During 1905–1934 physiology increasingly merged in vivo experimentation with quantitative analysis. Researchers advanced chamber-based models to observe vascular remodeling and blood flow in living tissues, coupled with cross-species growth studies that illuminated resource allocation and developmental scaling. Cellular physiology began linking mechanobiology and diffusion to tissue function, while investigations into respiration and blood oxygen transport across taxa highlighted universal strategies for gas exchange. This period thus established a unified framework combining observation, measurement, and mechanistic interpretation of living systems. Historical Significance: The period's innovations seeded a durable paradigm that treated physiological systems as integrated networks governed by transport and energy considerations. The concept that vascular networks optimize energy expenditure and the formalization of diffusion as a rate-limiting process shaped subsequent physiology and bioengineering. Simultaneous work linking muscular activity, respiration, and circulation fostered a holistic view of organismal function. Foundational demonstrations of nerve endings in muscle and early insights into neuromuscular communication established the basis for motor control and synaptic physiology, while cross-species growth and developmental trajectories informed theories of resource allocation and scaling.