Circulatory System and the Heart
BiologyΒ· 9.1, 9.2Β· 12 min read
1. 1. Structure of the Circulatory System (Core)β β ββββ± 3 min
Circulatory System
A system made of the heart, blood vessels and blood that transports oxygen, nutrients, waste products and hormones around the human body.
Humans have a double circulatory system, meaning blood travels through the heart twice for one full circuit of the body. The two separate, parallel circuits are:
- Pulmonary circuit: Carries deoxygenated blood from the heart to the lungs to pick up oxygen, then returns oxygenated blood to the heart.
- Systemic circuit: Carries oxygenated blood from the heart to all body tissues, then returns deoxygenated blood to the heart.
A student labels the pulmonary circuit on a circulatory system diagram as 'carries oxygenated blood to body tissues'. State the error the student has made, and correct the label.
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Step 1: Recall the function of the two distinct circulatory circuits
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Step 2: Identify the error: The pulmonary circuit does not carry oxygenated blood to body tissues; this is the role of the systemic circuit.
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Step 3: Correct label: The pulmonary circuit carries deoxygenated blood from the heart to the lungs, and returns oxygenated blood to the heart.
Exam tip:
Always clearly distinguish between the pulmonary and systemic circuits in exam answers, and explicitly name double circulation as a key feature of the human transport system to score full marks.
2. 2. Structure of the Heart (Core)β β β βββ± 4 min
The heart is a muscular organ that pumps blood around the body. It is divided into four chambers, separated by valves that prevent the backflow of blood. The heart wall is made of cardiac muscle, which contracts and relaxes continuously without fatigue.
Heart Chambers
Four hollow spaces in the heart: two upper atria (singular: atrium) that receive blood entering the heart, and two lower ventricles that pump blood out of the heart.
Right atrium: Receives deoxygenated blood from the body via the vena cava
Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery
Left atrium: Receives oxygenated blood from the lungs via the pulmonary vein
Left ventricle: Pumps oxygenated blood to the body via the aorta, has a thicker muscle wall to generate high pressure for blood flow around the whole body
Explain why the left ventricle wall is significantly thicker than the right ventricle wall.
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Step 1: Recall the function of each ventricle
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Step 2: The right ventricle only pumps blood a short distance to the lungs, so it does not need to generate high pressure.
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Step 3: The left ventricle pumps blood all around the body, so it needs to generate much higher pressure, requiring a thicker, stronger muscle wall.
Exam tip:
When labelling the heart, always view it as if you are looking at a person facing you: the right side of the heart appears on the left of your diagram, and the left side appears on the right.
3. 3. Cardiac Cycle (Extended Only)β β β β βExtended onlyβ± 3 min
Cardiac Cycle
The fixed sequence of events that occurs during one full heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles.
The cardiac cycle follows this fixed sequence:
- All chambers relax: Blood flows into the atria from the vena cava and pulmonary vein. Valves between atria and ventricles are open, valves leading out of ventricles are closed.
- Atria contract: Blood is pushed from the atria into the ventricles.
- Ventricles contract: Valves between atria and ventricles close to prevent backflow, valves leading to the aorta and pulmonary artery open, blood is pumped out of the heart to the lungs and body. The atria and ventricles then relax, and the cycle repeats for the next heartbeat.
Describe what happens to the valves in the heart when the ventricles contract.
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Step 1: Identify the two sets of valves in the heart: atrioventricular (between atria and ventricles) and semilunar (leading out of ventricles).
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Step 2: When ventricles contract, pressure inside the ventricles increases above the pressure in the atria, so the atrioventricular valves close to stop blood flowing back into the atria.
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Step 3: Pressure inside the ventricles is also higher than pressure in the aorta and pulmonary artery, so the semilunar valves open, allowing blood to flow out of the heart.
4. 4. Factors Affecting Heart Rate (Core + Extended)β β ββββ± 2 min
Normal resting heart rate for humans is between 60 and 80 beats per minute. Heart rate increases during exercise because muscles need more oxygen for aerobic respiration, and waste products like carbon dioxide need to be removed faster. The hormone adrenaline also increases heart rate, preparing the body for 'fight or flight' responses.
A runner's heart rate increases from 70 bpm to 150 bpm during a 100m sprint. Explain why this change occurs.
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Step 1: During exercise, leg muscles contract repeatedly and require more energy from aerobic respiration.
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Step 2: Aerobic respiration requires more oxygen, and produces more carbon dioxide waste.
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Step 3: A faster heart rate increases the rate of blood flow, delivering more oxygen and glucose to muscles, and removing carbon dioxide more quickly.
Exam tip:
You must link increased heart rate directly to oxygen supply for respiration and carbon dioxide waste removal in exam answers to get full marks.
5. 5. Coronary Heart Disease (Core)β β ββββ± 3 min
Coronary Heart Disease (CHD)
A disease in which the coronary arteries, which supply oxygenated blood to the heart muscle, become blocked or narrowed by a build-up of fatty deposits. This reduces the flow of blood, and therefore the supply of oxygen and glucose, to the heart muscle.
The coronary arteries branch from the aorta and carry oxygenated blood to the muscular wall of the heart itself. If they become narrowed or blocked, the heart muscle receives less oxygen and glucose for aerobic respiration, which can cause chest pain and, if blood flow is stopped, damage to the heart muscle.
You need to know the risk factors that increase a person's chance of developing coronary heart disease. The syllabus lists seven:
Diet: A diet high in saturated fat and salt increases fatty deposits in the arteries.
Lack of exercise: Reduces fitness and is linked to a higher risk of fatty deposits forming.
Stress: Long-term stress raises the risk of coronary heart disease.
Smoking: Chemicals in tobacco smoke damage the artery lining and raise the risk.
Genetic predisposition: Some people inherit a higher risk from their parents.
Age: The risk increases as a person gets older.
Sex: Males have a higher risk than females of developing coronary heart disease.
Two of these risk factors can be reduced by lifestyle choices. Eating a balanced diet that is low in saturated fat and salt reduces the build-up of fatty deposits in the coronary arteries. Taking regular exercise helps to keep the heart and blood vessels healthy and helps to control body mass, both of which lower the risk of coronary heart disease. Genetic predisposition, age and sex cannot be changed.
A doctor advises a patient with a high risk of coronary heart disease to change their diet and to exercise more. Explain how these two changes reduce the patient's risk of coronary heart disease.
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Step 1: Coronary heart disease is caused by fatty deposits blocking the coronary arteries that supply the heart muscle.
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Step 2: Eating a diet lower in saturated fat and salt reduces the build-up of these fatty deposits in the coronary arteries.
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Step 3: Taking regular exercise helps keep the heart and blood vessels healthy and helps control body mass, further lowering the risk of the arteries becoming blocked.
Exam tip:
If a question asks for the risk factors of coronary heart disease, learn the full syllabus list (diet, lack of exercise, stress, smoking, genetic predisposition, age and sex). Only diet and exercise can be changed to reduce the risk.
6. Common Pitfalls
Wrong move:
Stating all arteries carry oxygenated blood
Why:
The pulmonary artery is the only artery that carries deoxygenated blood, to the lungs
Correct move:
Specify that most arteries carry oxygenated blood, except the pulmonary artery
Wrong move:
Mixing up left and right sides of the heart in diagrams
Why:
The heart is viewed from the front, so the right side of the heart appears on the left of the diagram
Correct move:
Label the heart as if you are looking at a person facing you, so the right side is on the left of your page
Wrong move:
Stating the heart is made of normal skeletal muscle
Why:
The heart is made of cardiac muscle, which does not fatigue unlike skeletal muscle
Correct move:
Explicitly refer to heart muscle as cardiac muscle in all answers
Wrong move:
Describing atria and ventricles contracting at the same time (Extended)
Why:
Atria contract first to fill ventricles, then ventricles contract to pump blood out; simultaneous contraction would block blood flow
Correct move:
Follow the sequence: all chambers relax β atria contract β ventricles contract
Wrong move:
Saying increased heart rate during exercise only supplies more oxygen
Why:
It also removes extra carbon dioxide waste produced by increased cellular respiration
Correct move:
Mention both oxygen delivery and carbon dioxide removal when explaining exercise-induced heart rate increases
7. Quick Reference Cheatsheet
Component | Core Function | Key Feature |
|---|---|---|
Double circulatory system | Transports blood around the body | Pulmonary (lung) + systemic (body) circuits, blood passes heart twice per circuit |
Right atrium | Receives deoxygenated blood from body | Connected to the vena cava |
Right ventricle | Pumps deoxygenated blood to lungs | Connected to the pulmonary artery |
Left atrium | Receives oxygenated blood from lungs | Connected to the pulmonary vein |
Left ventricle | Pumps oxygenated blood to body | Thickest muscle wall for high pressure output |
Valves | Prevent backflow of blood | Located between atria/ventricles, and at the exit of ventricles |
Cardiac cycle (Extended) | Sequence of one full heartbeat | Relaxation β atrial contraction β ventricular contraction |
Coronary heart disease | Coronary arteries blocked by fatty deposits, reducing blood supply to heart muscle | Risk factors: diet, lack of exercise, stress, smoking, genetic predisposition, age, sex |
8. Frequently Asked
Is the human circulatory system single or double?
It is a double circulatory system: blood passes through the heart twice per full circuit of the body, once for the pulmonary (lung) circuit and once for the systemic (body) circuit.
Do all arteries carry oxygenated blood?
Almost all arteries carry oxygenated blood, except the pulmonary artery, which carries deoxygenated blood from the heart to the lungs.
Why is the left ventricle wall thicker than the right?
The left ventricle pumps blood all around the body, so it needs to generate much higher pressure than the right ventricle (which only pumps blood a short distance to the lungs), requiring a thicker muscle wall.
What's Next
Now that you understand the structure and function of the circulatory system and heart, you can move on to learning about the different types of blood vessels and their adaptations, followed by the composition and function of blood. These topics are all part of the Transport in Animals unit for CIE IGCSE Biology 0610, and are frequently tested together in both Core and Extended exam papers. Make sure you can label a heart diagram from memory, and practice explaining the key concepts of double circulation and factors affecting heart rate to score full marks on exam questions.