B6: The Heart and Blood
The circulatory system, blood vessels and the heart
📋 Key Concepts
The circulatory system is a double circulatory system. The right side pumps blood to the lungs (pulmonary circuit) and the left side pumps blood to the rest of the body (systemic circuit). Blood transports oxygen, nutrients, carbon dioxide and waste products.
Key Terms
- Double circulatory system - two separate circuits through the heart
- Pulmonary circuit - heart to lungs and back
- Systemic circuit - heart to body and back
- Arteries - carry blood away from the heart at high pressure
- Veins - carry blood to the heart at low pressure
- Capillaries - tiny vessels where exchange with tissues occurs
- Red blood cells - carry oxygen using haemoglobin
- White blood cells - fight infection
- Platelets - help blood clot
- Plasma - liquid part of blood that carries dissolved substances
📝 The Double Circulatory System
Humans have a double circulatory system. This means blood passes through the heart twice for each complete circuit of the body. The right ventricle pumps blood to the lungs (pulmonary circuit) where it picks up oxygen. The left ventricle then pumps this oxygenated blood to the rest of the body (systemic circuit) where oxygen is delivered to tissues.
Double circulatory system:
Circuit 1 (Pulmonary): Right ventricle → Lungs → Left atrium
Circuit 2 (Systemic): Left ventricle → Body → Right atrium
Blood passes through the heart TWICE per full cycle.
Circuit 1 (Pulmonary): Right ventricle → Lungs → Left atrium
Circuit 2 (Systemic): Left ventricle → Body → Right atrium
Blood passes through the heart TWICE per full cycle.
| Feature | Pulmonary Circuit | Systemic Circuit |
|---|---|---|
| From | Right ventricle | Left ventricle |
| To | Lungs | Rest of the body |
| Returns to | Left atrium | Right atrium |
| Blood type pumped out | Deoxygenated | Oxygenated |
| Blood type returning | Oxygenated | Deoxygenated |
| Pressure | Lower pressure | Higher pressure |
Example 1
Question: Explain why a double circulatory system is more efficient than a single circulatory system.
Answer: In a double circulatory system, the blood is pumped twice - once to the lungs and once to the body. This means the blood can be pumped at a higher pressure to the body after returning from the lungs. If the blood went straight from the lungs to the body (as in a single system), it would lose pressure passing through the lungs first. The higher pressure in the systemic circuit means oxygen reaches tissues faster and more efficiently.
📝 Heart Structure
The heart has four chambers: two atria (upper) that receive blood, and two ventricles (lower) that pump blood out. Valves prevent backflow of blood. The natural pacemaker (in the right atrium) controls the heartbeat.
Path of Blood Through the Heart
- Deoxygenated blood from the body enters the right atrium via the vena cava
- The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle
- The right ventricle contracts, pumping blood through the pulmonary valve to the lungs via the pulmonary artery
- Oxygenated blood returns from the lungs to the left atrium via the pulmonary vein
- The left atrium contracts, pushing blood through the bicuspid valve into the left ventricle
- The left ventricle contracts, pumping blood through the aortic valve to the body via the aorta
Why the left ventricle has a thicker wall: The left ventricle must pump blood all the way around the body (systemic circuit), so it needs to generate a much higher pressure. The right ventricle only pumps blood to the nearby lungs (pulmonary circuit), which requires less force. The thicker muscular wall of the left ventricle allows it to contract more powerfully.
Valves
- Valves prevent the backflow of blood
- Atrioventricular valves (tricuspid and bicuspid) separate the atria from the ventricles
- Semilunar valves are in the pulmonary artery and aorta
- Valves only open one way - they snap shut if blood tries to flow backwards
The Pacemaker
- The natural pacemaker is a group of cells in the right atrium called the sinoatrial node
- It produces electrical impulses that cause the heart muscles to contract
- These impulses coordinate the heartbeat, ensuring the atria contract before the ventricles
- Artificial pacemakers can be fitted if the natural one fails
Example 2
Question: Explain why the left ventricle has a thicker muscular wall than the right ventricle. (3 marks)
Answer: The left ventricle must pump blood all the way around the body (systemic circuit), which requires a high pressure to overcome the resistance of the long network of blood vessels. The right ventricle only needs to pump blood to the lungs (pulmonary circuit), which are much closer, so less pressure is needed. The thicker muscular wall of the left ventricle allows it to contract with more force and generate this higher pressure.
Example 3
Question: Explain the function of valves in the heart. (2 marks)
Answer: Valves prevent the backflow of blood. They ensure blood flows in one direction only - from the atria to the ventricles, and from the ventricles to the arteries. If blood tries to flow backwards, the valves snap shut.
📝 Blood Vessels
There are three types of blood vessel: arteries carry blood away from the heart, veins carry blood back to the heart, and capillaries are tiny vessels where exchange with body tissues takes place.
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Function | Carry blood away from the heart | Carry blood to the heart | Exchange of materials with tissues |
| Wall thickness | Thick walls (muscle and elastic tissue) | Thinner walls | One cell thick walls |
| Lumen size | Narrow lumen | Wide lumen | Very narrow lumen |
| Pressure | High pressure | Low pressure | Low pressure |
| Valves | No valves | Valves to prevent backflow | No valves |
| Blood flow | Pulsing (feels a pulse) | Steady flow | Slow flow |
| Adaptation | Thick walls withstand high pressure; elastic fibres allow stretch and recoil | Wide lumen allows easy flow at low pressure; valves prevent backflow | One cell thick for short diffusion path; large surface area for exchange |
Remember: Arteries carry blood AWAY from the heart (A for Away). Veins carry blood TO the heart. Most arteries carry oxygenated blood, but the pulmonary artery is an exception - it carries deoxygenated blood. Most veins carry deoxygenated blood, but the pulmonary vein is an exception - it carries oxygenated blood.
Example 4
Question: Explain why capillaries have walls that are only one cell thick.
Answer: Capillaries are the site of exchange between the blood and body tissues. Having walls that are only one cell thick creates a very short diffusion path, so oxygen, carbon dioxide, glucose and other substances can diffuse quickly between the blood and the surrounding cells. This makes exchange efficient.
📝 Blood Components
Blood has four main components: red blood cells, white blood cells, platelets, and plasma. Each has a specific function in transport, defence or clotting.
| Component | Function | Adaptations |
|---|---|---|
| Red blood cells | Carry oxygen from lungs to body tissues | Contain haemoglobin (binds oxygen); no nucleus (more space for haemoglobin); biconcave disc shape (increases surface area for oxygen uptake); flexible (can squeeze through narrow capillaries) |
| White blood cells | Fight infection and defend against pathogens | Phagocytes engulf pathogens (phagocytosis); lymphocytes produce antibodies; some produce antitoxins to neutralise toxins |
| Platelets | Help blood clot at wounds | Small cell fragments; clump together at wound sites to form a mesh; trigger conversion of fibrinogen to fibrin to form a clot |
| Plasma | Carries dissolved substances | Liquid part of blood; carries dissolved nutrients (glucose, amino acids), carbon dioxide, urea, hormones and antibodies |
Example 5
Question: Explain how red blood cells are adapted to carry oxygen. (4 marks)
Answer: Red blood cells contain haemoglobin, which binds to oxygen in the lungs and releases it in the tissues. They have no nucleus, which creates more space inside the cell for haemoglobin, allowing more oxygen to be carried. Their biconcave disc shape increases the surface area to volume ratio, speeding up oxygen uptake and release. They are flexible, allowing them to squeeze through narrow capillaries to deliver oxygen close to body cells.
Example 6
Question: Describe two ways white blood cells defend the body against infection.
Answer: 1) Phagocytes carry out phagocytosis - they engulf and digest pathogens. 2) Lymphocytes produce antibodies that are specific to the antigens on the pathogen's surface. Antibodies help destroy the pathogen by clumping them together or marking them for destruction. Some white blood cells also produce antitoxins to neutralise toxins produced by bacteria.
📝 Coronary Heart Disease
Coronary heart disease (CHD) occurs when fatty deposits (plaque) build up inside the coronary arteries, which supply the heart muscle with oxygenated blood. This narrows the arteries, reducing blood flow to the heart muscle.
How CHD Develops
- Fatty deposits (cholesterol) build up on the inner walls of the coronary arteries
- This process is called atherosclerosis
- The arteries become narrower, reducing blood flow
- The heart muscle receives less oxygen, causing chest pain (angina)
- If a coronary artery becomes completely blocked, it can cause a heart attack (myocardial infarction)
Treatments for CHD
| Treatment | How It Works | Advantages | Disadvantages |
|---|---|---|---|
| Stents | A wire mesh tube inserted into the artery to keep it open and maintain blood flow | Effective at restoring blood flow; quick procedure; lasts a long time | Risks of surgery; doesn't treat the underlying cause; risk of blood clots forming on the stent |
| Statins | Drugs that reduce blood cholesterol levels, slowing down the build-up of fatty deposits | Reduce the risk of CHD developing; can reduce the risk of other diseases; long-term benefit | Must be taken long-term; possible side effects (liver damage, muscle pain); doesn't work immediately |
Stents treat the symptom (narrowed artery), but statins treat the cause (high cholesterol). Lifestyle changes such as a balanced diet, regular exercise, stopping smoking and reducing alcohol also reduce the risk of CHD.
📝 Practical: Heart Dissection
Heart dissection allows you to observe the structure of the heart, including the four chambers, valves, and major blood vessels.
Key Observations
- The left ventricle wall is noticeably thicker than the right ventricle wall
- Valves between atria and ventricles can be seen and their one-way action demonstrated
- The aorta has a thicker, more muscular wall than the pulmonary artery
- Coronary arteries can be seen on the surface of the heart
Example 7
Question: During a heart dissection, a student observes that one ventricle has a much thicker wall than the other. Identify which ventricle has the thicker wall and explain why.
Answer: The left ventricle has the thicker wall. It must pump blood all the way around the body (systemic circuit) at high pressure, requiring strong muscular contractions. The right ventricle only pumps blood to the lungs (pulmonary circuit), which are nearby, so it needs less muscular force and has a thinner wall.
❓ Practice Questions
Q1: Describe the double circulatory system in humans. (3 marks)
Q2: Explain why the left ventricle has a thicker muscular wall than the right ventricle. (3 marks)
Q3: Compare the structure of arteries and veins. Explain how each is adapted to its function. (4 marks)
Q4: Describe how red blood cells are adapted to carry oxygen. (3 marks)
Q5: Explain how coronary heart disease develops and describe one treatment. (4 marks)
Q6: Name two substances carried in blood plasma and state where each is transported to. (2 marks)
✅ Answers
- Humans have a double circulatory system with two separate circuits. The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs to pick up oxygen, and returns oxygenated blood to the left atrium. The systemic circuit carries oxygenated blood from the left ventricle to the rest of the body to deliver oxygen, and returns deoxygenated blood to the right atrium.
- The left ventricle pumps blood around the whole body (systemic circuit), which requires high pressure to push blood through the extensive network of blood vessels. The right ventricle only pumps blood to the lungs (pulmonary circuit), which are close by and require less pressure. The thicker muscular wall of the left ventricle allows it to contract more powerfully and generate this higher pressure.
- Arteries have thick walls containing muscle and elastic tissue to withstand the high pressure of blood pumped from the heart; they have a narrow lumen to maintain high pressure. Veins have thinner walls because blood is at low pressure; they have a wider lumen to allow easy blood flow; and they contain valves to prevent backflow of blood. Both carry blood but arteries carry blood away from the heart while veins carry blood to the heart.
- Red blood cells contain haemoglobin which binds to oxygen in the lungs and releases it at the tissues. They have no nucleus, creating more space inside for haemoglobin so more oxygen can be carried. Their biconcave disc shape increases the surface area for oxygen diffusion.
- CHD develops when fatty deposits (cholesterol/plaque) build up on the inner walls of the coronary arteries, narrowing them and reducing blood flow to the heart muscle. This means the heart receives less oxygen. One treatment is a stent - a wire mesh tube inserted into the narrowed artery to keep it open and restore normal blood flow. Alternatively, statins are drugs that reduce blood cholesterol levels to slow the build-up of fatty deposits.
- Carbon dioxide - transported from body tissues to the lungs for excretion. Urea - transported from the liver to the kidneys for excretion. (Also acceptable: dissolved nutrients like glucose from the small intestine to body tissues; hormones from glands to target organs.)
🎯 Exam Tips
- Remember: Arteries carry blood Away from the heart (both start with A)
- The pulmonary artery and pulmonary vein are exceptions to the usual rule - learn them: pulmonary artery = deoxygenated, pulmonary vein = oxygenated
- Always explain WHY the left ventricle is thicker - it must pump to the whole body at high pressure
- For blood components, learn the adaptations AND the function - both are often required
- Red blood cells have NO NUCLEUS - this is a very common exam point
- When asked about valves, say "prevent backflow" not "keep blood flowing" - be specific about direction
- For CHD, distinguish between stents (treat narrowed arteries) and statins (reduce cholesterol)
- Capillaries are "one cell thick" not "one cell wide" - get the detail right
🔢 Maths Skills
Mathematical Skills
Cardiac output = heart rate × stroke volume
Units: cardiac output in cm³/min; heart rate in beats/min; stroke volume in cm³/beat
Units: cardiac output in cm³/min; heart rate in beats/min; stroke volume in cm³/beat
Maths Example
An athlete has a resting heart rate of 60 beats/min and a stroke volume of 80 cm³. Calculate their cardiac output.
Cardiac output = 60 × 80 = 4800 cm³/min = 4.8 dm³/min
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Arteries always carry oxygenated blood Correct: The pulmonary artery carries deoxygenated blood from the heart to the lungs — direction (away from heart) defines an artery, not oxygenation
2. Wrong: The heart pumps blood to itself Correct: The coronary arteries supply the heart muscle with oxygenated blood — they branch from the aorta, not from inside the heart chambers
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how the structure of an artery, a vein and a capillary relates to its function.
Arteries carry blood at high pressure away from the heart, so they have thick walls containing muscle and elastic tissue to withstand and accommodate this pressure. The elastic fibres allow the artery to stretch and recoil as blood is pumped through. Arteries have a narrow lumen to maintain high pressure. Veins carry blood at low pressure back to the heart, so they have thinner walls with less muscle and elastic tissue. They have a wide lumen to allow easy blood flow at low pressure, and contain valves to prevent backflow of blood. Capillaries are the site of exchange between blood and tissues, so their walls are only one cell thick, creating a very short diffusion path for oxygen, carbon dioxide and nutrients. They have a very narrow lumen (one red blood cell wide) to slow blood flow and maximise exchange.
Mark scheme: 1 mark for artery structure + function link; 1 mark for artery wall detail; 1 mark for vein structure + function link; 1 mark for vein valves; 1 mark for capillary structure + function link; 1 mark for one-cell-thick diffusion path
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Heart rate data was collected before and after exercise for two people:
| Resting HR (bpm) | Immediately after exercise (bpm) | After 2 min recovery (bpm) | ||
|---|---|---|---|---|
| Person A | 65 | 140 | 90 | |
| Person B | 72 | 165 | 110 |
(a) Calculate the increase in heart rate for each person during exercise.
(b) Which person is likely fitter? Explain your answer using the data.
Answers: (a) A: 140 − 65 = 75 bpm increase. B: 165 − 72 = 93 bpm increase. (b) Person A is likely fitter — they have a lower resting heart rate (stronger heart muscle, larger stroke volume), a smaller increase during exercise, and a faster recovery rate (back to 90 vs 110 after 2 minutes).
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