Final answer:
Afterload is the tension ventricles must generate to overcome the vascular resistance and open the semilunar valves, which are the aortic semilunar valve and the pulmonary semilunar valve. Thus, the correct answer is C. the aortic semilunar valve and the pulmonary semilunar valve.
Step-by-step explanation:
Afterload is the pressure that the ventricles must overcome to eject blood during systole. It is the tension required to pump blood effectively against the resistance in the vascular system. When ventricles contract, they exert force to open the semilunar valves, which include both the aortic semilunar valve and pulmonary semilunar valve, against this resistance. Hence, the pressure threshold that must be exceeded to open the semilunar valves is the afterload.
Conditions causing increased resistance, such as stenosis in the valves, result in a higher afterload, necessitating more force to open the valves and eject blood. Isovolumetric contraction is a phase in ventricular systole where the pressure within the ventricles rises but does not yet open the semilunar valves.
Following this phase, when the pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open, allowing blood to be ejected.
Therefore, the correct answer to the student's question 'Afterload is the pressure threshold that must be exceeded to open ______.' is C. the aortic semilunar valve and the pulmonary semilunar valve.