Small Intestine

Gross Anatomy

  1. Coiled tube filling most of abdominal cavity

  2. 3 regions

    1. Duodenum

    2. jejunum

    3. ileum

  3. Ileocecal Junction

    1. End of small intestine where ilium joins the cecum of large intestine

  4. Ileal Papilla -

    1. The small nipple-like structure located on the ileocecal junction that allows for the passage of digested food from the ileum into the cecum.

  5. Ileal Orifice

    1. Opening between the lips of the Ileal Papilla


Microscopic Anatomy

  1. Villi

    1. Projections on the inner lining of the intestine

    2. Largest in the duodenum then get smaller as it goes

    3. Covered into enterocytes (absorptive cells) and goblet cells

      1. Prevent digestive enzymes from leaking between them and eroding underlying tissue

    4. Core is filled with alveolar tissue and some smooth muscle

      1. Contains arteriole, blood capillaries, a venule, and a lymphatic capillary (lacteal)

      2. Blood capillaries absorb most nutrients

      3. Lacteal absorb most lipids

      4. Smooth muscle cell contractions enhances chyme mixing in intestinal lumen and pushes lymph down the lacteals to larger lymphatics

  2. Microvilli

    1. Smaller plasma membrane

    2. Form brush border on enterocyte surfaces

    3. Increase surface area

    4. Have brush border enzymes

      1. Carry out last stage of chemical digestion

      2. Chyme must contact brush border for this to occur = contact digestion

  3. Intestinal Crypts

    1. Pores between villi bases on the small intestine floor

    2. Made from enterocytes and goblet cells in upper half

    3. Stem cells make up lower half

    4. Paneth cells are at the end of each crypt

      1. Secrete lysozymes, defensins, and phospholipase

    5. Secrete 1 to 2L of intestinal juice per day

      1. 7.4 to 7.8 pH

      2. Has water, mucous, and little enzyme


  4. Duodenal Glands

    1. in the submucosa of the duodenum

    2. Secrete bicarbonate rich mucus

      1. Neutralizes stomach acids and shields mucosa from is erosion

    3. Contains signaling proteins for GI tract immune cells to tolerate food antigens and beneficial bacteria

Intestinal Mobility

  1. 3 Functions of contractions

    1. Mix chyme with intestinal juice, bile, and pancreatic juice

      1. Fluids neutralize acid and digest nutrients more effectively

    2. Churn Chyme and bring it into mucosa-contact

      1. Contact digestion and nutrient absorption

    3. Move residue to large intestine

  2. Segmentation

    1. Movement of stationary ring contractions along the intestine which relax when other contractions occur elsewhere

    2. Most common intestinal contraction

    3. Rhythm is set by pacemaker cells

    4. Intensity is modified by nervous and hormonal influences