Fundamentals Of Medical Physiology May 2026

E-1173 was now free in the interstitial space. This was a . Immediately, local smooth muscles in the vessel wall constricted ( vasospasm ). Circulating platelets, sensing exposed collagen, began to adhere, activate, and aggregate. They released ADP and thromboxane A₂, recruiting more platelets. A positive feedback loop had begun. Then, a cascade of inactive enzymes in the blood—the coagulation factors—catalyzed one another in a chain reaction, converting fibrinogen into sticky fibrin threads. Within minutes, a stable clot had formed, sealing the leak.

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. fundamentals of medical physiology

But a crisis loomed.

In the beginning, there was a void. Not an empty one, but a bustling, hypoxic darkness deep within the spongy red marrow of a human femur. Here, in the hematopoietic niche, a humble hematopoietic stem cell received a signal: a whisper of the cytokine erythropoietin, released by the kidneys because the blood’s oxygen levels had dipped slightly below a set point. E-1173 was now free in the interstitial space

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. Then, a cascade of inactive enzymes in the