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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Arteries, Veins and Capillaries
Your body contains about 100,000 kilometres of blood vessels - enough to circle the Earth twice! These vessels form a complex transport system that delivers oxygen and nutrients to every cell while removing waste products. Let's explore the three main types of blood vessels and how they're specially adapted for their roles.
Key Definitions:
Blood flows in a continuous circuit: heart โ arteries โ arterioles โ capillaries โ venules โ veins โ heart. This pathway ensures that oxygen-rich blood reaches all tissues and oxygen-poor blood returns to the heart to be pumped to the lungs for reoxygenation.
Blood pressure is highest in arteries (about 120/80 mmHg), drops significantly in capillaries (about 35 mmHg) and is lowest in veins (about 10 mmHg). This pressure gradient helps drive blood flow through the system.
Arteries carry blood away from the heart under high pressure. They must be strong and elastic to handle the surge of blood with each heartbeat.
Arteries have thick walls made up of three distinct layers:
The innermost layer consists of endothelium (flat cells) that creates a smooth surface for blood flow, reducing friction. It sits on a thin layer of elastic tissue.
The middle layer contains smooth muscle and elastic fibres. This is the thickest layer in arteries and gives them their strength and elasticity.
The outer layer is made of connective tissue that anchors the artery to surrounding tissues and contains nerves and small blood vessels.
Adaptations of Arteries:
Atherosclerosis is a condition where fatty deposits (plaques) build up in artery walls, narrowing the lumen and reducing blood flow. These plaques can rupture, causing blood clots that may lead to heart attacks or strokes. Risk factors include high cholesterol, smoking, high blood pressure and lack of exercise. In the UK, cardiovascular diseases linked to atherosclerosis cause about 170,000 deaths each year.
Veins return blood to the heart under low pressure. They need to be able to hold large volumes of blood and prevent backflow.
Veins have the same three layers as arteries, but with key differences:
Similar to arteries but forms valves at intervals. These pocket-shaped structures prevent the backflow of blood.
Much thinner than in arteries, with less muscle and elastic tissue. This makes veins less elastic but more expandable.
The thickest layer in veins (unlike in arteries), providing structural support to the thin-walled vessel.
Adaptations of Veins:
DVT occurs when a blood clot forms in a deep vein, usually in the leg. It can be caused by long periods of inactivity, such as during long-haul flights or hospital stays. If the clot breaks loose, it can travel to the lungs (pulmonary embolism) and be life-threatening. About 1 in 1,000 people in the UK are affected by DVT each year. Simple preventative measures include regular movement, staying hydrated and wearing compression stockings during long journeys.
Capillaries are the smallest blood vessels, with diameters so tiny that red blood cells must pass through in single file. They form extensive networks in tissues where exchange of substances occurs.
Capillaries have an extremely simple structure:
Capillaries are perfectly adapted for exchange:
Different tissues have different capillary densities based on their metabolic needs:
Here's a quick comparison of the three main types of blood vessels:
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction of blood flow | Away from heart | Towards heart | From arterioles to venules |
| Wall thickness | Thick | Thin | Very thin (one cell) |
| Lumen size | Small relative to wall | Large relative to wall | Very small (7-9 ฮผm) |
| Valves | Absent | Present | Absent |
| Blood pressure | High | Low | Medium to low |
| Main function | Transport under pressure | Return blood to heart | Exchange of substances |
If all your blood vessels were laid end to end, they would stretch about 100,000 kilometres - that's enough to circle the Earth more than twice! Most of this length comes from capillaries, which form extensive networks throughout your body. The average adult has about 5 litres of blood, but at any given moment, about 60% of this is in your veins, 15% in your arteries and 25% in your capillaries and heart.
The human circulatory system is a marvel of biological engineering. Each type of blood vessel is perfectly adapted to its role:
Together, these vessels form a complete circuit that keeps blood flowing continuously, ensuring every cell in your body receives what it needs to function and stays free of waste products.
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