Choose the most appropriate combination of components of respiratory membrane, from the following.
(A) Alveolar wall and ducts.
(B) Inner and outer pleural membranes and pleural fluid.
(C) Membranes of alveolar ducts and capillaries.
(D) Alveolar wall, alveolar capillary and interstitial space.

Hint: Respiration is defined as the exchange of O2 from the atmosphere with $CO_2$ produced by the cells. It is simply defined as intake of fresh air and removal of foul air. It includes inspiration and expiration processes . It involves biological oxidation of organic molecules.

Complete answer:
Based on the presence or absence of oxygen two types of respiration are present.
Aerobic respiration:- the respiration that occurs in the presence of oxygen and results in the formation of water , and energy.
Anaerobic respiration:- the respiration that occurs in the absence of oxygen and results in the formation of ethanol or sometimes lactic acid.

Though alveoli are the buildings that facilitate the gaseous exchange, the real gaseous exchange occurs via a layer known as the respiratory membrane. The membrane has countless layers which include - a layer of lining fluid that is formed of surfactant molecules, the epithelial layer, and the basement membrane; a very thin interstitial house current between the epithelial lining and the capillary membrane; a capillary basement membrane that every so often fuses with the alveolar basement membrane, and the capillary endothelium.

Alveoli are tiny sacs emerging from the bronchiolar extensions within the lungs that allow for the movement of oxygen and carbon dioxide between the lungs and bloodstream. They have thin walls that are surrounded by blood capillaries which also have simple squamous epithelium, thus forming a very thin membrane.

The gaseous exchange occurs in the alveolar sacs. During inhalation, oxygen has a higher concentration in the alveoli as compared to the blood and hence it diffuses into the blood through the thin layer of cells.

During exhalation, carbon dioxide is in a higher concentration in the blood than the alveoli and so it diffuses into the alveoli from the capillaries, again through this thin layer of cells.

Hence, the correct answer is option (D)

Note: Respiration is a multistep, enzyme mediated biochemical process of oxidative breakdown of organic compounds inside living cells releasing small packets of energy at various steps. It is, therefore, a catabolic and exergonic process. The organic compounds that undergo oxidative
breakdown in respiration are called respiratory substrates. The most common respiratory substrate is glucose.