How does the heart pump blood around two circuits, and how is blood adapted to its job?
The human double circulatory system, the structure and function of the heart, the differences between arteries, veins and capillaries, and the components of blood (red blood cells, white blood cells, platelets and plasma) and their functions.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on the circulatory system, covering the double circulation, the structure and function of the heart, arteries, veins and capillaries, and the components of blood and their functions.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
OCR wants you to describe the human double circulatory system, explain the structure and function of the heart, compare arteries, veins and capillaries, and state the components of blood and what each does.
The double circulatory system
A double circulation is more efficient than a single circulation because the heart re-pressurises the blood after it returns from the lungs. This lets oxygenated blood be pumped to the body at high pressure, so it reaches the cells quickly.
The heart
The heart has four chambers: two atria (top) and two ventricles (bottom). Its job is to pump blood around both circuits.
- The right side receives deoxygenated blood from the body (into the right atrium via the vena cava) and pumps it to the lungs (from the right ventricle via the pulmonary artery).
- The left side receives oxygenated blood from the lungs (into the left atrium via the pulmonary vein) and pumps it to the body (from the left ventricle via the aorta).
Valves between the chambers and in the major vessels make sure blood flows one way and does not flow backwards. The left ventricle has a thicker, more muscular wall than the right, because it must pump blood all the way around the body at high pressure.
Blood vessels
The three types of blood vessel are adapted to their jobs:
- Arteries. Carry blood away from the heart, at high pressure. They have thick, muscular, elastic walls and a small lumen to withstand and maintain the pressure.
- Veins. Carry blood towards the heart, at low pressure. They have thinner walls, a large lumen (to reduce resistance) and valves to stop blood flowing backwards.
- Capillaries. Tiny vessels with walls one cell thick, so substances (oxygen, glucose, carbon dioxide) can be exchanged between the blood and the cells by diffusion across the short distance.
The components of blood
Blood is a tissue made of cells suspended in a liquid called plasma:
- Red blood cells. Carry oxygen, using the red pigment haemoglobin which binds oxygen. They have no nucleus (more room for haemoglobin) and a biconcave disc shape (large surface area for oxygen to diffuse).
- White blood cells. Part of the body's defence against pathogens; they engulf pathogens (phagocytosis) and produce antibodies (you study this in B6).
- Platelets. Small cell fragments that help the blood to clot at a wound, forming a scab and preventing blood loss and infection.
- Plasma. The straw-coloured liquid that carries the cells, plus dissolved substances such as glucose, carbon dioxide, urea, hormones and antibodies.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksDescribe the path of a red blood cell through the double circulatory system, starting and ending at the right atrium, and explain why a double circulation is more efficient than a single circulation.Show worked answer →
A 6-mark extended response. Mark it for a clear, ordered path plus the efficiency point.
Path: blood enters the right atrium, passes into the right ventricle, and is pumped through the pulmonary artery to the lungs (where it picks up oxygen and loses carbon dioxide). It returns through the pulmonary vein to the left atrium, passes into the left ventricle, and is pumped through the aorta to the body. It returns through the vena cava to the right atrium. Reward the correct order of chambers and vessels.
Efficiency: in a double circulation, blood passes through the heart twice per circuit. The heart re-pressurises the blood after the lungs, so it can be pumped to the body at high pressure. This means oxygenated blood reaches the body's cells quickly and at high pressure, making it more efficient than a single circulation. Markers reward the idea that the blood is re-pumped so it travels to the body faster and at higher pressure.
OCR 20204 marksCompare the structure of an artery and a vein, and explain how each structure suits its function.Show worked answer →
A 4-mark Compare question with a structure-function link.
Artery: thick, muscular and elastic walls; a small lumen. This withstands and maintains the high pressure of blood pumped from the heart, and the elastic walls stretch and recoil to keep blood flowing.
Vein: thinner walls; a large lumen; valves. Blood here is at low pressure, so thick walls are not needed; the large lumen reduces resistance, and the valves prevent backflow so blood keeps moving towards the heart.
Markers reward paired structural differences (wall thickness, lumen size, valves) each linked to the pressure of the blood the vessel carries.
Related dot points
- Surface area to volume ratio and how it changes with size, why large organisms need specialised exchange surfaces and transport systems, and the adaptations of exchange surfaces such as alveoli, villi and root hairs.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on surface area to volume ratio and exchange surfaces, covering how the ratio changes with size, why large organisms need exchange surfaces and transport systems, and the adaptations of alveoli, villi and root hairs.
- Transport in plants by xylem (water and mineral ions) and phloem (dissolved sugars), the transpiration stream and translocation, the factors affecting the rate of transpiration, and the role of stomata and guard cells.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on transport in plants, covering xylem and phloem, the transpiration stream and translocation, the factors affecting transpiration rate, and the role of stomata and guard cells.
- Respiration as an exothermic series of reactions that releases energy from glucose in all living cells, the word and symbol equations for aerobic respiration, anaerobic respiration in animals (lactic acid) and yeast (ethanol and carbon dioxide), oxygen debt, and the uses of the energy released.
A focused answer to the OCR Gateway GCSE Biology A topic B1 on respiration, covering aerobic respiration and its equations, anaerobic respiration in animals and yeast, oxygen debt, and the uses of the energy released in cells.
- Diffusion, osmosis and active transport as ways substances move across cell membranes, the factors affecting the rate of diffusion, the effect of osmosis on plant and animal cells, and calculating percentage change in mass in the osmosis practical.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on diffusion, osmosis and active transport, covering the definitions, the factors affecting diffusion, the effect of osmosis on cells, the osmosis practical and percentage change calculations.
- Non-communicable diseases and their risk factors (diet, exercise, smoking and alcohol), the difference between correlation and cause, the effects of risk factors on the body, cardiovascular disease and its treatments, and the use of data to evaluate the impact of lifestyle on health.
A focused answer to the OCR Gateway GCSE Biology A topic B6 on non-communicable disease, covering risk factors such as diet, exercise, smoking and alcohol, the difference between correlation and cause, cardiovascular disease and its treatments, and using data to evaluate the impact of lifestyle on health.