The Heart
Chapter 18
The Heart
• Size – 250-350 grams
• Location – in mediastinum of thoracic cavity
• Function – generates pressure to pump blood through circulatory system
• Orientation – flat base is directed toward right shoulder, and pointed apex
points to left hip
Figure 18.1
Heart Coverings
Pericardium – the two layered, membranous sac in which the heart sits
Fibrous pericardium – the outer, thick layer composed of dense connective tissue,
for protecting, anchoring the heart in position, and preventing overfilling
Serous pericardium – the inner thin layer composed of a serous membrane
Visceral layer – membrane clinging to the outer heart surface
Parietal layer – membrane clinging to the inside of the fibrous pericardium
Pericardial cavity – serous fluid-filled space between the visceral and parietal
layers
Figure 18.2
Heart Layers
Epicardium – the epithelium clinging to the outer heart wall (is the visceral
pericardium)
Myocardium – the middle layer composed of cardiac muscle tissue
Endocardium – the epithelium clinging to the inner surfaces of the heart
chambers
Figure 18.2
The Heart
• 4 chambers: 2 atria, 2 ventricles
Atria :
•the superior chambers
• auricles – ear-like extensions of
the atria
• receiving chambers, limited
pumping means thin walls
Ventricles:
• the inferior chambers, the
majority of the heart volume
• pumping chambers, thick walls
Sulci:
• The indentations on the outer
heart surface, corresponding to
borderlines between chambers
• contain fat and vessels
• Ex – interventricular sulcus
Figure 18.4 a, b
The Heart
Septa:
• The internal walls that
divide the chambers
• Correspond to the external
sulci
•Ex’s- Interventricular
septum and Interatrial
septum
Figure 18.4e, f
The Atria
Right Atrium Entrances:
• Superior Vena Cava –
blood returning from above
the diaphragm
• Inferior Vena Cava –
blood returning from below
the diaphragm
• Coronary sinus – blood
returning from the heart
wall
Left Atrium Entrances:
• 4 pulmonary veins blood returning from lungs
Figure 18.4e
The Ventricles
Right Ventricle:
• Receives blood from the
right atrium
• Blood exits into the
pulmonary trunk – to lungs
Left Ventricle:
• Receives blood from the
left atrium
• Blood exits into the aorta
– to the body
Figure 18.4e
Circulation
Blood only passes through ½ of the heart at
a time, and therefore must pass through the
heart twice to complete circulation
Pulmonary circuit:
• the pathway from the heart to the lungs
and back
• is pumped by the right half of the heart
• blood leaves –O2 and returns +O2
Systemic circuit:
• the pathway from the heart to the body’s
tissues and back
• is pumped by the left half of the heart
• blood leaves +O2 and returns –O2
NOTE – arteries do NOT always carry
oxygenated blood, and veins do NOT always
carry deoxygenated blood
Figure 18.5
Circulation
• The Systemic circuit is a
longer circuit than the
Pulmonary circuit
• Greater pressure is
needed to pump blood
through the systemic circuit
• The left ventricle
therefore has more cardiac
muscle tissue than the right
ventricle
Figure 18.6
Coronary Circulation
Coronary circulation – the series of vessels that supply blood flow to
the wall of the heart, beginning at the aorta and ending at the right atrium
Anastomoses – the merging of blood vessels, providing more than one
way to deliver blood to one location…Why?
Myocardial Infarction – “heart attack” – due to a blockage of a coronary
artery, causing the death of cardiac muscle cells
Figure 18.7
Coronary Circulation
Coronary arteries – deliver oxygen and nutrient rich blood to the cardiac muscle
Figure 18.4b
Coronary Circulation
Cardiac veins – drain the oxygen and nutrient poor blood from the cardiac muscle
Coronary sinus – large vein on posterior side that empties into the right atrium
Figure 18.4d
Heart Valves
Function – ensure a unidirectional flow of blood through the heart
Types – there are 2 atrioventricular valves and 2 semilunar valves
Figure 18.8 a, b
Heart Valves
Atrioventricular valves:
• separate an atrium from a
ventricle
• prevent backflow into the
atrium
• Tricuspid (Right AV) valve
– separates right atrium from
right ventricle
•Bicuspid (Left AV) valve –
separates left atrium from left
ventricle. Also known as
mitral valve
• Flaps (cusps) of these
valves are supported by
papillary muscles and
chordae tendineae
Figure 18.8c
Heart Valves
Atrioventricular valves:
• Papillary muscles attach to valve flaps via chordae tendineae
• These muscles contract to prevent the valve flaps from inverting into the atria
Figure 18.9
Heart Valves
Semilunar valves:
• separate a ventricle from a
great vessel
• prevent backflow into the
ventricle
• composed of three cup-like
flaps
• Pulmonary semilunar
valve – regulates movement
of blood into pulmonary trunk
• Aortic semilunar valve –
regulates movement of blood
into the aorta
• Flaps (cusps) of these
valves are NOT supported by
papillary muscles and chordae
tendineae
Figure 18.8c
Heart Valves
Semilunar valves:
• during contraction, pressure forces these valves open, allowing blood into
the large vessels
• when the ventricles relax, the blood falls toward the ventricles, fills the cupshaped flaps, closing the valves
Figure 18.10
Microscopic Anatomy
Cardiac muscle tissue:
• Striated – a striped
appearance
• Involuntary – no conscious
control
Cardiac Muscle Fibers (cells):
• shorter than skeletal muscle
fibers, with central nuclei
• large amount of mitochondria
for endurance
• branched – fibers divide and
unite
• interconnected – fibers are
linked and work in unison
Figure 18.11
Cardiac Muscle
Cardiac Muscle Fibers (cells):
•Intercalated discs- junctions where adjacent fibers connect
• desmosomes – hold fibers tightly together
• gap junctions – allow fibers to share cytoplasm and contract
in unison
Figure 18.11a
Cardiac Muscle
Contraction
Autorhythmicity
The ability of cardiac muscle to
trigger its own contraction, not
needing a nervous system
impulse
Whole organ contraction
There is no partial contraction
of cardiac muscle, it is an all or
none event
Figure 18.11
Cardiac Muscle Contraction
• Action potentials in cardiac muscle involve 3 ions (Na+, K+ and Ca2+),
which produces a plateau phase of the impulse
• This prolongs the contraction to ensure efficient blood ejection
• Also increases the time of the absolute refractory period, ensuring
separate heart contractions (not prolonged muscle tetanus)
Figure 18.12
Intrinsic Conduction System
Intrinsic Conduction System:
• Stimuli that trigger cardiac
muscle contraction come from
within the heart itself
•Autorhythmic cells - specialized
heart cells that generate action
potentials which spread through
the heart to trigger contraction
• These cells have unstable
resting potentials, and
therefore depolarize regularly
• These pacemaker
potentials cause action
potentials in the cardiac
muscle fibers, triggering
contraction
Figure 18.13
Intrinsic Conduction System
Sinoatrial node – in the right atrium, the “pacemaker” whose cells
generate the sinus rhythm of 75 beats/min
Atrioventricular node - causes a delay of .1sec to allow atrial
contraction before ventricular contraction. Rhythm is 40-60 beats/min
Bundle of His – pathway into the interventricular septum
Figure 18.14
Intrinsic Conduction System
Bundle Branches – the right and left pathways through the
interventricular septum
Purkinje fibers – pathways to the walls of the ventricles
•The bundle of his, bundle branches and purkinje fibers would set a
rhythm of only 30 beats/min
Figure 18.14
Extrinsic Innervation
Cardiac centers – gray matter areas in
the medulla that can change heart rhythm
Parasympathetic innervation – via the
vagus nerve, slows heart rate
Sympathetic innervation – via the
sympathetic trunk, increases heart rate
Heart rate disorders:
Tachycardia – an abnormally fast heart
rate 100+ beats/min
Bradycardia – an abnormally low heart
rate 60- beats /min
Arrhythmias – irregular heart rhythms
Fibrillation – rapid, irregular contractions
that do not function to pump blood
Figure 18.15
Electrocardiogram ECG
ECG – a graphic
representation of all of the
action potentials in the heart
in a given time
P wave – shows the
depolarization of the atria
QRS complex – shows the
depolarization of the ventricles
T wave – shows the
repolarization of the ventricles
Figure 18.16
Electrocardiogram ECG
Note: the waves of the ECG graph correspond to the
spreading depolarization through the heart tissue.
Muscle contraction follows this depolarization
Figure 18.17
Electrocardiogram ECG
Figure 18.18
Heart Sounds
Heart sounds – the “lub dub”
“Lub” – the sound produced by
the closure of the AV valves
“Dub” – the sound produced by
the closure of the semilunar
valves
Murmurs – abnormal heart
sounds, indicating valve
problems
Figure 18.19
Cardiac Cycle
Cardiac cycle – the events of a
single heart beat
Systole – heart contraction
Diastole – heart relaxation
1. Ventricular filling – blood flows
through the atria into the
ventricles. The AV valves are
open, and the semilunar valves
are closed. Then, the atria
contract forcing the remaining
blood into the ventricles
2. Isovolumetric contraction –
ventricles contract, forcing the
AV valves closed. The volume in
the ventricles is now the end
diastolic volume (EDV)
Figure 18.20
Cardiac Cycle
3. Ventricular ejection – ventricular
pressure forces the semilunar
valves open and the blood
enters the great vessels
4. Isovolumetric relaxation –
ventricles relax, and the blood
within them is the end systolic
volume (ESV). The semilunar
valves close and the atria begin
to fill.
5. Ventricular filling – atrial
pressure forces the AV valves
open which restarts the cycle
Figure 18.20
Cardiac Output
Cardiac Output (CO) – the amount of
blood pumped by each ventricle in one
minute
CO = heart rate x stroke volume
Stroke volume (SV) = EDV - ESV
Practice Question:
If a person’s EDV is 125mL and their
ESV is 50mL, what is their CO if their
heart rate is 80 beats/min?
Answer:
SV = 125mL – 50 mL
SV = 75mL
CO = 75mL x 80 beats/min
CO = 6000mL/min = 6.0L/min
Figure 18.21
Fetal Heart Structures
Foramen ovale – a hole in the interatrial septum allowing blood to pass from
the right atrium to the left atrium. Its remnant is the fossa ovalis in adults
Ductus arteriosus – a passage from the pulmonary trunk to the aorta. Its
remnant is the ligamentum arteriosum in adults
Both structures allow the fetal blood to bypass the pulmonary circuit….Why?
Figure 18.24