APLN and acute respiratory distress syndrome: Given the regulatory roles of apelin/APJ in the cardiovascular system and its relation with ACE2 and COVID-19 patients’ symptoms, which are characteristic of dysregulation of apelin/APJ signaling pathways, such as ARDS, vascular dysfunction, thrombosis, and inflammation [11, 18, 19], this study aimed to evaluate the serum levels of apelin in patients with COVID-19 with and without the underlying diseases of hypertension (HTN), diabetes mellitus (DM), and obesity (OB).