Events in the cardiac cycle

Question 1

Transmural pressure

Answer 1

• Transmural pressure: the pressure exerted by blood on the wall of the vessels minus the pressure exerted by ISF on the wall of the vessels
• If the transmural pressure is positive the vessel expands, if it is negative the vessel collapses

Question 2

Resistance to flow 1

Answer 2

• Governed largely by vasoconstriction of arterioles
• Resistance is inversely proportional to diameter to the 4th power, thus as diameter is increased a little the resistance drops dramatically (and vice versa)
• The arterial resistance (Ra) and the venous resistance (Rv) can be used to calculate the capillary resistance (Rp) btwn them

Question 3

Resistance to flow 2

Answer 3

• The ratio Ra/Rv predicts what happens to capillary resistance
• If Ra/Rv increases (Ra goes up) then Rp will fall
• If Ra/Rv decreases (Ra goes down) then Rp will rise
• Mean arterial BP is equal to CO (cardiac output) times TPR (total peripheral resistance: MAP = COxTPR

Question 4

Events of the cardiac cycle: atrial activation (late diastole)

Answer 4

• Atrial activation (seen as a P wave) originates in the SA node and spreads to both atria, causing them to contract
• The depolarization travels from the atria thru internal pathways to the AV node where there is a delay
• There is not a significant amount of blood forced into the ventricle during atrial contraction
• This is because after systole there is a large volume of blood backed up in the atria which enters the ventricles as soon as the AV valves open
• It is this blood that represents the larges quantity of blood flowing into the ventricle

Question 5

Ventricular activation 1

Answer 5

• This process is seen as a QRS complex on an ECG
• As the ventricles fill during diastole, the pressure rises
• Right after atrial contraction there is maximum blood volume in the ventricles, such that the ventricular pressure exceeds atrial pressure
• This pressure difference causes the AV valves to close (papillary muscles keep these valves closed during systole)

Question 6

Ventricular activation 2

Answer 6

• AV valve closure is timed with the beginning of ventricular activation, in which the depolarization from the AV node spreads down the bundle of his to the right and left bundle branches to the apex
• Bundle branches branch into purkinje fibers which supply the ventricular muscle with the depolarization
• The depolarization causes the cardiac muscle to contract (isovolumetric), which leads to opening of the semilunar valves (from increased pressure) and ejection of blood (systole)

Question 7

Ventricular activation 3

Answer 7

• The volume of blood left in the ventricle after systole is the end systolic volume (ESV, min blood volume of ventricle) and the volume of blood in ventricles at the end of diastole is EDV (max blood volume of ventricles)
• Stroke volume (SV) is EDV-ESV
• Important to note that btwn closing the AV valves (ventricular pressure>atrial pressure) and opening of semilunar valves (ventricular pressure>arterial pressure) there is isovolumetric contraction of the cardiac muscle (EDV left in ventricles but both valves closed)

Question 8

QRS complex

Answer 8

• Q= depolarization of the IV septum, going from left side to the right
• Seen as a small downward deviation on lead 1 of ECG
• R= depolarization of ventricles, going from right to left
• Mostly dominated by left ventricle depolarization since it has more mass
• Seen as a large upward spike on lead 1
• S= depolarization of base of heart (right ventricle)
• The end of right ventricular depolarization (from left to right) can be seen as a slight downward deviation on lead 1

Question 9

Ventricular repolarization vs depolarization 1

Answer 9

• Important to note the directions of depolarization and repolarization
• Depolarization begins within the endocardium and goes outward to the epicardium (thus the positive end of the dipole is on the outside)
• Repolarization begins on the outside (epicardium) and goes inward to end within the endocardium
• Thus the positive dipole is also on the outside

Question 10

Ventricular repolarization vs depolarization 2

Answer 10

• Since the outside is last to depolarize and first to repolarize, the positive dipole doesn’t change from depolarization to repolarization
• This means that the depolarization (R) will be in the same direction as repolarization (T) (direction of the deviation depends on the orientation of the positive dipole relative to the positive end of the lead)
• A more paralel (+)dipole/(+)lead relationship leads to a larger spike in the positive direction

Question 11

Repolarization of ventricles (early diastole)

Answer 11

• Represented as a T wave on ECG (always should be in same direction as R in normal heart)
• Begins with closure of semilunar valves after enough blood is ejected from ventricles to make ventricular pressure less than arterial pressure
• At this point the AV valves open and all of the built-up blood from the atria during systole is dumped into the ventricle
• After the atrial blood is rapidly moved to the ventricle, early diastole ends and late diastole begins
• During isovolumetric relaxation both valves are closed

Question 12

Important calculations

Answer 12

• SV= EDV-ESV
• CO= SVxHR
• Pulse pressure= systolic BP-diastoic BP

Question 13

Components of heart sounds

Answer 13

• Lub: due to closure of AV valves and ejection of blood from ventricle
• Dub: due to closure of semilunar valves
• Mitral valve: AV valve on left side, 2 cusps (only valve w/ 2 cusps)
• Tricuspid: AV valve on right side, 3 cusps