How do nutrients get past the barriers in the body?

Explanation:

Your brain is like a car. A car needs gasoline, oil, brake fluid and other materials to run properly. Your brain also needs special materials to run properly: glucose, vitamins, minerals and other essential chemicals. For example, the fuel (energy) for your brain is glucose. You can get glucose by eating carbohydrates or other foods that can be converted to glucose.

Your brain must manufacture the right proteins and fats to do things such as grow new connections or add myelin, the fatty sheath to axons. You do this by digesting proteins and fats in food and using the pieces, that is, the amino acids and fatty acids, to make the new brain proteins and fats. Without the correct amount and balance of particular building blocks, your brain will not work properly. Too little (deficiency) or too much (overabundance) of the necessary nutrient can affect the nervous system. (For a table that shows the effects of too little or too much of specific nutrients, please see Nutrient Effects on the Nervous System.)

the major parts of the digestive system:

salivary glands.

pharynx.

esophagus.

stomach.

small intestine.

large intestine.

rectum.

accessory digestive organs: liver, gallbladder, pancreas.