CVS Mechanics Flashcards
Describe cardiomyocytes:
Rod-shaped and 100um long, with t-tubule invaginations that are 200nm in diameter, carrying action potentials to the sarcoplasmic reticulum, and lining up the Z-lines; many mitochondria located close to the myofibrils to ensure rapid ATP synthesis
What is EC-coupling in cardiomyocytes?
On excitation, depolarisation sensed by L-type Ca2+ channels, opening these to allow extracellular calcium to enter down concentration gradient to sarcoplasm (2mMol to 1nMol) - some calcium will activate myofilaments, but most binds to ryanodine receptors (aka SR Ca2+ release channel), inducing conformational change to open the RyR, causing Ca2+ efflux in calcium induced calcium release
What is the difference between cardiac and skeletal muscle?
DHPR = mechanical link, but does not exist in cardiomyocytes, so L-type calcium channel needed
What is the role of the Ca2+-ATPase pump?
It uses ATP to pump calcium against concentration gradient from cytoplasm to SR for storage
What is the role of the Na+/Ca2+ exchanger?
It uses downhill concentration gradient of Na+ to efflux calcium from cell - so same amount that entered to cause heartbeat is removed again
Summarise the movement of calcium in cardiac muscle contraction
AP opens L-type Ca2+ channels, leading to influx
Influx causes calcium induced calcium release by inducing conformational change in RyR that allows for an efflux from the sarcoplasmic reticulum
Calcium binds to troponin to move tropomyosin
Ca2+-ATPase uses ATP to pump calcium against the concentration gradient back to the SR - so all released by the SR is reabsorbed
Na+/Ca2+ exchanger uses downhill gradient of Na+ to efflux calcium from cell - so all that entered via the L-type Ca2+ channels exits
Result = same [Ca2+] as start
How are the physical properties of cardiac muscle different to skeletal muscle?
More resistant to stretch and less compliant than skeletal muscle, due to properties of the extracellular matrix/cytoskeleton - can’t be overstretched, and only ascending limb of relation important
What are the two types of contraction in cardiac muscle?
Isometric and Isotonic
What is isometric contraction?
There is no change in length but pressure increases in vesicles when valves closed
What is isotonic contraction?
Shortening of fibres leads to ventricular blood ejection when valves open
What is preload?
The weight stretching a muscle before it is stimulated to contract
What is afterload?
The weight not apparent to muscle in its resting state, only encountered after contraction initiates
What is the isometric-preload relationship?
Greater preload leads to greater force because stretches before contraction
What is the Isotonic-afterload relationship?
Greater afterload leads to reduced shortening and velocity
Heavier weight = reduced shortening but same force
What is the preload-afterload relationship?
A greater preload leads to a greater shortening for a given afterload
What are the in vivo correlates of Preload?
Blood filling in diastole stretches resting ventricular walls, and stretch determines the pre-load before ejection - dependent on venous return; measured by end-diastolic volume, end-diastolic pressure and right atrial pressure
What are the in vivo correlates of afterload?
Effectively diastolic blood pressure; afterload is load against which left ventricle ejects blood after opening of aortic valve - any increase decreases isotonic shortening and hence velocity; measured by DBP
How does hypertension affect afterload?
Higher DBP (afterload) means ventricle has to work harder to expel
Considering afterload and preload, what are the factors that affect heart contraction?
Amount of blood filling the ventricle (P)
Pressure in aorta needed to overcome to eject blood (A)
Pressure volume loops:
Name what happens at the four corners (A-D) where A is at the bottom right, going anti-clockwise:
A: Mitral valve closes
B: Aortic valve opens
C: Aortic valve closes
D: Mitral valve opens
Pressure volume loops:
What happens between A and B?
Isovolumic contraction
Pressure volume loops:
What happens between B and C?
Ejection
Pressure volume loops:
What happens between C and D?
Isovolumic relaxation
Pressure volume loops:
What happens between D and A?
Filling