H5.2 Pressure and volume changes during the cardiac cycle.
H5.1 Cardiac cycle.
The cardiac cycle is the repetitive sequence of events in which the heart chambers contract and relax in a co-ordinated manner to send blood continuously around the circulatory system. As with all cycles there is strictly speaking no one point that can be considered the beginning. The notes provided here follow the lead of other authors in terms of the beginning of the cycle and the associated diagrams.
Basic Heart Structure:
This diagram covers the basic internal structure of the heart.
The heart should be regarded as a double pump (left side and the right side).
Each side of the heart works in synchrony with the other side in the cardiac cycle.
The events on one side of the heart are mirrored by those on the other side.
The left ventricle is larger than the right ventricle in both the ventricular volume and the thickness of the surrounding muscular walls.
Heart Valves:
As with all pressurised fluid system there is a valve system to maintain the one way flow of blood.
The atrio-ventricular valves prevent backflow of pressurised blood from the ventricle to the atria.
The right atrio-ventricular valve has three cusps (tricuspid). The left side of the heart has two cusps and is also known as the bicuspid valve.
The semi-lunar valves prevent back flow from arteries to the ventricles when the heart relaxes (diastole).
Heart Sounds are caused by closure of the heart valves.
The cardiac cycle
Each diagram and accompanying notes are cross referenced ( with a graph ref) to the pressure and volume changes in section H5.2
Diastole: All heart muscle in state of relaxation.
Arterial blood pressure is greater than ventricle pressure.
The semilunar valves are closed
Ventricle pressures are still higher than atrial pressures
Atrio-ventricular valves closed.
Atrial Distention
At this point the atria are both filling with blood from both the pulmonary veins and the vena cava.
Graph ref: (10)
Atrial Distention:
The heart is still in a state of diastole.
All the valves are all closed.
The passive return of blood to the atria along veins causes the pressure to rise within the atria and the walls to distend. (swell)
Graph Ref (10 to 1 ) but rising pressure
Note that this stage is still diastole:
The pressure in the atria is greater than the pressure in the ventricles.
The pressure difference causes the atrio-ventricular valve to open.
The ventricle passively fills with blood.
Note the semi-lunar valves are still closed.
Graph ref (1)note the rise in ventricle pressure
Early Atrial Systole
The atria contract together (see myogenic contraction).
The contraction of the atrial muscle reduces the volume of the atria.
This volume reduction increases the pressure on the blood within the atria.
The pressure increases forces the additional volume of blood into the ventricle.
This stretches the ventricle walls (Starlings Law)
graph ref: peak of (1)
Systole: Contraction of the ventricles. graph ref (b)
The ventricle walls contract on both sides.
There is a sudden pressure increase within the ventricles.
The pressure in the ventricles is greater than the pressure in the atria
Atrio-ventricular valves close. (First heart sound "Lub").
Ventricle pressure is lower than arterial pressure.
Semi-lunar valves remain closed.
graph ref: (2)
Iso-volumetric contraction, in which the ventricle contracts increasing the blood pressure but the blood cannot yet pass out into the artery. This gives a sudden and large pressure increase.
graph ref (3)
Systole continues
Ventricle pressure is greater than arterial pressure.
The semi-lunar valves open.
Blood ejects into the arteries (a pulse).
graph ref (4)
Pressure in the ventricles peaks rapidly as we come to the end of systole.
Note the atrio-ventricular valves are still closed (graph ref (5))
As pressure falls in the ventricle the arterial blood will backflow closing the semi-lunar valves and creating the second heart sound, "dub".
graph ref: (6)
We now return to the first diagram in the cycle as we are back in a relaxed condition called diastole.
graph ref: (a) diastole
H5.2 Pressure and volume changes during the cardiac cycle.
The numbers on the diagram below have been cross referenced to the cardiac cycle diagrams above.
(a) Diastole (ventricles are relaxed)
(b) Systole (ventricles contracted)
(1) Atrial systole
(2) Ventricular Systole , atrio-ventricular valves close, first heart sound.
(3) Iso-volumetric contraction
(4) Opening semi-lunar valves
(5) Peak of systolic pressures
(6) End of ventricular systole, closure of the semi-lunar valves, second heart sound. Beginning of diastole
(7) Aorta pressure during ventricular diastole
(8) Falling ventricular pressure (Diastole)
(9) Passive atrial filling with blood/ compression from systole
(10) Passive filling blood by venus return
H5.3 The control of heart rate.
Myogenic Rhythm: The heart beat is initiated within the heart muscle itself.
Within the right atrium there are a specialise group of cells called the Sino Atrial Node.(SAN).
These cells can generate an electrochemical potential across the cell membrane and once threshold is reached this can be propagated across the other cells of both atria.
The speed of conduction across the two atria is fast enough that both effectively contract together.
However the cell membrane structure alters across a line rough consistent with the diagram and effectively prevents the conduction of the contraction stimuli traveling down into the ventricles.
a) The impulse is picked up by a specialised group of cells in the right atrium wall called the Atrio-Ventricular Node (AVN).
(b) The AVN conducts the impulse down through the central septum of the heart along specialised fibres called the Purkinje fibers. The fibres are insulated from the muscle and do not cause contraction.
c) The impulse emerges into the muscle at the apex of the heart so that the ventricular contraction begins at the apex.
d) The impulse travels on emerging into the heart muscle higher up the ventricle wall in this way the contraction spreads upwards.
Note that this direction of contraction pushes the blood towards the semi-lunar valve and also not that the transmission time down the Purkinje tissue creates a delay between atrial and ventricular systole. This delay maintains the correct directional flow of blood through the different chambers.
Myogenic Rhythm can be modified by the central nervous system to respond to cardiovascular demands.
Within the medulla region of the brain there are a specialised group of receptors and co-ordinators called the Cardiac Centre.
These are connected to the the SAN via the two sets of nerves.
a) Accelerator nerve that increases the rate SAN activity to produce faster heart rate.
b) Decelerator nerve that decreases the rate SAN activity to slow heart rate.
In addition the SAN is sensitive to hormones such as adrenaline that can directly stimulate heart rate.
The brain is sensitive to a wide range of stimuli including pH and CO2 levels which reflect the demand of the tissues for oxygen. As an example, exercise produces more CO2 in the plasma. Detected by the cardiac centre this stimulates the accelerator nerve
and therefore the SAN to increase heart rate. ie your heart beats faster when you exercise.
H5.4 Atherosclerosis and the causes of coronary thrombosis.
Atherosclerosis is a stage of arteriosclerosis involving fatty deposits (atheroma's) inside the arterial walls
The sequence of events in the build up of the atheroma:
1) damage to the arterial wall (perhaps due to high blood pressure)
2) attachment of phagocytes which secrete growth factors
3) enlargement of the muscle layer
4) permeability of the endothelium to low density lipoproteins (LDL)
5) deposition of cholesterol from LDL
6) crystallisation of cholesterol
7) reduction of the lumen space of the artery
8) decreased or interrupted flow
9) turbulence in blood flow can produce clots (thrombosis)
10) thrombosis in the coronary artery may block blood flow to heart muscle (myocardial infarction)resulting in tissue damage or death
H5.5 factors that affect the incidence of coronary heart disease.
