Unlike most capillary beds, the glomerular capillaries exit into efferent arterioles rather than venules. Prostaglandins normally dilate the afferent arterioles of the glomeruli. Blood flows into these capillaries through the afferent arterioles and leaves through the efferent arterioles. Paired arterioles are generally smaller than corresponding venules. The bulbar conjunctival microvasculature contains arterioles, meta-arterioles, venules, capillaries, and communicating vessels. Arteries branch into small passages called arterioles and then into the capillaries. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. They may be composed of abnormal aggregations of arterioles, capillaries or venules. Excessive vasodilation, or insufficient constriction of the blood vessels (mostly arterioles), causes hypotension. Constriction of the afferent arterioles causes a decrease in the intraglomerular pressure, reducing GFR proportionally. Immediately following the arterioles are the capillaries. Extraglomerular mesangial cells are located in the junction between the afferent and efferent arterioles. As blood flows from the arterioles into the capillaries a change in pressure occurs. Travelling through the arterioles blood moves into the capillaries where gas exchange can occur. Blood travels through the arterioles and moves into the capillaries where gas exchange can occur.