O-Level Biology · Secondary 3 and 4
Transport in Humans
The circulatory system carries oxygen, digested food, carbon dioxide, urea and hormones around the body. In this topic you study the parts of the blood and what they do, the structure of arteries, veins and capillaries, the heart and its chambers and valves, and the double circulation. It connects nutrition, respiration and gas exchange, and it is the basis for understanding coronary heart disease and how lifestyle affects health. Expect structure and function questions, and questions that trace blood through the heart.
What the syllabus expects
- State the functions of plasma, red blood cells, white blood cells and platelets
- Describe blood clotting and the roles of phagocytes and lymphocytes
- Relate the structure of arteries, veins and capillaries to their functions
- Describe the structure of the heart and the passage of blood through its chambers and valves
- Explain the double circulation and why the left ventricle wall is thicker than the right
- Describe how tissue fluid forms and how substances are exchanged at capillaries
- Describe coronary heart disease and the factors that increase its risk
Blood and its functions
Blood is made of plasma and cells. Plasma is mainly water and carries dissolved glucose, amino acids, carbon dioxide (mostly as hydrogencarbonate ions), urea, hormones and proteins such as fibrinogen. Red blood cells contain haemoglobin, which combines with oxygen in the lungs to form oxyhaemoglobin and releases it in respiring tissues. White blood cells defend the body. Phagocytes engulf and digest bacteria by phagocytosis. Lymphocytes make antibodies, which destroy pathogens or cause them to clump together. Platelets are cell fragments that start blood clotting. At a wound, damaged tissue and platelets release substances that cause soluble fibrinogen to be converted into insoluble fibrin threads. These form a mesh that traps red blood cells and forms a clot, which stops blood loss and keeps pathogens out.
Arteries, veins and capillaries
Arteries carry blood away from the heart at high pressure. They have thick walls with much muscle and elastic tissue, which stretch and recoil with each heartbeat and keep the blood flowing. Their lumen is relatively narrow. Veins carry blood back to the heart at low pressure. They have thinner walls, a wider lumen and valves that stop blood flowing backwards. Contracting skeletal muscles squeeze veins and help push blood towards the heart. Capillaries link arterioles and venules. Their walls are one cell thick and they form dense networks close to every cell, so the diffusion distance is short and exchange is fast. At the arterial end, pressure forces plasma without large proteins out through the capillary walls as tissue fluid. Oxygen and glucose diffuse from the tissue fluid into cells, and carbon dioxide and urea diffuse the other way.
The heart and double circulation
The heart is a muscular pump with four chambers. Deoxygenated blood from the body enters the right atrium through the vena cava, passes through the tricuspid valve into the right ventricle, and is pumped through the pulmonary artery to the lungs. Oxygenated blood returns in the pulmonary vein to the left atrium, passes through the bicuspid valve into the left ventricle, and is pumped out through the aorta to the body. Semilunar valves at the bases of the aorta and pulmonary artery stop blood flowing back into the ventricles. The septum keeps oxygenated and deoxygenated blood apart. This is a double circulation: blood passes through the heart twice in one complete circuit, once in the pulmonary circulation to the lungs and once in the systemic circulation to the body. The left ventricle has a thicker muscular wall because it pumps blood at higher pressure over a much longer distance.
Coronary heart disease
Heart muscle gets its own supply of oxygen and glucose from the coronary arteries. In coronary heart disease, fatty deposits containing cholesterol build up in the inner walls of these arteries. The lumen narrows and the walls become rough, which makes blood clots more likely. If a coronary artery is blocked, part of the heart muscle receives too little oxygen to respire aerobically, the muscle cells die, and a heart attack results. Risk factors include a diet high in saturated fat, smoking, lack of exercise, stress, high blood pressure and a family history of heart disease. Eating less saturated fat, not smoking and exercising regularly all reduce the risk.
Worked example
(a) A student counts 18 pulse beats in 15 seconds at the wrist. Calculate her heart rate in beats per minute. (b) Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. (c) Explain why the heart rate increases during exercise.
- One minute is 60 seconds, which is 4 lots of 15 seconds.
- Heart rate = 18 × 4 = 72 beats per minute.
- For (b), compare where each ventricle pumps blood: the right ventricle pumps only to the lungs, which are close to the heart, while the left ventricle pumps to the whole body through the aorta.
- More muscle produces a stronger contraction, so the left ventricle generates a higher pressure to push blood the longer distance through the systemic circulation.
- For (c), link to respiration: working muscles respire faster and need more oxygen and glucose and produce more carbon dioxide.
- A faster heart rate delivers oxygen and glucose to the muscles and removes carbon dioxide more quickly.
Answer: (a) 72 beats per minute. (b) The left ventricle has more muscle so it contracts more strongly, pumping blood at higher pressure to the whole body, while the right ventricle only pumps blood to the nearby lungs. (c) To supply oxygen and glucose to the muscles faster for increased respiration, and remove carbon dioxide faster.
Common mistakes
- Saying arteries carry oxygenated blood and veins carry deoxygenated blood. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. Define vessels by direction relative to the heart.
- Saying the left side of the heart is on the left of the diagram. Diagrams show the heart as if you are facing the person, so the left side appears on the right.
- Writing that capillaries have thin walls 'so blood can pass through'. Blood cells stay inside. It is substances such as oxygen and glucose that diffuse across the one-cell-thick wall.
- Saying platelets form the clot by themselves. Platelets trigger the conversion of fibrinogen to fibrin, and the fibrin mesh traps blood cells to form the clot.
- Saying phagocytes make antibodies. Phagocytes engulf pathogens. Lymphocytes make antibodies.
Exam tips
- When tracing blood through the heart, name every chamber, valve and vessel in order. Skipping a valve or calling the vena cava 'a vein' loses marks.
- For pulse or heart rate calculations, show the conversion to one minute clearly and give the unit, beats per minute.
- Structure and function answers need both parts: 'thick elastic wall' alone is not enough, add 'to withstand high pressure'.
Practice questions
1. Easy: Name the blood vessel that carries blood from the lungs to the heart and state whether the blood is oxygenated.
Show answer
The pulmonary vein. It carries oxygenated blood, because the blood has just picked up oxygen in the lungs.
2. Medium: State three differences between the structure of an artery and a vein, and give the reason for one of them.
Show answer
An artery has a thicker wall, more muscle and elastic tissue, and a narrower lumen. A vein has valves along its length, an artery does not. Reason: arteries have thick elastic walls to withstand and smooth out the high pressure of blood pumped from the heart.
3. Harder: Explain how a build-up of fatty deposits in a coronary artery can lead to a heart attack.
Show answer
The deposits narrow the lumen and roughen the wall, so less blood flows and a clot may form and block the artery. Heart muscle beyond the blockage gets too little oxygen and glucose for aerobic respiration, so it cannot release enough energy to contract. The muscle cells die, and the heart may stop pumping properly.
Related topics
Want a tutor for Biology?
Shortlist on WhatsApp within 16 hours. You choose the tutor. About O-Level Biology tuition
Get matched with a tutor