Cambridge O Level5090

Circulatory system

Biology 5090 Chapter Notes

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Circulatory systemHeartBlood vesselsBlood
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In this chapter

  1. 1Single and Double Circulatory Systems
  2. 2The Human Heart: Structure
  3. 3The Cardiac Cycle and Valves
  4. 4Blood Vessels: Arteries, Veins, Capillaries
  5. 5Composition and Function of Blood
  6. 6Coronary Heart Disease (CHD)

1. Single and Double Circulatory Systems

The circulatory system transports substances like oxygen, nutrients, and waste products around the body. Simpler vertebrates like fish have a 'single circulation' where blood passes through the heart once per circuit. Blood is pumped from the heart to the gills (to get oxygenated), then flows to the rest of the body at low pressure before returning to the heart. Mammals have a more efficient 'double circulation'. Blood passes through the heart twice per circuit: once when it is pumped to the lungs (pulmonary circulation) and a second time when it is pumped to the rest of the body (systemic circulation). This double pump system ensures that high-pressure, oxygenated blood is delivered to all body tissues.

Key term

Double Circulation: A circulatory system where blood passes through the heart twice for each complete circuit of the body, once to the lungs and once to the rest of the body.

Examiner insight

Examiners expect you to clearly state that the key advantage of double circulation is the ability to maintain high blood pressure to the systemic circuit.

Common pitfall

Stating that a single circulation is 'bad' without explaining why it is less efficient, which is due to the pressure drop in the gill capillaries.

Fun fact

An average red blood cell completes a full circuit of the human body, passing through the heart twice, in about 45 seconds.

Worked example 13 marks

Explain the main advantage of a mammal's double circulatory system over a fish's single circulatory system. [3]

  1. 1

    In a single circulation (fish), blood pressure is lost as it passes through the narrow capillaries of the gills.

  2. 2

    This means blood flows slowly and at a low pressure to the rest of the body's organs.

  3. 3

    In a double circulation (mammal), the blood returns to the heart after the lungs to be pumped again, which boosts its pressure, allowing for faster, more efficient delivery of oxygen and nutrients to body tissues, supporting a higher metabolic rate.

Recap

  • A circulatory system transports essential substances around the body.
  • Fish have a single circulation: Heart -> Gills -> Body -> Heart.
  • Mammals have a double circulation, consisting of a pulmonary circuit and a systemic circuit.
  • The pulmonary circuit transports blood between the heart and the lungs.
  • The systemic circuit transports blood between the heart and the rest of the body.
  • The main advantage of double circulation is maintaining high blood pressure and efficient oxygen delivery.

Quick check

  1. Name the two circuits that make up the double circulatory system in mammals.2 marks

2. The Human Heart: Structure

The human heart is a muscular organ that acts as a double pump. It has four chambers: two upper, thin-walled atria (singular: atrium) that receive blood, and two lower, thick-walled ventricles that pump blood out. The right side of the heart deals with deoxygenated blood, while the left side deals with oxygenated blood. A muscular wall called the septum completely separates these two sides, preventing the blood from mixing. Major blood vessels are connected to the heart: the vena cava brings deoxygenated blood from the body to the right atrium, the pulmonary artery takes it to the lungs, the pulmonary vein brings oxygenated blood from the lungs to the left atrium, and the aorta pumps this oxygenated blood to the rest of the body.

Key term

Septum: The muscular wall that divides the left and right sides of the heart, preventing the mixing of oxygenated and deoxygenated blood.

Examiner insight

You must be able to trace the path of blood through the heart, naming all chambers, valves, and blood vessels in the correct order.

Common pitfall

Confusing the left and right sides on a diagram. Remember diagrams are shown as if you are facing the person, so the left side of the diagram is the right side of the heart.

Worked example 14 marks

Starting from the left atrium, place the following structures in the correct order for one complete circuit of blood: lungs, right ventricle, aorta, pulmonary artery, vena cava, left ventricle, right atrium, body tissues, pulmonary vein. [4]

  1. 1
    1. Left atrium (starting point)
  2. 2
    1. Left ventricle
  3. 3
    1. Aorta
  4. 4
    1. Body tissues
  5. 5
    1. Vena cava
  6. 6
    1. Right atrium
  7. 7
    1. Right ventricle
  8. 8
    1. Pulmonary artery
  9. 9
    1. Lungs
  10. 10
    1. Pulmonary vein (returns to left atrium)

Worked example 23 marks

Explain why the wall of the left ventricle is thicker and more muscular than the wall of the right ventricle. [3]

  1. 1

    The left ventricle pumps blood to the entire body (systemic circulation), except the lungs.

  2. 2

    This requires generating a much higher pressure to overcome greater resistance over a longer distance.

  3. 3

    The right ventricle only pumps blood a short distance to the lungs (pulmonary circulation), which requires less force and lower pressure.

Recap

  • The heart has four chambers: two atria and two ventricles.
  • The right side handles deoxygenated blood; the left side handles oxygenated blood.
  • The septum is a wall that prevents the mixing of blood between the two sides.
  • The vena cava and pulmonary artery are linked to the right side of the heart.
  • The pulmonary vein and aorta are linked to the left side of the heart.
  • The left ventricle wall is the thickest as it pumps blood to the whole body.

Quick check

  1. Name the blood vessel that carries deoxygenated blood away from the heart.1 mark
  2. Which chamber receives oxygenated blood from the lungs?1 mark

3. The Cardiac Cycle and Valves

The cardiac cycle is the sequence of events in a single heartbeat. It involves contraction (systole) and relaxation (diastole) of the heart muscle. First, the atria contract, pushing blood into the relaxed ventricles. Then, the ventricles contract forcefully. This closes the atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) to prevent blood flowing back into the atria, and opens the semilunar valves, forcing blood into the aorta and pulmonary artery. Finally, the ventricles relax, the pressure drops, and the semilunar valves snap shut to prevent backflow from the arteries. The characteristic 'lub-dub' sound of a heartbeat is caused by the closing of these valves.

Key term

Valves: Structures in the heart and veins that ensure one-way blood flow by opening to allow blood through and closing to prevent it from flowing backwards.

Examiner insight

When describing the cardiac cycle, marks are awarded for using the correct sequence of events: atrial contraction, ventricular contraction, and the corresponding valve movements.

Common pitfall

Forgetting to mention the role of valves when describing blood flow. The contraction of chambers and the action of valves are equally important.

Worked example 13 marks

Describe how the heart forces blood into the aorta. [3]

  1. 1

    The muscular wall of the left ventricle contracts (systole).

  2. 2

    This increases the pressure inside the ventricle, forcing the bicuspid (mitral) valve to close and the semilunar valve in the aorta to open.

  3. 3

    The high-pressure contraction then ejects the blood from the left ventricle into the aorta.

Recap

  • The cardiac cycle is one complete heartbeat.
  • Atria contract first, pushing blood into the ventricles.
  • Ventricles then contract, pushing blood into the arteries.
  • Atrioventricular valves (tricuspid, bicuspid) prevent backflow into the atria.
  • Semilunar valves prevent backflow from the arteries into the ventricles.
  • The 'lub-dub' sound is made by the closing of heart valves.

Quick check

  1. What is the function of the semilunar valves?1 mark

4. Blood Vessels: Arteries, Veins, Capillaries

Blood travels through three types of vessels. Arteries carry high-pressure blood away from the heart. They have thick, muscular, and elastic walls to withstand and maintain this pressure, and a relatively narrow lumen (central channel). Veins carry low-pressure blood towards the heart. They have thinner walls, a wider lumen to reduce resistance, and contain valves to prevent the backflow of blood. Capillaries are the smallest vessels, forming vast networks in tissues. Their walls are only one cell thick, creating a very short diffusion distance for the efficient exchange of substances like oxygen, carbon dioxide, and glucose between the blood and body cells.

Key term

Capillary: A microscopic blood vessel with walls only one cell thick, forming a network to allow for the exchange of substances between the blood and tissues.

Examiner insight

Structure-function questions are very common. For full marks, you must explicitly link a structural feature (e.g., 'thin walls of capillary') to its function (e.g., 'to provide a short diffusion path for efficient exchange').

Common pitfall

Making the absolute statement that 'all arteries carry oxygenated blood'. The pulmonary artery is a critical exception that carries deoxygenated blood to the lungs.

Worked example 12 marks

Complete the table by placing a tick (✓) against the two structures that have valves. [2]

structure in circulatory systemhave valves
arteries
capillaries
heart
veins
  1. 1

    Valves are needed to prevent backflow in low-pressure areas. The heart has valves between chambers and at the exit of ventricles.

  2. 2

    Veins have low-pressure blood, so they need valves to ensure it continues flowing towards the heart, especially in the limbs.

  3. 3

    Solution:

    structure in circulatory systemhave valves
    arteries
    capillaries
    heart✓
    veins✓

Recap

  • Arteries carry blood away from the heart under high pressure.
  • Arteries have thick, muscular, and elastic walls.
  • Veins carry blood towards the heart under low pressure and contain valves.
  • Veins have thinner walls and a wider lumen than arteries.
  • Capillaries are tiny vessels with one-cell-thick walls for substance exchange.
  • The pulmonary artery is an exception, carrying deoxygenated blood.

Quick check

  1. State two features of an artery's structure related to its function.2 marks
  2. Why do capillaries have walls that are only one cell thick?1 mark

5. Composition and Function of Blood

Blood is a tissue composed of four main components suspended in a liquid called plasma. Plasma is a straw-coloured liquid that transports everything: blood cells, platelets, nutrients (like glucose), hormones, and waste products (like carbon dioxide). Red blood cells are biconcave discs without a nucleus that contain haemoglobin to transport oxygen. White blood cells are part of the immune system; they include phagocytes, which engulf and digest pathogens, and lymphocytes, which produce specific antibodies. Platelets are small cell fragments that are essential for blood clotting, a process that seals wounds and prevents blood loss and entry of pathogens.

Key term

Haemoglobin: The red, iron-containing protein in red blood cells that reversibly binds with oxygen to transport it around the body.

Examiner insight

Be specific about the functions of blood components. 'White blood cells fight infection' is not enough for full marks; you need to describe phagocytosis or antibody production.

Common pitfall

Confusing the roles of the two main types of white blood cells. Remember: Phagocytes 'eat' pathogens, Lymphocytes produce 'antibodies'.

Fun fact

If you have a cut, the clot that forms is a mesh of a protein called fibrin, which acts like a net to trap red blood cells and plug the wound.

Worked example 14 marks

Explain the roles of lymphocytes and phagocytes in defending the body against disease. [4]

  1. 1

    Phagocytes are a type of white blood cell that can move around the body and are attracted to pathogens.

  2. 2

    They defend the body by engulfing and digesting pathogens in a process called phagocytosis.

  3. 3

    Lymphocytes are another type of white blood cell that recognises specific antigens on pathogens.

  4. 4

    They defend the body by producing specific antibodies which neutralise or destroy the pathogen.

Recap

  • Blood consists of plasma, red blood cells, white blood cells, and platelets.
  • Plasma is the liquid matrix that transports dissolved substances and cells.
  • Red blood cells use haemoglobin to transport oxygen.
  • White blood cells (phagocytes and lymphocytes) are involved in immunity.
  • Platelets are cell fragments that initiate blood clotting.
  • Blood clotting involves converting soluble fibrinogen into insoluble fibrin.

Quick check

  1. What is the function of platelets?1 mark
  2. Name the protein in red blood cells that binds to oxygen.1 mark

6. Coronary Heart Disease (CHD)

Coronary Heart Disease (CHD) is a condition where the coronary arteries, the specific blood vessels that supply the heart muscle with oxygenated blood, become narrowed or blocked. This narrowing is usually caused by atherosclerosis, a build-up of fatty substances like cholesterol, forming plaques on the artery walls. If a plaque ruptures, a blood clot can form and completely block the artery. This cuts off the oxygen supply to an area of the heart muscle, causing the cells to die. This event is a heart attack. Risk factors for CHD include a diet high in saturated fat and cholesterol, smoking, lack of exercise, and genetic predisposition. It can be reduced by a healthy lifestyle and treated with drugs, stents, or bypass surgery.

Key term

Coronary Arteries: The arteries that branch from the aorta to supply the heart muscle itself with oxygenated blood.

Examiner insight

To get full marks for explaining a heart attack, you must clearly link the blockage of a coronary artery to the lack of oxygen supply for the heart muscle, which prevents respiration and causes cell death.

Common pitfall

Thinking that a heart attack is a blockage inside the main chambers of the heart. The blockage is in the coronary arteries on the outside of the heart.

Worked example 13 marks

A blood clot becomes stuck in a coronary artery. Explain what will happen to the heart muscle cells supplied by this artery. [3]

  1. 1

    The blood clot blocks the coronary artery, preventing blood flow to a region of the heart muscle.

  2. 2

    This means the heart muscle cells in that region will not receive oxygen and glucose.

  3. 3

    Without oxygen for aerobic respiration, the cells cannot release energy and will die, causing a heart attack.

Worked example 23 marks

List three actions people can take to reduce the risk of developing coronary heart disease. [3]

  1. 1
    1. Reduce intake of saturated fats and cholesterol by eating a balanced diet with more fruits and vegetables.
  2. 2
    1. Engage in regular physical exercise to maintain a healthy weight and improve circulation.
  3. 3
    1. Avoid smoking, as chemicals in tobacco smoke can damage artery linings and increase blood pressure.

Recap

  • CHD is the blockage of the coronary arteries.
  • Coronary arteries supply the heart muscle with oxygen.
  • Blockage is caused by a build-up of fatty plaques (atherosclerosis).
  • A complete blockage by a blood clot causes a heart attack.
  • Risk factors include poor diet, smoking, and lack of exercise.
  • A healthy lifestyle can significantly reduce the risk of CHD.

Quick check

  1. What is the name of the disease process that leads to the narrowing of arteries?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Name the valve located between the right atrium and the right ventricle.1 mark
  2. Describe two ways in which the structure of a vein differs from the structure of an artery.2 marks
  3. Explain why a person with a low red blood cell count may feel tired and weak.3 marks
  4. Explain the main advantage of a double circulatory system compared to a single circulatory system.2 marks
  5. A student's heart rate is 75 beats per minute at rest. During exercise, it increases to 135 beats per minute. Calculate the percentage increase in their heart rate.2 marks
  6. Explain why heart rate increases during exercise.4 marks
  7. Describe the process of blood clotting after a small cut to the skin.3 marks
  8. Explain how smoking and a diet high in saturated fats can increase the risk of coronary heart disease.4 marks
  9. A person has a faulty bicuspid (mitral) valve that does not close properly. Explain why this might cause blood to accumulate in the pulmonary circulation (lungs).3 marks
  10. Starting from a capillary in the big toe, trace the path of a deoxygenated red blood cell until it becomes oxygenated and is pumped into the aorta. Name all chambers, valves, and major blood vessels it passes through.6 marks

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