Liver, but not muscle, can provide neutral glucose to the bloodstream. The brain cannot tolerate glucose levels lower than about 2.2 mM for even short periods of time. The heart also makes no contribution to energy needs of other tissues. It relies on oxidation of fatty acids rather than glucose for its own needs, perhaps because the fatty acid supply is more reliable. In anaerobic operation, lactate is produced.
Energy is supplied through the liver.
It maintains blood glucose levels. The pancreas senses glucose levels and secretes glucagon when glucose is low and insulin when glucose is high. In times of glucose need, the liver provides glucose, and in time of glucose excess the liver converts it to glycogen. Glucose can be supplied from amino acids. This happens when the liver can no longer use all the Ac-CoA for fatty acid synthesis or for oxidation via the TCA cycle.