Week 7- Cardiovascular Mechanics Flashcards
How is the heart imaged?
MRI
What chemical is required to allow contraction of the heart?
Calcium
What are the dimensions of ventricular muscle cells?
100 microns long
15 microns wide
How big are the t tubule openings on ventricular cells?
200 nm
How are t tubules spaced on ventricular cells?
Approx 2 microns apart, lie alongside each Z line of every myofibril
What is the rough composition of ventricular cells?
45% myofibrils
35% mitochondria- v high content to provide lots of ATP
What is the calcium channel in ventricular cells called?
L type calcium channel
Where does most calcium bind after entering ventricular cells? What does it cause?
SR release channel, causes a conformational change allowing calcium release into the cytosol
What is the process of calcium release in ventricular cells called?
Calcium induced calcium release
What happens to calcium after it is used in ventricular cells?
Carried back to SR via ATP
Same amount that entered to trigger calcium release is pumped out via Na/Ca carrier (no ATP is required the gradient of sodium is used)
What happens to force generated as muscle is stretched?
Force generated increases
How does the length tension relation differ in skeletal vs cardiac muscle?
Much higher passive force generated in cardiac muscle so higher total force
Out of cardiac and skeletal muscle which is more resistant to stretch? Why is this?
Cardiac due to properties of ECM and cytoskeleton
What are the 2 forms of heart contraction?
Isometric
Isotonic
Describe isometric contraction
Muscle fibres dont change length but pressure in both ventricles increases
Describe isotonic contraction
Fibres shorten and blood is ejected from ventricles
Which comes first out of isotonic and isometric contraction in the cardiac cycle?
Isometric
What is the preload?
Weight that stretches muscle before it is stimulated to contract
What is the afterload?
Weight only encountered when the muscle has started to contract
As weight to lift increases what happens to muscle shortening?
It decreases
What happens to shortening as preloading increases?
Increased shortening
What determines the preload in the heart?
The amount of blood that returns to heart and fills the heart during diastole before excitation occours
What are some measures of preload in the heart?
End diastolic volume
End diastolic pressure
Right atrial presure
What is the afterload in the heart?
The load against which the left ventricle ejects blood after opening the aortic valve
What happens to isotonic shortening and velocity of shortening as afterload decreases?
Isotonic shortening: decreases
Velocity of shortening: decreases
How is afterload measured?
Diastolic blood pressure?
Define the Frank Starling relationship
Increased diastolic fibre length increases ventricular contraction
What are the 2 factors that cause the FS relationship?
Changes in no of myofilament cross bridges that interact
Changes in the calcium ion sensitivity of the myofilaments
Define stroke work
Work done by the heart to eject blood under pressure into the aorta and pulmonary artery
What is the equation for stroke work?
Volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)
Stroke work = SV x P
What is the law of LaPlace?
When the pressure in a cylinder is held constant, the tension in its walls increases as the radius is increased
What is the equation for wall tension?
Wall tension= pressure in vessel x radius of vessel
How does the LV generate more pressure than the RV according to the law of LaPlace?
Radius of the walls of LV is less than the walls of the RV so the LV can generate higher pressures with similar wall stress
Describe how muscle fibers in the heart shorten?
SAN depolarises Action potential travels down t tubules Ca2+ enters cells Ca2+ binds to ryanodine receptors Ca2+ enters cytoplasm from SR Ca2+ binds to troponin Myosin head binds to actin Muscle fibres shorten
What happens to the Frank Starling curve during exercise?
The graph is steeper as cardiac output increases, it also plateaus at a higher level
What happens to the Frank Starling curve during heart failure?
It is less steep as cardiac output falls and plateaus at a lower point
What is the equation for wall tension when you incorporate wall thickness?
Wall tension= (pressure in vessel x radius of vessel) / wall thickness
What happens to vessels during sustained hypertension according to LaPlace?
Tension increases, vessel walls thicken, lumen decreases in size, total radius of the vessel stays the same
