The effects of ischemia on a tissue vary depending on the oxygen and high energy compounds stores, the level of activity o, tissue, which determine the oxygen and metabolites consumption and the threshold for functionality cessation. In addition, the tissue capacity to generate ATP from anaerobic metabolism is important.

There is a large variability in all parameters between tissues. For example, the cardiac muscle has nearly no reserves of energy/oxygen stores. It extracts more oxygen from the blood flowing through the coronary circulation than any other tissue and is in constant high demand for oxygen and metabolites. The brain also lacks storage and is working hard even when seems idle. The brain has a very low tolerance for low oxygen supply and functionality will decrease or stop very soon after blood supply stops or becomes critically low. Nerves (axons), on the other hand, have little oxygen consumption, particularly when not activated.

Muscles, skin, fat and bone tissues, on the other hand, are much more resistive to blocking of the blood flow into them. Muscles are used to consuming more energy than is supplied to them. Energy is stored in the form of Creatine Phosphate which can easily transfer a phosphate to an ADP molecule and generate an ATP. In addition, oxygen is stored in the muscle as bound to a Heme component of the Myoglobin which can dissociate and become available when the tissue PO2 falls below 10 or so mm Hg. Also, there are some ATP stores available. Once the muscle PO2 falls sufficiently, glycogen stores are converted to glucose and anaerobic metabolism becomes active.

It should be noted that all the enzyme systems that are needed for these metabolic pathways are readily expressed in muscles. They provide energy when the muscles are active, and are available to prevent ischemic injury for quite some time.

It is generally accepted that blocking the arterial blood flow into a limb is safe for at least two hours. Abstracts shown below are a few of many publications that studied this issue in animal and human experiments.