Why does nitrous oxide have a very rapid onset?

The rapid onset of nitrous oxide can be attributed to the fact that very little of it is absorbed into the bloodstream when initially inhaled. This characteristic is related to its high lipid solubility and low solubility in blood. As nitrous oxide is inhaled, it quickly moves from the alveoli in the lungs into the bloodstream because it has a low partial pressure in the blood compared to the gas in the alveoli, which allows it to diffuse efficiently.

The rapid diffusion across the alveolar-capillary membrane is a primary reason for its quick effects, enabling a swift transition from the inhaled gas to the central nervous system. This leads to the immediate onset of sedation and analgesia that is often experienced in clinical settings. In contrast, if nitrous oxide were highly soluble in blood, it would take longer to achieve the desired concentration in the brain, resulting in a slower onset of action.