An abundant and continual supply of atp is necessary for all living cells. active muscle cells require an extraordinary amount of atp to permit strenuous exercise for prolonged periods. toxins, reduced blood flow, and a compromised respiratory system can interfere with the transport of oxygen to active cells. a runner in a marathon faces multiple obstacles to continue to produce sufficient atp to remain competitive. when oxygen delivery becomes insufficient to support a runner's aerobic metabolism, cells switch to an emergency mode in which