Three main types of blood vessels:

1. Artery (動脈 )

2. Vein (靜脈 )

3. Capillaries (微血管 )

Blood CirculationBlood Circulation

Heart

Venule ArterioleCapillary

Vein Artery

carries blood away from

the heart

returns blood to the heart

Artery

Vein

Cross section of a blood vesselCross section of a blood vessel

Smooth muscles & elastic fibres

Fibrous tissue

lining of wall

lumen

Cross section of Cross section of arteryartery

Cross section Cross section of veinof vein

Artery : thicker elastic wall

relatively smaller lumen

to withstand high blood pressure

Cross section of Cross section of arteryartery

Cross section Cross section of veinof vein

Artery : thicker smooth muscles

Heart drives blood forward

Not contract to force blood forward

arteries arterioles capillaries venules veins

Blood pressure changes along blood vessels

Blo

od

pre

ssu

re /

mm

Hg

40

80

100

120

0

90

Blood flow with pulse

Blood flow steadily with no pulse

Cross section of Cross section of arteryartery

Cross section Cross section of veinof vein

Valves present to prevent backflow of blood

Valve openblood can flow

Valve closedblood can’t flow back

How can blood return to heart through vein under low pressure?

Return of blood to heart is aided by contraction of body muscles as they squeeze the blood along the vein

contract

valve close

valve open

Demonstration of Demonstration of Venous Flow in Venous Flow in the Fore the Fore

ArmArm

What is the purpose of tying the arm with a piece of rubber tubing ?Ans: This makes the vein more conspicuous.

A

C D

B

vein

YX

SR

finger X pressing down

on R

finger Y squeezing blood towards S

finger X still pressing down

on R

finger Y removed

both fingers are removed

elbow joint

With reference to steps B & C, explain why the part of the vein between R and S has disappeared ?Ans: In step B, finger Y squeezes the vein towards point S.

Blood in this segment is therefore pushed along … Ans: On the other hand, finger X is still pressin

g down on point R which prevents blood flowing into R.

A

C D

B

vein

YX

SR

finger X pressing down

on R

finger Y squeezing blood towards S

finger X still pressing down

on R

finger Y removed

both fingers are removed

elbow joint

Ans: Since there are valves at point S, blood is prevented from flowing back …

What structure in the vein is indicated by the appearance of the bulge at S shown in diagram C ?

Ans: The valve in the vein.

A

C D

B

vein

YX

SR

finger X pressing down

on R

finger Y squeezing blood towards S

finger X still pressing down

on R

finger Y removed

both fingers are removed

elbow joint

Why is it necessary to take the rubber tubing away as soon as the demonstration has been completed ?

Ans: It is because we need to restore the normal blood flow for the arm as soon as possible.

A

C D

B

vein

YX

SR

finger X pressing down

on R

finger Y squeezing blood towards S

finger X still pressing down

on R

finger Y removed

both fingers are removed

elbow joint

CapillariesCapillaries

• allow exchange of materials between the blood and the body tissues by diffusion.

arterioles capillariesO2 ,

nutrients

CO2 ,

wastevenules

• the wall is permeable to O2, CO2, glucose, amino acids,

salts, water ... etc.cross section of capillarycross section of capillary

Specially adapted for exchange of materials

1 One-cell thickOne-cell thick walls walls diffusion distance between

blood and body tissue

2 Very small lumen but large total Very small lumen but large total cross sectional areacross sectional area blood flow

more time for exchange of materials

3 Greatly branchedGreatly branched surface area for efficient

diffusion of materials

cross section of cross section of capillarycapillary

very small lumen

one cell thick

CapillariesCapillaries

Same flow rate (Volume / time) V1 = V2

d1

d2

A1

A2

Total cross sectional area

A1 > A2

d2 > d1

v2 > v1

v2

v1

CapillarieCapillariess

arteriolearteriole

oxyhaemoglobincapillary

cell

Plasma proteinExchange of materials takes place at the capillaries only

Exchange of materials between Exchange of materials between the blood and the body cellsthe blood and the body cells

plasma

tissue fluid

O2 nutrientswater

white blood cells

Diffuse across the capillary wall

CO2waste products

Diffuse across the capillary

wall Squeeze through the wall