Structure and Function of Blood Vessels

Section 1 of 7Arteries

Job

• Job Arteries carry blood away from the heart at high pressure, with the pressure rising and falling with each heartbeat.

Thick muscular wall with elastic fibres

• Thick muscular wall with elastic fibres The thick wall withstands the high pressure without bursting.
• Elastic fibres stretch as a pulse of blood arrives and then recoil, which helps keep the blood moving and the pressure steady between beats.

Narrow lumen

• Narrow lumen A small central channel helps to maintain the high pressure.

Section 2 of 7Veins

Job

• Job Veins carry blood back to the heart. By the time blood reaches them, the pressure is low.

Wide lumen

• Wide lumen A large lumen offers little resistance to the slow-moving, low-pressure blood.

Valves

• Valves Valves stop blood flowing backwards, and contraction of surrounding skeletal muscles squeezes the veins and helps push the blood along.

Thin wall

• Thin wall Less muscle and elastic tissue is needed because the pressure is low.

Section 3 of 7Capillaries

Job

• Job Capillaries form tiny networks in tissues, where blood exchanges substances with the cells.

Walls one cell thick

• Walls one cell thick The very short diffusion distance lets oxygen, glucose and carbon dioxide pass quickly.

Leaky walls

• Leaky walls Some plasma leaks out and forms tissue fluid, which bathes the cells and carries substances to them.

Section 4 of 7Comparison

Feature	Artery	Vein	Capillary
Direction of flow	Away from the heart	Towards the heart	Between arteries and veins
Wall	Thick, muscle and elastic fibres	Thin	One cell thick
Lumen	Narrow	Wide	Very narrow, about one red cell wide
Valves	None (except at heart exits)	Present	None
Pressure	High	Low	Falling

Section 5 of 7Arterioles and venules

1Artery

• Artery Arteries divide into smaller branches as they move away from the heart.

2Arteriole

• Arteriole The narrow vessels that connect arteries to capillaries.

3Capillary

• Capillary Exchange of materials with the cells takes place here.

4Venule

• Venule The narrow vessels that connect capillaries to veins.

5Vein

• Vein Venules join to form larger veins that return blood to the heart.

Section 6 of 7Key terms

Term	Meaning
Lumen	The hollow space inside a blood vessel through which blood flows.
Elastic fibres	Stretchy protein fibres in an artery wall that allow it to expand and recoil.
Valve	A flap in a vein that prevents backflow of blood.
Tissue fluid	Plasma that has leaked out of capillaries and surrounds the body cells.
Arteriole	A small vessel that links an artery to capillaries.
Venule	A small vessel that links capillaries to a vein.

Section 7 of 7Exam technique

1

Saying that the artery lumen is wide. It is narrow, which helps keep pressure high; veins have the wide lumen.

2

Explaining the thick artery wall without mentioning pressure.

3

Forgetting that capillary walls are only one cell thick and relating this to short diffusion distance.

4

Suggesting arteries have valves to stop backflow. Valves are a feature of veins.

5

Confusing arterioles and venules. Arterioles come before capillaries and venules come after.