Transcript Capillaries

Three main types of blood vessels:
1. Artery (動脈)
2. Vein (靜脈)
3. Capillaries (微血管)
Blood Circulation
Vein
returns
blood
to the heart
Venule
Artery
Heart
Capillary
carries blood
away from
the heart
Arteriole
Artery
Vein
Smooth
muscles
& elastic
fibres
lumen
lining of
Fibrous
wall
Cross section of a blood vessel tissue
Cross section of
artery
Cross section
of vein
Artery : thicker elastic wall
to withstand high blood pressure
relatively smaller lumen
Cross section of
artery
Cross section
of vein
Artery : thicker smooth muscles
Not contract to force blood forward
Heart drives blood forward
Cross section
of vein
Cross section of
artery
Blood pressure changes along blood vessels
Blood pressure /
mm Hg
120
100
90
80
40
Blood flow steadily
with no pulse
Blood flow
with pulse
0 arteries
arterioles
capillaries
venules
veins
How can blood return to heart
through vein under low pressure?
Valve open
blood can flow
Valve closed
blood can’t
flow back
Valves present to
prevent
backflow of
blood
Return of blood to heart is aided by contraction
of body muscles as they squeeze the blood along
the vein
valve
open
contract
valve
close
Demonstration of
Venous Flow in
the Fore Arm
A
B
X
vein
elbow joint
C
D
S
R
Y
finger Y
squeezing
finger X pressing
blood
down on R
towards S
finger Y removed
finger X still
pressing down on R
both fingers are
removed
What is the purpose of tying the arm with a piece of
rubber tubing ?
Ans: This makes the vein more conspicuous.
A
B
X
vein
elbow joint
C
D
S
R
Y
finger Y
squeezing
finger X pressing
blood
down on R
towards S
finger Y removed
finger X still
pressing down on R
both fingers are
removed
With reference to steps B & C, explain why the part of the
vein between R and S has disappeared ?
Ans: Since
In step
On
thethere
other
B, finger
are
hand,
valves
Y squeezes
finger
at Xpoint
isthe
still
S,vein
blood
pressing
towards
is down
point
S. Blood
on
prevented
point in
R which
from
this segment
flowing
prevents
back
is blood
therefore
… flowing
pushed
into R.
along …
A
B
X
vein
elbow joint
C
D
S
R
Y
finger Y
squeezing
finger X pressing
blood
down on R
towards S
finger Y removed
finger X still
pressing down on R
both fingers are
removed
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
B
X
vein
elbow joint
C
D
S
R
Y
finger Y
squeezing
finger X pressing
blood
down on R
towards S
finger Y removed
finger X still
pressing down on R
both fingers are
removed
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.
Capillaries
• allow exchange of materials
between the blood and the
body tissues by diffusion.
arterioles
venules
O2 ,
nutrients
capillaries
CO2 ,
waste
• the wall is permeable to O2,
CO2, glucose, amino acids,
salts, water ... etc.
cross section of capillary
Capillaries
Specially adapted for exchange of materials
1 One-cell thick walls
–  diffusion distance between very
small
blood and body tissue
lumen
2 Very small lumen but large total
cross sectional area
–  blood flow
more time for exchange
of materials
cross section of
one cell
3 Greatly branched
capillary
thick
–  surface area for efficient
diffusion of materials
Capillaries
v1
A1
Same flow rate
(Volume / time)
V1 = V2
 Total cross
sectional area
d1
A2
arteriole
v2
d2
A1 > A2
 d2 > d1
 v2 > v1
Exchange of materials between
the blood and the body cells
white
Exchange of materials takes
blood
place at the capillaries
onlycells
water
Plasma protein
plasma
O2
tissue fluid
Diffuse across
the capillary
wall
capillary
nutrients
CO2 waste products
Squeeze through
the wall
cell
Diffuse across
the capillary
wall