Respiration and Gas Exchange

In birds, the air sacs permit unidirectional flow of air through the lungs. Unidirectional flow means that air moving through bird lungs is largely 'fresh' air & has a higher oxygen content. So, the correct option is 'birds'.

When the diaphragm contracts, it flattens and moves downward, which increases the vertical dimension and volume of the chest cavity, leading to inhalation.

Lungs are the main respiratory organs of the human respiratory system. The windpipe or trachea divides into two branches called bronchi, entering into each lung. Each bronchus in the lung further divides to form many bronchioles. These bronchioles end up in a sac-like structure called the alveoli.  Hence, the lungs are made up of bronchi, bronchioles and alveoli.

The lungs are protected by two membranes called the pleural membranes. The outer pleural membrane is called the pleura and the inner one is called the visceral or pulmonary layer. There is a space between the two layers called the intrapleural space. It is filled with fluid to keep the lungs protected from the shocks and injuries.

A column-shaped cell found in the respiratory and intestinal tracts, which secretes the main component of mucus. The mucus produced by the goblet cells traps the finest dust particles and filter out largest dust particles.

So, the correct option is 'II and III'.

This is due to the presence of transformed elastic cartilage which gives support to the initial bronchioles and prevents it from collapsing. The incomplete cartilaginous rings that are present in the trachea and bronchi are absent in the bronchioles. They are supported by the transformed elastic cartilage.

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.

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.

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.

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. Each terminal bronchiole gives rise to a number of very thin, irregular walled and vascularised bag-like structures called alveoli. The branching network of bronchi, bronchioles and alveoli comprise the lungs. Thus trachea and bronchi are present outside the lungs and alveoli are tiny bags of lungs.

A.Bronchus - a passage of airway in the respiratory system that conducts air into the lungs.

B.Alveoli - any of the many tiny air sacs of the lungs which allow for rapid gaseous exchange.

C.Bronchioles - any of the minute branches into which a bronchus divides.

D.Hilum - a part of an organ where structures such as blood vessels and nerves enter the body.

So, the correct option is ‘Bronchioles’