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However, when needed, active and or forced expiration can be completed by voluntary contraction of the abdominals and internal intercostal muscles,this further reduces the volume within the chest cavity and more air is exhaled.

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Expiration is generally a passive process that does not require muscle contraction.  As the muscles of inspiration relax and return to their resting positions air is forced out of the lungs as the pressure within them increases.

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As we breathe out the opposite happens as the diaphragm relaxes and pushes upwards,as seen in the right image above.  This reduces the size of the chest cavity making the pressure within the lungs higher and results in air being exhaled.

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(4)During exercise when inspiration increases, the external intercostal muscles are recruited to help with the increase in ventilation rate.  They work to lift the ribs up and outwards, further increasing the chest cavity and enabling more air to be inspired.

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(4)During exercise when inspiration increases, the external intercostal muscles are recruited to help with the increase in ventilation rate.  They work to lift the ribs up and outwards, further increasing the chest cavity and enabling more air to be inspired.

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(3)When we breathe in, the air enters the lungs, and the pressure in the lungs changes from high to low.

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(3)When we breathe in, the air enters the lungs, and the pressure in the lungs changes from high to low.

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The pressure changes are bought about by the movement of two sets of muscles.

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Ventilation requires pressure changes occur in the lungs,so that air can be forced in an out of the alveoli.

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Both of these actions decrease the volume inside the thorax,causing the pressure inside the thorax to increase.

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The muscle in the diaphragm relaxes and it bulges to due to pressure from the organs below.

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This lowers the ribs downwards and inwards.

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The internal intercostal muscles contract and the external intercostal muscles relax

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since atmospheric pressure is greater, air moves into the lungs and they inflate

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at the same time,the diaphragm contracts and flattens

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both of these actions increase the volume inside the thorax,causing the pressure inside the thorax to decrease

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the external intercostal muscles contract and the internal intercostals muscle relax, rising the ribs upwards and outwards