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VT 106
Comparative Anatomy and Physiology
Cardiovascular System
CARDIOVASCULAR SYSTEM – HEART
Function of the Heart
pump – circulates blood through the blood vessels
dual circulation
pulmonary circuit – deoxygenated (blue) blood from tissues  right
heart  lungs for oxygenation  left heart
systemic circuit – oxygenated (red) blood from pulmonary circuit  left
heart  body tissues  right heart
External Anatomy of Heart
apex – narrow caudal end
base – wide cranial end where major blood vessels emerge
lies in thoracic cavity within the mediastinum
sits just dorsal to the sternum
apex points to the left
lies between 2nd – 7th ribs (size varies with species)
pericardial sac (pericardium) – membrane surrounding heart
2 layers:
fibrous pericardium – outer dense irregular connective tissue
protects heart, prevents overstretching
attached to diaphragm
serous pericardium – secretes serous fluid
parietal pericardium – lines inner fibrous pericardium
visceral pericardium (epicardium) – lines heart
pericardial cavity – space between parietal and visceral layers
lubricated with pericardial fluid
Internal Anatomy of Heart
3 layers of heart wall:
epicardium – outermost; visceral pericardium
mesothelium + areolar connective tissue
myocardium – middle layer
cardiac muscle + endomysium
endocardium – innermost; thin, smooth layer
endothelium + areolar connective tissue
Chambers of the Heart – cavities within the heart
2 atria – thin-walled chambers at base of heart
right atrium, left atrium
auricles – extensions of atria
2 ventricles – thick-walled chambers forming apex of heart
right ventricle, left ventricle
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septa – walls separating the different heart chambers
interatrial septum – between right and left atria
interventricular septum – between right and left ventricles
right side of heart is completely separated from left
side of heart
atrioventricular septum – between atria and ventricles
atrioventricular (AV) valves – gates between atria
and ventricles
sulci – superficial grooves between heart chambers; contain adipose
tissue and blood vessels supplying the heart
coronary sulcus – between atria and ventricles
anterior and posterior interventricular sulci – between right
and left ventricles
Right Heart (pulmonary circuit) – receives deoxygenated blood from tissues and
pumps it to the lungs to be oxygenated
Right atrium – receives deoxygenated blood from tissues
cranial and caudal vena cava – large veins from body tissues
coronary sinus – venous sinus from heart tissue
right auricle – increases volume of right atrium
right atrium contracts
Right AV valve (tricuspid valve) – blood flows through from right atrium to
right ventricle
3 cusps (flaps) – connected to tendon-like chordae tendineae in ventricle
Right ventricle
trabeculae – raised bundles of muscle fibers in inner wall
papillary muscles – cone-shaped trabeculae which attach to chordae
tendineae
right ventricle contracts
Pulmonary valve (semilunar valve) – blood flows through into pulmonary trunk
 right and left pulmonary arteries  pulmonary capillaries 
pulmonary veins  left heart
(pulmonary arteries contain deoxygenated blood)
Left Heart (systemic circuit) – receives oxygenated blood from lungs and pumps it to
tissues throughout the body
Left atrium – receives oxygenated blood from pulmonary circuit
4-6 pulmonary veins carry oxygenated blood returning from lungs
left auricle – increases volume of left atrium
left atrium contracts
Left AV valve (mitral valve or bicuspid valve) – blood flows through from left
atrium to left ventricle
2 cusps – chordae tendineae
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Left ventricle
thickest myocardial wall
structure similar to right ventricle
left ventricle contracts
Aortic valve (semilunar valve) – blood flows through into ascending aorta 
aortic arch  descending aorta  arterial branches to all tissues 
systemic capillaries  systemic veins  right heart
Fibrous Skeleton of the Heart
dense fibrous connective tissue rings surround heart valves
continuous with dense fibrous connective tissue in atrioventricular septum
functions:
supports valves
origin for cardiac muscle
electrical insulation between atria and ventricles
os cordis – bone found in this region in cattle
Function of Heart Valves
one-way valves
open and close due to pressure changes as heart contracts
blood always flows from high pressure to low pressure
AV valves (tricuspid and bicuspid)
cusps connected to papillary muscles in ventricles (like parachutes)
low ventricular pressure – when ventricles are relaxed
blood flows through open valve into ventricles
high ventricular pressure – when ventricles are contracting
AV valves close
blood pushes cusps toward atria
papillary muscles and chordae tendineae prevent backflow
Semilunar valves (aortic and pulmonary valves)
3 cusps - crescent moon-shaped (form convex-concave shape)
high ventricular pressure – when ventricles are contracted
semilunar valves open (blood pushes between convex cusps)
blood flows into aorta and pulmonary trunk
low ventricular pressure – when ventricles relax
semilunar valves close
blood in aorta and pulmonary trunk flows back towards ventricles
filling concave cusps and pushing them closed
valvular incompetence – valve doesn’t close completely
regurgitation – blood leaks backwards
valvular stenosis – narrowing of valve, doesn’t open fully
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Cardiac Muscle Function
branched fibers, one nucleus, many large mitochondria
striated – thick and thin filaments overlap to form A & I bands
sarcomeres contract when an action potential causes calcium ions to flow into
cytoplasm from sarcoplasmic reticulum (SR) and through cell membrane
intercalated discs – connect neighboring muscle cells
gap junctions allow action potentials to conduct between cells
syncytium – cardiac muscle cells contract as a functional unit
Autorhythmicity – electrical stimulation comes from specialized cardiac muscle
cells (NOT the nervous system)
autorhythmic cells – 1% of cardiac muscle cells
spontaneously depolarize (ions leak in)
cells reach threshold at regular intervals
voltage-gated Ca+2 channels cause depolarization
pacemaker – sets rhythm for contractions (depolarizes fastest)
conduction system – pathway for propagation of pacemaker signal
ensures coordinated contractions
Conduction Pathway
1) sinoatrial (SA) node – normal pacemaker in right atrial wall
resting potential is not stable – spontaneously reaches threshold
faster than any other autorhythmic cells
threshold depolarization produces an action potential
action potential propagates through both atrial walls via gap
junctions
2) atrioventricular (AV) node – in atrial septum
3) atrioventricular (AV) bundle (bundle of His)
only site for conduction from atria to ventricles
(fibrous skeleton insulates other regions the of AV septum)
4) right and left bundle branches – in interventricular septum
conduct impulse to apex of heart
5) Purkinje fibers – conduct impulse from apex upward throughout
ventricular walls
AV node can act as a pacemaker, but depolarizes more slowly
other autorhythmic fibers fire even more slowly than AV node
ectopic pacemaker – abnormal site depolarizes too rapidly
electrolyte imbalances, hypoxia, toxin
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Cardiac Muscle Action Potential
1) depolarization
resting membrane potential is about -90mV
autorhythmic fibers cause threshold depolarization
voltage-gated sodium channels open
sodium ions rush in – rapid depolarization
sodium channels close
2) plateau – maintained depolarization
voltage-gated calcium channels open – calcium ions flow in from
SR and interstitial space
3) repolarization
Ca channels close
voltage-gated potassium channels open – potassium ions rush out
and restore negative charge inside cell
long refractory period – another action potential cannot occur until muscle
relaxation has occurred (prevents tetanus)
contraction is like skeletal muscle
calcium ions bind to troponin
actin and myosin filaments bind and slide past each other
Electrocardiogram (ECG or EKG) – composite recording of action potentials
generated by the heart
measured on body surface by electrodes in specific locations (leads)
normal sinus rhythm – generated by sinoatrial node
P wave – atrial depolarization
QRS complex – ventricular depolarization
T wave – ventricular repolarization
(atrial repolarization is masked by QRS)
size of waves and intervals between them can indicate condition
of the heart
Cardiac Cycle – time between 1 heartbeat and the next
systole – phase of contraction
atrial systole – atria contract together (follows P wave)
ventricular systole – ventricles contract together (follows QRS complex)
diastole – phase of relaxation
atrial diastole – atria relax (follows QRS)
ventricular diastole – ventricles relax (follows T wave)
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1 Cardiac Cycle:
atrial systole – atria contract
pressure increases in atria
blood flows through open AV valves into relaxed ventricles
end-diastolic volume – volume of filled ventricle
ventricular systole – ventricles contract
(simultaneous atrial diastole – atria relax)
pressure increases in ventricles
AV valves close
isovolumetric contraction – pressure in ventricle increases but volume
stays the same until pressure is greater than pressure in aorta or
pulmonary trunk
semilunar valves open
ventricular ejection – ventricles continue to contract
blood pumped into aorta/pulmonary trunk
end-systolic volume – volume remaining in ventricle
ventricular diastole – ventricles relax
pressure decreases in ventricles and semilunar valves close
isovolumetric relaxation – pressure in ventricle decreases but volume stays
the same until pressure is less than atrial pressure
AV valves open – ventricular filling begins
Blood Pressure – pressure in systemic circulation (pulmonary pressure is lower)
systolic pressure – due to maximum left ventricular contraction
diastolic pressure – during ventricular relaxation, pressure maintained
by smooth muscle in arteries
Heart Sounds
due to turbulence as blood flow patterns are altered
1st sound – closing of AV valves
2nd sound – closing of semilunar valves
3rd sound – opening of AV valves
4th sound – contraction of atria
murmurs – abnormal sounds (valve disorders, septal defects)
Circulation in the Fetal Heart
2 shunts – by-pass lungs (no oxygen in fetal lungs!)
foramen ovale – hole between right and left atria
shunts blood from right atrium to left atrium
fossa ovalis – closes following birth
ductus arteriosus – artery between pulmonary trunk and aorta
shunts blood from pulmonary trunk to aorta
ligamentum arteriosum – closes following birth
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CARDIODYNAMICS
stroke volume (SV) – ml of blood ejected by each ventricle/cardiac cycle
SV = end-diastolic volume(EDV) – end-systolic volume(ESV)
(eg. 130ml – 60ml = 70ml stroke volume)
heart rate (HR) – heart beats/minute (eg. 75 bpm)
Cardiac Output (CO) - volume of blood ejected by each ventricle/minute
(equal for right and left ventricles)
CO = stroke volume X heart rate
(eg. 70ml/beat X 75 beats/min = 5.25 liters/minute)
CO changes to meet body needs
cardiac reserve – difference between maximum cardiac output and resting
cardiac output
normally 4-5 times resting CO
Increasing Cardiac Output
Increase stroke volume
3 factors affect SV:
1) preload – stretch (fullness) of ventricle before contraction
>EDV = >preload = stronger contraction (Starling’s law)
pericardial sac prevents overstretching heart muscle
preload is affected by filling time and venous return
2) contractility – strength of contraction
positive inotropic agents – increase contractility
(most increase Ca entering cardiac muscle cells)
sympathetic NS, epinephrine, digitalis, Ca ions
negative inotropic agents – decrease contractility
parasympathetic NS, calcium-channel blockers,
beta-blockers (block sympathetic response)
3) afterload – pressure in aorta/pulmonary trunk
resists ejection of blood from ventricles
high blood pressure and atherosclerosis decrease SV
Increase heart rate – up to a maximum heart rate
(too high HR decreases filling time = decreases preload)
Regulation of Heart Rate
HR depends on:
tissue demands – basal metabolic rate and activity level
stroke volume – as SV decreases HR must increase
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Autonomic Nervous System Regulation
cardiovascular center – medulla oblongata
inputs – emotions (fear, excitement, anxiety)
sensory – proprioceptors – physical activity
chemoreceptors – oxygen, carbon dioxide levels
baroreceptors – blood pressure
output – sympathetic (norepinephrine)
cardiac accelerator nerves – beta receptors in SA and AV
nodes – increases depolarization rates
output – parasympathetic (acetylcholine)
vagus nerves – SA and AV nodes
decreases depolarization rates
predominates at rest
Chemical Regulation
hormones
adrenal medulla (sympathetic) – epinephrine, norepinephrine
increases heart rate
thyroid hormone – increases heart rate
Other Factors Affecting Heart Rate
body temperature
high (hyperthermia) – faster HR
low (hypothermia) – slower HR
fitness – decreases HR
electrolyte levels
Na+ and K+ decrease HR
Ca+2 increases HR
arrhythmia – abnormal heart rhythm
tachycardia – high resting heart rate
bradycardia – low resting heart rate
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