2.9 - Blood Vessels

The three types of blood vessel

• There are three main types of blood vessel, each with a different job.
• Blood leaving the heart passes through arteries → capillaries → veins, then back to the heart.

The job of each vessel

• Arteries - Carry blood away from the heart to the rest of the body.
• Veins - Carry blood back to the heart.
• Capillaries - Tiny vessels that link arteries and veins. They carry blood close to the body's cells so materials can be exchanged between the blood and the tissues.

How the vessels connect

1. Blood pumped out of the heart travels through arteries.
2. Arteries branch into ever smaller vessels - the smallest are called arterioles.
3. Arterioles lead into capillaries running through the tissues.
4. Capillaries join up into small vessels called venules.
5. Venules combine to form the veins that return blood to the heart.

Arteries and veins - Structure related to function

• Arteries carry blood away from the heart at high pressure - the heart pumps it out in powerful surges.
• Veins carry blood back to the heart at much lower pressure - the blood has lost pressure passing through the capillaries.
• The lumen is the central channel that blood flows through.

How each vessel's structure suits its job

Why arteries are built this way

• The muscle gives the wall the strength to withstand the high pressure.
• The elastic fibres let the artery stretch as a surge of blood passes, then spring back, smoothing out the flow.
• The thick wall and narrow lumen together suit a vessel carrying blood under high pressure.

Why veins are built this way

• The wide lumen reduces resistance to flow, helping the low-pressure blood move easily.
• The pressure is too low to keep blood moving forward on its own, so veins contain valves.
• These one-way valves shut if blood starts to flow backwards, keeping it moving in the correct direction towards the heart.

Capillaries

• Capillaries are the very small blood vessels found in the organs.
• They carry blood close to the body's cells so that substances can be exchanged between the blood and the tissues.
• This exchange happens by diffusion across the capillary wall.

What is exchanged

• Out of the blood - Substances the cells need, such as glucose and oxygen, pass from the blood into the tissues.
• Into the blood - Substances the cells produce, such as carbon dioxide, pass from the tissues into the blood.

Adaptations for exchange

• Very thin walls - Substances have only a short distance to diffuse across.
• Extensive networks - Capillaries spread throughout the tissues, so every cell is close to a capillary supplying it.