______ are thin-walled blood vessels that carry blood to the heart under relatively low pressure.
________ are small veins (microscopic in size) that collect blood from __________ and deliver it to larger veins.
In adults, all veins except for the _____________ carry deoxygenated blood. Venous walls are similar to arterial walls but their ________ have greater diameters.
Just like arteries, veins contain three distinct layers which are the _________, the ______ and the __________.
The tunica intima of the veins are composed of ___________.
The ___________ of the veins are poorly developed. They are composed of a thin layer of _________ and less __________ and their walls will _________ when empty.
The ________ is the outermost and most ________ layer of the veins. It contains much more _____ and _________.
Most veins that are larger than ___ millimeters in diameter have one-way ______ positioned in such a way that they can conduct blood against the force of _______.
Veins found in the ______ normally contain valves.
The valves of the veins are usually composed of 2 _______ formed by ________ of the __________. They are arranged to permit blood flow toward the heart and not backward in the opposite direction. In other words they prevent _________.