It s based on gravity. In lower doses of nitroglycerin, gravity is strong enough to open up the veins and cause pooling. Yes, this pooling is more effective when Nitro is taken standing up. When someone s supine there is less of a pooling effect and therefore Nitroglycerin is less effective. The arterioles still have sympathetic tone acting on them so they don t dilate as much as the veins.
Much higher doses can get the arterioles to dilate however.
There s yer standard in the movies sublingual nitroglycerin. It dissolves quickly in the venous bed down there at the bottom of your mouth and goes right to the heart. Kind of slimy. They looked about 5 x6 in the picture. Hey, the lipophillic nature of the nitrates is important in their absorption.