Answer: Increasing pressure in the arteries and elastic recoil causes the pulmonic and aortic valves to close.
Explanation:The pulmonic and aortic valves close due to two main factors: increased pressure and elastic recoil.
During ventricular systole, the contraction of the respective ventricles increases the pressure within the pulmonary artery (in the case of the pulmonic valve) and the aorta (in the case of the aortic valve).
Once the pressure in these arteries exceeds the pressure in the ventricles, the valves close to prevent blood from flowing back. Additionally, the elastic properties of the arteries contribute to the closure of the valves during ventricular diastole.