Muscular exercise leads to increased arterial output and is accompanied by capillary vasodilatation. Under these conditions the return circulation must adapt in order to fulfill its role of drainage. Primarily this is brought about by changing the properties of the vein walls: at the level of the venule there is a reduction in smooth muscle tone while at the level of the collecting veins there is an increase in vein wall elasticity. Secondly there is co-ordination of the muscle pump effect such that it matches the exerted effort. A special mode of flow results: at the level of the deep veins the flow becomes pulsatile. If the degree of muscular activity is prolonged and intesnse this "arterialisation" may reach the superficial veins. These adaptations require a venous system which is in perfect condition with regard to its ability to respond properly to the demands placed upon it.