The Respiratory System

The respiratory system includes the following (major) components:

The nose and mouth to breathe air in and out;
The pharynx (throat), larynx (voice box), trachea (windpipe) and the bronchi (two main branches) which lead down towards the lungs; and
The bronchioles (smaller branches), alveoli (air-sacs) and blood capillaries within the lungs.
While technically part of the muscular system, the diaphragm and intercostals (muscles of the ribs) are also employed for ventilation, as discussed below.

How Do We Breathe?

There are two main phases to breathing: inhalation and exhalation. In this cycle, air is exchanged between your lungs and the surrounding atmosphere, which is a result of muscle contractions on your part and Bernoulli's principle that air flows from a regions of high pressure to low. The average rate of respiration for adults at rest is 10-15 breathes per minute.

Inhalation

Also known as inspiration, inhalation involves the contraction of the of the intercostals and diaphragm. Found between the ribs, the intercostals lift the ribs higher while the diaphragm flattens in contraction. Subsequently, your thoracic (chest) cavity expands and more space is made available for your lungs to expand.

For those physics enthusiasts, they will know that such an increase in volume would coincide with a drop in pressure. This difference in pressure between your lungs and the outside will naturally force air to flow into your lungs through your nose and mouth. For this reason, breathing in humans and other animals that employ the same system is often referred to as "negative-pressure breathing".

Exhalation

Exhalation, or expiration, is essentially the reverse of the inhalation process. During this stage, the diaphragm and intercostals relax, pushing against the lungs and reducing the size of the thoracic cavity. This decrease in volume increases pressure in the lungs and the air is forced out from your lungs back to the atmosphere.

At rest, exhalation would be occur reflexively. However, during forced expiration (such as during exercise), your abdominal muscles and intercostals also contract to further decrease the volume of your chest cavity and force air out of your lungs more quickly

Respiratory Phase	Muscles Involved At Rest	Muscles Involved In Exercise
Inhalation	Diaphragm and Intercostals	Diaphragm and Intercostals
Exhalation	None (passive)	Abdominals and Intercostals

Hyperventilation

Hyperventilation is characterized by over-breathing and is typically triggered by sudden bouts of intense excitement, anxiety or injury. During hyperventilation, rapid deep-breathing quickly removes CO₂ from the body, leading to a change in blood pH often accompanied by nausea, light-headedness and tingling in the fingers and toes.

Individuals experiencing hyperventilation should be encouraged to sit and stay calm while try to reduce his/her breathing rate back to normal.

Valsalva Manoeuvre

Valsalva Manoeuvre is the technical term used to describe force exhalation against a closed airway. This is most commonly seen during exercise, especially weight training, when an individual holds his/her breath while exerting against a weight.

As a result, pressure is heightened in the thoracic cavity so that blood flow is stemmed, leading to dizziness, fainting or seeing spots. Accordingly, valsalva manoeuvre is especially dangerous to individuals with heart conditions or high blood pressure. Consideration should always be taken to ensure regular breathing in weight training and exercise.

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