Nutritive and non-nutritive blood flow: rest and exercise

There is growing evidence to support the notion of two vascular routes with in, or closely associated with skeletal muscle. One route is in intimate co ntact with muscle cells (hence is known as 'nutritive') and the other funct ions as a vascular shunt (and has had the interesting misnomer of 'non-nutr itive'). Recent findings suggest that the 'non-nutritive' route may, in par t, be those vessels in closely associated (interlacing?) connective tissue that nourishes attached fat cells, and may form the basis of 'marbling' of muscle in obesity. In addition, embolism studies using various size microsp heres indicate that the 'non-nutritive' vessels are likely to be capillarie s fed by terminal arterioles that branch from the same transverse arteriole s as those supplying terminal arterioles of the muscle capillaries (i.e. tw o vascular systems operating in parallel). The proportion of flow distribut ed between the two routes is tightly regulated and controls muscle metaboli sm and contraction by regulating hormone and substrate delivery as well as product removal. Because a high proportion of nutritive flow may elevate th e set point for basal metabolism, a low proportion of nutritive flow in mus cle at rest confers an evolutionary advantage, particularly when food is sc arce. In addition, the proportion of flow that is carried by the non-nutrit ive routes at rest affords a flow reserve that can be switched to the nutri tive route to amplify nutrient supply during exercise. Alternatively the no n-nutritive route may allow flow to escape when active muscle contraction c ompresses its nutritive capillaries. Thus rhythmic oscillation of blood flo w between the non-nutritive and nutritive networks may aid the muscle pump.