Why are valves found in veins, but not in capillaries or arteries?

Aside from preventing back flow of blood, valves in the veins aid in the return of blood from the extremities to the heart ( from feet and legs in particular).

Explanation:

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the answer is third world countries, take for instance china, they restricted each family with only one child,

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enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products