Cardiac physiology Flashcards
What are the purposes of the CVS?
- transport oxygen and co2
- transport nutrients and wastes
- transport WBCs and hormones
- distribute heat
what are the two circulations?
pulmonary and systemic
describe the components of the pulmonary circulation
Right Atrium: weak pump, receives blood from pulmonary circulation
Right ventricle: stronger pump, pumps blood to the pulmonary circulation for oxygenatino
Describe the elements of the systemic circulation
Left atrium: weak pump, recieves blood from the pulmonary circulation, primes the left ventricle
Left ventricle: strongest pump, pumps blood through the aorta to the peripheral circulation at very high pressure
What are the phases of heart contraction?
1: diastole
1. 5: atrial systole/ventricular diastole
2: systole
Describe the features of diastole in the heart
Diastole
- AV valves open
- semilunar valves closed
- atria and ventricles relaxed and passively filling with blood
Describe the features of atrial systole/ventricular diastole in the heart
Stage 1.5: A systole, V diastole
- AV valves open
- Semilunar valves close
- atria contract to push last bit of blood into the passive ventricles
Describe the features of systole
Systole
- AV valves close (ventricular P > atrial P)
- semilunar valves open (ventricular P > arterial P)
- relaxed atria
- contraction of ventricles
What is the governing rule about blood flow?
Blood follows the path of least resistance
What does flow equal?
Flow = change in pressure/resistance
How is resistance calculated?
It is inversely proportional to the radius of the blood vessel to the power of 4
Why do our blood vessels get smaller?
So that blood flows down to enable diffusion and nutrient exchange
Describe the path of normal signal propagation in the heart
1: SA node
2: AV node
3: Bundle of His
4: Left and Right bundle branches
5: Purkinje fibres
Describe the phases of the cardiomyocyte action potential
- Resting (-90mV)
- Rapid depolarisation (+20mV)
- Plateau (+10mV, 200-300ms)
- Rapid repolarisation (-90mV)
What ions are responsible for the cardiac AP?
K+, Na+, Ca2+
why is the resting membrane potential -90mV?
cardiomyocytes are not open to the free movement of ions, and so there is a concentration difference between the heart and the body.
K+ enters the heart with an electrical potential of -94mV, sodium is present as well which is +71mV, but does not have a huge role, so it evens out to about -90mV.
What does increasing the permeability to Na+ or Ca2+ in the heart do?
causes membrane depolarisation, by overriding the presence of the negative electrical potential of the potassium
What is the Nernst equation?
The electrical potential difference between the inside and outside of cell (E) at equilibrium for any ion
What is depolarisation caused and maintained by?
Caused by Na+, maintained by Ca2+
Describe the phases of the cardiac AP
0: rapid depolarisation (-70mV)
- Na+ influx
1: early repolarisation
- Na+ channels close
- K+ channels open and there is a K+ efflux
- (-)10mV
2: plateau
- Ca2+ channels open slowly at -40mV
- balance by K+ efflux
3: repolarisation
- Ca2+ channels close
- additional K+ channels open
- K+ efflux
What are the advantages of the long plateau and repolarisation phase in cardiac muscle?
- EC coupling: longer contraction allows tension and pressure to build to be able to pump blood to the whole body
- Refractoriness: cannot stimulate quickly again, prevents tetanus
Describe the phases of E-C coupling
Cell is resting, calcium is low…
1: local potential arrives
2: fast Na+ channels open
3: Ca2+ channels open
4: Ca2+ influx triggers further Ca2+ release from the SR (calcium induced calcium release)
Describe E-C coupling
AP and influx of calcium are coordinated, and simultaneously lead to contraction
What are the two refractory periods in the heart?
Effective/absolute
- ALL Na+ channels are open or locked
- CANNOT activate cell again
Relative
- Not all Na+ channels are available
- greater than normal stimulus required to generate another AP
How is the AP propagated throughout the heart?
SA node depolarises and this is detected by adjacent cells and is passed on, through intercalated disks and gap junctions.
The signal is NOT passed from the atrial to the ventricles due to the fibrous CT in the way. Signal must converge on the AV node before the ventricles can be activated
Where does the cardiac electrical signal come from?
Initiated in pacemaker cells located in the SA and AV nodes.
Describe the phases of the pacemaker potential
Phase 4: pacemaker potential
- leaky Na+ channel causes a slow increase in positive charge
- slow depolarisation
- Ca2+ channels contribute later on
Phase 0: depolarisation
- voltage gated Ca2+ channels open at -40mV
- Ca2+ influx
- membrane depolarises
Phase 3: repolarisation
- Ca2+ channels close
- K+ channels open, K+ efflux
- membrane repolarises
What are the main differences between the pacemaker potential and the cardiac potential
Pacemaker
- no plateuas
- no proper erst
- slower upstroke in phase 0 (slower depolarisation)
Describe the depolarising and repolarising currents in the pacemaker cells?
Depolarising (+ in)
- slow Na+ channels
- low and high threshold Ca2+ (T and L type)
Repolarising (+ out)
- variable K+ channels
Why isn’t the signal transmitted directly from the atria to the ventricles?
Too slow and uncoordinated
How can the ANS, drugs, hormones and electrolyte changes modify the heart rate?
Modify
- SA node firing
- speed of conduction incl delay at Av node
- force of contraction
How does the PNS (specifically the vagus nerve) decrease HR?
SA node - decrease rate
AV node - slows conduction, increases AV delay, can block it completlely
How does the SNS increase HR?
SA node - increase rate
AV node - speeds conduction, reduces AV delay
Speeds conduction
Increases contractility
Describe the process of PNS decreasing HR
Signal travels:
- Medulla
- PNS neurons
- ACh released, activates muscarinic receptors of autorhythmic cells
- increases K+ efflux and decreases Ca2+ influx
- hyperpolarisation of cell occurs
- increased time for depolarsiation
- decreased HR
Describe the process of SNS increasing HR
Signal travels from
- Medulla
- SNS neurons
- adrenal medula
- epinephrine released (or NA from another source)
- activates B1 receptors of auto-rhythmic cells
- increased Na+ and Ca2+ influx
- increased rate of depolarisation
- increased HR
