Red blood cells, also known as erythrocytes, develop in the fetal liver or the adult bone marrow via the process of erythropoiesis. Erythropoiesis is stimulated by the cytokine hormone, erythropoietin, which is produced in the kidney following detection of decreased circulating oxygen levels. The primary function of red blood cells is to bind oxygen via hemoglobin and deliver it to cells. Red blood cells are also capable of inducing vasodilation by mediating the effects of nitric oxide (NO) bioactivity in disease states such as pulmonary hypertension and sepsis. Specifically, red blood cells release S-nitrosothiols when hemoglobin molecules are deoxygenated, causing the smooth muscle in blood vessel walls to relax and promoting blood flow to oxygen-depleted tissues. Red blood cells can also promote vasodilation by releasing ATP under conditions of shear stress and induce an immune response when they are lysed by pathogens.