Chapter 18 Flashcards
(22 cards)
Anatomy of the heart: what is the pulmonary and systemic circuits (right and left side) ?
Right side of heart: receives deoxygenated blood from body tissues and then pumps it to the lungs where it picks up oxygenated blood and drops of carbon dioxide.
Blood vessels that carry blood to and from the lungs form the PULMONARY CIRCUIT
Left side of heart: receives oxygenated blood returning from the lungs and pumps it throughout the body to supply oxygen and nutrients to body tissues.
Blood vessels that carry blood to and from all body tissues form the SYSTEMIC circuit
Describe the location and orientation of the heart
The base of the heart (posterior surface) is directed towards the shoulder
The apex points inferiorly toward the left hip, an apical pulse could be found between the fifth and sixth ribs just below the left nipple, where it touches the chest wall
describe the coverings of the heart
The heart is enclosed by a double-walled sac called the pericardium
The loosely fitting superficial part of this sac is called the fibrous pericardium, this layer protects the heart, anchors the surrounding structures, prevents overfilling of the heart with blood
Deeper to the fibrous pericardium is the SEROUS PERICARDIUM: a thin slippery two layer serous membrane that closes a sac around the heart. The parietal layer lines the internal surface of the pericardium, it continues over the external heart surface where it now is the visceral layer (epicardium)
Between the parietal and visceral layers is the PERICARDIAL CAVITY, this cavity contains a serous fluid that lubricates the layers to prevent friction, allowing for the layers to glide smoothly against each other
Describe the layers of the heart
The superficial epicardium (visceral layer of the serous pericardium) is often infiltrated with fat, especially in older people
The middle layer is known as the MYOCARDIUM, which is composed mainly of cardiac muscles, BULK of the heart, this is the layer that contracts, branching cardiac muscles are tethered to one another by crisscrossing connective tissue fibres (CARDIAC SKELETON).
CARDIAC SKELETON reinforces the myocardium internally and anchors the cardiac muscle fibres, consisted mainly of collagen and elastic fibres. It also allows for AP to spread only via specific pathways in the heart
The third layer of the heart is called ENDOCARDIUM, found on the inner myocardial surface, lines the heart chambers and covers the fibrous skeleton of the valves, continuous with the endothelial linings of blood vessels leaving and entering the heart
Describe the chambers of the heart and the associated great vessels
Heart has FOUR chambers, two superior ATRIA and two inferior VENTRICLES
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ATRIA (receiving chambers): have small protruding appendages known as AURICLES that increase the atrial volume somewhat. Walls of the atria are thin because they only contract minimally to send the blood down to the ventricles.
Blood enters the right atrium via three veins:
Superior vena cava, returns the blood from body regions superior to the diaphragm
Inferior vena cava, returns blood from body areas below the diaphragm
Coronary sinus, collects blood draining from the myocardium
Four pulmonary veins enters the left atrium, these veins transports blood from the lungs and back to the heart
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VENTRICLES (discharging chambers):
Right ventricle form most of the heart’s anterior surface, pumps blood into the pulmonary trunk which routes blood to the lungs for gas exchange
Left ventricle form most of the heart’s posterior inferior surface, ejects blood into aorta, the largest artery in the body to send out blood to the rest of the body
Papillary muscles of the ventricle play a role in valve function, projects into the ventricular cavity
Walls of ventricle are much larger and it is the actual pump of the heart.
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The interatrial septum bears a shallow depression called the fossa ovalis, that marks the spot where a foramen existed in the fetal heart
The internal partition that divides the heart LONGITUDINALLY is called the INTERATRIAL SEPTUM, separates the atria from the INTERVENTRICULAR SEPTUM where it separates the ventricles
CORONARY SULCUS (atrioventricular groove) encircles the junction of the atria and ventricles like a crown
Which structures are responsible to the heart valves to make the blood flow in one direction? And describe the sequence of events that happen to ensure blood flows in one direction
The two atrioventricular valves (AVs), are located at each atrial-ventricular junction that prevent backflow into the atria when the ventricles contract.
The right AV valve, tricuspid valve, has three flexible cusps (flaps of endocardium reinforced by connective tissue)
The left AV valve, bicuspid valve, with two cusps called the mitrial valve
Attached to the AV valve flaps are tiny white collagen cords that anchor the cusps to the papillary muscles protruding from the ventricular walls called the chordae tendinae (heart strings)
The heartstrings and papillary muscles act as tethers that anchor the valve flaps in their closed position
- Blood returning to the heart fills atria, pressing against the AV valves.
- The increased pressure forces AV valves open. As ventricles fill, AV valve flaps hang limply into the ventricles.
- Atria contract, forcing additional blood into ventricles
AV valves open when atrial pressure is greater than ventricular pressure
- Ventricles contract, forcing blood against AV valve cusps
- AV valves close
- Papillary muscles contract and chordae tendineae tighten, preventing valve flaps from everting into atria
AV Valves closed; when atrial pressure is less than ventricular pressure
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The semilunar valves (SL), which is the AORTIC and PULMONARY valves which guard the bases of the large arteries issuing from the ventricles (aorta and pulmonary trunk, respectively) and prevent backflow into the associated ventricles.
Similar to the AV valves, the SL valves open and close in response to differences in pressure. When the ventricles contract and intraventricular pressure rises above the pressure in the aorta and pulmonary trunk, SL valves are forced open and their cusps flatten against the arterial walls as blood rushes past them. When ventricles relax, and intraventricular pressure falls, blood flows back from arteries, filling the cusps of semilunar valves and forcing them to close
How does blood flow from the atrium to the ventricle, and then to either the lungs or the rest of the body?
Coronary circulation is the function blood supply of the heart and the shortest circulation in the body.
The left and right coronary arteries both arise from the base of the aorta and encircle the heart in the coronary sulcus. The left coronary artery runs towards the left side of the heart and then divides into two major branches: anteriorventricular artery and circumflex artery
The right coronary artery: courses to the right side of the heart, where it also gives rise to two branches:
Right marginal artery and posterior interventricular artery
The coronary arteries provide an intermittent, pulsating blood flow to the myocardium. They deliver blood when the heart is relaxed, but are ineffective when the ventricles are contracting because they are compressed by the contracting myocardium.
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After passing through the capillary beds of the myocardium, the venous blood is collected by the cardiac veins, whose path roughly follow those of the coronary arteries, these veins empty blood into the right atrium
Compare and contrast the microscopic structure, metabolism of skeletal and cardiac muscle
Skeletal VS CARDIAC structure:
Skeletal: Striated, long, cylindrical, multinucleate
Cardiac: striated, short, branched, fat, interconnected, one or two centrally located nuclei per muscle cell
Cardiac muscle contains numerous of capillaries, connects cardiac muscle to cardiac skeleton, giving cells something they can pull against.
T tubules are wider and fewer in numbers, the SR is simpler than in skeletal muscle
Cardiac muscle cells have numerous large mitochondria that make up about 25-35% of the cell volume that enables fatigue resistance
Cardiac muscle cells have intercalated discs which are connecting junctions between cardiac cells that contain:
Desmosomes the hold cells together during contractions
Gap junctions that enables ions to pass from cell to cell and electrically couples adjacent cells and allows heart to work as FUNCTIONAL SYNCYTIUM (as one single unit)
Cardiac muscle uses aerobic respiration only (more mitochondria)
Contraction similarities and differences between skeletal and cardiac:
Similarities: both muscle contraction is preceded by depolarization action potential, depolarization wave travels down T tubules, triggering the release of calcium from the SR, excitation contraction coupling occurs which allows for the binding of calcium to troponin causing filaments to slide
Differences:
A) some cardiac muscle cells/myocytes are self excitable
Two kinds of myocytes: contractile cell (responsible for contraction) and pacemaker cells (noncontractile cells that spontaneously depolarize, and it initiates the depolarization of the entire heart. Does not need nervous stimulation
They spontaneously depolarize and this property is called automaticity (autorhythmicity). Whereas skeletal muscle needs neural input to contract and cutting the nerves result in paralysis.
B) the heart contracts as a unit,
due to the gap junction property of the intercalated discs, which ties the cardiac muscle cells together to form a functional syncyticum, the wave of depolarization travels from cell to cell across the heart to contract as a whole unit. Whereas the skeletal muscle is individually stimulated by nerve fibres and only contract in the designated area. Motor unit recruitment occurs in skeletal and does not occur in cardiac muscle.
C) the influx of calcium from extra cellular fluid triggers calcium release from the SR
Depolarization opens special calcium cancels in plasma membrane to be released. Slow calcium channels allow entry of 10-20% of the calcium needed for contraction. Once inside, the influx of calcium then triggers calcium sensitive channels in the SR to release another burst of calcium that accounts for the other 80-90% of calcium needed for contraction
D) tetanic contractions cannot occur in cardiac muscles
Cardiac muscle fibres have longer absolute refractory periods than skeletal muscle fibres
This allows the heart to relax and fill as needed to be an efficient pump
Prevents tetanic contractions
E) the heart relies almost exclusively on aerobic respiration
Cardiac muscle is abundant in mitochondria, therefore it relies heavily on oxygen and cannot function without it
Whereas skeletal muscle can go through fermentation when oxygen is not present as it does have FEWER mitochondria
How do the pacemaker cells trigger an action potential throughout the heart?
The intrinsic cardiac conduction system consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart, so that it depolarizes and contracts in an orderly sequential manner, pacemaker cells are also apart of this system
Cardiac pacemaker cells can spontaneously depolarize and pace the heart. Therefore cardiac fibres are autorythmic
Cardiac pacemaker cells have an unstable resting membrane potential, these spontaneous changing membrane potentials called pacemaker potentials or prepotentials initiate action potentials that spread throughout the heart to trigger rhythmic contractions
- Pacemaker potential
Due to special properties in sarcolemma.
Hyperpolarization of pacemaker cells at the end of an AP, both closes k channels and SLOWLY opens Na channels.
Sodium influx balances the loss of k ions, membrane interior becomes more positive - Depolarization
At threshold, approx -40Mv, calcium channels open and an influx of calcium flows in (instead of Na) that produces the rising phase of AP and reverses membrane potential - Repolarization
Calcium channels inactivate.
In other excitable cells, the falling phase of AP and repolarization reflect opening of K channels and k efflux from cell, once this stage is complete, k channels close, k efflux declines and the slow depolarization stage begins again
Describe the sequence of excitation
- Sinoatrial (SA) node:
-Typically generates impulses about 75 times every minute
-It sets the pace for the heart as a whole because no other region of the conduction system or myocardium has a faster depolarization rate.
- known as the HEARTS PACEMAKER and has a rhythm called SINUS rhythm that determines heart rate
———— - Atrioventricular (AV) node:
- depolarization from SA node travels via gap junctions throughout right atria via the intermodal pathway to the atrioventricular node
-impulse is delayed for .1 second here, to allow the atria to respond and complete their contraction before ventricles contract
-conducts impulses more slowly than the other parts of the system
-once through the AV node, signalling of impulse passes rapidly throughout the rest of the system
———— - Atrioventricular (AV) bundle
-From AV node, impulse travels to atrioventricular bundle (bundle of his)
-electrically connects the atria and ventricles as they do not have gap junctions to connect them
-the fibrous cardiac skeleton is non conducting and insulates the rest of the AV junction
———— - Right and left bundle branches
-the short AV bundles splits into left and right bundle branches which courses along interventricular septum toward heart apex
———— - Subendocardial conducting network
-few myofibrils
-AKA purkinjie fibres, completes the pathway through the interventricular septum, penetrates the apex
-the bundle branches excite the cells of the interventricular septum (depolarizes contractile cells of both ventricles), but the bulk of the ventricular depolarization depends on the large
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What is an arrhythmia
- defects in the intrinsic conduction system can cause irregular heart rhythms (arrhythmia)
- may cause uncoordinated atrial and ventricular contractions (fibrillation) , which is a condition of rapid and irregular or out-of-sync contractions in which control of heart rhythm is disrupted by rapid activity in other heart regions
What are the consequences of having a defective SA node?
- An ectopic focus (abnormal pacemaker) may appear and tame over the pacing of the heart rate, or the AV node may become the hearts pacemaker
- the junctional rhythm (pace set by the AV node) is about 40-60 bears per minute, slower the sinus rhythm but still adequate to maintain circulation
What does having a heart block mean ?
-Damage to AV node, AV bundle, and/or bundle branches
Would interfere with the ability of ventricles to receiving pacing impulses therefore failing heart contractions
How is the heartbeat modified by ANS via cardiac centres in the medulla oblongata
Cardioacceleratory center: sends signals through sympathetic trunk to increase both rate and force
-stimulates SA and AV nodes, heart muscle, and coronary arteries
Cardioinhibatory center: parasympathetic signals via vagus nerve to decrease rate
-inhibits SA and AV nodes via vagus nerves
Describe the action potentials of contractile cardiac muscle cells
- Depolarization
-opens a few fast voltage gated sodium channels in the sarcolemma allowing for extra cellular sodium to enter,
-this influx initiates positive feedback cycle that causes the rising phase of the action potential as well as reversal potential (from -90mV to 30mV)
-period of sodium influx is very brief because sodium channels quickly inactivate
———— - Plateau phase is due to calcium influx through slow calcium channels. This keeps the cell depolarizes because most k channels are closed
———— - Repolarization is due to calcium channels inactivating and potassium channels opening. This allows potassium efflux, which brings the membrane potential back to its resting voltage
What is an electrocardiogram (ECG)?
It is a graphic record of heart activity.
-it is a composite of all the APs generated by nodal and contractile cells at any given time
Describe the P wave
- Atrial depolarization, initiated by the SA node, causes the P-wave
- Approximately .1 seconds after the P-wave begins, the atria contract
Describe the QRS complex
- Ventricular depolarization begins at Apex, causing the QRS complex. Atrial repolarization occurs
- results from ventricular depolarization and precedes ventricular contraction
- lasts for about .08 seconds
Describe the T-wave
- ventricular repolarization begins at Apex, causing the T-wave
- Repolarization is slower than depolarization call muscle the T-wave is more spread out and has lower amplitude than the QRS complex
- lasts for about 0.16 seconds
Describe the PR interval
- is the time from the beginning of atrial excitation to the beginning of ventricular excitation.
- includes atrial depolarization and contraction as well as the passage of depolarization wave through the rest of the conduction system
Describe the ST segment
-when the action potentials of the ventricular myocytes are in their plateau phases, the entire ventricular myocardium is depolarized
