In an intact leg the blood is carried toward the heart step by step. This is caused by pressure and requires that the venous valves which are consecutively positioned close exactly and work together. If there is a build-up of pressure, caused e.g. by the change of position from lying to standing up, the blood, in the worst case, flows back only to the next venous valve.
With rising temperatures an enlargement of the veins is normal. An intact venous system compensates this phenomenon so that no blood becomes congested nor do orthostatic problems arise. But problems will occur when dilation is caused by the weakness of the connective tissue or the vascular wall of the veins. The venous valves then do not close properly, the blood “flows” in the wrong direction and finally collects in visible varicose veins.
Varicose veins: A widespread illness and its cause