The primary function of the circulatory system is to transport oxygen, nutrients, and waste products. Hemodynamics of the vessels are governed by a variety of physical properties and laws that explain blood flow through the vascular system. Blood flow is affected by pressure differences, radius of the vessel, length, and viscosity (Poiseuille's Law). Flow is directly proportional to the difference in pressure and inversely proportional to resistance. Any disturbance in blood flow in the arterial and venous system disrupts the delivery of oxygen and nutrients and the elimination of waste products. Disturbances of blood flow can be due to compression of the blood vessel (edema, hematoma, compartment syndrome), structural changes within the vessel (atherosclerosis), vasospasm (Raynaud's syndrome), vasodilatation (sepsis, distributive problems), or blood clot (thrombus or embolus). Decrease in perfusion will promote compensatory mechanisms such as vasodilatation, development of collateral vessels, and anaerobic metabolism. If compensatory mechanisms are unable to meet the oxygen demand by the tissues, ischemia develops, and eventually, tissue death occurs.