Lecture Quiz 7 Flashcards
What is autorhythmicity?
the ability to spontaneously generate an action potential
Describe the resting potentials in pacemaker cells
unstable
aka pacemaker potentials
initiate action potentials in the heart
Where are the pacemaker cells located?
sinoatrial node
atrioventricular node
right and left bundle branches - bundle of His
purkinje fibers aka subendocardial conducting network
What happens during the resting potential phase in pacemaker cells?
phase 4
unstable
slow leakage of Na+ ions causes the depolarization of the membrane at the end of the resting state
What happens during the depolarization phase in pacemaker cells?
phase 0
occurs due to the calcium influx (not sodium)
What do pacemaker potentials lack in comparison to cardiomyocytes?
early depolarization and plateau phases
What happens during the repolarization phase in pacemaker cells?
phase 3
occurs when K channels open allowing K+ to leave the cell
What are the rhythms at the SA node and what is the rate of conduction?
60-100 per minute
0.5 m/s
What are the rhythms at the AV node and what is the rate of conduction?
40-60 per minute
0.05 m/sec
What are the rhythms at the bundle branches and purkinje fibers and what is the rate of conduction?
20-40 per minute
bundles - 2 m/sec
purkinje - 4 m/s
What is the ultimate pacemaker in the heart and why?
SA node has the highest or fastest rhythm, therefore it sets the rate of contraction for the entire heart
How many impulses are generated on average?
SA node
70 impulses/min
Describe the pathway of an impulse in the heart
SA -> AV node
AV node delays impulse for 0.1 second
impulse passes to ventricles via bundle of His
AV bundle splits into two pathways in the interventriculur septum (bundle branches)
bundle branches carry impulse towards apex of heart
Purkinje fivers carry impulse to ventricular wall
Give an overview of the extrinsic innervation of the heart
heart is stimulated by the sympathetic cardio acceleratory center
heart is inhibited by the parasympathetic cardioinhibitory center
What does thryoid hormone do in the heart?
increases sensitivity of the heart to epinephrine
sympathetic stimulation
What does the P wave represent in an EKG?
depolarization of the SA node and atrial depolarization, followed by contraction
What does the QRS complex represent in an EKG?
ventricular depolarization followed by ventricular contraction
What does the T wave represent in an EKG?
ventricular repolarization followed by ventricular relaxation
What is the normal range for heartrate?
60-100 bpm
lower or higher and you are in bradycardia or tachycardia respectively
What does the cardiac cycle refer to?
All events associated with blood flow through the heart
What is systole?
contraction of heart muscle
What is diastole?
relaxation of heart muscle
How do conduction and contraction differ?
conduction of AP is an electrical event
contraction of the myocardium is a mechanical event
What parts of the heart contract at what time?
both atria and ventricles contract at the same time
How does blood move?
down the pressure gradient
from higher pressure to lower pressure
What controls the valves?
pressure changes make the valves open and close
What generates heart noise?
closure of valves
Describe what happens when the heart makes sound?
first sound - AV valves close, beginning systole
second sound - semilunar valves close, end of systole and start of diastole
What can be said about the cardiac systole regards to the ventricles?
cardiac systole includes isovolumetric contraction and cardiac ejection
it occurs between heart sounds 1 and 2
What can be said about cardiac diastole in regards to the ventricles?
cardiac diastole includes isovolumetric relaxation, ventricular filling, and atrial systole
occurs between heart sounds 2 and 1
What happens during ventricular filling in the left ventricle?
ventricular pressure drops below atrial pressure
mitral valve opens
blood flows from atrium into ventricle, filling it
blood pressure in the heart is low
blood enters the atria and flows into ventricles
What happens during atrial systole?
SA node fires
impulse spreads through the atrium, causing its depolarization (P wave)
atrium contracts last bit of blood into ventricle
impulse spreads to AV node
What happens during isovolumetric contraction?
impulse spreads to the ventricle causing its depolatization (QRS complex)
ventricles contract, causing pressure in the chamber to rise
mitral valves close (heart sound 1)
pressure in aorta higher so aortic valve remains closed
no blood flow in or out
What happens during cardiac ejection?
pressure in the ventricle rises
when it exceeds aortic pressure, aortic valve opens
blood is ejected from the heart
ventricular contraction is isotonic
ventricular muscle repolarizes (T wave) and begins to relax
pressure in ventricle slowly declines
What happens during isovolumetric relaxation?
pressure in the ventricle drops below aortic pressure
aortic valve closes (heart sound 2)
all valves are closed
relaxation continues and the ventricle pressure drops rapidly
What is cardiac output?
the amount of blood pumped by each ventricle in one minute
How is cardiac output measured?
ejected volume of blood/minute
heart rate * stroke volume
What is normal cardiac input and what could it increase to?
5 L/min
can increase to 30 L/min with exercise
What are two factors that affect cardiac output?
heart rate and stroke volume
What is heart rate?
the number of heart beats per minute
What determines heart rate?
the rate of spontaneous depolarization at the sinoatrial node
How is the heart controlled through the parasympathetic system?
vagus nerve acts on muscarinic receptors to slow the heart
negative chronotrpic effect
How is the heart controlled through the sympathetic system?
cardiac sympathetic fibers stimulate beta-adrenergic receptors and increases heart rate
positive chronotropic effect
What is stroke volume?
the amount of blood pumped out by a ventricle with each beat
determined by contractility, preload, and afterload
What is contractility?
the force of the ventricular contraction
depends on the preload
How does sympathetic stimulation affect contractility? Parasympathetic?
sympathetic stimulation increases the contractility of the myocardium (positive inotropic effect)
parasympathetic has the opposite effect
What is the preload?
the ventricular volume at the end of diastole
What is preload mainly dependent on?
the return of venous blood from the body
What can influence venous return in preload?
changes in position
intra-thoracic pressure
total blood volume and muscle tone in the venous wall
What is Sterling’s law of the heart?
the force of the ventricular contraction is proportional to the stretching of the fibers during diastole
an increased preload results in an increased stroke volume
What is the afterload?
the resistance to ventricular ejection
What causes the afterload and what is that known as?
resistance to flow in the systemic circulation
systemic vascular resistance
What determines the resistance in the afterload?
the diameter of the arterioles and pre-capillary sphincters
the narrower they are, the higher the resistance
What controls the level of systemic vascular resistance?
the sympathetic system
in turn, controls the tone of the muscle in the wall of the arterioles and hence the diameter