3). remodeling of pulmonary vessels vasoconstriction of vessels hypertrophy of smooth muscle cell 4). increased blood volume and increased blood VISCOSI hypoxia→RBC↑→ blood viscosity个 → resistance of blood flow个3). remodeling of pulmonary vessels vasoconstriction of vessels hypertrophy of smooth muscle cell 4). increased blood volume and increased blood viscosity hypoxia→ RBC  → blood viscosity  → resistance of blood flow 