8.3
Heart Action: Initiation & Control
Control of the basic heartbeat is myogenic, which means the heart will beat without any external stimulus
This intrinsic rhythm means the heart beats at around 60 times per minute
The sinoatrial node (SAN) is a group of cells in the wall of the right atrium. The SAN initiates a wave of depolarisation that causes the atria to contract
The Annulus fibrosus is a region of non-conducting tissue which prevents the depolarisation spreading straight to the ventricles
Instead, the depolarisation is carried to the atrioventricular node (AVN)
This is a region of conducting tissue between atria and ventricles
After a slight delay, the AVN is stimulated and passes the stimulation along the bundle of His
This delay means that the ventricles contract after the atria
The bundle of His is a collection of conducting tissue in the septum (middle) of the heart. The bundle of His divides into two conducting fibres, called Purkyne tissue, and carries the wave of excitation along them
The Purkyne fibres spread around the ventricles and initiate the depolarization of the ventricles from the apex (bottom) of the heart
This makes the ventricles contract and blood is forced out of the pulmonary artery and aorta
Stages in the cardiac cycle table
he Cardiac Cycle
The cardiac cycle is the series of events that take place in one heart beat, including muscle contraction and relaxation
The contraction of the heart is called systole, while the relaxation of the heart is called diastole
One cardiac cycle is followed by another in a continuous process
There is no gap between cycles where blood stops flowing
Volume and pressure changes
Contraction of the heart muscle causes a decrease in volume in the corresponding chamber of the heart, which then increases again when the muscle relaxes
Volume changes lead to corresponding pressure changes
When volume decreases, pressure increases
When volume increases, pressure decreases
Throughout the cardiac cycle, heart valves open and close as a result of pressure changes in different regions of the heart
Valves open when the pressure of blood behind them is greater than the pressure in front of them
They close when the pressure of blood in front of them is greater than the pressure behind them
Valves are an important mechanism to stop blood flowing backwards
Atrial systole
The walls of the atria contract
Atrial volume decreases
Atrial pressure increases
The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open
Blood is forced into the ventricles
There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria
The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole
Ventricular systole
The walls of the ventricles contract
Ventricular volume decreases
Ventricular pressure increases
The pressure in the ventricles rises above that in the atria
This forces the AV valves to close, preventing back flow of blood
The pressure in the ventricles rises above that in the aorta and pulmonary artery
This forces the semilunar (SL) valves open so blood is forced into the arteries and out of the heart
During this period, the atria are relaxing; atrial diastole coincides with ventricular systole
The blood flow to the heart continues, so the relaxed atria begin to fill with blood again
Diastole
The ventricles and atria are both relaxed
The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close
The atria continue to fill with blood
Blood returns to the heart via the vena cava and pulmonary vein
Pressure in the atria rises above that in the ventricles, forcing the AV valves open
Blood flows passively into the ventricles without need of atrial systole
The cycle then begins again with atrial systole
Valves during the cardiac cycle table
Stage in cardiac cycle | Atrioventricular valves | Semilunar valves |
Atrial systole | Open | Closed |
Ventricular systole | Closed | Open |
Diastole | Open | Closed |
The cardiac cycle graph can be analysed step by step
Analysing the cardiac cycle
The lines on the graph represent the pressure of the left atrium, aorta, and the left ventricle
The points at which the lines cross each other are important because they indicate when valves open and close
Point A - the end of diastole
The atrium has filled with blood during the preceding diastole
Pressure is higher in the atrium than in the ventricle, so the AV valve is open
Between points A and B - atrial systole
Left atrium contracts, causing an increase in atrial pressure and forcing blood into the left ventricle
Ventricular pressure increases slightly as it fills with blood
Pressure is higher in the atrium than in the ventricle, so the AV valve is open
Point B - beginning of ventricular systole
Left ventricle contracts causing the ventricular pressure to increase
Pressure in the left atrium drops as the muscle relaxes
Pressure in the ventricle exceeds pressure in the atrium, so the AV valve shuts
Point C - ventricular systole
The ventricle continues to contract
Pressure in the left ventricle exceeds that in the aorta
Aortic valve opens and blood is forced into the aorta
Point D - beginning of diastole
Left ventricle has been emptied of blood
Muscles in the walls of the left ventricle relax and pressure falls below that in the newly filled aorta
Aortic valve closes
Between points D and E - early diastole
The ventricle remains relaxed and ventricular pressure continues to decrease
In the meantime, blood is flowing into the relaxed atrium from the pulmonary vein, causing an increase in pressure
Point E - diastole
The relaxed left atrium fills with blood, causing the pressure in the atrium to exceed that in the newly emptied ventricle
AV valve opens
After point E - late diastole
There is a short period of time during which the left ventricle expands due to relaxing muscles
This increases the internal volume of the left ventricle and decreases the ventricular pressure
At the same time, blood is flowing slowly through the newly opened AV valve into the left ventricle, causing a brief decrease in pressure in the left atrium
The pressure in both the atrium and ventricle then increases slowly as they continue to fill with blood
Mammalian Heart Structure
The Walls of the Heart
The muscular walls of the atria are thinner than those of the ventricles
When the atria contract, the thin muscular walls do not generate much pressure, but enough to force blood down into the ventricles, through the atrioventricular valves
In contrast, the walls of the ventricles are thicker and more muscular
Following contraction of the atria, the ventricles contract and squeeze blood inwards, increasing its pressure and pushing it out of the heart through right and left semilunar valves
Left & right ventricle
The muscle of the left ventricle is significantly thicker than the right ventricle
This is because the blood leaving the right ventricle travels less distance than blood leaving the left ventricle
The blood pumped out from the right ventricle travels to the lungs, whereas blood leaving the left ventricle has to travel to the rest of the body to deliver oxygen for respiration.
To reach the rest of the body, the blood leaving the left ventricle must be under high pressure
This is generated by the contraction of the muscular walls of the left ventricle
The right ventricle generates less pressure from the contraction of its thinner walls, as blood only has to reach the lungsHeart structure
The human heart has a mass of around 300g and is roughly the size of a closed fist
The heart is a hollow, muscular organ located in the chest cavity
It is protected in the chest cavity by the pericardium, a tough and fibrous sac
The human heart has four chambers and is separated into two halves by the septum
The heart is divided into four chambers. The two top chambers are atria and the bottom two chambers are ventricles
The left and right sides of the heart are separated by a wall of muscular tissue, called the septum. The portion of the septum which separates the left and right atria is called the interatrial septum, while the portion of the septum which separates the left and right ventricles is called the interventricular septum
The septum is very important for ensuring blood doesn’t mix between the left and right sides of the heart
Valves in the heart
Valves in the heart:
Open when the pressure of blood behind them is greater than the pressure in front of them
Close when the pressure of blood in front of them is greater than the pressure behind them
Valves are important for keeping blood flowing forward in the right direction and stopping it flowing backwards. They are also important for maintaining the correct pressure in the chambers of the heart
The right atrium and right ventricle are separated by the atrioventricular valve, which is otherwise known as the tricuspid valve
The right ventricle and the pulmonary artery are separated by the pulmonary valve
The left atrium and left ventricle are separated by the mitral valve, which is otherwise known as the bicuspid valve
The left ventricle and aorta are separated by the aortic valve
There are two blood vessels bringing blood to the heart; the vena cava and pulmonary vein
There are two blood vessels taking blood away from the heart; the pulmonary artery and aorta
Coronary arteries
The heart is a muscle and so requires its own blood supply for aerobic respiration
The heart receives blood through arteries on its surface, called coronary arteries
It’s important that these arteries remain clear of plaques, as this could lead to angina or a heart attack (myocardial infarction)
The coronary arteries cover the outside of the heart, supplying it with oxygenated blood