Organ Systems Brons MT3 Heart 3 Lecture 10/22/13 (2 of 2) Flashcards
Preload or EDV (end diastolic volume) depends on
Venous filling pressure, duration of diastole, ventricular compliance
Does increased venous pressure (preload) increase or decrease stroke volume?
Increase
Does ventricular hypertrophy reduce or increase compliance? Does it reduce or increase stroke volume?
Reduces compliance
Diminishes stroke volume
Afterload
The systolic pressure needed to overcome the blood pressure within the aorta in order to eject blood. End systolic volume, ESV
Does afterload include isometric contraction? Isotonic contraction?
Isometric contraction to reach afterload pressure levels.
Isotonic contraction during ejection
Increased aortic pressure (afterload) __________the velocity of ventricular contraction (increases or reduces)?
Reduces. This in turn reduces how much blood is ejected during a cycle.
Does greater contractility increase or decrease velocity?
Increase.
How does the body compensate for decreased velocity?
By increasing contractility which increases the velocity of blood flow during ejection.
Does increased aortic pressure (afterload) increase or decrease stroke volume?
Decrease
Does increased ESV reduce or increase stroke volume?
Reduce
Contractility or Inotropism
The intrinsic ability of myocardial cells to develop force at a given preload and afterload
Increases in contractility (positive inotropy) are caused by any mechanisms that _________ Ca++ and its binding to ___________.
increase; troponin
What is/are involved in the regulation of myocardial contractility by the sympathetic nervous system? What is their effect?
Catecholamines (along with cardiac glycosides) increase contractility. Sympathetic = fight or flight
What is/are involved in the regulation of myocardial contractility by the parasympathetic nervous system? What is their effect?
Ach muscarinic receptors decrease contractility. Parasympathetic = Rest and Digest
Changes in contractility involve changes in
Contraction force
Latency of contraction and relaxation
Increased contractility by NE and EPI does what pressure and time of ventricular contraction?
Early onset, enhanced force, and early relaxation (start early, go higher, finish early)
What is an advantage of finishing contraction early?
Gives more time for refilling
Decreased contractility from Ach results in what?
Late onset, less force, late to normal finish.
What is the sequence of events in the excitation-contraction of cardiac muscle?
- AP opens L-type Ca++ channel. 2. This triggers release of Ca++ from the SR via the RyR channel. 3. Ca++ from the SR (not extracellular) binds to troponin complex and activates contractile apparatus.
How reliable is this cycle of events in producing contractions?
There are many places in the system that could have a mutation in one or multiple genes that could produce arrhythmias. Impact how faithfully the communication with Ca++ operates.
What is the mechanism by which NE and EPI accelerate onset and increase the magnitude of contraction (start early and go higher)?
cAMP activates protein kinase PK. cAMP-PK enhances Ca++ channel opening and subsequent release of Ca++ from SR.
What is the mechanism by which NE and EPI stop contraction sooner (finish early)?
cAMP-PK activates phospholamban to increase SR uptake of Ca++ (pumps Ca++ back into SR). Reduces Ca++ binding to troponin C, relaxing the myofibrils.
Stroke Volume (SV) = ?
SV = EDV - ESV
Does increased contractility expand or diminish stroke volume?
Expand
EDV=?
End diastolic volume.(preload)
Does increased ESV reduce or increase stroke volume?
Reduce. SV = EDV - ESV. Larger ESV will reduce SV.
Congestive heart failure (CHF) leads to a/an ________ in contractility, but is compensated for by __________ the preload.
Decrease in contractility; increasing the preload
Cardiomyopathy
Weak myocardial contraction
Effects of cardiomyopathy
Blood backs up and expands ventricular spaces. Overstretch the myocardium, hence high EDV.
Stretching muscles results in _____________ contractility?
Increased. Increase cross bridges, increase force, up to a point.
Overstretching muscles results in ______________ contractility?
Decreased
Heart function in early stage CHF?
Increased stretching compensates for the weak myocardium and low cardiac output with enhanced contractility, improving heart function.
Heart function in late stage CHF?
Further stretching reduces actin-myosin cross bridging and lowers contractility.
Four effects of cardiac glycosides (eg Digitalis, Digoxin) on cardiac function?
- increase vagal tone. 2. Sensitization of baroreceptors. 3. central vagal stimulation, 4. facilitation of muscarinic transmission to heart.
What is the effect of Digitalis on the Na+ K+ pump?
Slows it down
In the Na+ Ca++ exchanger, what comes in the cell and what goes out?
Na+ always goes into cell. Facilitates Ca++ going out.
What is the effect of Digoxin on Na-K ATPase? What does this do to contractility?
Digoxin increases contractility by inhibiting Na+ K+ ATPase
Inhibition of Na+K+ ATPase causes what?
Cell gains sodium, decreasing Na+ conc. gradient. Ca/Na exchange is reduced. Ca++ accumulates within cell to initiate contraction.
Cardiac work = ?
work (contractility) = force X distance
External work = ?
External work = ventricular pressure X stroke volume (muscle length)
External work is isotonic or isometric?
Isotonic
External work has high or low O2 requirement?
Low
Internal work has high or low O2 requirement?
High
Internal work is isotonic or isometric?
Isometric
Describe internal work
Isometrically builds ventricular pressure during isovolumic contraction and is released as heat. Increases to overcome aortic pressure. Not calculated, graphically estimated.
Aortic stenosis
Aortic valve calcified (fixed diameter). Only partially opens. Backflow. Takes more (isometric) work. Must build pressure to higher level than afterload. Increased aortic pressure.
Would the increased internal work involved in aortic stenosis require more O2 or less?
More
Increase in contractility _________ internal work and thus __________ O2 consumption
decreases: reduces
