Transcript double circulation

Double circulation
• Mammals have a
double circulation
• The right side of the
heart sends blood to the
lungs (pulmonary
circulation)
• Blood returns from the
lungs to the left side of
the heart which pumps it
into the systemic
circulation supplying
the rest of the body
Double circulation
• What is the
advantage of a
double circulation?
Double circulation
• A double circulation
has the advantage of
providing all body
organs with
oxygenated blood
at high pressure
In the single circulation of a fish, blood is pumped
Heart
Organs
Gills
Most of the pressure generated by the heart is lost
in the gills, and the organs receive low pressure
oxygenated blood.
Double circulation
• Double circulation is
made possible by
the complete
division of the heart
into separate left
and right sides
Pulmonary
circulation
• The left side receives
oxygenated blood from
the lungs, and pumps it
to the systemic
circulation; the right
side receives
deoxygenated blood
from the body, and
pumps it to the lungs.
Systemic
circulation
Structure of the heart
The diagram shows the heart at
diastole – relaxed, between
contractions.
Semilunar valves
Pulmonary
artery
Superior vena
cava
Pulmonary
veins
Aorta
Right atrium
Right atrioventricular valve
(tricuspid valve)
Inferior vena
cava
Left atrium
Left atrioventricular valve
(bicuspid valve)
Right ventricle
Septum
Tendon
Papillary muscle
Left ventricle
The cardiac cycle
At
systole
At
Atventricular
atrial
diastole
systole
(0.4
s):
(0.1 s):(0.3 s):
the
contract;
the
theventricles
atria
heartcontract;
muscle
is relaxed;
ventricular
pressureare
rises
the
thevein
atrioventicular
openings
valves
above
atrial,and
closing
constricted,
are open,
preventing
blood the
flows
atrioventicular
valves;
backflow
passivelyinto
from
theatria
veins
into
ventricles; pressure rises
ventricular
blood is forced
from atria into
above
arterial,
opening
the
ventricles
the semilunar
through
valves
the open
are
semilunar
valves;
atrioventicular
closed, because
valves;
arterial
pressure
is higher
blood
is forced
fromthan
ventricles
the semilunar
valves
remain
ventricular
pressure.
into
the because
pulmonary
artery and
closed,
arterial
aorta.
pressure is still higher than
ventricular pressure.
Papillary muscles adjust tension
in valve tendons
Pressure changes during the
cardiac cycle
This wave of arterial
pressure is felt as the pulse
This is the arteries’ elastic
Atrecoil
ventricular
systole the
As soon as ventricular
ventricle
In this sequence
we
will
look
at
pressure rises
above
walls
contract
As soon as ventricular
changesatrial,
in atrial,
ventricular
and
the atrioand
pressure falls below atrial,
aortic pressure
in
the
left
side
of
ventricular (bicuspid)
ventricular
the atrioventricular valve
the
heartvalve
during
one
cardiac
In atrial
systole,
atrial
closes,
making
The
aorticopens
semi- and blood flows
pressure
cycle.
pressure
assound
the
walls
As
bloodthe
isrises
forced
into
thesteeply
‘lubb’
lunar
valves
open from atria into
rises
passively
as ventricular
of the atria
contract …
ventricles
ventricular
pressure
ventricles
pressure
rises
rises too, but remains below
above aortic, then
atrial.
close as it falls
below again
Control of the cardiac cycle
• Contraction of cardiac muscle is
myogenic: cardiac muscle cells contract
of their own accord, without being
stimulated by nerves
• Isolated cardiac muscle cells will contract
in their own independent rhythm
• The cardiac cycle is initiated by waves of
electrical excitation generated by the sinoatrial node, a specialised patch of muscle
in the wall of the right atrium
Control of the cardiac cycle
0.1 s
0.4 s
0.3 s
The muscular walls of the atria are insulated
from the ventricle walls by a ring of fibrous
tissue, the annulus fibrosus: excitation passes
to the ventricles by being picked up by the
atrioventricular node (AVN) and sent down the
Bundle of His, passing to the ventricle walls via
the Purkinje fibres.
The ventricles contract from the apex
upwards, forcing blood upward into
the arteries.
Control of the cardiac cycle
• Because cardiac muscle contraction is
myogenic, if the heart’s nerve supply is
severed it will continue to beat
• The sino-atrial node is innervated by two
nerves, a sympathetic nerve and a
branch of the vagus nerve (cranial X), part
of the parasympathetic system
• Stimulation of the vagus nerve
(parasympathetic) slows the heart rate
• Sympathetic stimulation speeds the heart
rate
Coronary circulation
Aorta
The muscular wall of the
heart is provided with
high-pressure oxygenated
blood by the coronary
arteries, arising from the
base of the aorta.
Blood vessels
• Arteries carry blood from the heart
towards capillary beds, veins carry blood
from capillary beds towards the heart
• Arteries do not always carry oxygenated
blood, nor veins deoxygenated: the
pulmonary and umbilical arteries carry
deoxygenated blood, the pulmonary and
umbilical veins oxygenated blood
Arteries
Arteries characteristically have:
a narrow lumen, maintaining
high pressure
a thick wall to withstand high
pressure
a corrugated inner lining
(endothelium), allowing stretching
during systole
extensive elastic tissue, which
absorbs some of the energy given
to the blood at systole and then
returns it by recoiling during
diastole
muscle fibres, allowing the
artery to be constricted or dilated
to control the amount of blood
flowing through it (NB this muscle
does not propel the blood)
Artery structure
- single layer of epithelial cells
- on which epithelial cells rest
- absorbs energy and recoils
- vasoconstriction and vasodilation
- absorbs energy and recoils
- mostly collagen fibres, holding
the artery together
Veins
Veins characteristically have:
a wide lumen, giving minimum
resistance to low pressure flow
a smooth endothelium, again
giving minimum resistance
little elastic or muscular tissue
valves to prevent backflow of low
pressure blood
Action of valves in veins
Arteries, capillaries and veins
As arteries branch and become smaller, their muscle and elastic layers are reduced.
Arterioles still have muscle and a nerve supply, and control the blood supply to capillary
beds
Capillaries have no muscle or nerve supply, only a single cell layer (the endothelium)
As capillaries rejoin they form venules, which reunite to form veins
Capillary structure
The capillary wall is a single layer of squamous epithelial cells.
It is highly permeable, with gaps between cells and fenestrations through cells:
capillary beds are where exchange occurs between blood and tissues.
The lumen of a capillary is about
5 mm in diameter: red blood cells
(diameter 7 mm) pass through in
single file, squashed against the
capillary wall.
Nucleus of squamous epithelial
cell
Capillary lumen
Red blood cell
Basement membrane of
squamous epithelial cell – allows
small molecules through, keeps
plasma proteins in
Blood pressure and velocity
Only diastolic blood
pressure is shown
(blue line), i.e. without
the rise (pulse) at each
ventricular systole
The greatest drop in blood pressure occurs in the
arterioles. As the arterioles branch and form
capillaries, the total cross-sectional area of all the
vessels combined is about 1000x greater than that of
the arteries, leading to the pressure drop.
Where in the circulatory
system does the
greatest drop in blood
pressure occur? What
causes it?
Formation of tissue fluid
At the arterial end of a
capillary bed the blood is still at
high pressure (about 300 kPa).
Dissolved plasma proteins
give the blood a lower solute
potential than the fluid in the
tissues, but the difference in
hydrostatic pressure forces
water and solutes out.
At the venous end blood is at
lower pressure (about 120 kPa).
yp = 300 kPa
yp = 120 kPa
ys = -190 kPa
ys = -190 kPa
yw = 110 kPa
yw = -70 kPa
Tissue yw = -25 kPa
Plasma proteins are retained by
the capillary’s basement
membrane, so the blood’s solute
potential is effectively
unchanged. The blood’s water
potential is now lower than that
of the tissue fluid, and water
flows back in.
Reabsorption of tissue fluid
Some of the water in tissue fluid is reabsorbed by osmosis at the venous end of
the capillary bed, as explained in the previous slide, but the water potential
difference is greater at the arterial end and more fluid leaves the capillaries than
re-enters them.
The excess is absorbed by capillaries of the lymphatic system, which returns it to
the bloodstream at veins in the thorax (where blood pressure is lowest, especially
during inspiration).
Lymph vessel
Lymphatic capillaries
Vein
Arteriole
Artery
Tissue
Venule
Tissue
fluid
Tissue
The lymphatic system
After tissue fluid has entered the lymphatic
vessels it is called lymph.
The lacteals in the villi of the small intestine
are also branches of the lymphatic system:
the fat they absorb gives lymph a milky
appearance.
On its way back to the bloodstream lymph
passes through numerous lymph nodes,
concentrated masses of lymphocytes and
other leucocytes, which filter out microorganisms and other foreign particles.
The spleen, thymus, tonsils and adenoids
are also concentrated masses of lymphoid
tissue, as are Peyer’s patches in the gut
wall.
Practice questions
The micrograph shows a blood vessel in transverse
section
1. Identify the type of blood
vessel shown and give
two reasons for your
answer.
(3 marks)
2. Name two kinds of tissue
you would expect to find
in the layer labelled X,
and state one function for
each.
X
(2 marks)
Practice questions
A
B
3. Identify the stage of the cardiac
cycle shown in the diagram.
Give two reasons for your
answer.
C
(3 marks)
4. Identify the blood vessels
labelled A, B and C.
(3 marks)
X
Y
5. Explain the difference in
thickness of the muscular wall at
X and Y.
(2 marks)
Checklist
Make sure you
• understand the outline functions of the
circulatory system in the transport of respiratory
gases, metabolites, metabolic wastes and
hormones
• can describe the double circulatory system
• can describe the structure of the mammalian
heart and coronary circulation
• understand the cardiac cycle; myogenic
stimulation
• understand how the cardiac cycle is coordinated
Checklist
Make sure you
• can describe the structure and role
arteries, veins and capillaries
• can describe the interchange of materials
between capillaries and tissue fluid,
including the formation and reabsorption of
tissue fluid