Physiology Flashcards
Autorhythmicity
Hearts ability to beat rhythmically without a stimuli
Which node initiates heart beat
SA
Where is the SA node located
Upper right atrium, where SVC enters
Sinus rhythm
Heart controlled by SA node
How does cardiac excitation originate?
- SA node generates regular spontaneous pacemaker potentials
- Reaches threshold
- Action potential generated
Pacemaker potential
Due to:
Decrease in K+ efflux
Na and K + influx
Transient Ca ++ influx
Rising phase of action potential
Caused by:
Activation of long lasting Ca++ channels
Ca++ influx
Falling phase of depolarisation
Caused by:
Inactivation of Ca++ channels
Activation of K+ channels
K+ Efflux
How does cardiac excitation spread throughout the heart?
- SA node
- Passes to AV node by cell to cell conduction
- AV node allows delay to ensure atria are empty
- Passes down Bundle of His
- Into Purkinje Fibres
- Causes ventricles to contract
Only point of electrical contact between atria and ventricles
AV node
Where is the AV node
Vase of right atrium, at junction between atrium and vesicles
Phase 0
Fast Na+ influx
Phase 1
Closure of Na+ channels
Transient K+ efflux
Phase 2
Mainly Ca++ influx
Phase 3
Ca++ channels close
K+ efflux
Phase 4
Resting membrane potential
Ca++ influx stimulates
Systole
Bradycardia
HR <60
Tachycardia
HR >100
Neurotransmitter for heart
Acetylcholine acting through M2 receptors
Inhibitor for acetylcholine
Atropine - used in bradycardia to speed up heart
Stroke volume
Volume of blood ejected by each ventricle per heart beat
End diastolic - end systolic
Frank-Starling Law
The more the ventricle is filled with blood during diastole, the greater the volume of ejected blood will be during the resulting contraction
Starling law leads to increased …
SV in to the aorta
Afterload
Resistance to which the heart is pumping
Extra load
Load imposed after heart has contracted
Afterload increases
Heart unable to eject full SV
EDV decreased
Forced contraction due to Frank Starling mechanism
extrinsic neurotransmitter of stroke control
Noradrenaline
Increases force of contraction
Inotropic effect
Reduces duration of systole and diastole
Sympathetic nerve stimulation effect on Frank Starling Curve
Shift to left
Effect of parasympathetic nerves on ventricular contraction
Little innervation by vagus, little effect on SVC
Vagal stimulation influences rate not contraction here
Diastole
Heart ventricles relax and fill with blood
Systole
Heart ventricles contract and pump blood into aorta and pulmonary artery
Steps during cardiac cycle
- Passive filling
- Atrial contraction
- Isovolumetric ventricular contraction
4 Ventricular ejection - Isovolumetric ventricular relaxation
First heart sound (lub) caused by
shutting of AV valves (mitral and tricuspid) due to higher ventricular pressure than atrial
Systole begins
Second heart sound (dub) caused by
Pulmonary and aortic valves shutting as ventricle pressure lower than aortic/pulmonary
Diastole begins
Kortokoff sounds
Cuff placed > diastolic pressure and < systolic pressure
NO SOUND HEARD
Begin to release cuff
First sound heard
Systolic pressure
Release cuff until
Last sound heard
Diastolic pressure
5th Kortokoff sound
MAP
Diastolic + 1/3(systolic-diastolic)
= Diastolic + 1/3 Pulse pressure
Normal range of MAP
70-105
MAP needed to perfuse brain, kidneys etc
60
Baroreceptors preventing postural hypertension
- Person stands
- Venous return to heart decreases due to gravity
- MAP decreases
- Reduces firing rate in baroreceptors
- Vagal tone decreases, sympathetic tone increases
- Heart rate and stoke volume increase
- Systemic vascular resistance increases