Our vascular system comprises arteries, veins, and the capillaries that connect them. The capillary wall is made of a single layer of cells and is the only site where oxygen, carbon dioxide, and nutrients are exchanged. Krogh observed frog skin and muscle under a microscope and proved that capillaries open and close according to how much oxygen the tissue is using. He called this phenomenon "vasomotion" and suggested that muscle in the vessel wall and chemical signals were involved. His work greatly influenced the understanding of conditions in which blood-flow regulation is key, such as exercise physiology, diabetes, and ischemic disease. Today, concepts of microcirculatory control are also applied in brain-imaging techniques and drug-testing platforms.

Microcirculation comprises arterioles, capillaries, and venules whose diameters permit single-file passage of erythrocytes, thereby optimizing exchange surfaces. Using intravital microscopy of amphibian webs and mammalian skeletal muscle, Krogh quantified the recruitment of capillary networks during alterations in metabolic rate. He proposed the existence of precapillary sphincters that respond to local oxygen tension, carbon dioxide, pH, and metabolites such as adenosine. By calculating diffusion gradients around a single capillary—formulated later as the "Krogh cylinder"—he provided a mathematical framework for tissue oxygen transport. His measurements showed that only a fraction of capillaries are perfused at rest and that capillary density effectively increases during muscle contraction. This dynamic regulation minimizes cardiac work while ensuring adequate delivery and has been applied in clinical perfusion strategies. Krogh’s ideas foreshadowed contemporary research on endothelial NO signaling, capillary rarefaction in hypertension, and sepsis-induced microvascular dysfunction. The Nobel Committee recognized that revealing this fine-tuned regulatory system connected cellular metabolism to systemic circulation, a cornerstone of modern integrative physiology. The findings also influenced comparative biology, as Krogh demonstrated species-specific adaptations in capillary architecture relative to metabolic demands.

In his 1918 monograph "The Anatomy and Physiology of Capillaries," Krogh defined the "capillary motor regulating mechanism" as the periodic opening and closing of skeletal-muscle capillaries observed under intravital microscopy. Combining Fick’s first law with Poiseuille flow, he analyzed how capillary diameter shifts modulate tissue PO2 gradients, thereby delineating the boundaries between diffusion-limited and perfusion-limited oxygen delivery. The Krogh cylinder model, which treats a single capillary as the axis of a cylindrical tissue domain, allowed derivation of a critical diffusion radius Rc from the steady-state oxygen consumption rate (M0) and diffusion coefficient (D). Experimental measurements indicated that perfused capillary density is roughly 30–40 % at rest and approaches nearly 100 % during contraction, lending quantitative support to the concept of local regulation of peripheral vascular resistance by vascular smooth muscle. This framework became the theoretical basis for distinguishing flow-control and capillary-recruitment strategies in microvascular regulation. Modern fluorescence microscopy and photoacoustic imaging have validated Krogh’s observations in real-time 3-D, revealing molecular mechanisms whereby NO, PGI2, and EDHF modulate sphincter tone. The Krogh model now underpins simulations of oxygen transport, tumor vascular design, and analyses of muscle atrophy during spaceflight, among numerous other mathematical physiology applications. Within the expert community, Krogh’s "unified sphincter doctrine" is being re-examined through murine window-chamber studies and systems-biology network analysis, driving new hypothesis testing on heterogeneous perfusion.