Unit 7 Circulatory system
Key unit competence: Explain the structure and functions of the human circulatory and lymphatic systems and how to keep the heart healthy.
Key ideas
- Large animals need a transport system because diffusion is too slow over long distances and their surface area to volume ratio is small. An open circulation (insects) has blood in body spaces; a closed circulation (vertebrates, earthworm) keeps blood inside vessels.
- Single circulation (fish): blood passes through the heart once per circuit (heart → gills → body). Double circulation (mammals, birds): blood passes through the heart twice: pulmonary circulation (heart → lungs → heart) and systemic circulation (heart → body → heart). Double circulation keeps high pressure to the body.
- The heart has four chambers: right atrium (receives deoxygenated blood from the body through the vena cava), right ventricle (pumps it to the lungs in the pulmonary artery), left atrium (receives oxygenated blood from the lungs in the pulmonary vein), left ventricle (pumps it to the body in the aorta). The septum separates the two sides.
- The left ventricle has a thicker muscular wall because it pumps blood all round the body at high pressure. Valves prevent backflow: tricuspid (right side), bicuspid/mitral (left side) and semilunar valves at the bases of the aorta and pulmonary artery. Coronary arteries supply the heart muscle.
- Blood vessels: arteries carry blood away from the heart at high pressure; thick, muscular and elastic walls, narrow lumen, no valves (except at the base), pulse. Veins carry blood to the heart at low pressure; thin walls, wide lumen, valves. Capillaries: walls one cell thick, very narrow, where exchange with tissues happens.
- The pulmonary artery is the only artery carrying deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood.
- Blood = plasma (water with dissolved food, CO₂, urea, hormones, proteins) + red blood cells (biconcave, no nucleus, haemoglobin carries oxygen) + white blood cells (phagocytes and lymphocytes, defence) + platelets (blood clotting).
- Heart diseases: coronary heart disease (fatty deposits narrow coronary arteries, causing chest pain or heart attack), high blood pressure (hypertension), stroke. Risk factors: smoking, diet high in saturated fat and salt, lack of exercise, obesity, stress, diabetes, heredity. Prevention: exercise, balanced diet, no smoking, less salt and alcohol.
- The lymphatic system: tissue fluid that leaks from capillaries is drained into lymph vessels (with valves) and returned to the blood near the heart. Lymph nodes filter lymph and contain lymphocytes that fight infection. Lacteals absorb fats. Swollen lymph nodes show infection.
Common misconceptions
- "Arteries always carry oxygenated blood." No: arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood.
- "The right side of the heart pumps blood to the body." No: the left ventricle pumps blood to the body; the right pumps to the lungs.
- "Veins carry blue blood." No: deoxygenated blood is dark red; veins only look blue through the skin.
- "Capillaries have thick walls to stop leaks." No: their walls are one cell thick so substances can be exchanged.
Worked examples
Example 1. Trace the path of a red blood cell from the right atrium to the left ventricle.
Solution
Right atrium → tricuspid valve → right ventricle → semilunar valve → pulmonary artery → capillaries of the lungs (picks up oxygen) → pulmonary vein → left atrium → bicuspid valve → left ventricle.
Example 2. Explain why the wall of the left ventricle is thicker than that of the right ventricle.
Solution
The left ventricle pumps blood through the systemic circulation to the whole body, which needs high pressure. The right ventricle pumps only to the nearby lungs, at lower pressure (high pressure would damage lung capillaries). More muscle produces a stronger contraction.
Practice questions
Distinguish between single and double circulation, with an example of each.
Show answer
Single: blood passes through the heart once per circuit (fish). Double: blood passes through the heart twice, pulmonary and systemic circuits (mammals, birds).
Common mistake: Saying double circulation means two hearts.
Name the four chambers of the heart and the blood vessel connected to each.
Show answer
Right atrium: vena cava. Right ventricle: pulmonary artery. Left atrium: pulmonary vein. Left ventricle: aorta.
Common mistake: Swapping the pulmonary artery and vein.
Give three differences between arteries and veins.
Show answer
Arteries carry blood away from the heart, veins to it; arteries have thick muscular elastic walls, veins thin walls; arteries have narrow lumen and no valves, veins wide lumen and valves; arteries high pressure, veins low.
Common mistake: Saying all arteries carry oxygenated blood.
State the functions of the four components of blood.
Show answer
Plasma: transports dissolved food, CO₂, urea, hormones. Red cells: carry oxygen. White cells: defend against disease. Platelets: blood clotting.
Common mistake: Saying platelets carry oxygen.
Explain how capillaries are adapted for exchange.
Show answer
Walls one cell thick (short diffusion distance); very narrow so blood flows slowly; huge network gives large surface area; leaky walls allow tissue fluid to form.
Common mistake: Saying capillaries have valves.
State four risk factors for coronary heart disease and three ways to reduce it.
Show answer
Risk: smoking, fatty and salty diet, lack of exercise, obesity, stress, diabetes, heredity. Reduce: exercise, balanced low-fat diet, stop smoking, less salt and alcohol, manage stress.
Common mistake: Saying heart disease is infectious.
State three functions of the lymphatic system.
Show answer
Returns tissue fluid to the blood; absorbs fats through lacteals; filters lymph and fights infection in lymph nodes (lymphocytes).
Common mistake: Saying lymph vessels carry red blood cells.
Explain why the heart wall needs its own blood supply.
Show answer
Heart muscle contracts constantly and needs oxygen and glucose for respiration; it cannot absorb them from the blood inside its chambers, so coronary arteries supply it.
Common mistake: Saying the heart gets food directly from the chambers.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Cell fragments that help blood to clot are: a. left ventricle 2. The blood vessel carrying oxygenated blood from lungs to heart is the: b. pulmonary vein 3. The chamber that pumps blood to the whole body is the: c. capillary walls 4. Exchange of materials with tissues happens across: d. platelets Show answer
1 → d (platelets) · 2 → b (pulmonary vein) · 3 → a (left ventricle) · 4 → c (capillary walls)
- They seal wounds.
- It is a vein with oxygenated blood.
- It has the thickest wall.
- Capillary walls are one cell thick.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
aorta · septum · atrium · single circulation- Fish have .
- The largest artery in the body is the .
- The muscle wall separating the left and right sides of the heart is the .
Show answer
1: single circulation · 2: aorta · 3: septum
- Blood goes heart → gills → body → heart.
- It leaves the left ventricle.
- It stops oxygenated and deoxygenated blood mixing.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Lymph nodes contain: a. prevent backflow of blood 2. Valves in the heart: b. plasma 3. Deoxygenated blood from the body enters the heart through the: c. lymphocytes that fight infection 4. The liquid part of blood is: d. vena cava Show answer
1 → c (lymphocytes that fight infection) · 2 → a (prevent backflow of blood) · 3 → d (vena cava) · 4 → b (plasma)
- They filter lymph.
- Blood flows one way.
- It enters the right atrium.
- It transports dissolved substances.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
keratin · coronary heart disease · haemoglobin · is at low pressure- The pigment that carries oxygen in red blood cells is .
- Veins have valves because blood in them .
- Fatty deposits narrowing the coronary arteries cause .
Show answer
1: haemoglobin · 2: is at low pressure · 3: coronary heart disease
- It contains iron.
- Valves stop backflow.
- The heart muscle gets less oxygen.