The structural and functional unit of lung is

A. Alveoli

B. Trachea

C. Bronchiole

D. Bronchus

The lungs are the respiratory tract's main organs. They are suspended inside the thorax's pleural cavity. Two thin membranes called pleuras surround the lungs, which secrete a fluid that allows the lungs to pass freely inside the pleural cavity. This is important so that while breathing, the lungs can expand and contract.

Lung tissue is made up mostly of alveoli. The operating units of the lungs where gas exchange takes place are these tiny air sacs. There can be as much as 700 million alveoli in the two lungs, creating an immense overall surface area for the exchange of oxygen.

Currently, alveoli have as much surface space in the two lungs as half a tennis court! The alveoli fill up with air each time you breathe in, making the lungs expand.

The blood in the mesh-like network of tiny capillaries that surrounds each alveolus absorbs oxygen in the air within the alveoli. Carbon dioxide is therefore absorbed into the air inside the alveoli from the blood in these capillaries. Air escapes the alveoli and floods into the outer world each time you breathe out, bringing waste gases with it.

From two main outlets, the lungs receive blood. Deoxygenated blood is given to them from the heart. This blood consumes oxygen throughout the lungs and brings it back across the body to the heart to be injected into cells. The lungs also obtain oxygenated blood from the heart that supplies cellular respiration with oxygen to the cells of the lungs.

From these discussions we can conclude that alveoli is the structural and functional unit of the lung.

Therefore the correct answer is option (A).