The cardiac pressure and volume cycle with ions and action potentials Flashcards
What is important about cerebral circulation?
Brain maintains all vital functions
Constancy of flow & pressure
Autoregulation
Give the name of the circle of arteries found on the brains inferior surface
Circle of willis
Describe how renal circulation is an example of a portal system
Glomerular capillaries to peritubular capillaries
Which compounds are produced by the kidneys?
ACE and Renin
Give the function of ACE and Renin
Endocrine functions
Controlling blood volume
Responding to renal blood pressure
Which percentage of cardiac output does skeletal muscle use during strenuous exercise?
80%
How does skeletal muscle respond to adrenaline?
By causing vasodilatation
Describe some special aspects of skin circulation
Role in Thermo–Regulation
Perfusion can increase 100X
Arteriovenous Anastomoses- Primary role in thermoreg
Sweat Glands- Role in thermoregulation and Plasma ultrafiltrate
Response to Trauma- Red reaction, flare, wheal
Give the four sequential events of the cardiac cycle
Ventricular filling
Isovolumic* ventricular contraction
Ejection
Isovolumic ventricular relaxation
When does the aortic valve close?
Left ventricular pressure < aortic pressure
Describe the PV loop of mitral stenosis
Decrease preload
Decrease afterload
Describe the PV loop of aortic stenosis
Increased afterload
Describe the PV loop of mitral reguirgitation
Increase preload
Decrease afterload
Describe the PV loop of aortic reguirgitation
Increase in preload
What is S1?
Lub- first heart sound
AV valves close, normally loudest
What is S2?
Dub- Second heart sound
Semilunar valves close
Describe systolic murmur
Fluid leaves ventricle
AV regurgitation or SL stenosis
Describe diastolic murmur
Fluid enters ventricle
AV stenosis or SL regurgitation
What causes myocytes to contract?
The Cardiac Action Potential
When do voltage gated channels open?
When voltage becomes positive
List the two types of K+ channels
Delayed rectifier
Inward rectifier
Describe the action of delayed rectifier K+ channels
Open when membrane depolarises
But all gating takes place with a delay
Describe the action of inward rectifier K+channels
Open when Vm goes below -60 mV
Very unusual! More open when cells are at rest
Functions: to clamp membrane firmly at rest
K+ channel lets K+ out of cell, repolarising it
Describe the sequence of initial depolarisation of an action potential
The cell starts at rest (-70 mV)
Inward rectifier K+ channels are open, K+ flowing out is the dominant current
Resting membrane potential is near EK
Something causes the cell to become less negative
Depolarisation: inside the cell the voltage becomes less negative (or more positive)
Could be a nearby cell depolarising
Could be synaptic transmission where a neurotransmitter opens a ligand-gated channel
Describe the sequence of repolarisation
Due to the passage of time, 2 delayed-action events occur
Na+ channel inactivation causes ↓ Na+ current going in
Delayed rectifier K+ channels open causing ↑ K+ going out
These cause the membrane to be less positive and more negative inside
What is the refractory period?
Period of time during which neuron is incapable of reinitiating an AP,
The amount of time it takes for neuron’s membrane to be ready for a second stimulus once it returns to its resting state following an excitation
What is After-hyperpolarization?
at the end of an AP the voltage inside temporarily goes slightly more negative than at rest, followed by a return to the resting membrane potential
When the voltage goes below -60 mV, the inward rectifier K+ channels open again; they stay open until next depolarisation
These normally clamp the voltage toward EK, and are responsible for maintaining the resting membrane potential
During AHP: the ↑ K+ permeability and ↓ Na+ permeability, the membrane potential moves closer to EK
Describe the sequence of action potentials in a ventricular myocyte
Depolarisation: Na+ gates open in response to wave of excitation from pacemaker
Transient Outward Current: tiny amount of K+ leaves cell
Plateau phase: Inflow of Ca2+ just about balances outflow of K+
Rapid repolarization phase: Vm falls as K+ leaves cell
Back to resting potential
How do cardiac action potentials differ to skeletal action potentials?
Much longer: up to 500 ms
Varies in duration and size
long refractory period, no tetany
Describe the plateau phase
Dynamic equilibrium Ca2+ current in K+ current out Decreased Vm decreases the Ca2+ current Also for K+, but much less So decreased Ca2+ current leads to positive feedback: repolarisation by K+
The cardiac action potential varies in different parts of the heart. How does this relate to the ECG?
The timing of the different cardiac APs determines the ECG
List which parts of the cardiac action potential relate to the ECG
QT interval aligns with ventricular AP
QRS = ventricular depolarisation
T = ventricular repolarisation
Describe the sequence of a ventricular myocyte action potential
At rest, the inward rectifier K+ channel outward current stabilising membrane (phase 4)
The rapid rising phase (or upstroke) of the action potential is, exactly as in nerve and skeletal muscle, due to a transient increase in inward Na current (with positive feedback – phase 0)
Depolarization also leads to transient opening of time- and voltage-dependent Ca channels (phase 2)
The total K conductance decreases rather than increases upon depolarization
Repolarization is greatly delayed due to b) and c)
Describe the automaticity of the SAN
Sinoatrial (SA) node cells are autorhythmic
resting potential is unstable
resting potential is close to threshold
Cells independently beat @ 100 bpm
Increased by symp activity
Decreased by parasymp activity
SA node is normally the pacemaker
Other cardiomyocytes can be too
SA (pacemaker) nodal cells responsible for the initiation of a heart beat in the healthy heart because they have fastest rate
What is pacemaker potential otherwise known as?
Diastolic potential
What does the pacemaker potential relate to?
Voltage drifts positive between nodal beats
Instead of resting potential
Because cells lack inward rectifiers
In myocytes of SA node, AV node, conduction system only
Slope of PP determines rate of firing
Describe the If (funny current) and its actions
If makes the SA node cells spontaneously active - (HCN channel and autorhythmicity during the Pacemaker Potential)
If increases upon hyperpolarization
rather than depolarization
If leads to a net inward current
If —> a lot of Na+ current inward and a tiny K+ current outward
Depolarises cell toward 0 mV
What does blocking the Na+ channels do?
Decreases conduction velocity and can prevent arrythmias
What does blocking the Ca 2+ channels do?
Decreases heart rate and contractile force
