The heart and circulation

The heart is a muscular organ roughly the size of a fist, sitting in the chest slightly to the left. At rest it typically beats between 60 and 100 times a minute in adults, which adds up to around 100,000 beats a day and billions over a lifetime.

Its job is to push blood through the body, delivering oxygen and nutrients to every cell and carrying away carbon dioxide and waste. If the blood supply to the brain stops, consciousness is lost within seconds.

The heart has four chambers. The two upper chambers, the atria, receive blood; the two lower chambers, the ventricles, pump it out. Valves between the chambers and at the exits act as one-way doors, and their closing produces the familiar lub-dub sound heard through a stethoscope.

Blood travels in two linked loops. The right side of the heart pumps oxygen-poor blood to the lungs, where it picks up oxygen and releases carbon dioxide. The left side pumps that oxygen-rich blood to the rest of the body. Because the body loop is much longer, the left ventricle has the thickest, strongest wall.

• Arteries carry blood away from the heart; they have thick, elastic walls to withstand high pressure.
• Capillaries are microscopic vessels with walls one cell thick, where oxygen and nutrients pass into tissues.
• Veins return blood to the heart; they have valves that stop blood flowing backward.
• Laid end to end, an adult's blood vessels would stretch an estimated 100,000 kilometres.
• William Harvey described the continuous circulation of blood in 1628.

The heartbeat starts on its own. A small cluster of cells called the sinoatrial node, the heart's natural pacemaker, sends out regular electrical impulses. Each impulse spreads across the atria, pauses briefly at the atrioventricular node so the ventricles can fill, then races through the ventricles, making them contract.

The nervous system and hormones such as adrenaline speed the heart up or slow it down to match the body's needs, which is why the heart races during exercise or fear. An electrocardiogram, or ECG, records this electrical activity and helps doctors diagnose rhythm problems.

Blood pressure is the force of blood against artery walls. It is written as two numbers: systolic pressure, when the heart contracts, over diastolic pressure, when it relaxes between beats. Health guidelines generally regard a reading around 120 over 80 millimetres of mercury or lower as healthy for most adults.

Persistently high blood pressure, or hypertension, often causes no symptoms but strains the heart and damages blood vessels, raising the risk of heart attack, stroke and kidney disease. Because it can go unnoticed, regular checks are important.

Cardiovascular diseases are the leading cause of death worldwide, according to the World Health Organization. The most common problem is coronary artery disease, where fatty deposits called plaques build up in the arteries supplying the heart muscle. If a plaque ruptures and a clot blocks an artery, part of the heart muscle is starved of oxygen, causing a heart attack.

Major risk factors include smoking, high blood pressure, high cholesterol, diabetes, obesity, physical inactivity and an unhealthy diet, as well as age and family history. Not smoking, regular physical activity, a diet rich in vegetables, fruit and whole grains, and managing blood pressure greatly reduce risk. Anyone with symptoms such as chest pain, especially spreading to the arm or jaw, should seek emergency medical help immediately.