Blood Vessels
Blood vessels are a closed system of tubular vessels that contain blood and forms a branching network to distribute blood to all parts of the organism. There are different types of blood vessels: arteries, arterioles, capillaries and veins.
Arteries, arterioles and veins have the same basic layered structure from the outside inwards:
(i) a tough fibrous outer layer made from collagen, which resists pressure changes from outside and inside the blood vessel.

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Key terms in this lesson
- diffusion
- The net passive movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient.
More in Mass Transport in Animals
- The Cardiac Cycle
- Valves and Control of Blood Flow
- Pressure and Volume Changes
- Cardiac Output
- Arteries
- Arterioles
- Veins and Venules
- Capillaries
All 27 lessons in Mass Transport in Animals · All AQA AS-level Biology topics