Contents

Blood Vessels

What blood vessels do

Arteries

Capillaries

Summary quiz

Veins

Lungs

We can get an idea of what they could look like from considering the job they have to do.

Let us look at a small section of the circulatory system. There seems to be 33 types of blood vessel present.

oneone takes blood away

from the heart

oneone takes blood to the

heart

oneone joins the other two together

What blood vessels do

1.1.

Blood vessels that take blood away from the heart are known as ARTERIESARTERIES.

2.2.

Blood vessels that take blood back to the heart are known as VEINSVEINS.

Remember, aarteries (aaway)

Remember, veinins (ininto the heart)

Three broad types of vessels

3.3. Finally, if arteriesarteries take blood away from the heart and veinsveins take blood back to the heart, when does an artery turn into a vein?

Well, our third type of blood vessel links the two.

We find it present where an artery finishes and a vein is about to begin.

This third type of blood vessel is known as a CAPILLARYCAPILLARY..

and in between…

Contents

Blood Vessels

What blood vessels do

Arteries

Capillaries

Veins

Summary quiz

Artery cross-section

small lumen

thick muscle and elastic

fibres

The small size of the lumen means that blood will be under high pressure.

The thick fibrous wall strengthens the artery.

Arteriole pressure

Imagine using a water hose and putting your thumb half over the open end.

The water is released under higher pressure and it flows faster.

The elastic allows the artery to stretch under pressure whilst the muscle can contract to push the blood along.

thick arteriole wall

Properties of arteriole walls

lumen

This stretching of the artery is what allows us to feel a pulse.

blood flow

As the heart beats, it releases regular surges of blood. As these pass along the arteries, the vessel must stretch to allow the increased blood flow to pass.

This stretching pushes on the skin, which we sense as a pulse.

blood

flow

The human pulse

The blood flowing through an artery will be under high pressure and moving extremely fast.

There is no chance that the blood will turn around and start travelling in the opposite direction.

Therefore, there are no valvesno valves present in arteries.

Blood flow in arteries

Contents

Blood Vessels

What blood vessels do

Arteries

Capillaries

Veins

Summary quiz

large lumen

thin muscle and elastic fibres

Veins: cross-section

The lumen of a vein is much thicker than the wall that surrounds it.

There is less of a need to maintain a high pressure.

The walls still contain elastic and muscle but there is far less present compared to the artery.

You will appreciate that the blood is flowing far less quickly through veins compared to arteries.

Walls of veins

The veins are carrying blood back towards the heart.

Materials have been exchanged and now it is running out of energy.

It needs to return to the heart to receive another pump.

Unfortunately, this could lead to a real problem...

Getting another boost

leg

Q. What would happen if blood loses so much energy that it stops and turns around?

vein

This situation could arise where blood is flowing against gravity.

However, there is a solution to this problem and it comes in two parts.

Preventing backflow

Normal direction of blood flow

gravity

1.1. Firstly, veins have valves which act to stop the blood from going in the wrong direction.

(These valves are similar to those found in the heart)

valves open

blood

valves close

Valves

2.2. The second part of the solution has to get the blood flowing again whilst overcoming the problem of a lack of muscle in the lining of the vessel.

The solution comes in the form of general body muscles that surround the veins. When these muscles contract to move the body, they also squeeze the veins and push the blood along the vessel.

→

Muscular massage

Contents

Blood Vessels

What blood vessels do

Arteries

Capillaries

Veins

Summary quiz

These vessels link arteries with veins.

They are found all over the body and are essential for the exchange of materials between the blood and other body cells.

artery vein

capillaries

Capillaries: role and position

very narrow lumen

The wall of a capillary is only one cell thick!

Capillaries are so small that they can only be seen using a microscope.

A collection of capillaries is known as a capillary bedcapillary bed.

Capillary bed

body cell

Substances can diffuse across the lining of the capillary. This allows useful substances which are dissolved within the blood to move into surrounding cells whilst cellular waste moves in the opposite direction.

11 2233

The exchange of materials between the blood and the body can only occur through capillaries.

Diffusion and exchange

Exchange of dissolved gases

Oxygen and carbon dioxide exchange in the capillaries.

Vessel test. Part 1

Reaching all parts

Vessel test. Part 2

Blood characteristics

Contents

Blood Vessels

What blood vessels do

Arteries

Capillaries

Veins

Summary quiz

Multiple-choice quiz