As a respiratory physiologist, I can explain the process of oxygen entering the bloodstream. When we inhale, oxygen from the air travels down the trachea into the bronchi, which branch into smaller and smaller tubes until they reach the tiny air sacs known as alveoli in the lungs. These alveoli are surrounded by a network of tiny blood vessels called capillaries. The oxygen in the air within the alveoli diffuses across the thin walls of the alveoli and into the capillaries, where it binds to a protein in the red blood cells called hemoglobin. Hemoglobin has a high affinity for oxygen and can carry up to four molecules of oxygen at a time. This oxygen-hemoglobin complex is what gives arterial blood its bright red color. Once the oxygen is bound to the hemoglobin, the red blood cells transport it through the bloodstream to the body's tissues and cells. At the cellular level, oxygen is released from hemoglobin and diffuses into the cells where it is used for cellular respiration, which is the process by which cells produce energy. Here's the translation in Chinese: 作为一位呼吸生理学家，我可以解释氧气进入血液的过程。当我们吸气时，空气中的氧气通过气管进入支气管，支气管分支成越来越小的管子，直到它们到达肺部被称为肺泡的微小空气囊。这些肺泡被一个称为毛细血管的微小血管网络所包围。 肺泡内空气中的氧气穿过肺泡的薄壁扩散到毛细血管中，在那里它与红细胞中的一种叫做血红蛋白的蛋白质结合。血红蛋白对氧气有很强的亲和力，一次可以携带多达四个氧气分子。这种氧气-血红蛋白复合体赋予动脉血鲜红色。 一旦氧气与血红蛋白结合，红细胞就通过血液将氧气输送到身体的组织和细胞。在细胞层面，氧气从血红蛋白中释放出来，扩散到细胞中，在那里它被用于细胞呼吸，即细胞产生能量的过程。 read more >>

Oxygen from the air to cells. When we breathe in oxygen enters the small air sacs, called alveoli, in the lungs. Oxygen diffuses from there into the bloodstream. Oxygen is not carried in the plasma, but is carried by the red blood cells.read more >>