Fundamentals Of Medical Physiology 2021 -

In the lung’s alveolar capillaries, E-1173 experienced a transformation. It rolled to a stop, flattened against a thin endothelial wall. On the other side was a puff of inhaled air (partial pressure of O₂ ~100 mmHg). The air’s oxygen molecules, driven by the simple physics of , passed through the alveolar membrane, through the plasma, and into E-1173. There, oxygen bound cooperatively to the four heme groups of its hemoglobin. E-1173 turned from a dull maroon to a brilliant scarlet. It had been oxygenated . In return, it unloaded the waste product carbon dioxide (as bicarbonate, thanks to the enzyme carbonic anhydrase in its cytoplasm) back into the alveolus to be exhaled. The law of mass action was served.

And so, the story of medical physiology is not about one cell, but about the relentless, integrated, and beautiful logic of systems working in concert. It is the story of how the body, every second of every day, reads its internal environment and makes it right. fundamentals of medical physiology

E-1173, however, was trapped and doomed. A macrophage, the tissue’s resident sentinel, engulfed it in a quiet act of . The heme group was broken down into biliverdin, then bilirubin, which the liver would eventually excrete in bile. The iron atom was carefully saved, bound to transferrin, and shipped back to the bone marrow to build a new red blood cell. In the lung’s alveolar capillaries, E-1173 experienced a

Now bright and buoyant, E-1173 returned to the left heart and was launched into the systemic circulation. It traveled at breakneck speed through the aorta, then into arteries, then arterioles. The flow was not silent. It heard the faint, rhythmic thump-thump of each heartbeat—the —and felt the pressure wave that would be measured as 120/80 mmHg on a clinician’s cuff. The air’s oxygen molecules, driven by the simple

Finally, E-1173 arrived at its destination: a sleepy capillary bed in the gastrocnemius muscle of a jogging human. The environment here was hostile. The local pH was acidic from lactic acid. The temperature was high from muscular work. CO₂ partial pressure was elevated. All of these factors—the —were chemical insults screaming, “Unload your oxygen!”