Cardiac physiology

Question 1

What are the purposes of the CVS?

Answer 1

• transport oxygen and co2
• transport nutrients and wastes
• transport WBCs and hormones
• distribute heat

Question 2

what are the two circulations?

Answer 2

pulmonary and systemic

Question 3

describe the components of the pulmonary circulation

Answer 3

Right Atrium: weak pump, receives blood from pulmonary circulation

Right ventricle: stronger pump, pumps blood to the pulmonary circulation for oxygenatino

Question 4

Describe the elements of the systemic circulation

Answer 4

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

Question 5

What are the phases of heart contraction?

Answer 5

1: diastole
1. 5: atrial systole/ventricular diastole
2: systole

Question 6

Describe the features of diastole in the heart

Answer 6

Diastole

• AV valves open
• semilunar valves closed
• atria and ventricles relaxed and passively filling with blood

Question 7

Describe the features of atrial systole/ventricular diastole in the heart

Answer 7

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

Question 8

Describe the features of systole

Answer 8

Systole

• AV valves close (ventricular P > atrial P)
• semilunar valves open (ventricular P > arterial P)
• relaxed atria
• contraction of ventricles

Question 9

What is the governing rule about blood flow?

Answer 9

Blood follows the path of least resistance

Question 10

What does flow equal?

Answer 10

Flow = change in pressure/resistance

Question 11

How is resistance calculated?

Answer 11

It is inversely proportional to the radius of the blood vessel to the power of 4

Question 12

Why do our blood vessels get smaller?

Answer 12

So that blood flows down to enable diffusion and nutrient exchange

Question 13

Describe the path of normal signal propagation in the heart

Answer 13

1: SA node
2: AV node
3: Bundle of His
4: Left and Right bundle branches
5: Purkinje fibres

Question 14

Describe the phases of the cardiomyocyte action potential

Answer 14

1. Resting (-90mV)
2. Rapid depolarisation (+20mV)
3. Plateau (+10mV, 200-300ms)
4. Rapid repolarisation (-90mV)

Question 15

What ions are responsible for the cardiac AP?

Answer 15

K+, Na+, Ca2+

Question 16

why is the resting membrane potential -90mV?

Answer 16

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.

Question 17

What does increasing the permeability to Na+ or Ca2+ in the heart do?

Answer 17

causes membrane depolarisation, by overriding the presence of the negative electrical potential of the potassium

Question 18

What is the Nernst equation?

Answer 18

The electrical potential difference between the inside and outside of cell (E) at equilibrium for any ion

Question 19

What is depolarisation caused and maintained by?

Answer 19

Caused by Na+, maintained by Ca2+

Question 20

Describe the phases of the cardiac AP

Answer 20

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

Question 21

What are the advantages of the long plateau and repolarisation phase in cardiac muscle?

Answer 21

• 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

Question 22

Describe the phases of E-C coupling

Answer 22

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)

Question 23

Describe E-C coupling

Answer 23

AP and influx of calcium are coordinated, and simultaneously lead to contraction

Question 24

What are the two refractory periods in the heart?

Answer 24

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

Question 25

How is the AP propagated throughout the heart?

Answer 25

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

Question 26

Where does the cardiac electrical signal come from?

Answer 26

Initiated in pacemaker cells located in the SA and AV nodes.

Question 27

Describe the phases of the pacemaker potential

Answer 27

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

Question 28

What are the main differences between the pacemaker potential and the cardiac potential

Answer 28

Pacemaker - no plateuas - no proper erst - slower upstroke in phase 0 (slower depolarisation)

Question 29

Describe the depolarising and repolarising currents in the pacemaker cells?

Answer 29

Depolarising (+ in) - slow Na+ channels - low and high threshold Ca2+ (T and L type) Repolarising (+ out) - variable K+ channels

Question 30

Why isn't the signal transmitted directly from the atria to the ventricles?

Answer 30

Too slow and uncoordinated

Question 31

How can the ANS, drugs, hormones and electrolyte changes modify the heart rate?

Answer 31

Modify - SA node firing - speed of conduction incl delay at Av node - force of contraction

Question 32

How does the PNS (specifically the vagus nerve) decrease HR?

Answer 32

SA node - decrease rate AV node - slows conduction, increases AV delay, can block it completlely

Question 33

How does the SNS increase HR?

Answer 33

SA node - increase rate AV node - speeds conduction, reduces AV delay Speeds conduction Increases contractility

Question 34

Describe the process of PNS decreasing HR

Answer 34

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

Question 35

Describe the process of SNS increasing HR

Answer 35

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