Arteries differ anatomically from veins in that they have three layers of tissue. There is the inner layer or intima composed of smooth endothelial cells. The next layer is the media, or muscle layer composed of smooth muscle cells and collagen based tissues. Finally the outer layer or tunica aventita (or tunica externa) which is basically a fibrous sheath. Arteries are under high pressure, insofar as the circulatory system goes, and help to maintain overall blood pressure by contraction or expansion of the muscle layer. This muscle layer is controlled by the autonomic nervous system, the one you do not have conscious control over. Veins, comparatively are composed of collagen outer layer, with a very thin smooth muscle layer and finally an inner endothelial layer. Veins collapse when not filled with blood and they rely on muscle activity to keep the blood moving through them. They also have valves to prevent the blood from pooling due to gravity (imagine if we didn't have valves, then every time we stood upright, all our venous blood supply would pool at our feet and cause them to swell.) As to the WHY the arteries carry blood under high pressure, that is part of evolution. In terms of efficiency, the heart is a pump which makes an internal 'wave' each time the ventricles contract. This wave is preserved through the arterial system by the contraction of the smooth muscle to maintain a consistent pressure, somewhere about 90-130 mm Hg for a normal human during the contraction, or systolic, phase of the cardiac cycle. This wave is necessary to permit diffusion across capillary membranes (you need a gradient of high to low pressure to enable this to happen) this is most important in highly vascular organs, like our kidneys and liver. Additionally, our most important organ, the brain is unable to store any calorie reserves for functioning in the absence of adequate blood flow (unlike your muscles, heart, liver, etc.) No glucose, no cellular activity. When the heart is 'relaxed', between beats, a constant background pressure of at least 70 mm Hg is needed to keep your organ systems functioning (especially your brain). In fact, if your overall mean arterial pressure drops below 70 mm Hg, then generally your internal organs are starved for blood and cells in them start to die as the by-products of cellular metabolism build up. Veins, by contrast, are a low pressure system, because the blood flow through the capillaries is reducing the overall velocity, much like reeds in a river mouth calm raging waters. By the time it reaches the venous system, your blood pressure (or central venous pressure = CVP) averages around 5 mm Hg or so. Quite a difference from the arterial pressures. Your heart becomes an inefficient pump if the CVP is too high, this is due to what is called 'preload'. I'll avoid getting into too deep a detail of the cardiac cycle and stroke volumes and all their calculations, at this point. Yes, your veins cannot withstand the high pressure of an aterial flow, as they lack the muscular resistance needed to prevent that flow from causing the layers of the vessel to separate from the stress. (They don't really 'wear down' like they're being scoured) When they use veins to bypass diseased arteries in the heart, they reverse them (to make sure the valves are facing the 'right way'. However, the pressure gradient of flow in the coronary arteries is still much less than that of the aorta, which is why they don't use veins to repair aneurysms. An aneurysm is when the layers of the arteries separate from each other, like onion layers, and then the high-pressure blood flow can get trapped in one of these tissue pockets, resulting in rupture of the vessel. This can be very quickly fatal, due to that high pressure arterial flow rate.