B8: The Heart and Blood Vessels
The double circulatory system, heart structure, blood vessels, blood components, and coronary heart disease
Key Definitions
Double circulatory system — A circulatory system where blood passes through the heart twice per complete circuit: once to the lungs (pulmonary circuit) and once to the body (systemic circuit).
Artery — A blood vessel that carries blood AWAY from the heart (usually oxygenated, except pulmonary artery).
Vein — A blood vessel that carries blood TOWARDS the heart (usually deoxygenated, except pulmonary vein).
Capillary — A tiny blood vessel where exchange of substances between the blood and body cells occurs.
Coronary heart disease — A condition where coronary arteries become blocked by fatty deposits (atheroma), reducing blood flow to the heart muscle.
Artery — A blood vessel that carries blood AWAY from the heart (usually oxygenated, except pulmonary artery).
Vein — A blood vessel that carries blood TOWARDS the heart (usually deoxygenated, except pulmonary vein).
Capillary — A tiny blood vessel where exchange of substances between the blood and body cells occurs.
Coronary heart disease — A condition where coronary arteries become blocked by fatty deposits (atheroma), reducing blood flow to the heart muscle.
The Double Circulatory System
First circuit — Pulmonary circulation:
Right ventricle → Pulmonary artery → Lungs (gas exchange: O₂ in, CO₂ out) → Pulmonary vein → Left atrium
Second circuit — Systemic circulation:
Left ventricle → Aorta → Body (delivers O₂ and nutrients; collects CO₂ and waste) → Vena cava → Right atrium
Advantage of double circulation: Blood pressure is kept high for efficient delivery of oxygen and nutrients to body tissues. The heart can pump blood to the lungs at a lower pressure (to avoid damaging delicate lung tissue) and then pump it at a much higher pressure to the rest of the body.
Right ventricle → Pulmonary artery → Lungs (gas exchange: O₂ in, CO₂ out) → Pulmonary vein → Left atrium
Second circuit — Systemic circulation:
Left ventricle → Aorta → Body (delivers O₂ and nutrients; collects CO₂ and waste) → Vena cava → Right atrium
Advantage of double circulation: Blood pressure is kept high for efficient delivery of oxygen and nutrients to body tissues. The heart can pump blood to the lungs at a lower pressure (to avoid damaging delicate lung tissue) and then pump it at a much higher pressure to the rest of the body.
Heart Structure
Four chambers:
— 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 much thicker muscular wall than the right ventricle because it must pump blood at high pressure to the entire body (the right ventricle only pumps to the nearby lungs).
Valves:
— Atrioventricular valves — Between the atria and ventricles (tricuspid on right, bicuspid/mitral on left); prevent backflow of blood from ventricles to atria.
— Semilunar valves — In the aorta and pulmonary artery; prevent backflow of blood from arteries into ventricles.
Pacemaker: The septum divides the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing. The natural pacemaker is in the right atrium (sinoatrial node) — it controls the heart rate by sending electrical impulses that cause the heart muscle to contract. Artificial pacemakers can correct irregular heartbeats.
— 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 much thicker muscular wall than the right ventricle because it must pump blood at high pressure to the entire body (the right ventricle only pumps to the nearby lungs).
Valves:
— Atrioventricular valves — Between the atria and ventricles (tricuspid on right, bicuspid/mitral on left); prevent backflow of blood from ventricles to atria.
— Semilunar valves — In the aorta and pulmonary artery; prevent backflow of blood from arteries into ventricles.
Pacemaker: The septum divides the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing. The natural pacemaker is in the right atrium (sinoatrial node) — it controls the heart rate by sending electrical impulses that cause the heart muscle to contract. Artificial pacemakers can correct irregular heartbeats.
Example 1: Pathway of Blood Through the Heart
Deoxygenated blood enters the right atrium via the vena cava → passes through the tricuspid valve into the right ventricle → pumped through the semilunar valve into the pulmonary artery → travels to the lungs where it picks up oxygen → oxygenated blood returns via the pulmonary vein into the left atrium → passes through the bicuspid valve into the left ventricle → pumped through the semilunar valve into the aorta → delivered to the body.
Example 2: Why the Left Ventricle Has a Thicker Wall
The left ventricle must pump blood at very high pressure through the aorta and around the entire body, overcoming the resistance of the systemic circulation. The right ventricle only needs to pump blood the short distance to the lungs at a lower pressure (high pressure would damage the delicate lung capillaries). Therefore, the left ventricle has a much thicker muscular wall to generate the greater force needed.
Three Types of Blood Vessels
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Function | Carry blood away from the heart | Carry blood towards the heart | Exchange of substances with cells |
| Wall thickness | Thick walls (muscle and elastic tissue) | Thinner walls (less muscle/elastic tissue) | Wall is one cell thick (endothelium only) |
| Lumen size | Narrow lumen | Wide lumen | Very narrow lumen (one RBC wide) |
| Valves | No valves | Valves present (prevent backflow) | No valves |
| Blood pressure | High pressure (pulsing) | Low pressure (steady flow) | Very low pressure (slows for exchange) |
| Blood speed | Fast | Slower | Very slow (allows time for exchange) |
| Special features | Elastic fibres stretch and recoil to even out pressure surges | Valves ensure one-way flow back to heart against gravity | Large network gives huge surface area; short diffusion distance |
Exam tip: Arteries carry blood AWAY from the heart — not necessarily oxygenated blood. The pulmonary artery carries deoxygenated blood AWAY from the heart to the lungs. Veins carry blood TOWARDS the heart — the pulmonary vein carries oxygenated blood TOWARDS the heart from the lungs.
Four Components of Blood
| Component | Function | Adaptations |
|---|---|---|
| Red blood cells (erythrocytes) | Transport oxygen from lungs to body cells | Contain haemoglobin (binds O₂); biconcave disc shape (increases SA for gas exchange); no nucleus (more room for haemoglobin); flexible (squeeze through narrow capillaries) |
| White blood cells (leucocytes) | Defend the body against infection | Phagocytes: engulf and digest pathogens (phagocytosis); Lymphocytes: produce antibodies and antitoxins; have a nucleus |
| Platelets (thrombocytes) | Help blood clot at wounds | Cell fragments (not true cells); trigger the clotting cascade to form a fibrin mesh that seals wounds, preventing blood loss and pathogen entry |
| Plasma | Transport medium | Liquid part of blood; carries dissolved substances: CO₂, glucose, amino acids, urea, hormones, antibodies, and heat |
Example 3: How Red Blood Cells Are Adapted for Oxygen Transport
Red blood cells contain haemoglobin, which binds to oxygen in the lungs to form oxyhaemoglobin and releases it at body tissues. Their biconcave disc shape provides a large surface area for efficient gas exchange. They have no nucleus, freeing up more space for haemoglobin molecules so each cell can carry more oxygen. They are flexible, allowing them to squeeze through capillaries that are narrower than the cell itself.
Example 4: White Blood Cell Defence Mechanisms
Phagocytes (a type of white blood cell) recognise, engulf, and digest pathogens by phagocytosis — they change shape to surround the pathogen and use enzymes to break it down. Lymphocytes (another type) produce specific antibodies that match the antigens on the pathogen's surface, causing pathogens to clump together for easier destruction. Lymphocytes also produce antitoxins that neutralise toxins produced by bacteria.
Coronary Heart Disease and Treatments
Coronary heart disease (CHD) occurs when coronary arteries (which supply the heart muscle with oxygenated blood) become narrowed or blocked by a buildup of fatty deposits (cholesterol/plaque) called atheroma. This reduces blood flow to the heart muscle, causing:
— Angina — Chest pain during exercise (mild blockage, reduced blood flow).
— Heart attack — Complete blockage causes heart muscle to die (myocardial infarction).
Treatments:
— Stents — A wire mesh tube inserted into a blocked coronary artery to keep it open and restore blood flow. Quick recovery but does not treat the underlying cause.
— Statins — Drugs that reduce blood cholesterol levels, slowing the buildup of fatty deposits. Must be taken long-term; can have side effects (liver damage, muscle pain).
— Artificial hearts — Mechanical devices that temporarily replace a failing heart while waiting for a donor transplant. Not a permanent solution; risk of blood clots (requires blood thinners).
— Artificial heart valves — Replacement valves (mechanical or biological) for damaged valves; less invasive than full heart replacement.
— Angina — Chest pain during exercise (mild blockage, reduced blood flow).
— Heart attack — Complete blockage causes heart muscle to die (myocardial infarction).
Treatments:
— Stents — A wire mesh tube inserted into a blocked coronary artery to keep it open and restore blood flow. Quick recovery but does not treat the underlying cause.
— Statins — Drugs that reduce blood cholesterol levels, slowing the buildup of fatty deposits. Must be taken long-term; can have side effects (liver damage, muscle pain).
— Artificial hearts — Mechanical devices that temporarily replace a failing heart while waiting for a donor transplant. Not a permanent solution; risk of blood clots (requires blood thinners).
— Artificial heart valves — Replacement valves (mechanical or biological) for damaged valves; less invasive than full heart replacement.
Example 5: Evaluating Stents vs Statins
Stents provide an immediate solution by physically opening the blocked artery, restoring blood flow quickly. However, they only treat the specific blockage and do not prevent new blockages forming elsewhere. Statins reduce cholesterol levels throughout the body, reducing the risk of new blockages, but they take time to work and must be taken daily for life with potential side effects. The best approach often combines both: a stent for immediate relief and statins for long-term prevention.
Blood Groups
ABO blood group system:
— Blood group is determined by antigens on the surface of red blood cells and antibodies in the plasma.
— Group A: A antigens, anti-B antibodies
— Group B: B antigens, anti-A antibodies
— Group AB: A and B antigens, no antibodies (universal recipient)
— Group O: No antigens, anti-A and anti-B antibodies (universal donor)
If the wrong blood type is given, antibodies will attack the donated red blood cells, causing them to clump (agglutinate), which can be fatal.
— Blood group is determined by antigens on the surface of red blood cells and antibodies in the plasma.
— Group A: A antigens, anti-B antibodies
— Group B: B antigens, anti-A antibodies
— Group AB: A and B antigens, no antibodies (universal recipient)
— Group O: No antigens, anti-A and anti-B antibodies (universal donor)
If the wrong blood type is given, antibodies will attack the donated red blood cells, causing them to clump (agglutinate), which can be fatal.
Exam tip: When evaluating treatments for CHD, always consider both advantages AND disadvantages. Think about: effectiveness, recovery time, cost, side effects, long-term vs short-term benefits, and whether the treatment addresses the cause or just the symptoms.
Practice Questions
1. Foundation Describe the pathway of blood through the heart, naming the four chambers and four major blood vessels.
Deoxygenated blood enters the right atrium via the vena cava, passes into the right ventricle, and is pumped out via the pulmonary artery to the lungs. Oxygenated blood returns from the lungs via the pulmonary vein into the left atrium, passes into the left ventricle, and is pumped out via the aorta to the body.
2. Foundation Explain why the left ventricle has a thicker muscular wall than the right ventricle.
The left ventricle must pump blood at high pressure through the aorta and around the entire body, which requires greater muscular force. The right ventricle only pumps blood the short distance to the lungs, and at a lower pressure to avoid damaging the delicate lung capillaries.
3. Higher Compare the structure of arteries and veins and explain how each is adapted to its function.
Arteries have thick walls with muscle and elastic tissue to withstand and smooth out the high-pressure surges from the heart; a narrow lumen maintains high pressure. Veins have thinner walls and a wider lumen because blood is at low pressure; they contain valves to prevent backflow and ensure blood returns to the heart against gravity. Arteries do not need valves because the high pressure drives blood in one direction.
4. Higher Describe three adaptations of red blood cells for oxygen transport.
1) Haemoglobin binds to oxygen in the lungs and releases it at body tissues. 2) Biconcave disc shape increases surface area for gas exchange. 3) No nucleus — more room for haemoglobin, increasing oxygen-carrying capacity. Also acceptable: flexible to squeeze through capillaries.
5. Foundation Explain what coronary heart disease is and how stents can be used to treat it.
CHD occurs when coronary arteries become narrowed or blocked by fatty deposits (atheroma), reducing blood flow to the heart muscle. A stent is a wire mesh tube inserted into the blocked artery to keep it open, restoring normal blood flow to the heart muscle and relieving symptoms like angina.
🔢 Maths Skills
Mathematical Skills
Cardiac output = heart rate × stroke volume. For example, if heart rate = 72 beats/min and stroke volume = 70 mL/beat, then cardiac output = 72 × 70 = 5040 mL/min = 5.04 L/min. Make sure units are consistent and convert mL to L where required (1000 mL = 1 L).
⚠️ Common Misconceptions
Watch Out!
Students often think arteries always carry oxygenated blood. Wrong: All arteries carry oxygenated blood Correct: The pulmonary artery carries deoxygenated blood away from the heart to the lungs
Students often think the heart pumps blood to itself. Wrong: The heart muscle gets its blood from the blood inside its chambers Correct: Coronary arteries supply the heart muscle with oxygenated blood
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how artery, vein and capillary structure relates to function.
Arteries carry blood at high pressure away from the heart. They have thick walls containing muscle and elastic fibres to withstand and smooth out pressure surges; a narrow lumen maintains high pressure. Veins carry blood at low pressure towards the heart. They have thinner walls and a wider lumen because pressure is low; valves prevent backflow and ensure one-way return of blood to the heart against gravity. Capillaries are where exchange of substances occurs between blood and cells. They have walls that are only one cell thick, providing a very short diffusion distance for efficient exchange of oxygen, glucose, carbon dioxide and waste. Their very narrow lumen (one red blood cell wide) slows blood flow, giving more time for exchange, and their large network provides a huge surface area.
Mark scheme: 1 mark for each vessel's function linked to its key structural feature (artery × 2, vein × 2, capillary × 2); accept: thick walls/narrow lumen for arteries, valves/wide lumen/thin walls for veins, one cell thick/narrow lumen/large surface area for capillaries
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Calculate cardiac output from heart rate and stroke volume data. For example: a patient has a heart rate of 80 beats/min and a stroke volume of 65 mL/beat. Cardiac output = 80 × 65 = 5200 mL/min = 5.2 L/min. During exercise, heart rate increases to 140 beats/min and stroke volume to 90 mL/beat. Cardiac output = 140 × 90 = 12,600 mL/min = 12.6 L/min. Explain why cardiac output must increase during exercise: muscles need more oxygen and glucose for respiration, so more blood must be delivered per unit time.
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