Heart
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Structure of the Mammalian Heart
The heart is a muscular pump that drives blood around the body.
- Left and right atria - the upper chambers, which receive blood returning to the heart (the right atrium from the body via the vena cava; the left atrium from the lungs via the pulmonary vein)
- Left and right ventricles - the lower chambers, which pump blood out of the heart (the right ventricle to the lungs; the left ventricle to the body)
- Septum - the muscular wall dividing the heart into left and right sides
- Muscular wall - made of cardiac muscle, which contracts rhythmically without fatigue to provide the pumping force
- One-way valves - ensure blood flows in one direction only through the heart
- Coronary arteries - supply the heart muscle itself with oxygenated blood and nutrients, running over the surface of the heart
The heart's four chambers and valves work together to keep blood flowing in one direction
Direction of Blood Flow
- Blood is pumped away from the heart in arteries
- Blood returns to the heart in veins
Monitoring Heart Activity
- ECG (electrocardiogram) - records the electrical activity of the heart, showing the pattern of each heartbeat; abnormal patterns can indicate heart disease
- Pulse rate - each time the ventricles contract, a pressure wave passes along the artery walls; this pulse can be counted at the wrist or neck, and equals the heart rate in beats per minute
- Listening to valve sounds - a stethoscope detects the "lub-dub" sounds of the valves closing; abnormal sounds (murmurs) can indicate valve defects
Investigating the Effect of Physical Activity on Heart Rate
Heart rate can be measured by taking the pulse rate before and after a period of exercise (e.g. running on the spot for a set time).
- Result: heart rate increases during physical activity, then gradually returns to its resting rate afterwards
Coronary Heart Disease (CHD)
Coronary heart disease is a condition in which the coronary arteries, which supply the heart muscle with blood, become narrowed or blocked, reducing blood flow to the heart muscle.
Blockage is caused by a build-up of fatty deposits in the walls of the coronary arteries. If blood flow is severely restricted, part of the heart muscle may die, causing a heart attack.
- Diet - a high intake of saturated fats raises blood cholesterol levels, promoting fatty deposits in arteries; high salt intake raises blood pressure
- Lack of exercise - a sedentary lifestyle increases the risk of obesity, high blood pressure and high cholesterol
- Stress - raises blood pressure and may lead to unhealthy behaviours
- Smoking - reduces the oxygen-carrying capacity of the blood and damages artery walls
- Genetic predisposition - a family history of CHD increases risk
- Age - risk increases with age, as arteries naturally harden and narrow
- Sex - males are at higher risk at younger ages; risk in females increases after menopause
The risk of coronary heart disease can be reduced through diet and exercise:
- Healthy diet - reducing saturated fat lowers cholesterol; reducing salt lowers blood pressure; fibre and fresh fruit and vegetables help maintain a healthy weight
- Regular exercise - strengthens the heart muscle, lowers resting blood pressure, improves the balance of cholesterol in the blood, and helps maintain a healthy body weight
Valves of the Heart (Supplement)
| Valve | Position | Function |
|---|---|---|
| Atrioventricular (AV) valves | Between the atria and ventricles | Prevent the backflow of blood from the ventricles into the atria when the ventricles contract |
| Semilunar valves | At the exits of the ventricles, in the aorta and pulmonary artery | Prevent the backflow of blood from the arteries back into the ventricles after contraction |
Cardiac Cycle (Supplement)
One complete heartbeat consists of alternating contraction (systole) and relaxation (diastole) of the heart chambers:
- Atria contract - both atria contract simultaneously, pushing blood through the open AV valves into the relaxed ventricles
- Ventricles contract - both ventricles contract; pressure rises, forcing the AV valves shut and pushing blood through the open semilunar valves into the aorta and pulmonary artery
- Diastole (relaxation) - all chambers relax; the semilunar valves close, and blood fills the atria from the veins ready for the cycle to repeat
Common Mistakes
- ❌ Thinking the "lub-dub" heart sound has no biological meaning
- ✓ the "lub-dub" sounds are caused by valves closing - "lub" is the AV valves closing at the start of ventricular systole, and "dub" is the semilunar valves closing at the start of diastole
Explaining the Effect of Physical Activity on Heart Rate (Supplement)
- Muscles respire more rapidly during exercise, so the demand for oxygen and glucose increases, and CO₂ production increases
- The rising CO₂ concentration in the blood is detected by receptors
- The brain sends nerve impulses to the heart, so it beats faster and with greater force
- This increases cardiac output (the volume of blood pumped per minute), delivering more oxygen and glucose to the muscles and removing CO₂ faster
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