Electric + mechanical events of heart (wk6)

Question 1

What initiates a heart beat? (the circuitry of the heart)
-The SA node and pacemaker potentials

Answer 1

-The sinoatrial (SA) node has an intrinsic rate of approximately 100 beats per minute and can initiate cardiac contraction in the absence of any external control (hormonal or nervous)
-Create own action potentials. Can fire on rhythmic bases.

Question 2

What initiates a heart beat?
-Regulation of pacemaker potentials in the heart

Answer 2

-SA node cells initiate an action potential due to a unstable membrane potential that is continuously drifting towards threshold. -> The rate of firing depends on 1. The initial value of the membrane potential, 2. The slope of the drift towards the threshold.
-Sloping potential means that the action potential will fire and the slope means its inevitable. It fires without any external influence and fires due to the presence of sodium.
-Steeper the slope -> the faster the drift towards the threshold and the activation of the action potential
-Norepinephrine acts on the sodium channels which act on the depolarization of the sodium channels

Question 3

What initiates a heart beat?
-Regulation of pacemaker potentials in the heart (sympathetic and parasympathetic stimulation)

Answer 3

-Sympathetic stimulation to the SA-node leads to an increase in the slope of the drift due to an increase of the permeability of the Na +f current.
-Parasympathetic stimulation to the SA-node leads to a decrease in the slope of the drift due to an overall decrease of the inward current, and to the hyperpolarisation of the membrane due to increased K+ permeability.

Question 4

What stimulates the heart?
-The spread of the electrical signal across the heart

Answer 4

-Stimulation in the atria spreads from fibre to fibre through the gap junctions. Ventricular stimulation spreads initially via a network of conduits which begins at the atrioventricular (AV node) and terminates at cardiac muscle fibres via the Purkinje fibres)
-SA node -> atria -> AV node -> Bundle of His -> process of ventricles contracting

Question 5

The Electrocardiogram (ECG)
-Electrical events

Answer 5

1. Stimulus spreads through the muscle fibres and stimulates both atria almost simultaneously
2. The AV-node and the Bundle of His are the only pathway of the electric stimulus to travel from the atria to the ventricles and the signal is delayed at the AV-node in order to allow the atria to empty before ventricular contractions begin
3. The signal travels down the interventricular septum via the two bundle branches
4. The ventricles begin to contract as the stimulus spreads upwards depolarising muscle fibres via the Purkinje fibres
   -SA node -> atria (contraction of top part of the heart) -> AV node (slows down propagation of signal + stimulus reaches SA node where there is then a delay) -> Bundle of His (insulating layer) -> process of ventricles contracting (propagating into ventricular side of the heart)

Question 6

The Electrocardiogram (ECG)
-Magnitude of heart and different wave meanings

Answer 6

-The magnitude of the heart can tell about the state of the heart due to the electrical events that take part
-ECG can measure HR due to appreciation of the electrical events that take place in the heart
-Atrial excitation -> smaller amount of electrical activity - known as the P wave
-Ventricular contraction -> Q,R,S wave. There s an AV node delay near the Q wave which allows the ventricles in the heart to fill with blood, meaning that there is blood to eject when the heart contracts
-Ventricular relaxation -> T-wave. Involved in the relaxation of the heart

Question 7

The Electrocardiogram
-Uncoordinated heartbeat

Answer 7

-Tachycardia -> rapid heart beat (>100 beats/min)
-Bradycardia -> low heart rate (<60 beats/min)
-Ischemia -> lack of adequate blood supply to heart muscle, causing a shortage of oxygen
-Fibrillation -> rapid uncoordinated shuddering of heart muscle (a major cause of death from adult heart attacks)

Question 8

What causes valves to open and close (valve operation)
-Tricuspid and pulmonary valve

Answer 8

-Tricuspid valve -> Separates R atrium and R ventricle. Open when R atrial pressure is greater than R ventricular pressure. Closed when R atrial pressure is less than R ventricular pressure.
-Pulmonary valve -> Separates R ventricle and pulmonary artery (PA). Open when R ventricular pressure is greater than PA pressure. Closed when R ventricular pressure is less than PA pressure.

Question 9

What causes valves to open and close (valve operation)
-Mitral (bicuspid) valve and aortic valve

Answer 9

-Mitral -> Separates L atrium and L ventricle. Open when L atrial pressure is greater than L ventricular pressure. Closed when L atrial pressure is less than L ventricular pressure.
-Aortic valve -> Separates L ventricle and aorta. Open when L ventricular pressure is greater than aortic pressure. Closed when L ventricular pressure is less than aortic pressure.

Question 10

Mechanical events of systole and diastole
-Systole

Answer 10

-Systole -> Contraction of the heart. Beginning of phase, muscle is contracting but volume of blood in the heart stays the same and there are minimal differences (same in diastole beginning)
-Before ventricular ejection (isovolumetric ventricular contraction) -> AV valves and aortic and pulmonary valves closed
-During ventricular ejection (blood flows out of ventricles) -> AV valves closed and aortic and pulmonary valves open. Semilunar valve is open and blood is allowed to flow through the vessels in the heart

Question 11

Mechanical events of systole and diastole
-Diastole

Answer 11

Isovolumetric ventricular contraction -> AV valves and aortic and pulmonary valves closed. Semilunar valves close. All 4 valves close.
Ventricular filling (before atria contracts) -> AV valves open and aortic and pulmonary valves closed. Blood passively falls into the ventricles due to the gravity. The atrioventricular valves are big due to the aspect of gravity, which coincidentally allows lots of blood to fall through.
Ventricular filling (atria contracts) -> AV valves open and aortic and pulmonary valves closed

Question 12

The relationship of electrical activity, pressure, and volume during the heart cycle:

Answer 12

-Left side of the heart:
* Electrical activity – green line
* Pressure - Atrial pressure blue line, ventricular pressure orange line. When the v pressure is lower than the atrial pressure, the AV valves open and the heart fills with blood. Aorta pressure is greater than the ventricular pressure at stage 11-19. The aorta opens and blood pours out and exits the heart.
* Volume
* Heart sounds
-Focus on filling and heart with blood as key principles with cross-overs between aorta pressure and ventricular pressure etc. Every time there is a switch between pressures, valves either open or close.

Question 13

Pressure-volume curve in the heart
-Refer to diagram (02/11)

Answer 13

A – Atrium and ventricles relax. The mitral valve opens. (left to right, heart is filling. Right to left, heart is reducing in terms of blood)
B – Ventricles begin to contract and mitral valve shuts.
C – Ventricle pressure exceeds aortic pressure. Aortic valve opens.
D – Ventricles begins to relax, pressure falls below aortic pressure. Aortic valve shuts.

D-> A – Isovolumetric relaxation
B-> C – Isovolumetric contraction