the cardiac pressure and volume cycle

Question 1

explain the special aspects of the cerebral circulation? 3

Answer 1

• brain maintains all vital functions
• constancy of flow and pressure
• circle of Willis= arteries on the brains inferior surface organised into a circle, this means there is a redundancy of blood supply, so if one bridge of the circle is blocked, it can be supplied through another

Question 2

what are the special aspects of renal circulation? 4

Answer 2

• 20-25% of cardiac output when body is a rest
• kidneys only form a 0.5% of body weight
• portal system where glomerular capillaries join to peritubular capillaries
• makes both ACE and renin, which have endocrine functions, control blood volume and respond to renal blood pressure

Question 3

what are the special aspects of skeletal muscle circulation? 5

Answer 3

• androgenic input leads to vasodilation
• can use 80% of cardiac output during strenuous exercise
• 40% of adult body mass
• major site of peripheral resistance
• muscle pump augments venous return

Question 4

what are the special aspects of skin circulation? 4

Answer 4

• role in thermoregulation- perfusion can increase 100x
• arteriovenous anastomoses have a primary role in thermoregulation
• sweat glands have a role in thermoregulation, and produce plasma ultrafiltrate
• response to trauma= red reaction, flare, wheal

Question 5

what are the events in the cardiac cycle? 4

Answer 5

• ventricular filling
• isovolumic ventricular contraction
• ejection
• isovolumic ventricular relaxation

Question 6

what does isovolumic mean?

Answer 6

the volume of the ventricle doesn’t actually change

Question 7

where do the ECG waves come in relation to the ventricular pressure changes?

Answer 7

• precede them

Question 8

what is the isovolumic ventricular contraction affected by?

Answer 8

preload

Question 9

what is the ejection affected by?

Answer 9

afterload

Question 10

what is the isovolumetric ventricular relaxation affected by?

Answer 10

afterload

Question 11

what happens to the cardiac cycle during mitral stenosis? 2

Answer 11

• decreased preload

- decreased afterload

Question 12

what happens to the cardiac cycle during aortic stenosis?

Answer 12

• increased afterload

Question 13

what happens to the cardiac cycle during mitral regurgitation?

Answer 13

• increased preload

- decreased afterload

Question 14

what happens to the cardiac cycle during aortic regurgitation?

Answer 14

• increased preload

Question 15

what is the general rule for a systolic murmur? 2

Answer 15

• fluid leaves the ventricle

- AV regurgitation or SL (semilunar) stenosis

Question 16

what is the general rule for a diastolic murmur? 2

Answer 16

• fluid enters the ventricle

- AV stenosis, Sl regurgitation

Question 17

name 2 types of K+ channels? 3

Answer 17

• delayed rectifier K+ channels = open when the membrane depolarises, but all gating takes place without a delay
• inward rectifier K+ channels= open when Vm goes below -60mV, clamps the membrane firmly at rest

Question 18

explain the initial depolarisation of an AP? 2

Answer 18

• inward rectifier K channels open, K flows out

something causes the cell to become less negative

Question 19

explain the positive feedback of depolarisation? 4

Answer 19

• causes a few Na+ channels to open
• the additional current of Na+ leads to more depolarisation
• positive feedback loop
• once the voltage goes above the threshold (-50mV) the cell is committed to an AP

Question 20

explain repolarisation? 4

Answer 20

• voltage becomes less positive on the inside of the cell
• delayer NA+ channel inactivation
• Delayed rectifier K+ channels open
• this causes the membrane to be less positive and more negative inside

Question 21

explain the refractory period? 3

Answer 21

• period of time when the neuron is incapable of retaining an AP
• occurs mostly during hyperpolarization
• if a functional description

Question 22

explain hyperpolarisation? 2

Answer 22

• at the end of an AP, the voltage inside temporarily goes more negative than at rest followed by a return to the resting membrane potential
• when the voltage goes below -60mV, the inwards rectifier K channels open again and stay open until the next depolarisation

Question 23

describe the phases of the ventricular myocyte action potential? 5

Answer 23

• phase 0= depolarisation: Na+ gates open in response to a wave of excitation from the pacemaker
• phase 1= transient outward current: tiny amount of K+ leaves the cell
• phase 2=plateau phase: inflow of Ca2+ just about balances outflow of K+
• phase 3= rapid repolarization phase: Vm falls as K+ leaves the cell
• phase 4= back to resting potential

Question 24

compare neuron, skeletal muscle and cardiac action potentials? 3

Answer 24

• neurons= 1ms, always the same
• skeletal= AP completed before contraction begins, short refractory period means that repeated AP can cause tetany
• cardiac= much longer, up to 500ms, varies in duration and size, long refractory period so no tetany

Question 25

describe the plateau phase? 3

Answer 25

- dynamic equilibrium of Ca2+ in and K+ out - decreased Vm leads to decreased ca2+ current, not as much decrease for K+ - as there is a decrease in Ca2+ current, positive feedback causes repolarization by K+

Question 26

what determines the ECG recording?

Answer 26

- the cardiac AP varies in timing and shape in different regions of the heart

Question 27

describe the shape and timing of cardiac action potentials? 5

Answer 27

- SA node= pacemaker - AV node and bundle of His= potential pacemakers in case of atrioventricular conduction failure - QT interval aligns with ventricular AP - QRS= ventricular depression - T= ventricular repolarization

Question 28

describe the ionic basis for an action potential in ventricular myocytes? 4

Answer 28

- at rest, the inward rectifier K+ channel has an outward current which stabilizes the membrane (phase 4) - the rapid rising phase of the action potential is, exactly as in the nerve and skeletal muscle, due to a transient increase in inward Na current (with positive feedback- phase 0) - depolarisation also leads to transient opening of time and voltage dependent CA2+ channels (phase 2) - the total K+ conductance decreases rather than increases upon depolarisation

Question 29

explain AP in the SA node and the AV node? 4

Answer 29

- at rest it spontaneously depolarises, not stable at rest because there is no inward rectifier - the upstroke of the AP is due to a transient increase in inward CA2+, not due to NA+. nodal upstroke is slower than ventricular myocytes - the K+ conductance increases shortly after depolarisation which initiates repolarization - duration of nodal AP is around 300ms

Question 30

explain the automaticity of the SA node? 3

Answer 30

- the SA cells are autorhythmic, so resting potential is unstable and close to the threshold - cells independently beat at 100bpm, which can be increased by symp activity and decreased by parasymp - SA node is normally the pacemaker as it has the fastest rate

Question 31

explain pacemaker potential? 4

Answer 31

- in myocytes of the SA node, AV node and conduction system only - voltage drifts positive between nodal beats instead of resting potential because the cells lack inward rectifiers - the slope of the PP determines the rate of this firing - also called the diastolic potential

Question 32

explain the If (funny current)?5

Answer 32

- If makes the SA node cells spontaneously active - driven by HCN channel (responsible for Na+ in and K+ out) - increases upon hyperpolarization rather than depolarisation - leads to ent inward current, a lot of Na+ inwards - depolarises cell towards mV

Question 33

explain blocking ion channels of the cardiac AP? 3

Answer 33

- only block a % of channels otherwise you would kill the patient - Na+ channel block leads to a decrease in conduction velocity, changing the organisation fo firing in different regions of the heart, preventing arrhythmias, does not prevent depolarization of decrease HR - Ca2+ channel block can decrease the heart rate and contractile force