BIO 301
Human Physiology
Cardiovascular system
The Cardiovascular System:
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consists of the heart plus all the blood vessels
transports blood to all parts of the body in two 'circulations': pulmonary (lungs) &
systemic (the rest of the body)
Heart:
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hollow, muscular organ
4 chambers: 2 atria (right & left) & 2 ventricles (right & left)
Blood returning from the systemic (body) circulation enters the right atrium (via the inferior
& superior vena cavas). From there, blood flows into the right ventricle, which then pumps
blood to the lungs (via the pulmonary artery). Blood returning from the lungs enters the left
atrium (via pulmonary veins), then the left ventricle. The left ventricle then pumps blood to
the rest of the body (systemic circulation) via the aorta.
Heart walls - 3 distinct layers:
1 - endocardium - innermost layer; epithelial tissue that lines the entire circulatory
system
2 - myocardium - thickest layer; consists of cardiac muscle
3 - epicardium - thin, external membrane around the heart
Cardiac muscle tissue:
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striated (see photo below; consists of sarcomeres just like skeletal muscle)
cells contain numerous mitochondria (up to 40% of cell volume)
adjacent cells join end-to-end at structures called intercalated discs
Intercalated discs contain two types of specialized junctions:
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desmosomes (which act like rivets & hold the cells tightly together) and
gap junctions (which permit action potentials to easily spread from one cardiac
muscle cell to adjacent cells).
Cardiac muscle tissue forms 2 functional syncytia or units:
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the atria being one &
the ventricles the other.
Because of the presence of gap junctions, if any cell is stimulated within a syncytium, then the
impulse will spread to all cells. In other words, the 2 atria always function as a unit & the 2
ventricles always function as a unit. However, there are no gap junctions between atrial &
ventricular contractile cells. In addition, the atria & ventricles are separated by the
electrically nonconductive tissue that surrounds the valves. So, as will be discussed later, a
special conducting system is needed to permit transmission of impulses from the atria to the
ventricles.
In cardiac muscle, there are two types of cells:
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contractile cells &
autorhythmic (or automatic) cells.
Contractile cells, of course, contract when stimulated. Autorhythmic cells, on the other hand,
are self-stimulating & contract without any external stimulation. The action potentials that
occur in these two types of cells are a bit different:
On the left is the action potential of an autorhythmic cell; on the right, the action potential of a
contractile cell.
Autorhythmic cells exhibit PACEMAKER POTENTIALS. Depolarization is due to the
inward diffusion of calcium (not sodium as in nerve cell membranes). Depolarization begins
when:
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the slow calcium channels open (4),
then concludes (quickly) when the fast calcium channels open (0).
Repolarization is due to the outward diffusion of potassium (3).
Used with permission: http://mail.bris.ac.uk/~pydml/CVS/Heart/Cells/Electrics/APpmr.htm
In Contractile cells:
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depolarization is very rapid & is due to the inward diffusion of sodium (0).
repolarization begins with a slow outward diffusion of potassium, but that is largely
offset by the slow inward diffusion of calcium (1 & 2). So, repolarization begins with
a plateau phase. Then, potassium diffuses out much more rapidly as the calcium
channels close (3), and the membrane potential quickly reaches the 'resting' potential
(4).
Used with permission: http://mail.bris.ac.uk/~pydml/CVS/Heart/Cells/Electrics/APpmr.htm
Most of the muscle cells in the heart are contractile cells. The autorhythmic cells are
located in these areas:
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Sinoatrial (SA), or sinus, node
Atrioventricular (AV) node
Atrioventricular (AV) bundle (also
sometimes called the bundle of
His)
Right & left bundle branches
Purkinje fibers
Various automatic cells have different
'rhythms':
SA node - 60 - 100 per minute (usually 70
- 80 per minute)
AV node & AV bundle - 40 - 60 per
minute
Bundle branches & Purkinje fibers - 20 - 40 per minute
SA node = has the highest or fastest rhythm &, therefore, sets the pace or rate of contraction
for the entire heart. As a result, the SA node is commonly referred to as the PACEMAKER.
Spread of cardiac excitation:
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Begins at the SA node & quickly spreads through both atria
Also travels through the heart's 'conducting system' (AV node > AV bundle > bundle
branches > Purkinje fibers) through the ventricles
For efficient pumping:
o The atria should contract (& finish contracting) before the ventricles
contract. This occurs because of AV nodal delay (that is, the impulse travels
rather slowly through the AV node & this permits the atria to complete
contraction before the ventricles begin contraction).
o The atria should contract as a unit, & the ventricles should contract as a
unit. This occurs because the impulse spreads so rapidly that all myocardial
cells in the atria and ventricles, respectively, contract at about the same time.
The impulse spreads rapidly through the ventricles because of the conducting
system.
Refractory period of contractile cells:
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Lasts about 250 msec (almost as long as contraction period)
The long refractory period means that cardiac muscle cannot be restimulated until contraction
is almost over & this makes summation (& tetanus) of cardiac muscle impossible. This is a
valuable protective mechanism because pumping requires alternate periods of contraction &
relaxation; prolonged tetanus would prove fatal.
Electrocardiogram (ECG) = record of spread of electrical
activity through the heart
P wave = caused by atrial depolarization
QRS complex = caused by ventricular depolarization
T wave = caused by ventricular repolarization
ECG = useful in diagnosing abnormal heart rates, arrhythmias, & damage of heart muscle
Heart Valves:
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Atrioventricular (AV) valves - prevent backflow of blood from ventricles to atria
during ventricular systole (contraction)
o Tricuspid valve - located between right atrium & right ventricle
o Mitral valve - located between left atrium & left ventricle
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Semilunar valves - prevent backflow of blood from arteries (pulmonary artery & the
aorta) to ventricles during ventricular diastole (relaxation)
o Aortic valve - located between left ventricle & the aorta
o
Pulmonary valve - located between right ventricle & the pulmonary artery
(trunk)
All valves consist of connective tissue (not cardiac muscle tissue) and, therefore, open &
close passively. Valves open & close in response to changes in pressure:
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AV valves - open when pressure in the atria is greater than pressure in the ventricles
(i.e., during ventricular diastole) & closed when pressure in the ventricles is greater
than pressure in the atria (i.e., during ventricular systole)
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Semilunar valves - open when pressure in the ventricles is greater than pressure in the
arteries (i.e., during ventricular systole) and closed when pressure in the pulmonary
trunk & aorta is greater than pressure in the ventricles (i.e., during ventricular diastole)
Mechanical Events of the Cardiac Cycle:
(also check www-
medlib.med.utah.edu/kw/pharm/hyper_heart1.html)
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the cardiac cycle has two phases: systole (contraction) & diastole (relaxation)
'Electrical' events are correlated with the 'mechanical' events:
o P wave = atrial depolarization = atrial systole
o QRS complex = ventricular depolarization = ventricular systole (& atrial
diastole occurs at the same time)
o T wave = ventricular repolarization = ventricular diastole
What happens in the heart during each 'mechanical' event:
o Atrial systole (labeled AC below):
 no heart sounds (because no heart valves are opening or closing)
 a slight increase in ventricular volume because blood from the atria
is pumped into the ventricles
o Ventricular systole:
 the first heart sound (lub) (labeled S1 below) - this sound is generated
by the closing of the AV valves (& this occurs because increasing
pressure in the ventricles causes the AV valves to close)
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o
initially there is no change in ventricular volume (called the period
of isometric contraction) because ventricular pressure must build to a
certain level before the semilunar valves can be forced open & blood
ejected. Once that pressure is achieved, & the semilunar valves do
open, ventricular volume drops rapidly as blood is ejected.
Ventricular diastole:
 the second heart sound (dub) (labeled S2 below) - this sound is
generated by the closing of the semilunar valves (& this occurs because
pressure in the pulmonary trunk & aorta is now greater than in the
ventricles & blood in those vessels moves back toward the area of
lower pressure which closes the valves)
 ventricular volume increases rapidly (period of rapid inflow) - this
occurs because blood that accumulated in the atria during ventricular
systole (when the AV valves were closed) now forces open the AV
valves (because the pressure in the atria is now greater than the
pressure in the ventricles). & flows quickly into the ventricles. After
this 'rapid inflow', ventricular volume continues to increase, but at
a slower rate (the period of diastasis). This increase in volume occurs
as blood returning to the heart via the veins largely flows through the
atria & into the ventricles.
Used with permission: http://mail.bris.ac.uk/~pydml/CVS/Heart/Whole/CardCyc/CCprvo.htm
Cardiac output:
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volume of blood pumped by each ventricle
equals heart rate (beats per minute) times stroke volume (milliliters of blood pumped
per beat)
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typically about 5,500 milliliters (or 5.5 liters) per minute (which is about equal to total
blood volume; so, each ventricle pumps the equivalent of total blood volume each
minute under resting conditions) BUT maximum may be as high as 25 - 35 liters per
minute
Cardiac reserve:
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the difference between cardiac output at rest & the maximum volume of blood the
heart is capable of pumping per minute
permits cardiac output to increase dramatically during periods of physical activity
What factors permit variation in cardiac output?
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Changes in heart rate:
o Parasympathetic stimulation - reduces heart rate
o Sympathetic stimulation - increases heart rate
Effect of parasympathetic stimulation on the heart:
Increased parasympathetic stimulation > release of acetylcholine at the SA node > increased
permeability of SA node cell membranes to potassium > 'hyperpolarized' membrane > fewer
action potentials (and, therefore, fewer contractions) per minute
a = sympathetic stimulation, b = normal heart rate, & c = parasympathetic stimulation
Effect of sympathetic stimulation on the heart:
Increased sympathetic stimulation > release of norepinephrine at SA node > decreased
permeability of SA node cell membranes to potassium > membrane potential becomes less
negative (closer to threshold) > more action potentials (and more contractions) per minute
Regulation of Stroke Volume:
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intrinsic control ==> related to amount of venous return (amount of blood returning to
the heart through the veins)
extrinsic control ==> related to amount of sympathetic stimulation
Intrinsic control:
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Increased end-diastolic volume = increased strength of cardiac contraction = increased
stroke volume
This increase in strength of contraction due to an increase in end-diastolic volume (the
volume of blood in the heart just before the ventricles begin to contract) is called the
Frank-Starling law of the heart:
o
Increased end-diastolic volume = increased stretching of of cardiac muscle =
increased strength of contraction = increased stroke volume
Source: http://www.sci.sdsu.edu/Faculty/Paul.Paolini/ppp/lecture21/sld006.htm
Extrinsic control:
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Increased sympathetic stimulation > increased strength of contraction of cardiac
muscle
Mechanism = sympathetic stimulation > release of norepinephrine > increased
permeability of muscle cell membranes to calcium > calcium diffuses in > more crossbridges are activated > stronger contraction
Flow rate through blood vessels
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directly proportional to the pressure gradient
inversely proportional to vascular resistance
Flow = Difference in pressure/resistance
Pressure Gradient = difference in pressure between beginning & end of vessel (pressure =
force exerted by blood against vessel wall & measured in millimeters of mercury)
Resistance:
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hindrance to blood flow through a vessel caused by friction between blood & vessel
walls
major determinant = vessel diameter (or radius)
is inversely proportional to radius to the fourth power (so, for example, doubling the
radius of a vessel decreases the resistance 16 times which, in turn, increases flow
through the vessel 16 times)
Source: http://www.oucom.ohiou.edu/CVPhysiology/H003.htm
Arteries:
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serve as passageways for blood from heart to tissues
act as pressure reservoirs because the elastic walls collapse inward during ventricular
diastole (when there is less blood in the arteries):
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blood pressure averages 120 mm Hg during systole (systolic pressure) & 80 mm Hg
during diastole (diastolic pressure) (& the difference between systolic & diastolic
pressures is called the pulse pressure)
Arterioles:
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distribute cardiac output among systemic organs (whose needs vary over time)
Resistance (&, therefore, blood flow) varies as a result of VASODILATION &
VASOCONSTRICTION
Factors that influence radius of arterioles:
o intrinsic (or local) control
o extrinsic control
Intrinsic (local) control:
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changes within a tissue that alter the radius of blood vessels & adjust blood flow
especially important in skeletal muscles, the heart, & the brain
increased blood flow in an active tissue results from active hyperemia:
Increased tissue (metabolic) activity > increases levels of carbon dioxide & acid in the tissue
& decreases levels of oxygen > these changes in the concentrations of acid, CO2, & O2 cause
smooth muscle in the walls of the arterioles to relax & this, in turn, causes vasodilation of the
arterioles > vasodilation reduces resistance with the vessel &, as a result, blood flow through
the vessel increases
So, blood flow increases when a tissue (e.g., skeletal muscle) becomes more active & the
increased blood flow delivers the needed oxygen & nutrients.
Extrinsic control occurs via:
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sympathetic division of the Autonomic Nervous System
parasympathetic division of the Autonomic Nervous System
The sympathetic division innervates blood vessels throughout the body while the
parasympathetic division innervates blood vessels of the external genitals. Varying degrees of
stimulation of these two divisions, therefore, can influence arterioles (& blood flow)
throughout the body.
Capillaries:
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site of exchange of materials between blood & tissues
exchange may occur by simple diffusion
diffusion enhanced by:
o thin capillary walls (just one cell thick)
o narrow capillaries (so the red blood cells &
plasma are close to the walls)
large numbers (the human body has 10 - 40 billion capillaries!) which
translates into a tremendous amount of surface area through which exchange
can occur
o relatively slow flow of blood (providing more time for exchange to occur)
exchange also occurs through pores (located between the cells the form the capillary
walls), by vesicular transport (e.g., pinocytosis), & by bulk flow
o
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BULK FLOW:
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protein-free plasma filters out of capillaries, mixes with surrounding interstitial fluid,
& is then reabsorbed. Plasma filters out at the arteriole end of capillaries because
hydrostatic (blood) pressure (an outward force) exceeds osmotic pressure (an inward
force). At the venous end of capillaries, the filtrate tends to move back in because
osmotic pressure now exceeds hydrostatic pressure.
because the outward force at the arteriole end exceeds the inward force at the venous
end, more plasma filters out than moves back in to the capillaries. So, fluid tends to
accumulate in the tissues. The lymph vessels pick up this fluid & transport it back to
the blood.
BULK FLOW:
1 - not very important in exchange (much more exchange occurs by way of diffusion)
2 - important in regulating the 'distribution' of fluids between the plasma & interstitial
fluid (which is important in maintaining normal blood pressure)
Veins:
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serve as low-resistance passageways to return blood from the tissues to the heart
serve as a BLOOD RESERVOIR (under resting conditions nearly two-thirds of all
your blood in located in the veins) &, therefore, the veins are important in permitting
changes in stroke volume
Related links:
NOVA: Cut to the Heart
Gross Physiology of the Cardiovascular System
The Electrocardiogram: Basics
Cardiac Cycle
Cardiovascular Physiology
The Circulatory System
Coronary Heart Disease Risk Calculator
Valvular Heart Disease
Back to BIO 301 syllabus
Lecture Notes 1 - Cell Structure & Metabolism
Lecture Notes 2 - Neurons & the Nervous System I
Lecture Notes 2b - Neurons & the Nervous System II
Lecture Notes 3 - Muscle
Lecture Notes 4 - Blood & Body Defenses I
Lecture Notes 4b - Blood & Body Defenses II
Lecture Notes 6 - Respiratory System