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Respiratory Physiology: An Introduction

The respiratory system is responsible for incorporating the oxygen in the environment for the utilization of energy from the organic compounds and for the elimination of carbon dioxide formed in the above process. This process can be subdivided into:

  1. Passage of air in between the lungs and the external environment
  2. Exchange of gases in between the alveoli and the blood in the pulmonary capillaries
  3. Transport of oxygen and carbon dioxide in blood
  4. Diffusion of oxygen and carbon dioxide between the cells and the capillaries
  5. Cellular respiration

1. Passage of Air In Between the Lungs and the External Environment

Air flows as a bulk, in and out of the lungs through the upper respiratory tract to come into contact with the blood in the pulmonary capillaries. The flow of air is dependent on the differences of pressure created in between the environment and the thoracic cavity due to the contraction of the respiratory muscles causing movements of the chest wall and the diaphragm.

2. Gaseous Exchange at the Lungs

Oxygen diffuses along a partial pressure gradient from the alveolar air spaces in to the pulmonary capillaries through the lining of the alveoli (simple squamous epithelium), the thin interstitium and the endothelium of the pulmonary capillaries, which is collectively known as the blood-gas barrier. Carbon-dioxide diffuses in the opposite direction through the blood-gas barrier in to the alveoli.

3. Transport of Oxygen and Carbon Dioxide in Blood

Oxygen which enters the blood stream by simple diffusion through the alveolar respiratory membrane is transported mainly bound to haemoglobin. A small percentage of oxygen is transported dissolved in the plasma. Carbon-dioxide is transported mainly in the dissolved form in plasma and the formed bicarbonate ions are transported within the cytoplasm of the red blood cells.

4. Diffusion of Gases In Between the Cells and the Capillaries

Oxygen is released from the haemoglobin to which it is bound and diffuses along a concentration gradient towards the cells in the peripheral tissues. Carbon dioxide produced as a by-product of cellular respiration diffuses in the opposite direction and is dissolved in the plasma of the blood and the cytosol of the red blood cells.

5. Cellular Respiration

The organic substances undergo oxidation by losing electrons during the passage of tricarbolic acid cycle and the electrone transport chain. In the process oxygen acts as an electrone and hydrogen acceptor and is converted to water. During the process, carbon dioxide is produced as a by-product.

Physiological Anatomy of the Respiratory System

The respiratory system is made up of:

  1. Upper respiratory tract (nose, pharynx and larynx)
  2. Lower respiratory tract (trachea and the divisions of the airways)

1. Upper Respiratory Tract

The upper respiratory tract is formed by the nose, pharynx and the larynx. The upper respiratory tract is responsible for the conduction of air, which is in the external environment, to the lower respiratory tract. In the process of conduction, the air is filtered of any macro-particles, is humidified and warmed to the body temperature. Large particles are prevented from reaching the lower respiratory tract by adhesion to the mucus in the nasal cavity and the pharynx and the hair in the nasal cavity. In addition, certain irritants are expelled by sneezing.

The pharynx is common to the digestive and the respiratory tracts and therefore, is incorporated with a defense mechanism (gag-reflex) to prevent food from entering the respiratory tract.


The larynx has an epiglottis (a covering cartilaginous flap) preventing aspiration. It also has vocal cords responsible for phonation, which meet at the glottis, which also can be closed tightly to prevent aspiration of substances. The glottis dilates during inspiration and constricts during expiration. The larynx is supplied by a sensory branch of the vagus nerve which can initiate the cough reflex, preventing any aspirated and irritant substances (if inhaled accidentally) form reaching the trachea.

2. Lower Respiratory Tract

The lower respiratory tract commences at the trachea, which has a diameter of 2.5 cm and divides in to two bronchi, supplying air to each lung. The bronchi further subdivide up to 16 divisions forming the conducting airways. The first eleven divisions have a cartilaginous wall but the next five divisions, known as bronchioles, is mainly muscular and therefore are subjected to collapse easily.


The 17th to 19th divisions of the lower respiratory tract, which are known as respiratory bronchioles, further divide to form alveolar ducts and alveolar sacs. These alveolar sacs communicate with each other through Kohn’s pores. Each lung comprises approximately 150 – 300 million alveoli and the total surface area is larger than a tennis court (70m2). The alveoli have a conformation of a honey-comb, which prevents collapse of individual alveoli and are lined by two types of cells. The predominant type (known as type I alveolar cells) is a simple squamous epithelium, across which the gases easily diffuse to the rich network of pulmonary capillaries lying underneath the thin basement membrane. The second type of cells is the type II alveolar cells, which secrete surfactant (a phospholipid responsible for decreasing the surface tension in the alveoli, so that they would be prevented from collapsing).

The alveoli are separated from each-other by a thin inter-alveolar septum, which is formed only of pulmonary capillaries. The pulmonary capillaries bring poorly oxygenated blood to the alveoli.

The physiology of the respiratory system and respiration is discussed in detail in this series of articles. However, the respiratory system preforms some non-respiratory functions in addition to its main function. These will be discussed in a separate article.

Learn more about the non-respiratory functions of the respiratory system


Chanaka Kahathuduwa (author) from Sri Lanka on July 23, 2013:

I believe the word "snot" has been used for organisms / germs.

Infection of the sinuses, known as sinusitis is mostly a due to bacteria. Most of the time, sinusitis is caused by bacteria, which are dwelling in the nasal cavity and the pharynx (throat) - known as commensals. When mucus (secretions) get accumulated in the sinuses, these organisms get multiplied in that medium and cause sinusitis.

tvs290 from India,bangalore on July 19, 2013:

This is interesting and all. Can you tell me why and from where all the snot comes from what makes a sinus infection.