chapter 13 part 2

Cardiac Cycle

Includes all events associated with blood flow through the heart during one complete heartbeat.
Involves changes in pressure, blood flow, and valve functions.
Topics include the pumping action of the heart, valve opening and closing, atrial pressure, ventricular pressure, aortic pressure, and ventricular volume.

Major Phases

Systole: Ventricular contraction.
Diastole: Ventricular relaxation.
When systole and diastole are mentioned, it refers to ventricular activity.
Atrial contraction and relaxation are not the main events and aren't directly involved in pumping blood to tissues.
Diastole is longer, occupying 65% of the cardiac cycle, allowing the heart muscle to relax, ventricles to fill, and preventing fatigue.

Key Principles

Blood moves from high to low pressure areas.
Unidirectional flow is maintained by valves.
Valve opening and closing ensures one-way blood flow.
Ventricular contraction and relaxation drives pressure changes.

Heart Valves

Ventricular Relaxation: AV valves open, semilunar valves close, and ventricles fill with blood.
Ventricular Contraction: AV valves close to prevent backflow into atria, and blood is forced through open semilunar valves into the aorta or pulmonary trunk.
Ventricular Relaxation (Again): Semilunar valves close to prevent backflow into ventricles.

Specific Phases of the Cardiac Cycle

1. Ventricular Filling (Diastole)

Atrial pressure exceeds ventricular pressure.
Venous return: Blood returns from veins to atria, driven by higher pressure in veins than in atria.
Blood enters relaxed atria, AV valves are open, and blood passively flows into ventricles.
Semilunar valves are closed.
Late diastole involves atrial contraction, driving more blood into ventricles.
Some textbooks separate atrial contraction as a fifth phase.

2. Systole (Ventricular Contraction)

Two parts: Isovolumetric contraction and ventricular ejection.

Isovolumetric Contraction (Early Systole)

Ventricular pressure rises and exceeds atrial pressure, closing AV valves.
Pressure is not high enough to open semilunar valves.
All valves are closed, and ventricular volume is constant.
Ends when ventricular pressure opens semilunar valves.

Ventricular Ejection (Late Systole)

Ventricular pressure opens semilunar valves, and blood ejects into the aorta and pulmonary trunk.
Blood exiting causes ventricular pressure to drop.
Aortic pressure exceeds ventricular pressure, closing semilunar valves and concluding systole.

3. Isovolumetric Relaxation (Early Diastole)

Ventricular myocardium relaxes, and ventricular pressure is intermediate.
Semilunar valves are closed, but ventricular pressure is too high for AV valves to open.
Both sets of valves are closed, and ventricular volume is constant.
Ventricular pressure drops below atrial pressure, transitioning back to ventricular filling.

Ventricular Filling (Return)

Ventricular pressure drops below atrial pressure, and AV valves open.
Blood moves from atria to ventricles, passively filling them.

Additional Details on Cardiac Cycle Phases

Early systole involves isovolumetric contraction, where ventricles contract, increasing pressure above atrial pressure, closing AV valves, but pressure remains insufficient to open semilunar valves.
Later systole involves ventricular ejection; ventricles contract further, increasing pressure to open semilunar valves, ejecting blood into the pulmonary trunk and aorta.
Pressure drop in ventricles causes semilunar valves to close, transitioning to diastole.
Diastole begins with isovolumetric relaxation; semilunar valves are closed, and AV valves remain closed as ventricular pressure is still higher than atrial pressure.
Ventricular filling occurs when ventricular pressure drops below atrial pressure, opening AV valves and allowing passive blood flow into ventricles.
Atrial contraction at the end of diastole pushes remaining blood into ventricles.

Ventricular Filling (End of Diastole)

Heart at rest with blood returning to atria from veins (venous return).
AV valves are open; semilunar valves are closed as blood moves to ventricles (passive filling).
Late stage includes atrial systole (contraction) triggered by the SA node.
Atrial pressure remains higher than ventricular pressure during filling.
End-diastolic volume (EDV) represents the total blood volume in ventricles at the end of diastole.

Systole Phases

Isovolumetric Ventricular Contraction (Beginning of Systole)

Ventricles contract, increasing pressure above atrial pressure, but semilunar valves remain closed.
Arterial trunk pressure exceeds ventricular pressure.
Ventricular volume remains constant as all valves are closed.

Ventricular Ejection (Later Systole)

Increased ventricular pressure exceeds arterial trunk pressure, opening semilunar valves, and ejecting blood into the aorta and pulmonary trunk.
Ventricular pressure remains higher than atrial pressure, maintaining AV valve closure.
Stroke volume: The amount of blood ejected during contraction; end-systolic volume (ESV) is the remaining blood in the ventricle after systole.
$EDV = Stroke Volume + ESV$

Isovolumetric Ventricular Relaxation

Beginning of ventricular relaxation and diastole.
Both semilunar and AV valves are closed.
Ventricular pressure is lower than arterial trunk pressure, closing semilunar valves, but still higher than atrial pressure, preventing AV valve opening.
Ventricular volume remains constant.
Pressure eventually drops below atrial pressure, reopening AV valves, restarting the cycle with ventricular filling.

Heart Sounds

Events displayed to describe the cardiac phases in relation to valve function.

Late Diastole

Passive filling of ventricles while AV valves are open and semilunar valves are closed.
Atrial systole forces remaining blood into ventricles.

Isovolumetric Ventricular Contraction

Ventricular pressure exceeds atrial pressure, closing AV valves.
Pressure is insufficient to open semilunar valves.

Ventricular Ejection

Ventricular pressure exceeds aortic and pulmonary trunk pressure, opening semilunar valves, and ejecting blood.
AV valves remain closed.

Isovolumetric Relaxation Phase

Aortic and pulmonary trunk pressure exceeds ventricular pressure, closing semilunar valves to prevent backflow.
Ventricular pressure is still higher than atrial pressure, maintaining AV valve closure.

Late Stage of Diastole

The ventricles relax and their pressure drops below the pressure of the atria, reopening AV valves, filling the ventricles with blood is allowed.

Pressure and Volume Changes

Cardiovascular Pressures

Pressures in heart chambers or vasculature measured in millimeters of mercury (mmHg).
Measurements are relative to atmospheric pressure (760 mmHg at sea level).
Atmospheric pressure considered zero for cardiovascular pressures.
If blood pressure is 100 mmHg, it's 100 mmHg above atmospheric pressure (860 mmHg absolute).

Atrial and Ventricular Pressure

During ventricular filling (phase 1), ventricular pressure is slightly below atrial pressure, opening AV valves for passive filling.
Pressure increases slowly in both ventricles and atria.
Atrial contraction causes a spike in atrial pressure, leading to increased ventricular pressure.
Systole (phases 2 and 3) causes a large increase in ventricular pressure.
Ejection phase (phase 3) has the highest ventricular pressure, opening semilunar valves.
Pressure drops as blood ejects.
Phase 4 involves a significant drop in ventricular pressure during the relaxation phase.
Diastole continues, returning to phase one as ventricular pressure drops below atrial pressure, reopening AV valves for passive filling.

Aortic Pressure

During diastole (phase 1), semilunar valves are closed; blood leaves the aorta, decreasing aortic pressure.
Aortic pressure reaches a minimum at the end of diastole (diastolic pressure).
Semilunar valves open at the start of phase 3, and aortic pressure rises.
Maximum aortic pressure (systolic pressure) is reached during the ventricular ejection phase (phase 3).
Blood enters the aorta faster than it leaves during the ejection phase.
Aortic valve closure at the end of systole causes a brief spike in aortic pressure (dicrotic notch).
Process returns to ventricular filling, gradually decreasing aortic pressure.
Mean Arterial Pressure (MAP): Average aortic pressure during the cardiac cycle, representing the driving force of blood through the systemic circuit.
Aorta stores pressure and energy in its walls.
Left ventricle pumps blood into the aorta which stretches its walls storing pressure and energy generated during ventricular contraction.
Aorta acts as a pressure reservoir.
Elastic tissue recoil releases stored energy, driving blood flow during diastole.

Ventricular Volume

Mid-to-late diastole (phase 1) involves rising volume in the left ventricle due to passive filling through open AV valves.
A spike in ventricular volume occurs at the end of phase 1 due to atrial contractions.
End-Diastolic Volume (EDV): Maximum ventricular volume reached at the end of phase 1.
Volume is constant during the isovolumetric contraction phase (phase 2).
Ventricular ejection (phase 3) involves a decrease in ventricular volume as blood enters the aorta via the open aortic valve.
Minimum volume of blood in the left ventricle is reached at the end of phase 3.
End-Systolic Volume (ESV): Minimum ventricular volume reached at the end of ventricular ejection, maintained during isovolumetric relaxation (phase 4).
AV valves open during the shift to phase 1 to allow blood enter.
Stroke Volume: Volume of blood ejected during one heartbeat, calculated as $EDV - ESV$ .
Average stroke volume is 70 mL.
End-systolic volume can be altered by autonomic nervous system and hormones.
Increased contraction force leads to a smaller minimum volume, raising stroke volume.
Ejection Fraction: Fraction of end-diastolic volume ejected during a heartbeat, calculated as $Stroke Volume / EDV$ .
Normal ejection fraction at rest is 54% (70 mL / 130 mL).

Pressure-Volume Curve

The x-axis shows the Left ventricular volume and the y-axis show the Left ventricular pressure.
Plot of left ventricular volume (x-axis) against left ventricular pressure (y-axis) forming a loop representing the cardiac cycle.
Phase one, ventricular filling, shows small increase in pressure and increase in ventricular volume from end-systolic volume (60 mL) to end-diastolic volume (130 mL).
Phase two, isovolumetric contraction, shows all the valves are closed, the volume remains constant at and the increase in pressure is caused by the start of systole.
The transfer frome one phase to the other it's caused by ventricles' pressure exceeds the the aortic valve pressure.
Phase three, ventricular ejection, shows valves, and volume from end-diastolic to it's minimum, end-systolic volume value, begins to decrese, the ventricles are empting and reaching the low volume point.
The transfer frome one phase to the other it's caused by the droping in in the ventricles to a value below the aortic artery.
Phase four, isovolumetric relaxation, shows as volume remains at minimum, end-systolic volume value, a dramtic drop in pressure from pressure that was reached during systole, to a low value when the AV valves opend and phase one is reached again.

Compliance

Phase 1 slope measures the compliance of the left ventricle.
Compliance indicates how easy the left ventricle can expand relative to pressure.
A greater slope indicates a high compliance.
Compliance reduction is caused by aging.

End-Systolic Volume

End-systolic volume or a minimum left ventricles amount of blood is a measure of the ventricular contractility.
A shift left means less blood left is higher a contractility.
A shift right means more blood left and lower the contractility.

Heart sounds

Occur at the start of systole and diastole.
Described as lub dub.
Heart sound number 1, the lub, It corresponds with closure of the AV valves.
Heart sound number 2, the dub, It corresponds with closure of the semilunar valves.
These sounds are caused by the rush of blood through the the narrowing valves as they were about to close.