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Roughly, what I know is, when we eat food it goes into our: Stomach > Small Intestine > Large Intestine > Rectum. So, it just moves through a digestive pipe.

What I don't understand is, what part of the food is responsible for blood's color and how does the food that we eat mixes with blood?

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up vote 13 down vote accepted

The red colour of blood isn't actually to do with food at all. The primary purpose of the blood is to carry oxygen to all the cells that require it to release energy. Red Blood Cells are filled with an iron containing pigment called haemoglobin. When it has oxygen bonded to it, haemoglobin has a bright red colour - it is this that gives blood its red colour.

In terms of how food enters the bloodstream, it is first broken down into extremely small constituents. This is done by mechanical action (i.e. chewing and the squeezing movement of your digestive system) and by chemical action through the use of enzymes. Enzymes are proteins that are secreted by various glands in the mouth (and are therefore contained in saliva), stomach and both intestines. These chemically break large food molecules down into small products such as glucose (sugar) and amino acids (protein-building blocks) amongst other things. These then move across the wall of the intestines and into the bloodstream, which are separated by only one cell:

Diagram of villus

Unfortunately that's the best image I could find. The intestine is everything outside of the pink layer (which is the wall of the intestine). The small molecules are able to pass through the pink layer and straight into the blood stream. The picture shows a fold in the wall of the gut, which increases its surface area so allows more molecules to diffuse across.

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Ok, I understand the intestine part. So, the iron in the food is responsible for creating hameoglobin which is bounded with oxygen to form red blood cells? Please feel free to correct me. –  user132314 Jun 8 '12 at 12:45
    
@user132314 yep, that's pretty much it :) –  Rory M Jun 8 '12 at 18:45
    
Just one slight correction - red cells aren't coated in haemoglobin; the haemoglobin is to be found inside the cells. –  Alan Boyd Jun 30 '13 at 21:56
    
@AlanBoyd thanks for the spot –  Rory M Jun 30 '13 at 22:46
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In essence, some of your food contains iron (especially red meat). This is taken up by your body and integrated into the haemoglobin protein which red blood cells contain in high amounts.

The iron in haemoglobin can bind oxygen (it does this when it passes through the lungs) which enables the red blood cell to carry it through your blood circulation.

The colour of blood comes from this haemoglobin. When the iron is binding oxygen, it is red. Otherwise, blueish-red. (http://www.ebi.ac.uk/interpro/potm/2005_10/Page1.htm)

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