The respiratory system deals with ventilation and perfusion of tissues through the gas exchange process. It is composed of several essential structures contributing to its functionality.It involves the upper and lower airways, the lungs, the bony thorax, and the respiratory muscles.

a. The upper airways – these includes the nose, mouth, and the laryngo- pharynx and larynx. The purpose of these structures is to filter and humidify the air that is inhaled. The importance of epiglottis during eating and drinking prevents aspiration of food and fluids into the lower airways.

b. The lower airways – trachea, bronchi, and the lungs are considered to be in the lower airways. The trachea is the passageway of air towards the two bronchi of the lungs. The bronchi on the other hand are comprised of lobar bronchi, tertiary bronchi, terminal and respiratory bronchioles, alveolar ducts and the alveoli – which is the gas-exchange unit of the lungs. The alveoli are made up of two types of cells called Type I and Type II cells. Type I cells forms the walls where the gas exchange happens while Type II cells produces surfactant (essential in expanding and collapsing the alveoli uniformly).

The lungs are wrapped with what is called pleura which allows the layers of the lungs to slide smoothly as it expands and contracts during respiration. The lungs are divided into two, the right and left lungs. The left lung is composed of two lobes which are smaller than the right lung. On the contrary, the right lung is much bigger than left lung and composed of three lobes; the upper, middle, and the lower.

c. The bony thorax – it is comprised of bony structures such as the clavicles, sternum, scapula, the ribs, and the thoracic vertebrae. The ribs protect the lungs and allow the chest to expand and contract properly. These ribs are attached to vertebrae and the sternum. The floating ribs, also known as the 11th and the 12th ribs are not attached to any bones in front.

d. The respiratory muscles – these muscles pertains to the diaphragm and the external intercostals muscles. They contract during inspiration and expand during exhalation. The accessory muscles involve the trapezius, strenocleidomastoid, and the scalene muscles. Sometimes a person uses his abdominal muscles when there is an obstruction in the airway. These accessory muscles are evident during difficulty in breathing.

Physiology of Respiration

            Respiration can be considered into two, the internal respiration and the external respiration. The external respiration happens inside the lungs while the internal respiration occurs within the tissues.  There are three mechanisms during external respiration: (1) ventilation which refers to gas exchange in the airways, (2) pulmonary perfusion, which speaks itself to the adequate perfusion of blood in the lungs, and (3) diffusion, which pertains to the movement of gas from an area of greater pressure to an area of lesser pressure through a semi-permeable membrane- it is through diffusion that internal respiration happens.

            The respiratory process is controlled in the medulla oblongata of the brain. It adjusts with the alveolar ventilation and the demands of the body. Here is what happening during breathing:

1. During inhalation, the respiratory muscles contract and the diaphragm is pushed downward. The pressure is then negative which allows the inhalation of air into the upper to the lower airways or up to the lungs. The lungs expand to accumulate adequate amount of air.

2. During exhalation, the muscles relax making the lungs contract. The diaphragm moves upwards maintaining positive pressure and letting the air leave the lungs.

3. During at rest, the respiratory muscles relax and there is no movement of air.


Bullock, B. (1996). Pathophysiology: Adaptations and Alterations in Function 4th Edition. Lippincott- Raven Publishers.
Critical Care Nursing Made Incredibly Easy 3rd Edition. (2012). Lippincott Williams & Wilkins.