When erythrocytes undergo shear stress in constricted vessels, they release ATP, which causes the vessel walls to relax and dilate so as to promote normal blood flow.[17]
When their hemoglobin molecules are deoxygenated, erythrocytes release S-nitrosothiols, which also acts to dilate vessels,[18] thus directing more blood to areas of the body depleted of oxygen.
It has been recently demonstrated that erythrocytes can also synthesize nitric oxide enzymatically, using L-arginine as substrate, just like endothelial cells.[19] Exposure of erythrocytes to physiological levels of shear stress activates nitric oxide synthase and export of nitric oxide,[20] which may contribute to the regulation of vascular tonus.
Erythrocytes can also produce hydrogen sulfide, a signalling gas that acts to relax vessel walls. It is believed that the cardioprotective effects of garlic are due to erythrocytes converting its sulfur compounds into hydrogen sulfide.[21]
Erythrocytes also play a part in the body's immune response: when lysed by pathogens such as bacteria, their hemoglobin releases free radicals, which break down the pathogen's cell wall and membrane, killing it.[22][23]