Different types of pulmonary volumes.

1. Tidal volume : It is the air normally inspired or expired. It is approx 500 ml.

2. Inspired reserve volume : It is the additional volume of air inspired with maximum possible effort after normal inspiration. It is approximately 2500 ml.

3. Inspiratory capacity (I.C.) : It is the total amount of air a person can inspire after a normal expiration.  It is the sum of tidal volume and inspiratory reserve volume. 

4. Expired reserve volume (ERV) : It is the amount of air expired with maximum possible effort after normal expiration. It is 1100 ml.

5. Vital capacity : It is the maximumvolume of air a person can breathe in after forced expiration or the maximum amount of air a person can breathe out after forced inspiration. It is the sum total of tidal volume, inspired reserve volume and expired reserve volume.

6. Residual volume (RV): It is the amount of  air left in the lungs after forcible expiration. It is 1100–1200 ml.

7. Functional residual volume : It is the amount of air left in lungs after normal expiration. FRV = ERV +RV.

8. Total lung capacity : It is the amount of air in lungs and respiratory tract after maximum inhalation effort. It is equal to the sum of vital capacity and the residual volume.

Total lung capacity = Vital capacity + Residual volume

9. Dead space : It is the space which contains a part of inspired air during inspiration and a part of expired air during expiration. 

Partial pressure : It is the pressure exerted by individual gas in a mixture of gases.

Air is a mixture of gases like O₂, CO₂, N₂ etc. The partial pressure exerted by O₂ is designated as pO₂. It is called partial pressure of O₂. Similarly partial pressure of CO₂ is written as pCO₂.

Due to pressure gradient  and difference in the partial pressure the gase move from the region of their higher partial pressure to the region of their lower partial pressure.

Thus, O₂ from alveolar air (High pO₂) diffuses into blood (having low pO₂₎ till pO_2. 
CO₂ from blood (High pCO₂₎ diffuses into alveoli ( having low pCO₂₎ . Thus, partial pressure helps in gaseous exchange.

Functions of respiratory tract :

1. The cartilages in the trachea keep the respiratory tract extended, allowing for the unobstructed passage of air between the outside atmosphere and the alveoli of the lungs.

2. The mucus lining keeps the respiratory passage moistened and traps particles as dust in the air preventing them from reaching the alveoli.

3. The diameter of the respiratory passages may be altered by muscles in their walls, thus regulating the volume of air entering the lungs.

4. Cells in connective tissue protect against infection and inhaled foreign particles not trapped by mucus. Lymphocytes and plasma cells produce antibodies in the presence of antigens, and macrophages and polymorphonuclear leukocytes are phagocytic. 

5. The respiratory tracts maintains the temperature of the air. 

Pharyngitis : It is an inflammation of pharynx often called sore throat. It may accompany the common cold and it usually involves the tonsils (tonsilitis).

Laryngitis : It is an inflammation of larynx that often accompanies pharyngitis. In laryngitis inflammation of larynx disturbs the vibration of vocal cords and the person has difficulty in speaking.

Bronchitis : It is an inflammation of the bronchi. The lining of bronchi swells and produces excess mucus.

Asthma : It is more correctly called bronchial asthma. It is  caused due to an allergic reaction to foreign substances that affect the respiratory tract. Allergens stimulate the release of histamine from the mast cells. Histamine has several effects, one of which is to cause bronchial smooth muscle to contract. 

Any surface that promotes the diffusion of gases must have following four basic features :

1. The surface area must have a relatively large surface area. Diffusion is a slow process, and the larger the area, the more is diffusion. Large surface area is maintained by extensive tissue projections and foldings.

2. The surface must be moist at all times, because oxygen and CO₂ must be dissolved in water to diffuse across cell membranes. Aquatic organisms easily meet this requirement, but it is difficult for the organisms living on land to maintain a moist surface. Many adaptations in terrestrial organisms minimize but can never totally prevent water loss during respiration.

3. A surface for gas exchange must be close to the active cells of the body or to the fluid that transports gases between these cells and the surface. Diffusion is slow to carry gases very deeply into the body of an organism.

4. The membrane should be thin and permeable to the gases