AS Module 1
10.8 The Function of the heart
After this tutorial you should be able to:
| Label the structure of the heart and relate it to its function. |
| Explain how the blood pressure and volume changes during the cardiac cycle, and how it effects valve movement. |
| Explain the myogenic stimulation of the heart, and understand the roles of the sinoatrial node, atrioventricular node and the bundle of His. |
| Describe the effect of exercise on the cardiovascular system. |
The structure of the heart
External structure
Internal structure
How the hearts structure aids its function
The heart is the organ that moves blood around the body. It is a complex, four-chambered muscular bag. It is found in the chest, protected by the ribs and sternum.
The following bullets on the relationship between the hearts form and function should be referred to the diagram above.
Structure |
Function |
| Cardiac muscle | This muscle has special properties which enable it to carry out regular contraction without fatigue. |
| Aorta. | The major artery of the body. It forms the aortic arch and other major arteries branch off from it. |
| Semilunar Valves. | Prevent blood from flowing back into the ventricles. |
| Pulmonary Vein. | Carries oxygenated blood back from lungs to the left side of the heart at a relatively low pressure as the blood has travelled through the extensive capillaries of the lungs. |
| Right atrium | This chamber receives the deoxygenated blood from the great veins at relatively low pressure. It retains the blood until the pressure builds up and opens the tricuspid valve so that the ventricle can fill with blood. When both atrium and ventricle are filled with blood, the atrium contracts, forcing the blood into the ventricle. The atrium has thick muscular walls as the blood it receives is at low pressure and it needs to exert relatively little pressure to move it into the ventricle. One-way valves at the entrance to the atrium stop back flow of blood. |
| Right Ventricle | This is filled with deoxygenated blood by the atrium. Shortly afterwards, the ventricle also contracts, its thicker muscular walls allowing it to force blood out of the heart into the pulmonary artery and onto the lungs. |
| Left atrium | This performs the same function as the right atrium. |
| Left ventricle | This is filled with oxygenated blood as the left atrium contracts it forces the blood out of the heart into the aorta. The muscular wall of the left side of the heart is much thicker than that of the right because the lungs are relatively close to the heart, but the left side has to produce sufficient force to move the blood under pressure to all the extremities of the body. |
| Bicuspid valve | These are made up of two flaps. It is also an atrioventricular valve. It functions by allowing blood through from the atrium to the ventricle but closing as the ventricle contracts to prevent a back flow of blood. |
| Tricuspid valves. | This is made up of three flaps. It performs the same function of the bicuspid valve. |
| Septum | This is a thick muscular wall which divides the two halves of the heart. |
To see these structures in action, you can use the Visible Heart. When you have selected the icon you must scroll down to the Visible Heart icon. Then select the Frontal view with blood.
Click here.
Valve movements
In the heart there are many types of valve in both the right and left sides. For an animated tutorial go to both Left and Right Heart Valves in the visible heart.
Pressure and volume changes in the heart
During the cardiac cycle, the pressure of and the volume of the blood changes with each pumping stage.
For an online tutorial, click on
Compare ventricular volume and pressure on the Visible Heart site.Heart stimulation
The heart beats continually through life, with an average of 70 beats per minute. This rhythm is maintained by a wave of electrical excitation similar to a nerve impulse which spreads through special tissue in the heart muscle.
The sinoatrial node (SAN) or pacemaker sets up a wave of electrical excitation which causes the atria to start contracting and also spreads to an adjacent area of similar tissue. This is the P wave (see later).
The atrioventricular node (AVN) is excited as a result of the SAN’s excitation and from here the excitation passes into the bundle of His. This is a bundle of nodal tissue fibres made from Purkyne tissue (see picture). This penetrates through the septum between the ventricles.
As the excitation travels through the tissue it sets off the contraction of the ventricles, starting at the bottom and so squeezing blood out. This is the Q,R,S and T waves on an electrocardiogram (ECG). The speed at which the excitation spreads makes sure that the atria have stopped contracting before the ventricles start.
To look at an ECG of the heart beat and see how the wave of electrical activity occurs, go to the
Compare ventricular cycle and electrocardiogram, on the Visible heart.Effects of exercise
Definitions
Cardiac Output
The pumping ability of the heart is the number of beats per minute (cardiac rate) multiplied by the volume of blood ejected per beat (stroke volume). The cardiac rate and stroke volume are regulated by autonomic nerves and by mechanisms intrinsic to the cardiovascular system.
Cardiac output = cardiac rate x stroke volume
Pulmonary ventilation
The amount of air passing into and out of the lungs per minute is known as pulmonary ventilation. The tidal volume of air is the volume of air (cm3) that enters and leaves the lungs during each breath. This is multiplied by the breathing rate, which is the number of breaths per minute, to give the pulmonary ventilation.
Pulmonary ventilation = tidal volume x breathing rate
What do you think the changes in cardiac output and pulmonary ventilation will be with exercise ?
During strenuous exercise tidal volume can increase greatly as can the breathing rate. So pulmonary ventilation increases greatly.
Also during exercise the cardiac rate can increase, as you know from feeling your heart beat after running, as can the stroke volume by more efficient contraction of the ventricles. So cardiac output increases with exercise.
Nervous control of heart rate
The muscle that makes up the walls of the heart is unusual in that it does not require nervous stimulation in order to contract. In the very early embryo, cells that are destined to become the heart begin contracting rhythmically long before the organ forms. They have intrinsic rhythmicity. An adult heart removed from the body will continue to contract as long as it is bathed in a suitably oxygen rich fluid. The intrinsic rhythm is the rate at which it beats when isolated from the nervous and hormonal control of the body and is about sixty beats per minute.
However, this rate can increase during periods of exercise or stress and decrease with rest and relaxation. These changes are under nervous control. A nerve from the sympathetic nervous system speeds up the heart rate and the vagus nerve from the parasympathetic system slows it down. Other factors such as hormones (e.g. adrenalin), pH changes and temperature all effect heart rate.
Redistribution of blood during exercise
| Organ or tissue | Blood flow at rest (ml/min) |
Blood flow during strenuous exercise (ml/min) |
| Brain | 700 |
750 |
| Heart | 200 |
750 |
| Lungs | 100 |
200 |
| Kidneys | 1100 |
600 |
| Liver | 1350 |
600 |
| Skeletal muscles | 750 |
12500 |
| Bone | 250 |
250 |
| Skin | 300 |
1900 |
| Thyroid gland | 50 |
50 |
| Adrenal gland | 25 |
25 |
| Other tissue | 175 |
175 |
| Total | 5000 |
17800 |
Which organs have gained the greatest increase in blood flow ?
Which organs have stayed relatively stable?
Now click on this to test your knowledge!
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