The structure of the heart is shown in the diagram below:
The structure is closely related to its function.
Mammals have a double circulation, which means that the right hand side of the heart pumps deoxygenated blood to the lungs in the pulmonary artery to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left hand side of the heart in the pulmonary vein.
From there the blood is pumped to the body in the aorta, eventually returning to the right hand side of the heart in the vena cava to start the cycle again.
Since the right side pumps to the lungs which are situated close to the heart, the walls are much thinner than the left side which has to pump blood out of the heart to the body.
The heart has 4 chambers; 2 on the right hand side (the left as you look at it in the screen) and 2 on the left. The top chamber on each side is called the atrium; the bottom is called the ventricle. The atria receive blood as it enters the heart and pumps it into the ventricles. The ventricles pump blood out of the heart.
Due to this, the walls of the atria are much thinner than the walls of the ventricles.
Inside the heart and at the base of the vessels that leave the heart are valves. These valves only open one way, which ensures that there is no backflow of blood.
The valves are held open or closed by tendons (chordae tendinae or perhaps better known as heart strings), which are attached at the other end to the papillary muscles in the ventricle walls. The valves open to let blood through and then snap shut. This sound of the valves closing is the 'lub dub' sound of the heartbeat.
The muscle of the heart is called cardiac muscle and is made of tightly connecting cells. This close contact allows rapid ion transport from cell to cell. This then allows smooth, efficient waves of depolarisation to produce contractions (and repolarisation to bring about relaxation), which pass through the heart.
The tissue is said to be myogenic, i.e. it does not need electrical impulses from a nerve to make it contract. If the cardiac muscle is supplied with oxygen and nutrients (a task carried out by the coronary arteries which you can see running over the surface of the heart) it will continue to contract at a steady pace.
Nerves supplying the heart, though they are not needed to start the contractions, can bring about an increase or decrease in the rate of contractions when appropriate.
One cardiac cycle consists of the atria and then the ventricles contracting so that the blood that has entered the heart is pumped out. This occurs about 70 times every minute and is continuous. The periods of contraction are called systole. The periods of relaxation are called diastole.
We shall start when the atria and ventricles are in diastole.
Blood at a low pressure in the veins flows into the atria. This increases the pressure inside the empty atria as they fill. Some of the blood trickles through the open atrioventricular valves into the relaxed ventricles below.
When the atria are full, they go into atrial systole, their walls contract and blood is pushed through the valves into the ventricles. The pressure in the atria is increased due to the contractions and the pressure is increasing in the ventricles as they fill with blood.
When the atria contract, blood cannot flow back into the veins because the pressure of the blood pushes on the valves in the veins to shut them.
After a short delay the ventricles contract from the apex (base) upwards. The pressure inside the ventricles increases due to the ventricular systole. As the pressure increases to a higher level than the pressure in the atria, blood pushes against the atrioventricular valves, shutting them (the first heart sound) and preventing backflow.
The semilunar valves open under the pressure and blood leaves the heart.
The ventricles relax - ventricular diastole - and the semilunar valves snap shut behind the blood (the second heart sound).
To work out from a graph what stage of the cycle the heart is in, it is important to look at the relative pressure of the atria and ventricles.
The heartbeat is initiated in a specialised area of muscle in the right atrium called the sinoatrial node (SAN) or the pacemaker. The SAN starts the waves of depolarisation, which results in contraction.
The waves spread out over the 2 atrial walls so that they contract. There is a band of fibres between the atria and ventricles, which have a high electrical resistance so the waves cannot spread from the atria to the ventricles.
There is an area, however, which does conduct in the septum, and the waves can pass from here through the ventricles. This specialised area is called the atrioventricular node (AVN) and will pass on the waves of depolarization after about 0.1s.
It would be disastrous if the ventricles contracted at the same time so that is why there is a short period of delay before the ventricles contract.
The AVN passes them on to the Purkinje (also called Purkyne) fibres in the inter-ventricular septum. The excitation is passed to the apex of the heart and then through the ventricle walls. This causes the ventricles to contract from the base upwards ensuring that the blood is forced up and out in the vessels leaving the heart.
Regulation of the cardiac cycle by the heart by other factors
The total amount of blood pumped by the heart in lone minute = cardiac output
Cardiac output = stroke volume x number of beats per minute (Stroke volume is the volume pumped in one beat)
Increasing the stroke volume therefore can increase cardiac output. A larger volume might enter the atria through the veins during exercise because the vessels become dilated to enable more blood to flow to the muscles to supply more oxygen and nutrients. The atria are stretched more than normal; the heart detects this and responds by beating faster and with more force.
Increasing the number of beats per minute can also increase the cardiac output.
The effect of hormones.
This occurs when adrenaline is released from the adrenal medulla, flows in the blood and affects the SAN. The SAN is stimulated, works faster, increasing the heart rate.
The effect of nervous stimulation.
One nerve, the accelerator nerve, runs from the cardioacceleratory centre in the medulla of the brain to the SAN.
Another, the vagus nerves, runs from the cardioinhibitory centre in the medulla of the brain to the SAN.
These nerves are stimulated in various situations, e.g. during exercise, the accelerator nerve is stimulated. It releases noradrenaline at the SAN resulting in the heart rate increasing due to a decreased delay at the AVN and increasing the force of the contractions.
If the vagus nerve is stimulated, acetylcholine is released at the SAN. The delay at the AVN increases and the cardiac output falls.
Blood pressure also affects the cardiac output. Some blood vessels (e.g. the aorta and carotid arteries) have baroreceptors (also called stretch receptors) in their walls. These detect the pressure and send impulses to the cardiac centre in the medulla.
If the pressure is too high: the cardioinhibitory centre is stimulated, impulses are sent down the vagus nerve, the heart rate is slowed and the pressure will fall.
If the pressure is too low: the cardioacceleratory centre is stimulated, impulses are sent down the accelerator nerve, the heart rate is increased and the pressure will rise.