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10.05 Small Intestine

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  1. Structure
    1. Characteristics
      1. 1” diameter
      2. 6’ long in life
    2. Location – abdominal cavity
    3. Divisions
      1. Duodenum
        1. First 10 inches
        2. “C”-shaped (the C is the location of pancreas)
      2. Jejunum –  ~2 feet
      3. Ileum – Final 3 feet
        1. Peyer’s patches (lymphoid tissues)
        2. Ends at cecum of large intestine
          1. Ileocecal valve – Prevents contents of large intestine from regurgitating into small intestine
  2. Functions
    1. Completion of physical digestion and mixing of digested materials
    2. Completion of chemical digestion
    3. Site of major absorption
      1. Nutrients
      2. Salts
      3. Water
      4. Vitamins

Nursing Points


  1. Cell types in small intestine
    1. Simple columnar epithelium
      1. Secretes most digestive enzymes
    2. Goblet cells – secrete mucus
    3. Paneth cells – in glands
      1. Secrete peptidases & lysozymes
  2. Tissue layers of small intestine
    1. Tunica serosa (visceral peritoneum)
      1. Serous membrane
      2. Double layer on posterior side – mesentery
        1. Keeps small intestine suspended in normal anatomical position
    2. Tunica muscularis – smooth muscle
      1. Outer longitudinal (runs pylorus → cecum)
      2. Inner circular
    3. Tunica submucosa
      1. Areolar CT
      2. Connects tunica muscularis to inner lining
    4. Tunica mucosa
      1. Cells
        1. Simple columnar epithelium
        2. Goblet cells
      2. Plicae circularis – circular folds around inner border to increase surface area of small intestine
      3. Villi – fingerlike projections extending off plicae circularis – also to increase surface area
        1. Mucosal lining – 1-cell layer thick
        2. Tissues inside villi
          1. Tunica submucosa – areolar tissue
          2. Lacteal – lymphatic capillary
            1. Site of lipid absorption
          3. Blood capillary networks around lacteal
        3. Crypt of lieberkuhn (gland)
          1. Depression at base of villi
          2. Secretes watery fluid which contains peptidases and lysozymes (Paneth cells)


Betts, J.G., et al. (2017). Anatomy and physiology. Houston, TX: OpenStax, Rice University. Retrieved from

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Video Transcript

In this lesson we are going to talk about the small intestine
The small intestine is about 6 feet long and has a 1-inch diameter and is located in the abdominal cavity. The small intestine is broken up into three sections, duodenum, jejunum, and ileum. The duodenum is the first 10 feet, the jejunum is approximately the next two feet, and then the ileum makes up the final three feet. The ileum ends at the cecum which is in the large intestine. Encourage you to check out the large intestine lesson to get a feel for what’s going on there. But another important part of the ileum is that it contains specific cells called peyer’s patches which are really important lymphoid tissue.

The main purpose of the small intestine is that is a major location for absorption. It’s responsible for nutrient, salt, water, and vitamin absorption. It’s also the process where physical and chemical digestion are completed, and absorption takes over.

Now the small intestine is made up of a bunch of different cell types. At the top we’ve got simple columnar epithelium and that’s going to secrete most of the digestive enzymes that are responsible for that process in the small intestine. There are also goblet cells which produce mucus, and then there are these things called paneth cells and plant cells are found in glands and they are responsible for releasing peptidases and lysozymes which help to digest food.

And just like all of the other lessons, we focus on the layers of the GI tract. There are four, the serosa, muscularis, submucosa, and mucosa. Starting on the outermost edge you have this thing called the Tunica serosa and is a serous membrane. There’s something called a double layer and it happens on the posterior side. As you can see in this picture, There’s this connective tissue called a mesentery, and it wraps around the layers of the small intestine and that helps to keep the small intestine in the correct anatomical position. It acts essentially like a sling and keeps it where it’s supposed to be.

With the Tunica muscularis, there are two layers to this particular layer. The outermost layer of this is made of smooth muscle and it runs longitudinally, and then there is an inner circular smooth muscle layer. This layer of the small intestine helps to propel food via peristalsis.

On the innermost layer is the Tunica mucosa, and it’s made of simple columnar epithelium and goblet cells. What’s interesting about the layer in the small intestine are these things called up like a circulares or circular folds. They’re essentially these hills and valleys that occur inside the small intestine and they helped to increase the surface area. Now let’s get down to more detail about this layer since this is the main area of absorption in the small intestine

With the plicae circulares or those circular folds, they create these Hills and Valleys and they increase its surface area. Along each circular fold are these ridges called Villi, and they are finger-like projections on those Plica circulares. They also help to increase the surface area. So imagine all of the tissue being flat there’s very minimal surface area, but if you create ridges and on each of those ridges you create more ridges you’re increasing the surface area. This allows for better absorption. On each one of these Villi are microvilli, and those also help an absorption.

When we look inside the Villi, underneath is the Tunica submucosa, and there’s also this thing called the lacteal. A lacteal is a lymphatic capillary and it’s responsibility is to absorb lipids. What happens is that bile is released by the gallbladder, and helps to increase the absorption through these lacteals. There’s also something called the Crypt of lieberkuhn and it is where watery fluid is produced. Inside this watery fluid are peptidases and lysozyme and those are part of digestion.

Okay so let’s recap.

The small intestine is made up of those three sections, the duodenum, the jejunum, and the ileum.

In the small intestine that is where at the majority of absorption happens.

There are four layers to the small intestine, just like the rest of the GI tract. You’ve got the Tunica serosa, Tunica muscularis, Tunica mucosa, and Tunica mucosa.

The folds and Villi, and even the microvilli help to increase the surface area inside the small intestine for absorption.

The lacteals play an important role in lipid absorption as they are right underneath all of those villi and help to get all of the fats absorbed in the digestive tract.
That’s it for a lesson on the small intestine. Make sure you check out all the resources attached to this lesson. Now, go out and be your best self today, and as always, happy nursing!