The principle of conversation of matter that in closed system the total amount of matter?

Veins have a much lower blood pressure than arteries. which of these prevents backflow of blood in veins? a. pressure applied by the heart b. one–way valves in veins c. thin muscular walls of veins

How do all types of diffusion/passive transport actually ‘work’ without using even the smallest amount of cellular energy?

Body temperature is tightly regulated in mammals for example when external temperatures drop too much the body of a mammal response by in order to it's core temperature.