Heart 7 Flashcards
Four factors that determine cardiac output
Heart rate, afterload, preload, and contractility
Preload
The muscle load BEFORE contraction is initiated; dependent on ventricular filling
Afterload
Muscle load AFTER contraction initiation; determined by arterial pressure
What is the difference in muscle tension between preload and afterload?
Preload will generate passive tension on the heart while the ventricles are filling; afterload will be any force within the heart that resists muscle shortening
Contractility
The inherent ability of actin and myosin to form cross-bridges and generate contractile force
What does contractility have to do with preload and afterload?
Nothing
What primarily determines contractility?
Intracellular calcium concentration
Contraction
Process by which muscle generates tension or force
Two types of muscle contraction
Isometric contraction and isotonic contraction
Isometric contraction
Contraction without shortening; usually occurring when the heart can’t generate enough force to meet the afterload
Isotonic contraction
Contraction with shortening and constant force; occurs whenever the heart is able to meet the afterload
Contraction types during a normal cardiac cycle
Initially the muscle generates isometric contractions until the semilunar valves blow open, in which case they become isotonic contractions
Length-Tension relationship of cardiac muscle
Increase in resting cardiac muscle length will increase contraction strength
Which cardiac factor is related to the length-tension relationship?
Preload
Resting tension
Amount of tension that develops passively by stretching the muscle (i.e., increasing the preload).
Graphic view of compliance
Slope of the resting tension curve is determined by this
Active tension
Amount of isometric tension that is developed by muscle contraction at a particular muscle length
Graphic view of contractility
Slope of the active tension curve on the length-tension graph
Compliance
Change in volume in relation to a change in pressure: primary determinant of resting cardiac tension
Which type of muscle has a lower compliance: skeletal or cardiac?
Cardiac
What is another word for contractility?
Inotropy
What would changes in inotropy do to the length-tension graph?
Positive inotropic effect: active curve goes up and to the left
Negative inotropic effect: active curve goes down and to the right
How does an increase in preload cause an increase in tension development?
Creates more optimal overlap between thick and thin filaments;
increases calcium sensitivity of myofilaments
Changes in preload and their effect on muscle shortening
Increase in preload: more muscle shortening
Decrease in preload: less muscle shortening
Changes in afterload and their effect on muscle shortening
Increase in afterload: less muscle shortening
Decrease in afterload: more muscle shortening
Increase in contractility achieved by sympathetic nerve stimulation increases what three things?
Amount of muscle shortening, velocity of shortening, and rate of relaxation
Force generated by the muscle determines . . .
velocity of shortening.
Index of contractility
Maximal velocity of shortening with no load”
Maximal isometric force
When the muscle is unable to meet the afterload
Increasing preload has what effect on the force-velocity graph?
It has no effect on Vmax, but will increase the velocity of shortening, moving the X-intercept over to the right
Increasing contractility has what effect on the force-velocity curve?
Both increases Vmax and the velocity of shortening, increasing both the X- and Y-intercepts
