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Lesson 6 - The Cardio-Respiratory System


The cardio-respiratory system consists of the cardio vascular system (heart and blood vessels) together with respiratory system (lungs and air ways). These systems work to transport oxygen to the muscles and organs of the body and remove waste products including carbon dioxide.

The Heart

The heart is a double pump. "Oxygen-poor" blood enters the heart from the vena cava to the right atrium, and flows down to the right ventricle. The first pump pumps "oxygen poor" blood to the lungs from the right ventricle of the heart via the pulmonary artery where it returns as "oxygen rich blood" via the pulmonary vein to the left atrium. It flows through to the left ventricle, where the second pump of the heart pumps the oxygen-rich blood to all the other parts of the body, via the aorta.

There are four valves that ensure blood flows in the correct direction, these are the pulmonary valve, the tricuspid valve, the mitral valve and the aortic valve.

The number of times the heart beats per minute, otherwise known as the pulse rate, is affected by the age and fitness of a person and their current level of activity. The heart's muscle wall is called the myocardium. Oxygen for the heart muscle itself is provided by blood vessels wrapped around the surface of the heart, not the blood flowing through it.

The Heart diagram of blood flow through the heart

The Lungs


The total lung capacity is approximately 5 litres - this is the maximum amount of air the lungs can contain.

The tidal volume is the volume of air breathed in and exhaled during normal breathing and is approximately 500 ml.

Residual volume is the volume of the air remaining in the lungs after the maximum amount has been exhaled. It is around 1.5l.

The vital capacity is the maximum amount of air that can be exhaled after a maximal intake of breath.

Composition of Inhaled and Exhaled Air

Inhaled Air

Nitrogen 79%
Oxygen 20%
Inert Gases 1%
Carbon Dioxide 0.03%

Exhaled Air

Nitrogen 79%
Oxygen 16%
Inert Gases 1%
Carbon Dioxide 4%

Oxygen is taken up by haemoglobin to form oxyhaemoglobin when the blood flows around the capillaries of the alveoli, and carbon dioxide is simultaneously returned to the alveoli.

trachea, bronchus, bronchi, bronchioles, alveoli

The lungs are like two huge sponges in the thorax who's job is to oxygenate blood and remove carbon dioxide from it.

Pharynx - part of the digestive system situated behind the mouth but in front of the oesphagus.

Larynx - the voice box

Epiglottis - a flap of cartilage that seals the trachea when food is swallowed, so that food cannot enter the lungs.

Trachea - the wind pipe.

Facts & Figures
About 500 ml or air is inhaled and exhaled 15 times per min.