Which statement is the best description of respiration?A. Respiration is the same thing as inhaling and exhaling oxygen.B. Respiration is the process of breathing in oxygen through the lungs.C. Respiration is the exchange of oxygen and carbon dioxide in the alveoli.D. Respiration includes inhaling, exhaling, and reactions in cells using oxygen.

Correct option is d . 

Explanation: Our lungs are the portal where CO2, via tidal air, is chemically released from the hemoglobin contained in the red blood cells, component of the blood in the pulmonary artery coming from a heart ventricle (right one).

The air (O2) incoming to the lungs has maximum oxygen level, which enables oxygen molecules to the hemoglobin molecules who were just made available. (Actually, the CO2 didn’t leave so willingly—there was a little “pushing and shoving” going on, but happily, the oxygen won out in the end).

Now, firmly on board and seated in the “oxygen express”, these lifesaving molecules are routed via the left side of the heart, to the entire body, first through large arteries, but eventually into the tissues via capillaries.

Carbon, from sugars in the body, is reduced to the form of Pyruvate, which is the molecule that originally feeds into the Krebs’ Cycle, which decomposes it to CO2 (and energy).

Diffusion is the mechanism by means of which the oxygen goes from bloodstream to cell, and the opposite for CO2. Luckily, water doesn’t impede this movement too much.