Carbohydrates must undergo active transport to be absorbed into the epithelial cells of the small intestine, because they cannot passively pass through the membrane.
Amino acids also must undergo active transport into the epithelial cells of the small intestine because many of them are charged. This is important, because they cannot passively be absorbed as many amino acids go against an electrochemical gradient.
Small lipids can passively diffuse through the membrane into intestinal cells because they are small and nonpolar.
The epithelial cells of the small intestine are the first layer of cells that absorb nutrients. They must pass the absorbed nutrients through to intestinal capillaries.
The intestinal capillaries bring oxygenated blood to the small intestine, but also carry away the nutrients absorbed. Because blood entering the small intestine through these capillaries have low concentrations of nutrients, a concentration gradient is established that facilitates the movement and absorption of these nutrients into the blood.
Hepatic portal circulation is used to transport nutrients to the liver for processing.
The liver is the processing center for many nutrients, except for most fats. Most fats bypass first-pass metabolism in the liver and enter the circulation.
Larger lipids cannot diffuse through the membrane into the intestinal cells, so they are broken down into triglycerides and form chylomicrons for transport.
The lacteals are lymphatic capillaries that absorb dietary fats in the villi of the small intestine. These lacteals transport chylomicrons through the lymphatic system into blood circulation.
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