Cardiac Cycle

Question 1

what is the SAN and what is its role?

Answer 1

• a group of cells located in the wall of the RA.
• sets the rhythm of the heart and so is known the the hearts natural pacemaker
• it can spontaneously produce an AP that travels through the heart via the electrical conduction system.

Question 2

the rate of AP production, and therefore heart rate is influenced by what?

Answer 2

the nerves that supply it

Question 3

what is the AVN and what is its role?

Answer 3

• Part of the electrical conduction system of the heart that coordinates the top of the heart, connected by conductive fibres to the SAN.
• Electrically connects the right atrium and right ventricle, delaying the impulse/spread of depolarisation so that the atria have time to eject their blood into ventricles before ventricular contraction.
• conductive signal carried to the apex of the heart through the bundle of HIS and conductive fibres.

Question 4

what is the resting potential of SA cells and when will they depolarise? (salty banana analogy)
what voltage gated channels do they have?

Answer 4

With SA cells, Na+ conc high on the outside, and K+ conc high on the inside (3 Na out, 2 K in).

• the SAN has Na+/K+/Ca2+ voltage gated channels
• resting potential of -70mV but they are unstable, and depolarise at when voltage decreases to -40/-50 mV.

Question 5

3 SAN phases:

Answer 5

phase 4:

• without an external stimulus Na+ is trickling in, this is called the funny current.
• starts to depolarise

phase 0:

• when voltage is at -30/-40 mV, VGCC open, rapid influx of Ca2+, making the inside of the cell less negative.
• VGCC rapidly depolarises the cell, gives an AP across the cell
• membrane potential increased

phase 3:

• VGCC close and VGKC open due to increase in MP and depolarisation.
• repolarisation and K+ efflux
• membrane potential established

Question 6

Explain the phases of Atrial/Ventricular AP’s (different to the SAN ones):

Answer 6

phase 0 (rapid depolarisation):

• depolarisation stimulus received from SAN
• VGSC open, influx of Na+, rapid depolarisation
• VGCC open very slowly, some Ca2+ entering

phase 1 (early repolarisation):

• Na+ channels close, cells begin to repolarise
• cell starts to repolarise slightly but not fully as Ca2+ channels are opening slowly still, which halts repolarisaton
• Ca2+ starts to influx in, stopping inside from becoming too negative

phase 2 (plateau phase):

• CICR, high levels of calcium stimulates release of calcium from SR.
• VGCC open fully, Ca2+ influx halts repolarisation (prevents repolarisation by Na+/K+ pump)
• VGCK’s start to open slowly

phase 3 (rapid repolarisation):

• Ca2+ channels close and K+ channel open fully
• K+ efflux
• re-establishing the resting MP

phase 4 (resting phase):

• Na+/K+ pump, 3 Na+ out and 2 K+ in
• inside is -70mV compared to outside

Question 7

electrical activity is generated in the SAN, how does it spread and what happens?

Answer 7

electrical activity generated in the San spreads out via gap junctions into the atria, leading to atrial contraction and the pushing of blood into ventricles.

Question 8

why is conduction slightly delayed at the AVN, and what does the conduction occur down?

Answer 8

slightly delayed to allow correct filling of the ventricles, and conduction occurs rapidly down the bundle of HIS which is made up of Purkinje fibres. Spreads quickly through the ventricles.

Question 9

where does ventricle contraction begin?

Answer 9

ventricular contraction begins at the apex of the heart, as the atria contract the ventricles relax

Question 10

what is generated at the SAN?

Answer 10

electrical activity

Question 11

what is the electrical activity generated at the SAN converted into?

Answer 11

electrical activity is converted into myocardial contraction which creates pressure changes within chambers.

Question 12

what pressure is the aorta maintained at?

Answer 12

120/80

Question 13

summary of flow of blood in the heart?

Answer 13

Blood cell is heading back to the heart, enters vena cava and go into RA, which relaxes and then contracts, so blood cell moves through tricuspid valve into RV, contracts, pulmonary valve into pulmonary arteries, lungs, pulmonary veins, back into LA then into LV through bicuspid/mitral valve. Ventricles fill with blood and contract, through aortic valve into aorta.

Question 14

the opening and closing of valves depends on what?

Answer 14

pressure changes in the chambers

Question 15

relationship between blood flow and pressure:

Answer 15

blood flows from an area of high pressure to an area of low pressure, unless the flow is blocked by a valve.

Question 16

cardiac cycle, chamber and volume changes

Answer 16

1. DIASTOLE- ventricular filing/atrial contraction
   - blood enters atria, moves into ventricles
   - atrial pressure>ventricular pressure
   - mitral and tricuspid valves open, aided by atrial contraction
2. SYSTOLE- ventricular isovolumetric contraction
   - pressure in full ventricles>atria
   - mitral/tricuspid valves close
   - contraction on closed ventricle, so pressure rises
3. SYSTOLE- ventricular contraction and ejection/atrial filling
   - pressure in ventricles> pressure in aorta/pulmonary artery
   - aortic/pulmonary valves open, ejection of blood
   - blood enters atria
4. DIASTOLE- ventricular isovolumetric relaxation
   - pressure in aorta/pulmonary artery>ventricles
   - aortic/pulmonary valves close
   - close ventricle is ready to receive blood

Question 17

explain isovolumetric contraction:

Answer 17

• One-way valves close due to the high pressure in the ventricles.
• As the ventricles squeeze together, there is a period where the volume inside the ventricle isn’t changing, but due to more compression the pressure inside the ventricles starts to rise.

Question 18

where are the mitral valve and aortic valve located?

Answer 18

mitral valve is between the LA and LV

aortic valve is between the ventricle and the aorta

Question 19

describe the changes in pressure that occur in the LV:

Answer 19

• contraction of left atrium, ventricle is relaxed, atrium pressure higher than ventricle pressure, blood moves down
• ventricles fill up, pressure rises and when ventricle pressure>atrial pressure mitral valve closes, ventricles start to contract- isovolumetric contraction. Pressure increases a lot. Ventricle pressure>aortic pressure, aortic valve opens and blood ejected from the heart.
• ventricle starts to empty and pressure falls. When ventricle pressure drops below aortic pressure the aortic valves slam shut.
• ventricle is nearly empty and starts to relax, pressure drops so its lower than atrial pressure.
• mitral valves open again and empties blood back into the ventricles

Question 20

explain the changes in volume that occur in the LV:

Answer 20

volume goes down during systole, and increases during diastole

Question 21

ventricular pressure-volume loop:

• what does the area inside the loop indicate
• what is the “work”

Answer 21

• the area inside the loop is the amount of stroke work done/amount of work done in a heartbeat
• work= change in ventricle pressure x change in volume

Question 22

when does the amount of work done change?

Answer 22

• the amount of work done changes as you get older and with heart disease
• if the heart uses too much energy in order to be a normally you compromise how the heart works

Question 23

when does the loop change?

Answer 23

changes in exercise and also in heart disease

Question 24

the pressure in the vena cava is the same as the pressure in what?

Answer 24

the same as the pressure in the jugular vein which is same as the pressure in the right atrium

Question 25

what happens with congestive heart disease?

Answer 25

the jugular veins become distended when observing patients. Pressure on RHS of heart is high, bulging jugular vein.

Question 26

when talking about heart sounds, what are vibrations induced by?

Answer 26

vibrations are induced by closure of the cardiac valves

Question 27

what is the s1 phase?

Answer 27

the LUB phase

• ventricles squashing together
• closure of tricuspid/mitral valves at the beginning of ventricular systole, as ventricular pressure started to rise and atria empty

Question 28

what is the s2 phase?

Answer 28

the DUB phase

-closure of aortic/pulmonary valves at the end of ventricular systole

Question 29

what is the s3 phase?

Answer 29

the occasional one

• during diastole it can be heard, its a swooshy sound
• turbulent blood flow into ventricles detected near the end of the first 1/3 diastole, especially in older people

Question 30

what is the s4 phase?

Answer 30

it is pathological in adults

• happens when the ventricle doesn’t expand well
• forceful atrial contraction against a stiff ventricle, less so in young people

Question 31

explain the Lub dub sound:

Answer 31

due to closure of tricuspid valves as the ventricles start to contract and aortic pulmonary valves ventricles end the contraction.
-the pulse is the lub

Question 32

summary of cardiac cycle:

Answer 32

• atrial contraction
• isovolumetric ventricular contraction
• rapid ventricular ejection
• reduced ventricular ejection
• isovolumetric ventricular relaxation
• rapid ventricular filling
• reduced ventricular filling

Question 33

Stroke volume?

Answer 33

The amount of blood pumped by the LV in one contraction.