Cardiovascular mechanics Flashcards
Which ion do ventricular cells require for contraction?
Calcium
What is the shape of ventricular cells?
Rod shaped
What is the distinctive difference regarding the heart v skeletal muscle for contraction?
The heart will not beat without external calcium difference to skeletal muscle which is independent
What is the 3 stages for ventricular contraction?
Electrical event (action potential) Calcium transient (Amount of calcium in sarcoplasm has increased for short period of time) Contractile
What is the length and width of a ventricular cell?
100microm,
15 Microm
Describe the position of T-tubules respect to the Z-discs of a sarcomere?
The t-tubules is intermediate between each z-line of Myofibrils, transmitting surface depolarisation deep into the cell
SAQ: What organelle is prevalent in ventricular cells?
Mitochondria
SAQ: Why is there a high prevalence of mitochondria within ventricular cells? (2 marks)
Supply adequate amount of ATP (1)
To supply the sliding filament theory (2), myosin globular heads require ATP binding to reconfigure in high energy state.
Which type of channel opens in response to the action potential within the cardiomyocyte?
L-type calcium channel
Why do L-type calcium channels open?
Upon excitation, the depolarisation is sensed by the ion channel within the cardiomyocyte, opening in response
How does extracellular calcium enter into the cardiac cell?
Passive diffusion across concentration gradient
What is the fate of the diffused intracellular calcium?
Minor proportion directly activates the contraction of sarcomere, binding onto TnC of actin filaments, causing contraction
Majority bind to ryanodine receptors on SR to cause Ca efflux from SR.
Which receptor does Calcium ions bind onto within the sarcoplasmic reticulum?
Ryanodine receptor
Upon ligand calcium activation of ryanodine receptor, what occurs?
Receptor undergores conformation change, opening the ryanodine receptor, thus enabling calcium efflux from sarcoplasmic reticulum
Which complex does calcium bind to on the actin filaments?
Troponin-C.
Upon TnC-Ca binding, what occurs to the actin filaments?
Myosin-binding site becomes exposed, through withdraw of tropomyosin complex, enables myosin head to bind, and undergo power stroke, sarcomere shortening is activated
How is ionic balance achieved within ventricular cells upon relaxation phase?
Calcium is actively pumped out into stored position by Ca2+ ATPase channels of sarcoplasmic reticulum.
Same
Calcium-induced calcium release via sodium-calcium exchanger
How much calcium is effluxed from the sodium-calcium exchanger?
Same amount of calcium that came in = same amount effluxed
Is calcium-induced released passive or active?
Passive
Why is calcium-induced release passive?
Does not require energy, since energy is transferred through the passive diffusion of sodium ions into the cell to expel calcium
What is the relationship between contractile force and cytoplasmic calcium concentration?
Sigmoidal relationship
As intracellular cytoplasmic calcium increases, the force exerted by muscles increases
What is the optimum cytoplasms calcium concentration sufficient to produce maximum force?
10micrometers
What is the relationship between muscle length and baseline force?
Direct proportionality
What is the relationship shown by the active force production line?
Cardiac preparation increases, muscular force increases
What is the relationship with muscle length and passive force?
Direct proportionality
Why does passive force increase as muscle length increases?
Elastic components (elating) stretch, passive tension is produced; cytoskeletal components of cells stretch, elastic potential energy is stored, during no sarcomere shortening of the muscle isometric contraction
What is an isometric contraction?
Tension provided does not cause muscular shortening, exerts pulling force
Why is there a limit of proportionality for active force line?
Stretching point
Further stretching cannot occur to generate force, insufficient overlap between actin and myosin filament within the A band
Why is passive force greater in cardiomyocytes than myocytes?
Cardiac is less compliant, and resilient to stretch, thereby can withstand greater isometric contractions before sarcomere shortening proceeds.
Why are cardiac muscles less compliant?
Due to its extracellular matrix and cytoskeleton
Why is there a descending limb in the muscle force relationship?
Overstretch of muscle beyond the actin and myosin filament overlap will result in a decrease in force, behaviour is exhibited in skeletal muscle.
What is passive force?
based on resistance to stretch of the muscle
What is the total force produced?
Active force + Passive force
Why is there not an descending limb of length-tension curve?
Important in physiological circumstance in cardiac muscle; the descending limb does not occur in physiological condition because pericardium restricts stretching
What is an isometric contraction?
Sarcomeres and muscle fibres do not change length, however pressures increase in both ventricles, attributed to the passive force of cytoskeleton stretch.
What is an isotonic contraction?
Shortening of fibres, and blood is subsequently ejected from ventricles.
What is preload?
Degree to which cardiomyocytes are stretched from filling of the ventricles prior to contraction. Therefore, preload is a way of expressing end-diastolic volume. Increasing ventricular filling, increases EDV, and cardiac muscle is stretched to a greater degree.
What influences preload directly?
Venous return and end diastolic volume
What is end-diastolic volume?
The volume of blood resident within the ventricles before ventricular systole
How does tachycardia affect preload?
Decreases, reduced duration of ventricular diastole, thus diastolic volume decreases, shorter filling, lowering preload
How does a reduced preload due to tachycardia overcome?
Increased contractility, raises the stroke volume
What is the Frank-Starling relationship?
Ventricular stretch and contraction. Within physiological limits, the force of heart contraction is directly proportional to initial length.
Greater stretch = more preload = contractility increased = greater stroke volume
What is cardiac contractility?
Cardiac contractility: Tension developed and velocity of shortening (Strength of contraction) of myocardial fibres at a given preload, and afterload.
How does sympathetic activity influence preload?
Increased venous return to heart, contributes to ventricular filling, EDV + preload.
What is afterload?
The load against which the left ventricle ejects blood after opening the aortic valve.
Refers to the tension/force that the ventricle must develop to pump blood effectively against the resistance in the vascular system.
Which conditions increase afterload?
Vasoconstriction, stenosis, atherosclerosis
HYPERTENSION
Why does vasoconstriction, stenosis and hypertension cause greater afterload?
Greater backforce on aortic valves, thus greater pressure is required to open the aortic semi-lunar valves for ventricular systole to occur (Magnitude of isotonic contraction must increase)
What effect does an increased afterload have on isotonic shortening?
Decreases isotonic shortening, and contractility (velocity of shortening)
Which factors decrease vascular resistance?
Vasodilation (Increased radius)
What are the measure of afterload?
Diastolic arterial blood pressure
Afterload is blood pressure
Why do shorter muscle lengths produce less tension?
Reduces actin-myosin filament overlap, fewer cross-bridges can be made.
Which two factors influence cardiac contractility?
Changes in the number of myofilament cross-bridges that interact
Changes in calcium sensitivity of the myofilaments
How does ventricular stretching influence contractility in respect to calcium?
Ventricular stretching subsequently increases the contact between the myosin heads with the myosin binding sites presented by the thin actin filaments, lattice-spacing decreases.
Decreasing myofilament lattice spacing increases the probability of forming strong-binding cross-bridges; providing more force for the same amount of activating calcium.
How does calcium sensitive influence contractility?
Ca2+ required for myofilament activation, troponin C (TnC), is thin filament protein that binds Ca2+, subsequently causing tropomyosin to expose the myosin binding sites, regulating the formation of cross-bridges between actin and myosin.
At longer sarcomere lengths, the affinity of TnC for Ca2+ is increased due to conformational change in protein; thereby less Ca2+ is required for equivalent amount of force.
What is stroke work?
Work done by the heart to eject blood under pressure into aorta and pulmonary artery
What is stroke work?
Area beneath pressure volume loop
Stroke volume x pressure
What is stroke volume?
End diastolic volume - end systolic volume
Which factors affect stroke work?
Preload
Contractility
Afterload
What is Law of Laplace?
Assuming pressure within a cylinder is held constant, the tension exerted onto the walls increases with increasing radius. (h=Wall thickness)
Increase radius = Increase tension
πΎπππ π»ππππππ=π· π πΉ
π»=π·πΉ/π
How is the left ventricular adapted to cope with systemic circulation (afterload increased) in regards to Law of Laplace?
Radius of curvature is comparatively less than RV, allowing LV to generate higher pressures with equivalent wall stress
