle itself, vital because the heart supplies blood to the whole body.
• Organs like the brain need a steady flow of fresh, oxygenated blood to function.
• The heart acts as a double pump, ensuring continuous flow to the lungs (pulmonary circuit) and the rest of the body (systemic circuit).

These two circuits expose two facts that contradict everyday assumptions and are therefore favourite exam traps: the pulmonary artery is the only artery that carries deoxygenated blood, and the pulmonary veins are the only veins that carry oxygenated blood. Following each loop step by step makes the pattern stick.

In the pulmonary circuit, deoxygenated blood leaves the right ventricle through the pulmonary artery and travels to the lungs, where it releases carbon dioxide and takes up oxygen, then returns as oxygenated blood to the left atrium through the pulmonary veins. In the systemic circuit, the freshly oxygenated blood leaves the left ventricle through the aorta, is carried through arteries and capillaries to supply oxygen and nutrients to every tissue, after which the now-deoxygenated blood is returned through the vena cava to the right atrium.

• Systemic circuit: left ventricle → aorta → body tissues (supplies O₂ and nutrients) → vena cava → right atrium.

With the circuits clear, attention turns to the pipes that carry the blood — the three kinds of blood vessel.

Blood travels through three kinds of vessel that differ in structure according to their job: arteries carry blood away from the heart under high pressure and so have thick, elastic, muscular walls; veins carry blood back to the heart at low pressure and have thinner walls with valves to stop backflow; and capillaries are the tiny, one-cell-thick vessels where the actual exchange of gases, nutrients and wastes takes place between blood and tissues. Comparing them side by side is one of the highest-yield tables in the whole topic.

The asterisks mark the famous exceptions noted earlier — the pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood — so the rule "arteries carry oxygenated blood" holds everywhere except the pulmonary vessels. Knowing how fast and how hard this blood is pushed brings us to the normal values.

The pumping action can be measured, and examiners expect the standard adult resting figures. A normal adult heart beats about 72 beats/min at rest, pumping roughly 70 mL of blood with each beat (the stroke volume) and so moving about 5 litres/min (the cardiac output). The pressure of blood against the artery walls is the blood pressure, with a normal value of about 120/80 mmHg; the higher figure (systolic) is the pressure when the ventricles contract, the lower (diastolic) when they relax.

• Normal blood pressure is 120/80 mmHg, measured with a sphygmomanometer.
• Cardiac output is about 5 litres/min, roughly the body's whole blood volume each minute.
• The heart's electrical activity is recorded as an ECG (electrocardiogram); readings persistently above normal are called hypertension.

Because the heart muscle itself depends on a constant supply of oxygenated blood through the coronary arteries, anything that interrupts that supply is dangerous, and the difference between two often-confused emergencies is a frequent question.

• Blockage of an artery can cause a heart attack or damage to the muscle of the heart.
• A heart attack is distinct from a cardiac arrest — cardiac arrest is a sudden loss of heart function usually caused by electrical disturbances of the heart's rhythm.

These facts, together with the structure and circulation above, cover the heart as it appears in the general-awareness paper, and the quick revision below gathers the points that come up most often.