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Arteries carry blood away from the heart under high pressure and have thick muscular walls with elastic fibres. Free GCSE Biology notes and 14 FAQs.
Blood Vessels and Their Functions is part of Animal tissues, organs and organ systems in GCSE Edexcel Higher Biology.
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Arteries carry blood away from the heart under high pressure and have thick muscular walls with elastic fibres. Free GCSE Biology notes and 14 FAQs.
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Use this lesson to stay inside Animal tissues, organs and organ systems while connecting the detail back to the wider Topic 8: Exchange and transport in animals topic.
Read the core explanations from Blood Vessels and Their Functions before testing yourself from memory.
Your body has over 100,000 kilometres of blood vessels - enough to wrap around Earth twice. But not all roads are the same. Motorways handle fast traffic, country lanes allow local access, and service stations let you refuel. Your circulatory system works exactly like this transport network, with three specialist vessel types doing very different jobs.
Think of your heart as a powerful pump station. Blood shoots out at high speed through thick-walled motorways (arteries), slows down for deliveries through tiny country lanes (capillaries), then returns via wider, slower roads (veins). Each vessel type has evolved the perfect structure for its specific function.
Picture a fire hose under full pressure - that's what arteries deal with every second. When your heart contracts, it forces blood into arteries at pressures up to 120 mmHg. Without the right structure, they'd burst like a balloon.
Arteries have three key structural features. The thick muscular wall (smooth muscle) can contract to control blood flow to different organs. The elastic fibres stretch when blood surges in, then recoil to keep blood moving smoothly between heartbeats. The narrow lumen (internal space) maintains high pressure - like putting your thumb over a hosepipe.
Trap: Many students think arteries always carry oxygenated blood. Actually, the pulmonary artery carries deoxygenated blood from the heart to the lungs. Arteries are defined by direction (away from heart), not oxygen content.
If arteries are motorways, capillaries are the final delivery vans reaching every house. They're so narrow that red blood cells must squeeze through single file. This isn't a design flaw - it's perfect engineering for their job.
The wall is just one cell thick (about 0.5 micrometers). This creates the shortest possible diffusion distance for oxygen and glucose to reach your cells, and for carbon dioxide and waste to enter the bloodstream. The narrow diameter also slows blood flow, giving more time for exchange to occur.
Capillaries form dense networks called capillary beds in active tissues like muscles and brain. Every cell in your body is within 50 micrometers of a capillary - closer than the width of a human hair.
By the time blood reaches veins, it's like a river that's travelled hundreds of miles - the pressure has dropped dramatically. Veins face the opposite challenge to arteries: how to get low-pressure blood back to the heart, often against gravity.
Veins solve this with three clever adaptations. Thin walls reduce resistance to blood flow (no need for thick walls at low pressure). Large lumens act as blood reservoirs - veins hold about 60% of your blood volume. Most importantly, valves prevent backflow when blood tries to flow backwards due to gravity.
Veins also get help from surrounding muscles. When you walk, leg muscles squeeze veins like a pump, pushing blood upward. This is why long flights can cause blood clots - without muscle movement, blood pools in leg veins.
Blood flow rate is a compound measure - it combines volume and time. Just like calculating speed (distance ÷ time), we calculate flow rate as volume ÷ time. This helps doctors diagnose circulation problems and monitor heart function.
Worked example: Calculating Cardiac Output
A person's heart beats 70 times per minute. Each beat pumps 70 cm³ of blood. Calculate the cardiac output in dm³/min. [3 marks]
Answer: 4.9 dm³/min
Notice how we converted units in the calculation. Examiners often give data in different units to test your conversion skills. Always check if your final answer makes sense - a resting cardiac output of about 5 litres per minute is typical for adults.
The three vessel types work as an integrated system. Blood leaves the heart through arteries at high pressure and speed. It slows down dramatically in capillary beds where exchange occurs. Finally, it returns via veins as a low-pressure, high-volume flow.
This creates a perfect circulation system. Arteries deliver blood quickly to where it's needed. Capillaries allow time for exchange. Veins act as a blood reservoir and return system. Each structure is precisely adapted to its function - a masterpiece of biological engineering.
Examiners love asking how structure relates to function - it's a classic 6-mark question. The key is linking each structural feature to a specific functional advantage. Never just describe structure; always explain why that structure helps the vessel do its job.
Command word alert: 'Explain' questions need the word 'because' or 'so that'. Don't just describe what happens - explain why it happens. 'Compare' questions need both similarities AND differences, with specific examples from each vessel type.
For rate calculations, watch out for unit traps. Data might be given in beats per minute but the answer needed in beats per second. Always convert units step by step and show your working - that's where the marks are.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Arteries carry blood away from the heart under high pressure and have thick muscular walls with elastic fibres. Veins return blood to the heart at low pressure and have thin walls, large lumens and valves. Capillaries have walls one cell thick, where exchange happens between blood and tissues.
Cardiac output = heart rate × stroke volume. For a heart rate of 70 beats per minute and 70 cm³ per beat, volume per minute = 70 × 70 = 4900 cm³/min. Since 1000 cm³ = 1 dm³, this gives a cardiac output of 4.9 dm³/min.
A normal resting cardiac output for adults is about 5 litres per minute. For example, a heart rate of 70 beats per minute with 70 cm³ per beat gives 4.9 dm³/min.
The pulmonary artery carries deoxygenated blood from the heart to the lungs. Arteries are defined by direction, carrying blood away from the heart, not by oxygen content.
A capillary bed is a dense network of capillaries in active tissues like muscles and brain. Every cell in the body is within 50 micrometers of a capillary, closer than the width of a human hair.
Blood flow rate = volume ÷ time, a compound measure like speed. Cardiac output is calculated as heart rate × stroke volume, for example 70 beats/min × 70 cm³ = 4900 cm³/min = 4.9 dm³/min.
Capillaries have walls just one cell thick, creating the shortest possible diffusion distance for oxygen and glucose to reach cells and for carbon dioxide and waste to enter the blood. Their narrow diameter slows blood flow, giving more time for exchange.
The lumen is the internal space of a blood vessel that blood flows through. Arteries have a narrow lumen that maintains high pressure, while veins have large lumens that act as blood reservoirs.
Arteries have elastic fibres that stretch when blood surges in from a heartbeat, then recoil to keep blood moving smoothly between heartbeats. This helps arteries withstand pressures up to 120 mmHg.
Arteries are adapted to carry blood under high pressure. Their thick muscular wall can contract to control blood flow, elastic fibres stretch when blood surges in then recoil to keep blood moving smoothly, and a narrow lumen maintains high pressure.
Veins are adapted to return low-pressure blood to the heart. Thin walls reduce resistance to blood flow, large lumens act as blood reservoirs, and valves prevent backflow due to gravity, while surrounding muscles squeeze veins like a pump during walking.
Veins have valves to prevent backflow of blood. Blood in veins is at low pressure and often has to return to the heart against gravity, so the valves stop it flowing backwards.
Capillary walls are one cell thick, about 0.5 micrometers. This creates the shortest possible diffusion distance for oxygen and glucose to reach cells, and for carbon dioxide and waste to enter the blood.
Long flights can cause blood clots because without muscle movement, blood pools in the leg veins. When walking, leg muscles squeeze the veins like a pump, pushing blood upward.