Physiology
Lec: 4 د. زيـد الاطرقجيCardiovascular system
The mechanical events of the cardiac cycle:
Contraction, relaxation and the resultant changes in blood flow through the heart are brought about by the rhythmic changes in cardiac activity.The heart alternately contracts to empty and relaxes to fill with blood.
The cardiac cycle consists of alternate periods of:
A. systole means contraction and emptying.
B. diastole means relaxation and filling.
Contraction results from the spread of excitation across the heart.
Relaxation follows the subsequent repolarization of the cardiac musculature. The atria and ventricles go through separate cycles of systole and diastole.
Mid-ventricular diastole
During most of ventricular diastole the atrium is still also in diastole. This stage corresponds to the T-P interval on the ECG [the interval after ventricular repolarization and before another atrial depolarization].
Because of the continuous inflow of blood from the venous system into the atrium atrial pressure slightly exceeds ventricular pressure even though both chambers are relaxed.
Because of this pressure differential the A-V valve is open and blood flows directly from the atrium into the ventricle throughout ventricular diastole. As a result of this passive filling the ventricular volume slowly continues to rise even before atrial contraction takes place
Late-ventricular diastole
Late in ventricular diastole the S-A node reaches threshold and fires. The impulse spread throughout the atria which appear in the ECG as the P-wave. Atrial depolarization brings about atrial contraction raising the atrial pressure and squeezing more blood into the ventricle. Throughout atrial contraction atrial pressure still slightly exceeds ventricular pressure so the A-V valve remains open.
Ventricular diastole ends at the onset of ventricular contraction by this time atrial contraction and ventricular filling are completed. The volume of blood in the ventricle at the end of diastole is known as the end-diastolic volume [EDV]. Which average about 135ml. No more blood will be added to the ventricle during this cycle.
Therefore the end diastolic volume is the maximum amount of blood that the ventricle will contain during this cycle.
Ventricular-excitation and onset of ventricular systole:
After atrial excitation the impulse travels though the A-V node and specialized conduction system to excite the ventricle. By the time ventricular activation is complete atrial contraction is already over. The QRS complex represents this ventricular excitation which induces ventricular contraction. The ventricular pressure sharply increases after the QRS complex signaling the onset of ventricular systole.
The slight delay between the QRS complex and the actual onset of ventricular systole is the time required for the excitation-contraction coupling process to occur. As ventricular contraction begins ventricular pressure immediately exceeds atrial pressure. This backward pressure differential forces the A-V valve close.
After ventricular pressure exceeds atrial pressure and the A-V valve has closed. And to open the aortic valve the ventricular pressure must continue to increase until it exceeds aortic pressure. Therefore after closing of the A-V valve and before opening of the aortic valve there is a brief period of time when the ventricle remains a closed chamber. Because all valves are closed no blood can enter or leave the ventricle during this time. This interval is termed the period of iso-volumetric ventricular contraction [iso-volumetric means constant volume and length]. Because no blood enters or leaves the ventricle the ventricular chamber stays at constant volume and the muscle fibers stay at constant length. During iso-volumetric ventricular contraction ventricular pressure continues to increase as the volume remains constant.
Ventricular ejection
When ventricular pressure exceeds aortic pressure the aortic valve is forced open and ejection of blood begins.
The amount of blood pumped out of each ventricle with each contraction is called the stroke volume
The aortic pressure rises as blood is forced into the aorta from the ventricle faster than blood is draining off into the smaller vessels at the other end. The ventricular volume decreases substantially as blood is rapidly pumped out.
Ventricular systole include both:
1. The period of iso-volumetric contraction.
2. The ventricular ejection phase.
End of ventricular systole:
The ventricle does not empty completely during ejection. Normally only about half the blood within the ventricle at the end of diastole is pumped out.
The amount of blood left in the ventricle at the end of systole when ejection is complete is the end-systolic volume [ESV] which average about 65ml.
This is the least amount of blood that the ventricle will contain during this cycle.
Systolic volume = end diastolic volume - end systolic volume.
SV= EDV-ESV
Stroke volume=135-65=70ml
Ventricular repolarization and onset of ventricular diastole:
The T-wave signifies ventricular repolarization at the end of ventricular systole. As the ventricle starts to relax on repolarization ventricular pressure falls below aortic pressure and the aortic valve closes.
No blood leaves the ventricle during this cycle because the aortic valve has closed. When the aortic closes the A-V valve is not yet open because ventricular pressure still exceeds atrial pressure. So no blood can enter the ventricle from the atrium. Therefore all the valves once again closed for a brief period of time known as iso-volumetric ventricular relaxation. The muscle fiber length and chamber volume remain constant. No blood leaves or enters as the ventricle continues to relax and the pressure steadily falls.
When ventricular pressure falls below atrial pressure the A-V valve opens and ventricular filling occurs again. Ventricular diastole includes both the period of: 1. iso-volumetric ventricular relaxation. 2. The ventricular filling phase.
Atrial repolarization and ventricular depolarization occur simultaneously so the atria are in diastole throughout ventricular systole. Blood continues to flow from the pulmonary veins into the left atrium. As this incoming blood pools in the atrium atrial pressure rises continuously.
When the A-V valve opens at the end of ventricular systole blood that accumulated in the atrium during ventricular systole pours rapidly into the ventricle. Ventricular filling thus occurs rapidly because of the increased atrial pressure resulting from the accumulation of blood in the atria. Then ventricular filling slows down as the accumulated has already been delivered to the ventricle and atrial pressure starts to fall. During this period of reduced filling blood continue to flow from the pulmonary veins into the left atrium and through the open A-V valve into the left ventricle.
During late ventricular diastole when the ventricle is filling slowly the S-A node fires again and the cardiac cycle starts over.
When the body at rest one complete cardiac cycle lasts 800msec with 300msec devoted to ventricular systole and 500msec taken up by ventricular diastole. Significantly much of ventricular filling occurs early in diastole during the rapid filling phase.
During rapid heart rate diastole length is shortened much more than systole length. Because much of ventricular filling occurs early in diastole during the rapid filling phase, filling is not seriously impaired when diastolic time is reduced as a result of an increase in heart rate.
Normally ventricular rate do not exceed 200beats per minute because the relatively long refractory period of the A-V node will not allow impulses to be conducted to the ventricles more frequently than that.
Heart sounds:
1. First heart sound is low pitched soft and relatively long, sound like lub.
2. Second heart sound has a higher pitch and is shorter and sharper often said to sound like dup.
The first heart sound is associated with closure of the A-V valves whereas the second heart sound is associated with closure of the semi lunar valves.
Opening of valves does not produce any sound.
The sounds are caused by vibrations setup within the walls of the ventricles and major arteries during valve closure not by the valves snapping shut.
Because the A-V valves close at the onset of ventricular contraction when ventricular pressure exceeds atrial pressure the first heart sound signals the onset of ventricular systole.
The semi lunar valve close at the onset of ventricular relaxation as the left and right ventricular pressures falls below the aortic and pulmonary artery pressures respectively.
The second heart sound therefore signals the onset of ventricular diastole.
The pouring of blood from atrium toward ventricle during mid-diastole produce sound called third heart sound.
Atrial contraction during late diastole push additional blood to the ventricle which is already contain blood this pouring of blood produce a sound called fourth heart sound.
So the second, third and the fourth heart sounds all occurs in diastole
Murmurs: are abnormal sounds due to turbulence of blood flow.
Stenotic valve is stiff narrowed valve that does not open completely. Blood must be forced through the constricted opening at tremendous velocity resulting in turbulence that produce an abnormal whistling sound similar to the sound produced when you force air rapidly through narrowed lips to whistle.
Insufficient or incompetent valve:
Is the one that cannot close completely usually because the valve edges are scarred and does not fit together properly. Turbulence is produced when blood flows backward through the insufficient valve and collides with blood moving in the opposite direction creating a swishing gurgling murmur.
An insufficient heart valve is often called a leaky valve because it lets blood leaks back through at a time when the valve should be closed.
Murmur between the first and second heart sound LUP-murmur-dup is a systolic murmur.
Diastolic murmur occurs between the second and first heart sound LUP-DUP murmur.
The sound of the murmur characterizes it as either a stenotic [whistling] murmur or an insufficient [swishy] murmur.