Fundamentals Of Medical Physiology -

The stem cell heeded the call. It divided, differentiated, and extruded its nucleus, transforming into a biconcave disc of pure hemoglobin. Thus was born Erythrocyte E-1173, a cell with no organelles, no ambitions, and only one purpose: to carry oxygen.

E-1173’s first challenge was to leave the marrow. It squeezed, deforming its flexible membrane (a property called ) through a tiny pore in the sinusoidal wall. It was now adrift in a raging river: the venous bloodstream. The current was driven by the right ventricle of the heart, a four-chambered marvel of hemodynamics . E-1173 was swept through the vena cava, into the right atrium, through the tricuspid valve, and into the right ventricle. With a coordinated electrical impulse from the sinoatrial node—a cardiac action potential —the ventricle contracted. Lub . E-1173 was shot through the pulmonary artery toward the lungs. fundamentals of medical physiology

This was the first law of physiology: —the body’s fierce, unyielding drive to maintain stability. The stem cell heeded the call

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!” E-1173’s first challenge was to leave the marrow