Blood cannot consume anything. This is just a colloidal solution. Only chemical reactions or mechanical reactions can occur in the solution. The blood cannot know and understand that it needs food or some elements, it is not alive. And the brain cannot control the blood. Nutrition needs only the cells that form the body. Therefore, all living organisms and feed. If the solution needs vitamin or iron, as you write, then it is placed there by cells, either according to a program recorded in their DNA or by receiving a command from the brain and with only one purpose - TO BLOOD REMOVE POWER TO CELLS. The process itself as iron enters the blood does not make sense to describe. But only through the cells. By the way, why do you need iron or vitamin or glucose in the blood, why do you need their population, where they are spent according to yours.
If it is not clear imagine drinking water in the faucet. To clean it, let's say add some kind of reagent and it becomes drinking. But the water itself does not need it. This is from the outside, a person makes her drinkable and with only one purpose, to DRINK.
A cell is life, and everything else exists thanks to cells, which are forced to create and maintain the necessary conditions for their existence in the whole organism.
And such a supplement if you want to eat, then your cells do not receive nutrition. Who eats a lot? Fat people. Why? Because the power to the cells does not reach. And the cells send signals to the brain: WE WANT TO EAT! But they get nothing. And the man eats and gets fat. If food from the stomach through blood, lymph and extracellular space does not reach the end point of the cell, then it should accumulate somewhere - that’s the result of fat and excess weight.