Diaphragm: The Relationship between Blood Supply Regulation and Characteristics of the Contractile Function

Abstract

The diaphragm is a unique skeletal muscle; it is active throughout the lifetime and, therefore, differs from locomotor muscles in the properties of muscle fibers and the mechanisms of blood supply control. The aim of this review was to survey the structural characteristics of diaphragm muscle tissue, which provide its integral contractile properties; to compare the activity of vascular tone control mechanisms in the diaphragm and locomotor muscles; and to explore their relationship with the regulation of contractile function. The diaphragm differs from the majority of skeletal muscles by a high content of both slow fatigue-resistant muscle fibers of type I and fast fibers of type IIb, which provides endurance and high force-velocity characteristics of the diaphragm. The muscle fibers in the diaphragm are smaller, and the density of capillarization is much higher than in locomotor muscles. Arteries and arterioles that regulate blood supply to the diaphragm capillary bed combine the properties of arteries from muscles composed mainly of oxidative or mainly of glycolytic fibers. Such variety provides blood flow in the diaphragm adequate to its functional load with various patterns of activity. The mechanisms of vasoregulation in the diaphragm can qualitatively differ in the proximal and distal parts of the vascular bed. The functional properties of the proximal arteries can in part be explained by their proximity to the aorta and their small length. The contractile characteristics and blood supply of the diaphragm in various conditions should be considered when conducting respiratory muscle training in sports and rehabilitation medicine.