Blood pathways and vessels
Trace blood through the double circulation and compare arteries, veins and capillaries.
On this page
Blood Circulation · about 25–35 min
What you need to understand
Artery and vein are named by direction relative to the heart, not by oxygen content.
Definitions
- artery
a vessel carrying blood away from the heart
For example: Arteries have walls suited to higher pressure.- vein
a vessel carrying blood towards the heart
For example: Many veins have valves that reduce backflow.- capillary
a very narrow vessel with a one-cell-thick wall for exchange
For example: Capillaries provide a short exchange distance.
Key idea
- LookTrace a red blood cell from the heart to the lungs and back before it travels to the body.
- ThinkArtery and vein are named by direction relative to the heart, not by oxygen content.
- DoOrder the heart–lung pathway and link thick walls, valves or thin exchange surfaces to function.
Explanation
Follow one complete route: right side of heart → lungs → left side of heart → body → right side of heart. The two sides belong to the same heart. Blood passes through the heart twice on this complete journey, which is why this is called double circulation.
The right side pumps oxygen-poor blood through the pulmonary artery to the lungs. There, oxygen passes from air into the blood and carbon dioxide passes from blood into the air. Oxygen-rich blood returns through the pulmonary veins to the left side of the heart, which pumps it through arteries to the body.
At body tissues, oxygen passes from blood to cells for respiration. Carbon dioxide produced by the cells passes into blood. Blood then returns through veins to the right side of the heart. Oxygen-poor, also called deoxygenated, means it contains less oxygen; it does not mean it contains none.
Arteries carry blood away from the heart. Their thick muscular walls withstand the higher pressure of blood pumped from the heart; elastic tissue lets the walls stretch and recoil. Veins return blood towards the heart at lower pressure and have thinner walls. Many veins have valves: flaps that open for flow towards the heart and close against reverse flow. A valve controls direction; it is not a pump.
Capillaries connect the arterial and venous parts of the circulation. Their walls are only one cell thick, so substances have a short distance to cross between blood and surrounding tissues. This suits exchange, rather than withstanding the high pressure close to the heart. Compare the vessel drawings in the diagram, while remembering that they use different views and scales.
Pause and say it: Artery and vein are named by direction relative to the heart, not by oxygen content.
Common mistake
Tempting wrong idea: Blood loses oxygen instead of gaining it during gas exchange at the lungs.
Why it fails: Blood arriving at the lungs has come from body tissues that used its oxygen, so it carries little oxygen. The air in the lungs has more, so oxygen moves into the blood and carbon dioxide moves out of it.
Use this instead: At the lungs, oxygen enters the blood and carbon dioxide leaves it; dissolved food is absorbed into blood at the intestine.
Practical work
What to show: Order the heart–lung pathway and link thick walls, valves or thin exchange surfaces to function.
Before you finish: The pulmonary vein is likewise named because it returns blood towards the heart.
Practical link: The topic investigation is taught in “Pulse response and recovery”
6. Worked Examples
Modelled example 1
Trace one circuit
Problem
Trace a red blood cell from the right side of the heart, through the lungs and body, and back to its starting point. Include the oxygen exchange at each site.
Study the worked solution
Reason from the evidence
Method
Start at the heart and trace every journey through an exchange surface and back.
Reason
Artery and vein are named by direction relative to the heart, not by oxygen content.
Working
The right side of the heart pumps it through the pulmonary artery to lung capillaries.
At the lungs, oxygen enters the blood and binds to haemoglobin in the red cell; carbon dioxide leaves the blood.
It returns through the pulmonary veins to the left side of the heart, which pumps it through arteries to body capillaries.
At body tissues, it releases oxygen for cells to use. Carbon dioxide from the cells enters the blood.
It returns through veins to the right side of the heart. It still contains some oxygen.
State the conclusion
Working
heart → lungs → heart → body → heart is the complete double-circulation pattern.
Common misconception 2
Direction defines arteries and veins
Learner response
“Every artery contains oxygenated blood and every vein contains deoxygenated blood.” Correct the rule and use the lung circulation as evidence.
State the direction rule and the exception
View solution step by step
Use direction from the heart
Method
Define vessels by the direction of blood flow.Reason
Oxygen content changes at the lungs and body, so it cannot define every vessel.
Working
Arteries carry blood away from the heart; veins return blood to the heart. The pulmonary artery carries deoxygenated blood from the heart to the lungs, while the pulmonary vein carries oxygenated blood back to the heart.
Challenge 3
Identify vessels from their structures
Problem
Vessel P has a thick elastic wall and a relatively narrow space. Vessel Q has valves and returns blood at lower pressure. Vessel R has a wall one cell thick. Identify each vessel type and explain how one stated feature suits its function.
Match P, Q and R, then explain
Hints
Hint 1: pressure
Which vessel receives blood pumped away from the heart?
Hint 2: exchange
Which wall gives a short path for substances moving between blood and tissues?
View solution step by step
Match structure to function
Method
Identify P as artery, Q as vein and R as capillary.
Reason
Each feature suits the pressure, direction or exchange role of the vessel.
Working
P is an artery; its thick elastic wall withstands and smooths high-pressure flow from the heart. Q is a vein; valves help prevent backflow as blood returns at lower pressure. R is a capillary; its one-cell-thick wall gives a short exchange distance.
Guided practice
Try it with support
Trace one complete circulation and link one feature each of an artery, vein and capillary to function.
Use heart → lungs → heart → body → heart.
Do not classify vessels only by oxygen content.
Check the guided answer
Answer: Blood follows heart → lungs → heart → body → heart. Arteries have thick elastic muscular walls for higher pressure. Many veins have valves that reduce backflow. Capillaries have one-cell-thick walls, giving a short distance for exchange.
Check: Arteries carry blood away from the heart and veins towards it.
Practise and continue
Practise this
Why is the pulmonary artery still an artery even though it carries blood with less oxygen?
Need a hint?
Use the direction-based definition.
Check your answer
Answer: It carries blood away from the heart towards the lungs, so it is an artery; oxygen content does not define the vessel name.
Check: The pulmonary vein is likewise named because it returns blood towards the heart.
Think like a scientist
Why does a circulation diagram show useful relationships without representing every vessel?
Check the reasoning
It preserves the order and direction between heart, lungs and body while omitting the enormous branching network, scale and continuous three-dimensional flow.
Remember: Use the model to trace a route, not to count real vessels.
One-minute check
Hide the page. Trace blood from the right side of the heart through the lungs and body, back to where it started.
State the direction of oxygen and carbon-dioxide movement at the lungs, then at body tissues.
Correct this common mistake: “Blood loses oxygen instead of gaining it during gas exchange at the lungs.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: