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Cardiovascular System: The Heart (Homeostatic Imbalance (Note: Defective…
Cardiovascular System:
The Heart
Two side-by-side pumps
Right
Side receives oxygen-poor blood from tissues. Pumps blood
to
lungs to get rid of CO2, pick up O2, via
pulmonary circuit
Left side receives oxygenated blood from lungs. Pumps blood to body tissues via
systemic circuit
Pathway of Blood through heart
Equal volumes of blood are pumped to pulmonary and systemic circuits. Pulmonary circuit is short, low-pressure circulation. Systemic circuit is long, high-friction circulation. Anatomy of ventricles reflects differences (i.e. left ventricle walls are 3x thicker than right - pumps with greater pressure)
Anatomy
Heart Wall
Myocardium
Endocardium
Epicardium
Pericardial cavity contains fluid which allows heart to expand without much resistance
Chambers
Atria
: The receiving chambers. Small, thin-walled, located at the top
Right Atrium
receives deoxygenated blood from body
Anterior portion is smooth-walled
Posterior portion contains ridges formed by
pectinate muscles
Posterior and anterior regions are separated by
crista terminallis
Three veins empty into right atrium
coronary sinus
returns blood from coronary veins
Inferior vena cava
: returns blood from body regions below the diaphragm
superior vena cava
: returns blood from body regions above the diaphragm
Left Atrium
: receives oxygenated blood from lungs
Pectin muscles only found in auricles
Four pulmonary veins return blood from lungs
Auricles
: Appendages that increase atrial volume
Ventricles
: The discharging chambers. Make up most of the volume of the heart. Thicker walls than Atria. Actual pumps of heart
Right Ventricle
: Most of anterior surface. Pumps blood into
pulmonary trunk
Left Ventricle
: Posteroinferior surface. Pumps blood into
aorta
Trabeculae carneae
: Irregular ridges of muscle on ventricular walls
Papillary muscles
: project into ventricular cavity
Anchor
chordae tendineae
that are attached to heart valves
Septum
Interatrial septum: Separates atria
Fossa Ovalis
: Remnant of foramen ovale of fetal heart
Interventricular septum: Separates ventricles
Valves
Types
Semilunar valves
: located between ventricles and major arteries. Prevent backflow from major arteries back into ventricles. Each valve consists of three cusps that roughly resemble a half moon. Open and close in response to pressure changes.
pulmonary semilunar valve
: located between right ventricle and pulmonary trunk
Aortic semilunar valve
: located between left ventricle and aorta
Atrioventricular valves
located between atria and ventricles. Prevent backflow into atria when ventricles contract
Mitral valve
(left AV valve, bicuspid valve): made up of two cusps.
Chordae tendineae
Anchor cusps of AV valves to papillary muscles that function to:
-Hold valve flaps in closed position
-Prevent flaps from everting back into atria
Tricuspid valve
(right AV valve): made up of three cusps and lies between right atria and ventricle
Ensure unidirectional blood flow. Open and close in response to pressure changes
No valves found between major veins and atria; not a problem because
Inertia of incoming blood prevents backflow
Heart contractions compress venous openings
Cardiac Muscle Fibers
Cardiac muscle cells: striated, short, branched, fat, interconnected
Rest of volume composed of sarcomeres (Z discs, A bands, and I bands)
T tubules are wider, but less numerous (enter cell only once at Z disc)
SR (sarcoplasmic reticulum) simpler than in skeletal muscle; no triads
Contain numerous large mitochondria (25-35% of cell volume) that afford resistance to fatigue
One central nucleus (2 at most)
Intercalated discs: connecting junctions between cardiac cells that contain
Desmosomes hold cells together; prevent cells from separating during contraction
Gap junctions allow ions to pass from cell to cell; electrically couple adjacent cells (allows heart to be a functional syncytium, a single coordinated unit)
Homeostatic Imbalance (Note: Defective valve can be replaced with mechanical, animal or cadaver valve)
Incompetent Valve: Blood backflows so heart has to repump same blood over and over
Valvular stenosis: stiff flaps that constrict opening. Heart needs to exert more force to pump blood
Angina Pectoris: Thoracic pain caused by fleeting deficiency in blood delivery to myocardium. Cells are weakened.
Myocardial infarction (heart attack): Prolonged coronary blockage - areas of cell death are repaired with noncontractile scar tissue
Arrhythmias: irregular heart rhythms
Uncoordinated atrial and ventricular contractions
Fibrillation: rapid, irregular contractions
Heart becomes useless for pumping blood, causing circulation to cease; may result in brain death
Treatment: defibrillation interrupts chaotic twitching, giving heart "clean slate" to start regular, normal depolarization
The Intrinsic Conduction System: Heart depolarizes and contracts without nervous system stimulation, although rhythm can be altered by autonomic nervous system
Action Potential initiation by pacemaker cells
Cardiac pacemaker cells have unstable resting membrane potentials called pacemaker potentials or prepotentials
3 parts of action potential
Pacemaker potential: K+ channels are closed, but slow Na+ channels are open, causing interior to become more positive
Depolarization: Ca2+ channels open (around -40mV), allowing huge influc of CA2+, leading to rising phase of action potential
Repolarization: K+ channels open, allowing efflux of K+, and cell becomes more negative
Sequence of excitation: Cardiac pacemaker cells pass impulses, in following order, across heart in ~0.22 seconds
Atrioventricular (AV) bundle,
Right and left bundle branches,
Atrioventricular (AV) node,
Subendocardial conducting network (Purkinje fibers)
Sinoatrial (SA) node,
Coordinated heartbeat is a function of
Presence of gap junctions
Intrinsic cardiac conduction system
Network of noncontractile (autorhythmic) cells
Initiate and distribute impulses to coordinate depolarization and contraction of heart