Respiration and Gas Exchange

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’

Breathing involves expansion and contraction off lungs to adjust the partial pressure of relative gases for exchange. The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction and chest wall expansion) which results in a decrease in lung pressure in comparison to the atmosphere. Thus, air rushes in the airway. The process of exhalation occurs due to an elastic recoil of the lung tissue which causes a decrease in volume, resulting in increased pressure in comparison to the atmosphere. Thus, air rushes out of the airway. 

A bronchus, also known as a main or primary bronchus, is a passage of airway in the respiratory tract that conducts air into the lungs. There is a right bronchus and a left bronchus and these bronchi branch into smaller secondary and tertiary bronchi which branch into smaller tubes, known as bronchioles. No gas exchange takes place in the bronchi. The trachea (windpipe) divides into two main or primary bronchi, the left and the right, at the level of the sternal angle and of the fifth thoracic vertebra or up to two vertebrae higher or lower, depending on breathing, at the anatomical point the carina of trachea. An alveolus is an anatomical structure that has the form of a hollow cavity. Thus, option B is correct.

The vertebral column, also known as the backbone or spine, is a bony skeletal structure found in vertebrates. Skull is present in head. The rib cage is formed by the vertebral column, ribs, and sternum and encloses the heart and lungs. In humans, the rib cage, also known as the thoracic cage, is a bony and cartilaginous structure which surrounds the thoracic cavity and supports the pectoral girdle (shoulder girdle).

• The process of breathing ) is divided into two distinct phases, inspiration and expiration occur simultaneously.
  
• Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure,
•  Inspiration is initiated by the contraction of a diaphragm which increases the volume of the thoracic chamber in the anteroposterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorsoventral axis.
• The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases the intrapulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs. Hence, When the pressure of air within the lungs is less than the atmospheric pressure, air will not flow out of the lungs.
  

The bronchioles or bronchioli are the passageways by which air passes through the nose or mouth to the alveoli (air sacs) of the lungs. The bronchus divides into smaller branches known as segmental bronchi, which divide again into bronchioles, and then again into terminal bronchioles. 

Bronchi are tubes that branch off from the trachea into each lung. Bronchioles are smaller tubes that branch off from each bronchus. There are only two bronchi compared with the hundreds of bronchioles.

The bronchioles or bronchioli are the passageways by which air passes through the nose or mouth to the alveoli (air sacs) of the lungs, in which branches no longer contain cartilage or glands in their submucosa.  The diameter of the bronchioles is often said to be less than 1 mm, though this value can actually range from 0.3 mm to 5 mm.

The cartilage rings in the trachea prevent the trachea from collapsing during the absence of air and also protect it. These cartilaginous rings are C-shaped.

Alveoli are tiny, balloon-like structures that significantly increase the total surface area of the lungs, facilitating efficient gas exchange between air and blood.

From the larynx, air moves into lower respiratory tract, starting with the trachea and down to the intersection that branches to form the right and left primary (main) bronchi. Each of these bronchi branch into secondary (lobar) bronchi that branch into tertiary (segmental) bronchi that branch into smaller narrow tube airways called bronchioles that eventually connect with tiny specialized structures called alveoli that function in gas exchange.

Ventilation in mammals is achieved by changing the volume of the thoracic cavity, which is controlled by the contraction and relaxation of the diaphragm and the intercostal muscles.

The large number of alveoli maximizes the surface area available for the diffusion of oxygen and carbon dioxide, which is essential for efficient respiration.