Tag: gas exchange and smoking

Total lung capacity (TLC) is the maximum amount of air the lungs can hold when they are fully inflated. The average total lung capacity of an adult human male is about 6 litres of air. It can be calculated by using the formula TLC = TV + IRV + ERV + RV where TV is tidal volume(500 ml), IRV is inspiratory reserve volume(3100 ml), ERV is expiratory reserve volume (1200 ml) and RV (1200  ml) stands for residual volume.

Alveoli are the site of respiration inside the lungs. The human lungs have 300 million alveoli with a total area of about ${100 m^2}$ which is 50 times than that of the skin surface.

• The left lung is divided into two lobes, an upper and a lower, by the oblique fissure, which extends from the coastal to the mediastinal surface of the lung both above and below the hilum.
•  The left lung, unlike the right, does not have a middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the "lingula". 
• Its name means "little tongue". Hence, the Lingula structure originates from the Superior side of the left lung.
  

Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings, which prevents them from collapsing. Each terminal bronchiole gives rise to a number of very thin, irregular walled and vascularised bag-like structures called as alveoli. The branching network of bronchi, bronchioles and alveoli comprise the lungs.

The lungs of mammals including those of humans are soft, spongy, elastic organs with the smooth shining surface. Lungs are not hollow like balloons but are made of spongy, flexible tissue that inflates, when filled with air.

The walls of the alveoli are lined with simple squamous epithelial cells known as alveolar cells. This single layer of squamous cells is ideal for the diffusion of gases in the lungs.

In mammals, the nasal chambers are called as nasal vestibules. The nasal vestibules are formed by invagination of skin in embryonic stage and hence nasal vestibules have hair and sebaceous glands. In this manner dust particles cannot cross nasal vestibules. The remaining part of nasal chamber is surrounded by three types of bones- nasal, maxilla and ethmoids. At several places in the nasal passage these bones form outgrowths. These outgrowths are called turbinal bones. Due to these outgrowths, the nasal passage is long and spiral. This helps in making the temperature of air equal to body temperature before it reaches the lungs and does not allows unwanted particles like dust to reach lungs.

The trachea branches to give rise to two primary (main) bronchi. These then branch successively to give rise in turn to secondary and tertiary bronchi. These then branch to give rise to several orders of progressively smaller airways called bronchioles, the smallest of which are called terminal bronchioles. These are the last components of the conducting portion of the respiratory system. Terminal bronchioles give rise to respiratory bronchioles, which ultimately lead to the alveoli.

Alveoli are small pouches or sacs. Large number of alveoli occur inside each lung. The whole surface area of an alveolus functions as a respiratory surface. Due to large number of  alveoli in each lung, a very large area of respiratory surface becomes available (about $80{ m }^{ 2 }$) for exchange of gases. Thus, alveoli are designed to maximize the exchange of gases.