Structure of the cardiovascular system
If you clench your hand into a fist, this is approximately the same size as your heart. It is located in the middle of the chest and slightly towards the left.
The heart is a large muscular pump and is divided into two halves – the right-hand side and the left-hand side.
The right-hand side of the heart is responsible for pumping deoxygenated blood to the lungs.
The left-hand side pumps oxygenated blood around the body.
Each side of the heart consists of an atrium and a ventricle which are two connected chambers.
The atria (plural of atrium) are where the blood collects when it enters the heart.
The ventricles pump the blood out of the heart to the lungs or around the body.
The septum separates the right-hand and left-hand side of the heart.
The tricuspid valve is located between the right atrium and right ventricle and opens due to a build-up of pressure in the right atrium.
The bicuspid valve is located between the left atrium and left ventricle and likewise opens due to a build-up of pressure, this time in the left atrium.
The semilunar valves stop the back flow of blood into the heart. There is a semilunar valve where the aorta leaves the left ventricle and another where the pulmonary artery leaves the right ventricle.
Blood vessels leading into and out of the heart
There are four main blood vessels that take blood into and out of the heart.
The aorta is the largest artery in the body. It carries oxygenated blood away from the left ventricle to the body.
The vena cava is the largest vein in the body. It carries deoxygenated blood from the body back to the heart.
The pulmonary artery carries deoxygenated blood away from the right ventricle to the lungs.
The pulmonary vein returns oxygenated blood from the lungs to the heart.
Arteries carry oxygenated blood away from the heart (except for the pulmonary artery which carries deoxygenated blood away from the right ventricle to the lungs).
The main artery is the aorta.
The main vein is the vena cava.
Functions of the cardiovascular system
The cardiovascular system has four main functions:
- transport and deliver oxygen, nutrients and hormones to the body
- remove waste products such as carbon dioxide and lactic acid
- protection against disease and infection
- maintain body temperature
Maintaining body temperature
In the heat, blood vessels close to the surface of the skin enlarge. This process is called vasodilation. This allows more heat to be lost from the blood.
When a person takes part in exercise their face can become pink due to vasodilation of the blood vessels close to the skin’s surface.
In the cold, blood vessels at the skin’s surface close. This process is called vasoconstriction and takes blood away from the surface of the skin to help prevent it from losing heat.
Blood pressure
When the heart contracts it pushes blood into blood vessels which creates blood pressure.
A blood pressure reading consists of two values:
- systolic value – blood pressure while the heart is squeezing
- diastolic value – blood pressure while the heart is relaxing
Systolic is when the heart contracts and diastolic is when the heart relaxes.
The average blood pressure for an adult is 120/80 mmHg. The first number is the systolic value and the second number is the diastolic value.
Blood pressure is determined by Q (cardiac output) and the resistance to the blood flow (R). Resistance to blood flow is caused both by the diameter of the blood vessels and by the thickness of the blood. Furthermore, if a person has a condition called atherosclerosis (plaque in the arteries), their resistance to blood flow will increase and so will blood pressure. This can have serious health implications such as causing chronic high blood pressure, angina or even heart attack or stroke.
The heart’s performance as a pump
The heart’s function is to pump the blood and circulate it round the body. We assess the heart’s performance by measuring how much blood it pumps out each minute. This is called cardiac output. To calculate cardiac output, we also need to know about heart rate and stroke volume.
Cardiac values
Cardiac output (Q) is the amount of blood pumped from the heart every minute and can be calculated by multiplying heart rate (HR) by stroke volume (SV).
Heart rate (HR) is the number of times the heart beats in one minute. The average number of beats is 72 beats per minute.
Stroke volume (SV) is the volume of blood pumped out of the heart with every beat. The average amount of blood per beat is 0.07 litres.
Cardiac output = stroke volume × heart rate
Q = SV × HR
4.9 litres per minute = 0.07 litres × 70 beats per minute
The fitter you are, the larger your stroke volume and the lower your heart rate, therefore your cardiac output stays the same.
During exercise, tidal volume increases as the depth and rate of breathing both become greater. This has the effect of taking more oxygen into the body and removing more carbon dioxide.
| Rest | Exercise | |
|---|---|---|
| Heart rate | 72 beats per minute | 120 beats per minute |
| Stroke volume | 0.07 litres | 0.2 litres |
| Cardiac output | 5.04 litres per minute | 24 litres per minute |
Structure of blood and blood vessels
Blood is carried through three different types of blood vessels in the body:
- arteries
- capillaries
- veins
All blood vessels are specifically structured to perform their function. For example, a capillary is microscopically thin to allow gases to exchange, the arteries are tough and flexible to cope with high pressure blood flow and the veins contain valves to prevent the blood from travelling backwards when at low pressure. All vessels feature varying lumensize. The lumen is the hollow opening or the space inside the blood vessel.
| Artery | Vein | Capillary | |
|---|---|---|---|
| Function | Carry blood away from the heart (usually oxygenated blood, except for the pulmonary artery) | Carry blood towards the heart (usually deoxygenated blood, except for the pulmonary vein) | Allows diffusion of gases and nutrients from blood into the body cells |
| Wall | Thick, muscular | Thinner | Very thin, one cell thick |
| Lumen | Small | Large | Very small, only allows blood to pass through one cell at a time |
| Other features | Thick muscular walls to withstand blood flowing at high pressure as it leaves the heart; the largest artery is the aorta | Contain valves to prevent back flow of blood | Walls are made of semi-permeable membrane to allow transport of gases and nutrients into and out of the blood |
Blood
The main function of blood is to transport nutrients and oxygen to the cells of the body.
Blood is made up of four components:
- red blood cells – these transport oxygen around the body
- white blood cells – these fight infection
- platelets – these clot to prevent blood loss during injury
- plasma – this is the liquid part of blood
Red blood cells are very important for sport and physical activity because they contain haemoglobin. Haemoglobin allows them to carry oxygen from the lungs to the working muscles.
Red blood cells are disc-shaped cells with no nucleus. They are very small but their flattened shape gives a relatively large surface area which allows rapid diffusion of oxygen.
Cardiovascular system and exercise
Any changes to heart rate, stroke volume and cardiac output are determined by the intensity and duration of exercise.
Changes to heart rate during exercise
Heart rate is measured in beats per minute (bpm). During exercise the heart rate increases so that sufficient blood is taken to the working muscles to provide them with enough nutrients and oxygen. An increase in heart rate also allows for waste products to be removed.
Maximal heart rate can be worked out by the following equation:
Maximum HR = 220 – age
Change to stroke volume during exercise
Stroke volume increases which means more blood is pumped out of the heart each time it contracts.
Changes to cardiac output during exercise
At rest a person’s cardiac output is approximately 5 litres per minute, while during exercise it can increase to as much as 30 litres per minute as both their heart rate and stroke volume increase.
During exercise there is a greater cardiac output because the athlete requires more blood and oxygen to be transported to the working muscles. The increase in the amount of blood also helps with the removal of waste products lactic acid and carbon dioxide.
Changes to blood pressure during exercise
As exercise increases, cardiac output (Q) also increases. This has the effect of increasing blood pressure.
A typical blood pressure reading for a person at the start of exercise would be around 160/85 mmHg.