Physiology of the
Cardiovascular System
1
The Cardiovascular (Circulatory)
System
The cardiovascular system (CVS) is a
closed system in which the blood
circulates throughout the body.
It consists of the heart (pump) and the
blood vessels.
2
Components of the CVS
Heart: It is a pump composed of 4 chambers (2 atria & 2
ventricles.
2. Blood Vessels: The blood vessels are systems of tubes
including:
a) Arteries and arteriols which carry the blood from the
heart to all parts of the body.
b) Venules and veins which carry the blood back from the
tissues to the heart.
c) Blood capillaries which form a network of fine vessels
connecting the arteriols with the venules. The blood
capillaries are the sites of exchange of gases (O2 & CO2),
nutrients and waste products between blood and tissues.
1.
3
• The walls of arteries and veins have three layers.
• The inner layer is composed largely of endothelium, with a
basement membrane that has elastic fibers.
• The middle layer is smooth muscle tissue
• The outer layer is connective tissue (largely collagen
fibers).
Arteries have a thicker wall than veins because they have a
larger middle layer than veins.
• Veins are larger in diameter than arteries, so that
collectively veins have a larger holding capacity than
4
arteries.
Blood vessels
5
The Arteries
• Arteries and arterioles take blood away from the
heart.
• The largest artery is the aorta.
• The middle layer of an artery wall consists of smooth
muscle that can constrict to regulate blood flow and
blood pressure.
• Arterioles can constrict or dilate, changing blood
pressure.
6
Veins
• Veins: relatively large
vessels that carry
deoxygenated blood from
the body tissue to heart, with
one exception of the four
pulmonary veins which carry
oxygenated blood from
lungs to left atrium.
• Veins contain valves that aid
in venous return
7
The Capillaries
Capillaries have walls only one cell thick to
allow exchange of gases and nutrients
with tissue fluid.
Capillary beds are present in all regions of
the body but not all capillary beds are
open at the same time.
Contraction of a sphincter muscle closes off
a bed and blood can flow through an
arteriovenous shunt that bypasses the
capillary bed.
8
Anatomy of a capillary bed
9
Passage of Blood Through
the Heart
Blood follows this sequence through the
heart: superior and inferior vena cava →
right atrium → tricuspid valve → right
ventricle → pulmonary semilunar valve →
pulmonary trunk and arteries to the lungs →
pulmonary veins leaving the lungs → left
atrium → bicuspid valve → left ventricle →
aortic semilunar valve → aorta → to the
body.
10
11
Internal view of the heart
12
• The pumping of the heart sends out
blood under pressure to the arteries.
• Blood pressure is greatest in the aorta; the wall
of the left ventricle is thicker than that of the right
ventricle because it pumps blood to the entire
body.
• Blood pressure then decreases as it travels
through the arteries and then arterioles.
13
Path of blood through the heart
14
The Pulmonary Circuit
• The pulmonary circuit begins with the
pulmonary trunk from the right ventricle
which branches into two pulmonary arteries
that take oxygen-poor blood to the lungs.
• In the lungs, oxygen diffuses into the blood,
and carbon dioxide diffuses out of the blood
to be expelled by the lungs.
• Four pulmonary veins return oxygen-rich
blood to the left atrium.
15
The Systemic Circuit
The systemic circuit starts with the aorta carrying O2-rich blood
from the left ventricle.
The aorta branches giving arteries going to each specific organ.
Generally, an artery divides into arterioles which in turn divide to
capillaries. Capillaries then lead to venule and eventually to
vein.
The vein that takes blood to the vena cava often has the same
name as the artery that delivered blood to the organ.
In the adult systemic circuit, arteries carry blood to the tissues
that is relatively high in oxygen and low in carbon dioxide, while
veins carry blood back to the heart that is relatively low in
oxygen and relatively high in carbon dioxide.
This is the reverse of the pulmonary circuit.
16
 The coronary arteries serve the heart muscle itself;
they are the first branch off the aorta.
 Since the coronary arteries are so small, they are
easily clogged, leading to heart disease.
 Coronary artery disease (CAD) also known as
atherosclerotic heart disease, coronary heart
disease, or ischemic heart disease (IHD), is
the most common type of heart disease and
cause of heart attacks. The disease is caused by
plaque building up along the inner walls of the
arteries of the heart, which narrows the arteries
and reduces blood flow to the heart.
17
Double Pump and Double
Circulation
18
MAIN FUNCTIONS OF THE
CIRCULATORY SYSTEM
1. Transport and distribute essential substances
to the tissues.
2. Remove metabolic byproducts.
3. Adjustment of oxygen and nutrient supply in
different physiologic states.
4. Regulation of body temperature.
5. Endocrine function by releasing atrial
natriuretic peptide.

ANP is a powerful vasodilator released by atrial myocytes in
response to high blood pressure.
19
The Heart
Anatomical Properties of Cardiac Muscle Fibers
• Involuntary
• Branched and interdigitated
• Cell-cell cohesion forming what is called intercalated disk
• Contain one nucleus
• Contain many mitochondria
20
• Striated
Intrinsic Control of Heartbeat
The SA (sinoatrial) node, or pacemaker,
initiates the heartbeat and causes the atria to
contract on average every 0.85 seconds.
The AV (atrioventricular) node conveys the
stimulus and initiates contraction of the
ventricles.
The signal for the ventricles to contract travels
from the AV node through the atrioventricular
bundle to the smaller Purkinje fibers.
The intrinsic rate of discharge of the SA node
is 100 beats per minute
21
Conduction system of the
heart
22
The Conductive System of the
Heart
23
CONDUCTION SYSTEM
Sequence of excitation
1.
2.
3.
4.
sinoatrial (SA) node - spreads to both atria
atrioventricular (AV) node
atrioventricular (AV) bundle (bundle of His)
right & left bundle branches
•
in the interventricular septum
5. Purkinje fibers
•
conduction myofibers
24
Maintaining the Heart’s Rhythmic Beat
• Some cardiac muscle cells are self-excitable,
meaning they contract without any signal from the
nervous system
• The sinoatrial (SA) node, or pacemaker (leader) ,
sets the rate and timing at which cardiac muscle
cells contract.its intrinsic rate is 100 beats per
minute but it is modified by the effect of autonomic
nervous system.
• Impulses that travel during the cardiac cycle can
be recorded as an electrocardiogram (ECG
pronounced like EKG)
25
1
SA node
(pacemaker)
2
AV
node
3
Bundle
branches
4
Heart
apex
Purkinje
fibers
ECG
26
ECG Waves and Intervals
QRS length
R
T
P
Q S
Normal: PR interval: 0.12-0.2 sec
QRS length: <0.10 sec
QT interval: 0.3-0.4 sec
P-R
interval
Q-T interval
27
ECG Waves
• P wave
– atrial depolarization
• QRS complex
– ventricular depolarization
– onset of ventricular contraction
• T wave
– ventricular repolarization
– just before ventricles start to relax
• Atrial repolarization is usually not visible
– masked by larger QRS complex
28
Electrocardiogram
29
The heart contracts and relaxes in a rhythmic cycle
called the cardiac cycle. The duration of it is 0.85
seconds at heart rate of 70 beats per minue.
• The contraction, or pumping, phase is called
systole
• The relaxation, or filling, phase is called diastole
If the heart rate is 70 beats/min and the duration of
the cardiac cycle is 0.85 sec the duration of
systole will be 0.3 sec and diastole 0.55 sec.
If the heart rate increased to 200 beats/min the
duration of the cardiac cycle will be reduced to 0.3
sec . 0.15 sec systole and 0.15 sec diastole.So
shortening occurs more in diastole .
30
Figure 42.8-3
2 Atrial systole and ventricular
diastole
1 Atrial and
ventricular diastole
0.1
sec
0.4
sec
0.3 sec
3 Ventricular systole and atrial
diastole
31
The heart rate, also called the pulse, is the number
of beats per minute.
A pulse is the rhythmic bulging of artery walls with each
heartbeat
The stroke volume is the amount of blood pumped
in a single contraction
The cardiac output is the volume of blood pumped
into the systemic circulation per minute and
depends on both the heart rate and stroke volume
• Cardiac output equals stroke volume (SV) times
heart rate (HR)
CO = SV x HR
32
• Heart rate (HR) = 75 beats/min, one cycle requires
0.8 sec
• Stroke volume (SV) blood volume ejected per beat
from each ventricle (70 ml).
• Cardiac output = stroke volume (SV) ml/beat X
heart rate (HR) beat/min= 70 X 75 = 5.25 L/min.
• Cardiac index = is the cardiac output per square
meter of body surface area. At rest it is about
3L/mint/m2 . If body surface area is 1.7m2
33
33
Heart Sounds
•
Auscultation
–
•
•
•
•
•
•
act of listening to heart sounds
Sound of heart valves closing
Four sounds but only two loud enough to be
heard by stethoscope (S1 and S2)
S1 = lubb = long, booming sound
atrioventricular (AV) valves closing
S2 = dupp = short, sharp sound Semilunar (SL)
valves closing
S3 blood turbulence during ventricular filling
S4 blood turbulence during atrial systole
34
Blood Flow Velocity
• Physical laws governing movement of fluids through
pipes affect blood flow and blood pressure
• Velocity of blood flow is slowest in the capillary beds,
as a result of the high resistance and large total crosssectional area
• Blood flow in capillaries is necessarily slow for
exchange of materials
35
Area (cm2)
Velocity
(cm/sec)
50
40
30
20
10
0
Pressure
(mm Hg)
5,000
4,000
3,000
2,000
1,000
0
120
100
80
60
40
20
0
Systolic
pressure
Diastolic
pressure
36
Some Properties of Hemodynamics: Peripheral
Resistance
• As the blood flows in the arterial
side toward venous side of the
circulation, it meets resistance
because of the smaller caliber of
the vessels and the viscous nature
of the blood. This is called the
peripheral resistance.
• It is an important factor in
generating and maintaining the
arterial blood pressure.
Vasoconstriction of the small
vessels increases the peripheral
resistance, which in turn elevates
the arterial blood pressure. Whilst
vasodilatation decreases the
resistance and lowers the
pressure.
37
Some Properties of Hemodynamics: Velocity of
Blood Flow
• As the blood flows in
the arterial side, the
velocity of the blood
decreases with the
increase of the total
cross sectional area
38
Extrinsic Control of Heartbeat
• A cardiac control center in the medulla
oblongata speeds up or slows down the
heart rate by way of the autonomic nervous
system branches: parasympathetic system
(slows heart rate) and the sympathetic
system (increases heart rate).
• Hormones epinephrine and norepinephrine
from the adrenal medulla also stimulate
faster heart rate.
39
Control of Heart Rate
Sympathetic effect
•
cardiac accelerator nerves
– Release of Norepinephrine/ Noradrenaline (NA) that
bind to beta 1 receptors.
– An increase in norepinephrine from the sympathetic
nervous system increases the rate of contractions.
1. increases spontaneous firing of SA node & conduction in AV
node.
2. increases Ca++ to contractile fibers
Parasympathetic effect
•
vagus nerve
– Release of acetylecholine
1. causes hyperpolarization (open K+ channels)
2. slows spontaneous depolarization of intrinsic fibers
40
Control of Heart Rate
• Cardiovascular center of medulla oblongata
• Sensory inputs:
– movement as monitored by proprioceptors increase
input to cardiovascular center.
– chemical changes in the blood, monitored by
chemoreceptors. The chemical changes include
arterial PO2 and PCO2 and arterial H+ ion
concentration.
– blood pressure changes, monitored by baroreceptors
(in aortic arch & carotid)
– The carotid baroreceptors are innervated by
glosopharyngeal nerve ; the aortic baroreceptors
41
innervated by vagus .
Blood Pressure
• Blood flows from areas of higher pressure to areas of
lower pressure.
• Blood pressure is the pressure that blood exerts against
the wall of a vessel.
• Blood pressure is generally measured for an artery in
the arm at the same height as the heart.
• Blood pressure for a healthy 20 year old at rest is 120
mm Hg at systole and 70 mm Hg at diastole.
42
Sphygmomanometer
Blood pressure reading: 120/70
1
3
2
120
120
70
Artery
closed
Sounds
audible in
stethoscope
Sounds
stop
43
Factors Affecting BP
•
•
•
•
Age
Gender
Exercise
Smoking: refrain from smoking at least 30
minutes before having a blood pressure
measurement taken.
• Stress
• Daytime and night time
44
Changes in Blood Pressure During
the Cardiac Cycle
• Systolic pressure is the pressure in the arteries during
ventricular systole; it is the highest pressure in the
arteries
• Diastolic pressure is the pressure in the arteries during
diastole; it is lower than systolic pressure
• Pulse pressure is the pressure difference between
systolic and diastolic pressure.
• Mean pressure is diastolic pressure + 1/3 pulse pressure.
45
Regulation of Blood Pressure
• Blood pressure is determined by cardiac output and
peripheral resistance due to constriction of arterioles
• Vasoconstriction is the contraction of smooth muscle
in arteriole walls; it increases blood pressure
• Vasodilation is the relaxation of smooth muscles in the
arterioles; it causes blood pressure to fall
46
• Vasoconstriction and vasodilation help maintain
adequate blood flow as the body’s demands
change
• Nitric oxide is a major inducer of vasodilation
• The peptide endothelin is an important inducer of
vasoconstriction
47
Blood Flow in Veins
Venous blood flow is dependent upon:
a. skeletal muscle contraction
b. presence of valves in veins
c. rhythmic contractions of smooth muscles in the wall
of veins and venules.
d. respiratory movements: the change in pressure
during inhalation causes the vena cava and other
veins to expand and fill with blood.
Compression of veins causes blood to move forward
past a valve that then prevents it from returning
backward.
48
• When a person is standing, gravity helps pull the blood
downward to the lower extremities. Without gravity,
blood tends to remain closer to the heart.
• The force of gravity also makes it more difficult for the
blood to flow upward to return to the heart and lungs for
more oxygen.
• Our bodies have evolved to deal with the ever-present
downward force of gravity; our leg muscles function as
secondary pumps to help in the process of venous
return which is blood flow back to the heart, also
referred to as cardiac input).
• During walking or other leg movements, the muscles
contract, forcing blood up through the veins of the calf
toward the heart. The valves in the veins are arranged
so that blood flows only in one direction. This
mechanism effectively counteracts the force of gravity.
49
+
50
Venous Return
• It is the major determinant of cardiac output
and normally,the heart pumps all blood
returned to it,normally at rest is about 5.5
L/min.The force of contraction of the left
ventricle ejecting blood into the aorta is not
sufficient to push the blood through the
arterial and venous circulation and back to
the heart.Other factors are involved.These
are :
1. Position of the body : standing and lying.
2. Muscular conrtraction : Skeletal Muscle
Pump .
3. Respiratory Pump.
51
Frank-Starling law of the Heart
• The amount of blood pumped by the heart
each minute is determined almost entirely by
the rate of blood flow into the heart from the
veins which is called venous return. The
heart in ,turn, automatically pumps this
incoming blood into arteries. This increased
ability of the heart to adapt to increasing
volumes of inflowing blood into the heart is
called Frank-Starling law of the heart. In other
words ( within physiologic limits , the heart
pumps all the blood that returns to it by the
ways of veins).
52
Blood pressure regulation
1. Short-term blood pressure regulation .
2. Long-term blood pressure regulation.
53
Short-term blood pressure
regulation
The cardiovascular centre ( CVC ) is a
collection of inter-connected neurons in
the medulla and Pons of the brainstem.
The CVC receives, integrates and
coordinates inputs from :
1. Arterial baroreceptors (Carotid and aortic)
2. Arterial chemoreceptors (carotid and
aortic bodies)
3. Higher centres in the brain e.g
54
Hypothalmus.
Changes in arterial BP or
Higher centre
e.g Hypothalamus
Arterial baroreceptors detect
changes in B.P
Heart changes in rate and
stroke volume
Blood vessels
vasoconstriction or
vasodialation
Arterial chemoreceptors
detect changes in
PO2,PCO2 and H+ in
arterial blood.
Cardiovascular centre in Medulla and Pons
55
Long-term blood pressure
regulation
• It involves regulation of blood volume by
the Kidneys and the renin-angiotensinaldosterone system.
56