Vasodilation can be described as a widening of the veins and blood vessels which results from the relaxation of smooth muscle cells within the vessel walls. It is particularly prevalent in the large arteries and small arterioles. The primary function of vasodilation is to increase blood flow in the body to tissues that need it most. In essence, this process is the opposite of vasoconstriction, which is the narrowing of blood vessels.
This effect is typically very difficult to consciously perceive but often results in a bloodshot red eye effect and relief from glaucoma.  
Vasodilation is often accompanied by other coinciding effects such as decreased blood pressure. It is most commonly induced under the influence of moderate dosages of cannabinoid compounds, such as cannabis, JWH-018, and THJ-018. However, it can also occur under the influence of poppers and viagra.
- Cardiovascular Effects of Cannabis | http://www.idmu.co.uk/canncardio.htm
- Is Marijuana an Effective Treatment for Glaucoma? | http://medicalmarijuana.procon.org/view.answers.php?questionID=000140