Alcohol has a high affinity for water and is therefore found in blood, urine, saliva and any other body tissue that contains water.  The blood carries the absorbed alcohol to the various organs and tissues of the body and will deposit alcohol in proportion to the water content of the various organs and tissues.  The more water - the more alcohol.  The brain has high water content so it receives a substantial share of the distributed alcohol.  Muscle tissue also has a relatively high water content, but fat tissue contains very little water, so very little alcohol is distributed to fatty tissue.  Thus, assume 2 people of equal size with markedly different amounts of fatty tissue consume precisely the same amount of alcohol under the same circumstances, the one with more fatty tissue will rise higher because there is less water in which to distribute the alcohol - so the concentration of alcohol will be higher.  This is an important consideration when dealing with gender differences, because generally speaking, women have more fatty tissue than men. 

Below is a table showing the distribution ratios for alcohol in body fluids and tissues compared to that in whole blood - which has been assigned a value of 1.00. 

Distribution Ratios Relative to Whole Blood

Whole Blood 1.00
Urine 1.35
Blood Plasma 1.12 (approx)
Saliva 1.12
Liver 0.92
Brain 0.86 (approx)
Fat 0.019