Cardiac Contractility and Frank-Starling Law of the Heart

diastole

cardiac muscle at rest

systole

cardiac muscle during contraction

isovolumic ventricular contraction

1st phase of ventricular contraction when AV valves are closed by pressure within the ventricles

Semi-lunar valves = CLOSED

ventricular ejection

pressure within ventricles increase forcing the semi-lunar valves to open; allows ejection of blood into pulmonary or aortic circuit

isovolumic ventricular relaxation

as blood ejection computes and ventricles relax, blood falls back into semi-lunar valves to snap them closed

S1 sound

1st heart sound = AV valves closing

S2 sound

2nd heart sound = semi-lunar valves closing

what is the contraction cycle

1. late diastole = both chambers relax and ventricles fill passively

2. atrial systole = atria contract = top off filling of ventricles

3. isovolumic ventricular contraction = AV valves close

4. Ventricular ejection = ventricular pressure increase, semi-lunar valves open and blood ejected out

5. isovolumic ventricular relaxation = pressure decrease, blood flows back in cusps of S.L. valves and snaps closed

preload

degree of myocardial stretch before contraction begins (EDV)

afterload

the pressure the heart works against to eject blood during systole; the combined load of EDV and arterial resistance

relationship between afterload and heart pumping effort

the larger the afterload = the harder the heart is pumping

contractility

intrinsic ability of a cardiac muscle fiber to contract at any given fiber length

increased contractility

wringing the heart out

decreased contractility

passive squeeze of the heart; sponge analogy

__ in length of muscle exposes more sites for _ binding and for actin-myosin interaction

increase

Ca2+

__ stretch also affects greater release of Ca2+ from the _

increase

SR

describe the Frank-Starling graph

EDV = x-axis

SV = y-axis

graph is linear until a max volume is reached and plateaus

equation for SV

SV = EDV - ESV

what primarily determines venous return by the heart

EDV

4 factors that increase venous return

1. skeletal muscle pump

2. respiratory pump

3. SNS innervation of veis

4. blood volume

what happens if blood volume is decreased by diuretic

increase EDV

beta-adrenergic receptor activation causes what

increased contractility

positive inotrope vs negative inotrope on Frank-Starling curve

(+) = increases the curve

(-) = decreases the curve

if SV increases, explain effects on EDV, ESV and EF

EDV = the same

ESV = decrease

EF = increase

negative chronotrope

blocks Ca2+ entry in SA node = decrease in the curve

what happens if arterial pressure decreases

more blood is ejected quickly

increase SV and ESV

**because the ESV is decreasing, there’s a new EDV = decreases preload and afterload

define (+) inotrope

higher ventricular function curve = increase cardiac work for given level of ventricular filling pressure

vasodilators

better ventricular function curves and decrease cardiac filling pressure

diuretics

improve symptoms of congestive HF and decrease cardiac filling pressure along the same ventricular function curve

digoxin

selective inhibitor of Na+/K+ ATPase

• decrease Na+ gradient for Na+/Ca2+ exchanger

• increase Ca2+ in cytosol is pumped into SR

• improves cardiac performance