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3.4: Small Intestine

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    1080
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    The small intestine is the primary site of digestion. It is divided into three sections: the duodenum, jejunum, and ileum (shown below). After leaving the stomach, the first part of the small intestine that chyme will encounter is the duodenum.

    Figure 3.41 .png

    Figure 3.41 Three sections of the small intestine1

    The small intestine consists of many layers, which can be seen in the cross section below.


    Figure 3.42.png

    Figure 3.42 Cross section of the small intestine2

    Examining these layers closer, we are going to focus on the epithelium, which comes into contact with the chyme and is responsible for absorption. The lumen is the name of the cavity that is considered “outside the body” that chyme moves through.

    Figure 3.43.png

    Figure 3.43 Cross section of small intestine with the structures labeled2

    The organization of the small intestine is in such a way that it contains circular folds and finger-like projections known as villi. The folds and villi are shown in the next few figures.

    Figure 3.44.png

    Figure 3.44 Folds in the small intestine2

    Figure 3.45.png

    Figure 3.45 Villi in the small intestine3

    Figure 3.46.png

    Figure 3.46 Villi line the surface of the small intestine2,4

    If we were to zoom in even closer, we would be able to see that enterocytes (small intestine absorptive cells) line villi as shown below.

    Figure 3.47.png

    Figure 3.47 Enterocytes line villi4

    The side, or membrane, of the enterocyte that faces the lumen is not smooth either. It is lined with microvilli, and is known as the brush border (aka apical) membrane, as shown below.

    Figure 3.48.png

    Figure 3.48 Enterocyte, or small intestinal absorptive cell is lined with microvilli. This lined surface is referred to as the brush border membrane.

    Together these features (folds + villi + microvilli) increase the surface area ~600 times versus if it was a smooth tube5. More surface area leads to more contact with the enterocytes and thus, increased absorption.

    Going even closer, we discover that the surface of the microvilli is covered by the hair-like glycocalyx, which is glycoproteins and carbohydrates as shown below.

    Figure 3.49.png

    Figure 3.49 Glycocalyx lines the microvilli

    Now that you have learned about the anatomy of the small intestine, the following subsections go through the different digestive processes that occur there.

    References & Links

    1. commons.wikimedia.org/wiki/Im...atal%C3%A0.png
    2. Author unknown, NCI, http://visualsonline.cancer.gov/deta...m?imageid=1781
    3. digestive.niddk.nih.gov/ddiseases/pubs/celiac/
    4. commons.wikimedia.org/wiki/Image:Gray1061.png
    5. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.


    This page titled 3.4: Small Intestine is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Brian Lindshield via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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