2 - Cardiovascular Mechanics 1 Flashcards
What is the shape of the force-calcium relationship?
sigmoidal
What concentration of calcium is sufficient to generate maximum contraction?
10 micromolar
What are the features of a single ventricular cell?
- requires Ca2+ for contraction
- rod shaped
- 100 μm x 15 μm
What is isometric contraction, with an example?
- muscle fibres don’t change length but pressures increase in both ventricles
- isovolumetric contraction
What is isotonic contraction, with an example?
- shortening of fibres and blood is ejected from ventricles
- ventricular contraction ejecting blood out
What is the length-tension relationship?
- increase in muscle length causes increase in force
- as muscle keep stretching, point reached where no more force is generated regardless of stretch (not enough overlap between filaments to produce force)
Which 3 factors increase pre-load?
- decreased HR (more time to fill)
- increased central venous pressure
- increased venous return
How is cardiac muscle affected by the length-tension relationship?
- more resistant to stretch and less compliant than skeletal muscle
- doesn’t overstretch as encased in pericardium
What is preload?
- weight that stretches muscle before it is stimulated to contract
- more preload = more force
How is preload used in the heart?
- blood filling heart during diastole stretches resting ventricular walls
- stretch (filling) determines preload on ventricles before ejection
- preload dependent on venous return
What are some measures of preload?
- end-diastolic volume
- end-diastolic pressure
- right atrial pressure
What is afterload?
weight not apparent to muscle in resting state, only encountered when muscle has started to contract
What is the result of more afterload?
- less shortening
- lower velocity of ejection (as more pressure so there’s a lowered stroke volume)
What is a measure of afterload?
diastolic blood pressure
How is afterload used in the heart?
- load against which left ventricle ejects blood after opening of aortic valve
- pressure in aorta that left ventricle needs to overcome
What is the Frank-Starling relationship?
- increased diastolic fibre length increases ventricular contraction
- as filling of heart increases, force of contraction also increases
What is an example of the mechanism of the Frank-Starling relationship?
- increased venous return leads to increased preload
- more receptors of ventricle fibres
- increased force of contraction
- increased stroke volume
- CO = HR x SV (so CO also increases)
What 2 factors affect the Frank-Starling relationship?
- changes in no. of myofilament cross bridges that overlap
- changes in Ca2+ sensitivity of myofilaments (2 hypotheses)
What is stroke work?
- work done by heart to eject blood under pressure into aorta and pulmonary artery
- stroke work = stroke volume x pressure
What is the stroke volume?
volume of blood ejected during each stroke
What is the law of LaPlace?
when pressure within a cylinder is held constant, the tension on its wall increases with increasing radius
What are the equations for wall tension?
- wall tension = pressure x radius (T = P x r)
- T = (P x r)/h (h is wall thickness)
How is the law of LaPlace used in relation to the heart?
- radius of curvature LV walls less than RV
- LV can generate higher pressure with similar wall stress
- failing hearts become dilated so increases wall stress
How are pressure-volume loops used in conjunction with preload and afterload?
- if preload (length) is increased then for given afterload (force to overcome) amount of force created will increase
- increasing preload will increase muscle shortening
How does the number of myofilament cross bridges affect the Frank-Starling relationship?
- at shorter lengths than optimal, actin filaments overlap
- no. of myosin cross bridges reduced
- more stretch = more optimum interdigitation of actin and myosin filaments that can be achieved
- too much overlap can also reduce no. of cross bridges
How do changes in calcium sensitivity of myofilaments affect the Frank-Starling relationship?
- at longer sarcomere lengths, affinity of troponin c for Ca2+ is increased due to conformational change in protein (increased length = increased Ca2+ sensitivity)
- spacing between myosin and actin filaments decreased with stretch (decreased lattice spacing means probability of forming strong binding cross-bridges increases)
