What causes the pump
During higher-rep training, blood flow into the working muscle outpaces venous return, and metabolic byproducts accumulate and draw fluid into the muscle cells. The result is a temporary swelling, the sensation lifters call the pump, driven largely by an increase in intracellular fluid.
The effect is transient and subsides as circulation normalizes after the set.
Cell swelling as a possible growth signal
Cell swelling has been proposed as one of several mechanisms that may contribute to muscle growth. The idea is that the stretch on the cell membrane is detected by mechanosensors that can influence signaling pathways linked to protein synthesis. This is a plausible and actively studied mechanism rather than a settled, primary driver of hypertrophy.
Current evidence points to mechanical tension and training volume as the better-established drivers, with cell swelling a secondary and uncertain contributor. This is general educational information summarizing exercise physiology, not medical advice.