Cardiac mechanics Flashcards
What are the dimensions of ventricular cells?
100 μm long and 15 μm wide
What are T tubules (transverse tubules)?
finger-like invaginations of the cell surface
What is the diameter of T tubule openings?
Up to 200 nm
Describe the positioning of t tubules
Spaced (approx. 2 μm apart) so that a T-tubule lies alongside each Z-line of every myofibril
What is the purpose of t tubules?
To carry surface depolarisation deep into the cell
What is the relationship between force production (% max) and cytoplasmic Ca2+?
Sigmoid curve
Which is more resistant to stretch, cardiac or skeletal muscle?
Cardiac
Which is more compliant, cardiac or skeletal muscle?
Skeletal
Why is cardiac muscle more resistant to stretch and less compliant than skeletal muscle?
Due to properties of the extracellular matrix and cytoskeleton
What are the names of the two forms of contraction the heart uses?
Isometric and isotonic
Describe isometric contraction
Muscle fibres do not change length but exert force so pressures increase in both ventricles
Describe isotonic contraction
Shortening of fibres and blood is ejected from ventricles
What is preload?
Weight that stretches muscle before it is stimulated to contract
What is afterload?
Weight not apparent to muscle in resting state; only encountered when muscle has started to contract
What are in vivo correlates of preload?
As blood fills the heart during diastole, it stretches the resting ventricular walls
This stretch (filling) determines the preload on the ventricles before ejection
Preload is dependent on venous return
Measures of preload include end-diastolic volume, end-diastolic pressure and right atrial pressure
What are in vivo correlates of afterload?
Afterload is the load against which the left ventricle ejects blood after opening of the aortic valve
Any increase in afterload decreases the amount of isotonic shortening
Measures of afterload include diastolic blood pressure
Define fs relationship
Increased diastolic fibre length increases ventricular contraction
What are the consequences of the fs relationship?
Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return
What is the fs relationship thought to be caused by?
- Changes in the number of myofilament cross bridges that interact
- Changes in the Ca2+ activity of the myofilaments
Define stroke work
Work done by heart to eject blood under pressure into aorta and pulmonary artery
How do you calculate stroke work?
Volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)
What greatly influences SV?
Preload and afterload
What greatly affects pressure (P)?
Cardiac structure
Define law of LaPlace
When the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius
What is the formula for wall tension?
T=PxR
or
T= (PxR)/h
P= pressure
R= radius of vessel
h= wall thickness
Why is wall stress increased in failing hearts?
increased dilation
