Describe the action of the tricuspid valve when the ventricle is full

It prevents backflow of the blood

Blood enters from the right atrium to the right ventricle through the tricuspid valve also called right atrioventricular valve. Ventricles are filled during diastole (relax) period and blood leaves the ventricle during systolic (when the ventricle contracts) period. During systole, this tricuspid valve closes and prevents the backward flow of blood into the right atrium. This causes all blood to move towards the pulmonary arteries.

when it is at an intermediate population density

a metamorphic rock would be formed.