The Cardiac Cycle Flashcards
As a recap, what is convection and what are the main functions of the heart, arteries, capillaries and veins in the context of circulation?
Convection is the mass movement of fluid caused by pressure differences.
HEART: driving force (creates large pressures)
ARTERIES: distribution (mostly in parallel to alter blood flow)
CAPILLARIES: exchange (found in huge number, thin for ease of movement)
VEINS: reservoir (2/3rd of the blood volume stored in veins and venules)
Describe the Sinoatrial Node (SAN) and its functions.
The SA node is a group of cells located in the walls of the right atrium.
It has the ability to spontaneously produce action potentials that travel through the heart via the electrical conduction system.
It sets the rhythm of the heart, and so is known as the heart’s natural pacemaker.
The rate of action potential production (and therefore the heart rate) is influenced by nerves that supply it.
Describe the Atrioventricular Node (AVN) and its functions.
The AV node is a part of the electrical conduction system of the heart that coordinates the top of the heart.
It electrically connects the right atrium and the right ventricle, delaying impulses
So that the atria have time to eject their blood into ventricles before ventricular contraction.
Describe the phases of the SAN pacemaker potentials.
PHASE 4: PACEMAKER POTENTIALS - “funny current (If)”
At the end of an SA action potential, the membrane repolarises below the If threshold (approximately -40/50 mV), then the funny current is activated and supplies inward current.
If-hyperpolarisation activates Na+ channels, causing Na+ influx - a slow depolarisation that is responsible for starting the diastolic depolarisation phase.
PHASE 0: VOLTAGE-GATED CA2+ CHANNELS
As the cell depolarises, it reaches a threshold for voltage-gated Ca2+ channels leading to a Ca2+ influx, making the inside less negative. This causes RAPID depolarisation. Voltage-gated Na+ channels are not involved as in normal depolarisation.
PHASE 3: REPOLARISATION
The calcium channels switch off. There is also activation of voltage-gated K+ channels, causing a K+ efflux, repolarising the cell.
Describe the phases of atrial/ventricular action potentials.
PHASE 0: RAPID DEPOLARISATION
Voltage-gated Na+ channels open, causing a Na+ influx. Voltage-gated Ca2+ channels start to open slowly.
PHASE 1: EARLY REPOLARISATION
Na+ channels close. Beginning to re -polarise
PHASE 2: PLATEAU PHASE
Voltage-gated calcium channels are now fully open, causing a Ca2+ influx. The voltage-gated K+ channels start to open slowly.
PHASE 3: RAPID REPOLARISATION
Voltage-gated calcium channels close. K+ channels open fully, causing a K+ efflux.
PHASE 4: RESTING PHASE
The Na+/K+ pump works to get Na+ out and K+ in. The membrane is impermeable to Na+ but permeable to K+, which helps establish the equilibrium.
Describe the electrical conduction through the heart.
1) Electrical activity generated in the SA node spreads out via the gap junctions into the atria.
2) At the AV node, conduction is delayed to allow the correct filling of the ventricles.
3) Conduction occurs rapidly through the bundle of His into the ventricles.
4) Conduction occurs through the Purkinje fibres and spreads quickly throughout the ventricles.
Ventricular contraction begins at the apex.
There are many parts to an ECG. List what is represented by the P wave, PR segment, QRS complex, ST segment, T wave and TP interval
P WAVE: atrial depolarisation
PR SEGMENT: AV node delay
QRS COMPLEX: ventricular depolarisation (atria repolarising simultaneously)
ST SEGMENT: time during which ventricles are contracting and emptying
T WAVE: ventricular repolarisation
TP INTERVAL: time during which ventricles are relaxing and filling
What are some general principles of the cardiac cycle regarding electrical conductivity, pressure and valves?
- electrical activity is generated at the SA node and conducted throughout the heart
- electrical activity is converted into myocardial contraction which creates pressure changes within chambers
- blood flows from an area of high pressure to low pressure, unless the flow is blocked (by a valve, for example)
- valves open and close depending on the pressure changes in the chambers
- events of the right and left sides of the heart are the same, but pressures are lower on the right
Describe the movement of blood through the heart by listing all the structures it goes through/past.
1) Venae Cavae to Right Atrium
2) Past the Tricuspid Valve
3) Into Right Ventricle
4) Past Pulmonary (Semilunar) Valve
5) Into Pulmonary Arteries
6) Goes through Lung Circulation
7) Comes back in Pulmonary Veins
8) Enters Left Atrium
9) Past the Bicuspid (Mitral) Valve
10) Into Left Ventricle
11) Past Aortic (Semilunar) Valve
12) Into Aorta
13) Into Systemic Circulation
Describe the cardiac cycle in terms of pressure and valve opening/closing.
1) VENTRICULAR FILLING/ ATRIAL CONTRACTION
Blood enters the atria and moves into the ventricles. The pressure in the atria is greater than in the ventricles, so the mitral/tricuspid valve opens, aided by atrial contraction.
2) ISOVOLUMETRIC CONTRACTION
The pressure in the full ventricles is greater than in the atria. This closes the mitral/tricuspid valves. There is a contraction of the closed ventricles, so the pressure rises.
3) EJECTION
The pressure in the ventricles is greater than in the aorta/pulmonary artery. This causes the aortic/pulmonary valves to open, ejecting the blood. Blood enters the atria.
4) ISOVOLUMETRIC RELAXATION
The pressure in the aorta/pulmonary artery is greater than in the ventricles, so the aortic/pulmonary valves close . The closed ventricles relax, ready to receive blood.
Describe the ventricular pressure-volume loop, and include the equation learnt for work.
Work = change in ventricle pressure x change in volume
The loop relates to the amount of energy consumption during the cardiac cycle. The area inside the loop is equal to the amount of stroke work done.
Describe heart sounds, and list the four of them.
They are vibrations in ventricular chambers induced by the closure of cardiac valves or by turbulent blood flow through valves.
S1 - “LUB”
Closure of tricuspid/mitral valves at the beginning of ventricular systole
S2 - “DUB”
Closure of aortic/pulmonary valves (semilunar valves) at the end of ventricular systole
S3 - OCCASIONAL
Turbulent blood flow into ventricles, detected near the end of first 1/3 diastole, especially in older people
S4 - PATHOLOGICAL IN ADULTS
Forceful atrial contraction against a stiff ventricle, less so in young people
Describe the Left Ventricular PRESSURE Changes
- Contraction of the left atrium - during ventricular diastole - ventricular pressure rises slightly
- Pressure Rises during isovolumetirc contraction
- When Ventricular pressure > Aorta
The aortic valve opens and blood is ejected - Ventricle empties and ventricular pressure < Aortic valve
there fore the valve closes and we get isovolumetric relaxation
Large pressure drop below that of atrium
mitral valve opens
Describe the Left Ventricle VOLUME Changes
- Filling ventricle due to contraction of atria - 120ml
- Full ventricle has higher pressure therefore closes the mitral valve
Systole begins - no change in volume - Ventricular pressure overcomes Aortic Valve and blood is ejected
- When ventricular pressure falls the aortic pressure closes aortic valve - Isovolumetric relaxation
Describe stages A-D in ventricular pressure volume loop
A - Diastole, ventricle relaxed and filling so pressure remains low but volume increases
B - mitral valve closes and ventricle contracts - volume doesn’t change but pressure increases since both mitral and aortic valves remain closed
C - pressure becomes high enough to open Aortic valve and blood begins to leave the ventricle. Volume decreases as pressure continues to rise because heart is still contracting
D - As ventricle empties the aortic valve closes and the ventricle relaxes so the pressure falls dramatically
