Small Intestine

The small intestine is the primary site of nutrient absorption. It is divided into three sections: the duodenum, jejunum, and ileum.

A anatomical illustration of the stomach, small intestine, and colon or large intestine.

The small intestine shown here in yellow, blue, and pink is divided into three sections. The duodenum, jejunum, and the ilium.

FYI: The pancreas is an accessory organ that produces a secretion called bicarbonate. When chyme leaves the stomach it is acidic which would harm the small intestine. Bicarbonate is added to the chyme to neutralize its acidity prior to entering the small intestine.

Although appearing as a smooth circular tube from the exterior, from the inside the small intestine has circular folds and finger-like projections known as villi demonstrated in the following video.

The following video depicts nutrient absorption in the small intestine.

To sum up important information from the video: the structure of the small intestine (the circular folds and villi) increases surface area to maximize nutrient absorption. Additionally carbohydrates, lipids, and proteins are absorbed differently, but most of the absorption for all three macronutrients happens in the small intestine.

FYI: The liver is another accessory organ. In addition to filtering blood as part of the circulatory system, it produces bile which is stored in the gall bladder. Bile emulsifies fats which means it allows them to enter the water-based blood stream so that the body can use the lipids we eat. Bile is added to the chyme where the stomach and small intestine meet.