Mechanisms that determine the transport of substances from capillary blood to myocytes during changes in oxygen metabolism.

Transport of substances by convection in myocardium is mainly controlled by autoregulation and metabolic vasodilation. It can be increased by a factor 3-4 during increased oxygen demand and probably even more during pharmacologic coronary vasodilation. Other factors such as the systolic compression, neural and humoral factors, and endothelium may alone or in combination modulate local blood flow rate. Resistance to movement of molecules across the capillary membrane is well known in measurement of capillary permeability, and it seems likely that these phenomena operate in the interstitium as well. The rate of which hydrophilic substances find their way through the interstitial space between matrix fibers depends mainly on molecular size, i.e., the pathways will offer relatively more resistance to the progress of larger molecules as compared with small molecules. Despite the central role played by the microcirculation in supplying the myocytes with oxygen and nutrients, we remain relatively ignorant about many aspect of its functional and morphological responsiveness to ischemia.