Best: Fundamentals Of Medical Physiology

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.

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. fundamentals of medical physiology

But a crisis loomed.

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. And so, the story of medical physiology is

The jogger felt nothing. A single cell had been lost, a thousand more had been born. The heart continued its electrical rhythm. The kidneys balanced pH. The lungs exchanged gases. The brain, unaware of the drama, sent a new signal down a motor neuron: Lift the foot. It rolled to a stop, flattened against a

And E-1173 obeyed. The oxygen disassociated from its hemoglobin and diffused down its concentration gradient into a muscle cell. Inside that muscle cell, the oxygen was immediately consumed by the in the mitochondria, the final step of aerobic respiration, to produce ATP. The jogger’s leg contracted. Homeostasis, at this microscopic level, was being maintained.