Question

opening and closing of the heart valves is caused by ________.

Explanation:

The opening and closing of heart valves are caused by pressure differences. When the pressure in one chamber of the heart is higher than in the adjacent chamber or vessel, the valve opens; when the pressure gradient reverses, the valve closes. This is a key concept in the study of the heart's physiology, which falls under the subfield of Biology (within Natural Science).

Answer:

pressure differences (or pressure gradients between heart chambers and adjacent vessels/chambers)