S2: Heart - Cardiac Cycle

Question 1

Why do we need a CVS?

Answer 1

We need a CVS because passive diffusion is fast over short distances (less than 1mm) but very slow over longer distances (over 1mm) so is useless for body O2 transport.
So we use passive diffusion to transport oxygen at the lungs and at the capillaries (where we also transport nutrients and other substances).

Question 2

What is convection transport?

What organ aids this?

Answer 2

Convection is the “mass movement of fluid caused by pressure difference” and is fast over long distances.
- This helps get O2 to tissues quickly and remove CO2 quickly

The heart uses a lot of energy to create blood at high pressure and therefore create a pressure difference, with the arteries under high pressure and veins delivering blood back to the heart under low pressure. The pressure difference between these is what drives blood flow.

Question 3

What is the role of heart, arteries, capillaries and veins?

Answer 3

Heart – Is the driving force, creating large pressures
Arteries – Involved in distribution of blood and alter blood flow
Capillaries – Involved in exchange, are present in huge numbers and only one cell thick
Veins – Act as a reservoir, 2/3rds of blood volume, we can access if needed (e.g. during exercise)

Question 4

What are the phases of pacemaker potentials in SAN and AVN?

Answer 4

Phase 4: The funny current If channels activated by hyperpolarisation. There is an inward diffusion of Na+ ions causing slow depolarisation and an unstable resting membrane potential.

Phase 0: Depolarisation by the activation of voltage gated Ca2+ channels causing Ca2+ influx

Phase 3: Repolarisation where there is activation of voltage gated K+ channels causing K+ efflux

Question 5

What are the phases of the atria/ventricular action potentials?

Answer 5

Phase 0: Rapid depolarisation caused by VgNA+ channels. VGCC start to open slowly

Phase 1: Early repolarisation where Na+ channels close

Phase 2: VGCC fully open and there is Ca2+ influx for sustained depolarisation. VGK+ channels slowly open.

Phase 3: Rapid repolarisation where VGCCs close and VgK+ channels open fully causing K+ efflux.

Phase 4: This is the resting phase with the Na+/K+ pump. Membrane impermeable to Na+, membrane permeable K+.

Question 6

Explain electrical conduction through the heart

Answer 6

First electrical activity is generated in the SAN which spreads out via gap junctions causing the atria to contract simultaneously.

Electrical impulses reaches AVN (which is fibrous and insulating) it causes a delay which allows correct filling of ventricles.

Then there is conduction which occurs rapidly through the bundle of His into the ventricles. Finally conduction through the Purkinje fibres spreads rapidly throughout the ventricles causing contraction.

Contraction begins at the apex of the heart.

Question 7

What can represent the electrical activity and conduction of the cardiac cycle?

Answer 7

Electrocardiogram (ECG)

Question 8

Describe a ECG wave

Answer 8

P wave = atrial depolarisation
PR segment = AV node delay
QRS complex = Ventricular depolarisation and contraction
ST segment = Time during which ventricles are contracting and empty
T wave = Ventricular repolarization
TP interval = Time during which ventricles are relaxing and filling

Question 9

General principles of the heart

Answer 9

Electrical activity is generated at the sino-atrial node and conducted throughout the heart. The electrical activity is then converted into contraction of the myocardium (muscle) which creates pressure changes within chambers.
Blood flows through the heart from high pressure to low pressure, unless flow is blocked by a valve. Valves open and close depending on pressure changes in the chambers. When pressure upstream of the valve is higher, it will cause the valve to open, when the pressure downstream the valve is higher, it will cause the valve to close.

Events on the right and left sides of the heart are the same, but pressures are lower on the right. The let side of the heart needs to be under higher pressure as it needs to pump blood around the whole body, the right side of the heart is under lower pressure enabling better oxygen exchange at the lungs. The heart is a dual-circulatory system, what is happening on the right side of the heart is happening on the left and the two must be in balance. If not in balance (congestion), then this causes problems.

Question 10

Path of blood through heart

Answer 10

Blood enters right atrium from SVC and IVC -> Tricuspid valve -> Right ventricle -> Pulmonary semilunar valve -> pulmonary arteries -> Lung circulation -> Pulmonary veins -> Left atrium -> Mitral (bicuspid) valve -> Left ventricle -> Aortic semilunar valve -> Aorta -> Systemic circulation

Question 11

Describe the cardiac cycle (4)

Answer 11

1. It starts off with diastole (second part) where you have the ventricles being filled and atrial contraction.
2. Then systole (part 1) occurs where there is isovolumetric contraction
3. Then part 2 of systole where ventricular contraction and ejection occurs and blood is pushed out of the ventricles into the arteries. At the same time atrial filling occurs.
4. Finally there is the first part of diastole where there is ventricular isovolumetric relaxation.

Question 12

Describe ventricular filling/ atria contraction

Answer 12

Blood is returning to the heart into the atria. The tricuspid and mitral valves are open so blood flows into the ventricles (general filling). This occurs because the pressure in the atria (returning venous blood) is greater than the ventricles which are relaxing and getting bigger. Eventually the ventricles reach maximum relaxation but continue filling up with blood. However, the pressure in the ventricles increases which would reduce pressure difference and could stop blood flow. So to aid the movement of blood into ventricles, there is atrial contraction.

Question 13

Describe isovolumetric contraction

Answer 13

The ventricles are now full and under high pressure - greater pressure in ventricles than atria which leads to closure of the tricuspid and mitral valves.
The aortic/pulmonary valves are also closed so the ventricles are closed chamber. There is then contraction of closed chambers so pressure in the ventricles is greater than the aorta/pulmonary artery so the valves will open.

It is therefore called isovolumetric contraction as volume doesn’t change but due to contraction the pressure increases.

Question 14

Describe Ejection

Answer 14

Very quickly, the pressure rises in ventricles and gets above that in the aorta/pulmonary artery, therefore the aortic/pulmonary valve opens. This allows blood to move out of the heart into the pulmonary/systemic circulation, due to the great pressure difference between the ventricles and arteries.
You also have blood now entering the atria, ready for the next beat

Question 15

Describe isovolumetric relaxation

Answer 15

Ejection continues to happen, but as the blood leaves, pressure goes down and eventually pressure in ventricles is less than in the arteries and kinetic energy of blood is lost and the semilunar valves close. So now you have the case where the semilunar valves are closed and the tricuspid and mitral valves are closed, again you have a closed chamber but with far less volume. You then get relaxation, pressure decreases, pressure in atria is increasing and then blood starts to flow again into the ventricle once pressure in atria is greater and cuspid valves open.

Question 16

Describe left ventricular volume changes in a cardiac cycle using a graph

Answer 16

• Red line is the volume of blood in the ventricles, black line is the velocity of blood going through the aorta.
• Start: Just finished off filling the ventricles and we get atrial contraction. What’s left is the final ventricular volume known as the ‘End diastolic volume (EDV)’. It is the volume of blood in the ventricles at the end of diastole, about 120 mm. The pressure in the ventricles is greater than in the atria so tricuspid and mitral valves close.
• Then isovolumetric contraction occurs (volume in ventricle doesn’t change but the pressure is increasing). The aortic valve opens at the end of isovolumetric contraction and ejection of blood from the heart occurs.
• There is rapid ejection at the start due to the huge pressure difference between ventricles and arteries seen through the massive spike in aortic velocity.
• Over time, there is a slowing of ejection as we start to get pressure in the arteries greater than in the ventricles so ejection slows. Two phases of ejection: the rapid and slow phase (aorta can expand and recoil).
• In the end, we reach the point where the aortic valve closes, this corresponds to a bit of aortic backflow that helps close the aortic valve (this is seen as the dip in the aortic velocity). The volume we are left with in the ventricle is the End Systolic Volume (40ml), so not all the blood is ejected.

Question 17

What is EDV-ESV?

Answer 17

SV= EDV -ESV. This is usually around 80ml.

Question 18

What is SV/EDV?

Answer 18

Ejection fraction (EJ) = SV/EDV. The normal value is about 2/3 or more (so usually about 2/3 of blood is pumped out) and lower values indicate heart failure.

Question 19

Describe left ventricular pressure changes through the cardiac cycle using a graph

Answer 19

• Black at top is aortic pressure and below is atrial pressure. Red is left ventricular pressure.
• Initially pressure in ventricles is low and it starts to increase as the atria contracts and pushes blood into the left ventricle. Eventually pressure is greater than in the atria and the mitral valve closes. We are left with EDV in a closed chamber.
• Then there is isovolumetric contraction and pressure rapidly starts to increase due to contraction in a closed chamber.
• It then becomes greater than pressure in the aorta, the aortic valve opens and this signals the end of isovolumetric contraction (as no longer a closed chamber), now we are ejecting the blood. During ejection, the pressure increases initially as we are still contracting, we have the rapid phase of ejection and then the slow phase as pressure decreases in the ventricle.
• In fact the ventricular pressure drops below that of the aorta (where red line dips under the black), but the valves don’t close yet. This is because the blood flow as momentum/kinetic energy, it may not be under the high pressure but it still has a high enough momentum to get out (although slower).
• Eventually however we lose this kinetic energy and the aortic valve closes (we get incisura, small increase in pressure as valves close), now we have the ESV, we have a closed chamber so we have isovolumetric relaxation. The pressure drops rapidly (red line falling rapidly). It eventually falls below that in the atria and the mitral valve opens and blood (that has been flowing into the atria) starts to flow into the left ventricle and it starts refilling.

Question 20

What two factors decide what closes the aortic valve?

Answer 20

• Momentum of blood.

- Pressure.

Question 21

Describe a ventricular pressure-volume loop

Answer 21

• If we start at where the mitral valve opens, we now have the filling phase (which we can see along the bottom). When mitral valve opens we have the ESV and this volume starts to increase as blood flows into the ventricles.
  We still have the decrease in pressure in ventricles (red line dips down still) because they are still relaxing, however relaxation then finishes and we are filling up the left ventricle that is now at a set volume. The pressure starts to increase, so that the pressure in the left ventricle is now greater than in the atria and the mitral valve closes. We have EDV (120ml).
• We now have a closed chamber and get isovolumetric contraction, the volume doesn’t change but the pressure increases rapidly as we contract on the closed chambers.
  Eventually pressure in left ventricle is higher than in aorta, aortic valve opens, we get ejection phase now. There is a rapid phase of ejection, volume of blood decreases rapidly and then a slower phase as pressure starts to decrease.
  Eventually the aortic valve closes, now we have the closed chamber again and are at our ESV.
• We get isovolumetric relaxation, the volume (ESV) stays the same but pressure decreases rapidly and eventually it becomes lower than the atria and mitral valve opens and cycle starts again.

Question 22

How can the ventricular volume-pressure loop tell us about ‘work’?

Answer 22

• Work = change in ventricle pressure x change in volume.
• So if we look at the area of the loop, which encompasses the total change in pressure and volume during one cardiac cycle, this gives us the amount of stroke work.
• The stroke work tells us the amount of energy used (oxygen used) in order to produce this cardiac cycle.
  Hence we can look at the amount of oxygen/energy we are using in comparison to how much stroke volume we are getting.
• If we are putting in a lot of work and getting small stroke volume, this isn’t good and is seen in heart disease, where the heart has to work harder for less stroke volume.

Question 23

Describe atria and jugular venous pressure changes

Answer 23

• We can see at first it is under a lot lower pressure compared to the ventricles and that it is made up of a number of phases.
• We have the A wave first which is an increase in venous pressure caused by atrial contraction. Then we get the AV valves closing, pressure starts to reduce but as the valves swing back and close they give a brief increase in pressure in the great veins, this is called “C”.
• Now we have “X” wave, this a big drop in atrial/venous pressure caused by contraction in the ventricles, leading to atria getting larger and reducing pressure in the atria. This is good because you want the atria to fill.
• The drop in pressure is followed by an increase as blood starts to flow into the atria, this is the “V” wave. Pressure returns enough that the AV valves start to open, blood leaves the atria and enters into the ventricles so pressure decreases.
• You get atrial contraction and pressure increases again, back to “A” wave.
• This is useful in clinical examination as you see a persons jugular contracting and relaxing, corresponding to the X and Y decreases in pressure and A and V increases.

Question 24

What are the 4 types of heart sounds?

Answer 24

S1- lubb
S2 - dupp
S3 - Occasional
S4 - Pathological in adults

Question 25

What causes heart sounds?

Answer 25

The heart sounds are caused by vibrations induced by closure of the cardiac valves. You may also hear other sounds, like vibrations in the ventricular chambers or turbulent flow through the valves.

Question 26

What are S1 heart sounds?

Answer 26

They are caused by the closure of the tricuspid/mitral valve and denote the beginning of ventricular systole.

Question 27

What are S2 heart sounds?

Answer 27

They are caused by closure of the aortic/pulmonary valves, denotes end of ventricular systole and beginning of ventricular diastole.

Question 28

What are S3 heart sounds?

Answer 28

They are often seen in young adults, caused by turbulent blood flow into the ventricles, detected near the end of first 1/3 of diastole.

Question 29

What are S4 heart sounds?

Answer 29

Found later on in diastole, you have forceful atrial contraction but you have a stiff ventricle caused by old age. So you have blood entering into a stiff ventricle that isn’t compliant, causes a turbulent sound.