Parts of the small intestine

Parts of a Small Intestine- A Quick Guide


The small intestine is the part that lies between the stomach and the large intestine. It is about 6 meters long and has a diameter of about 4 – 7cm. This intestine has three layers of muscles and is separated into three parts or areas.

Each part of the small intestine performs a different function, with the duodenum being the most crucial indigestion. It is here that the majority of the vitamins and minerals are absorbed. The small intestine is divided into three major parts: the duodenum, jejunum, and ileum.

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The Duodenum

The duodenum is more than twenty inches long and lies just beneath the stomach. It extends from the pyloric sphincter of the stomach and ends at the pyloric sphincter of the jejunum. It is sometimes referred to as a “first stomach.” In Latin, the word “duodenum” means twelve fingers long.

The duodenum is divided into three parts: the bicaval, the right, and left hepatic ducts. The bicaval is V-shaped, with the curvature pointing toward the small intestine. It may be divided into four parts in some babies and has characteristics of both the bicaval and the hepatic ducts. It has smooth muscle walls that help with peristalsis to move food along.

The hepatic ducts on each side also have smooth muscles that help the bile flow. Bile is made by the liver and stored in the gall bladder. It helps break down fats and is stored in the gall bladder until needed for digestion.

Figure: Structure of the duodenum

How the Bile Works

Bile contains bile salts that break up fat globules. They combine with fats and emulsify them into tiny droplets, making them easier to digest. The bile then enters the duodenum through the hepatic duct, after which it mixes with pancreatic juice and digestive enzymes from the pancreas. Together, they form one of the most powerful digestive enzymes.

In addition to bile and pancreatic juice, the duodenum also has mucus that helps break down food. The duodenal mucosa produces the mucus. It lubricates and protects the delicate lining of the duodenum, which is only one cell thick. This means it can be easily damaged by indigestion, alcohol and chemotherapy drugs, and radiation therapy for cancer treatment. When the mucus is lost, the lining of the duodenum becomes inflamed and ulcerated.

Food in the Duodenum

Food passes into the duodenum from the stomach through the pyloric sphincter, opening to allow food to enter and closing as soon as it passes through. It is controlled by a hormone called gastrin, which is secreted in response to the presence of food. When it opens, most of the food can enter the duodenum and start moving through it.

The juice produced by the liver, gallbladder, and pancreas mix with food in the duodenum so that its enzymes become activated. The enzymes start digesting the food, converting it into usable energy. Digestion also makes the nutrients in food available to the body.

The duodenum is one end of the gastrointestinal system where the liver and pancreas release pancreatic juice-containing enzymes. These juices help to break down food into usable energy.

The Villi in the Duodenum

Figure: Structure of the villus

The duodenum is lined with millions of tiny villi that increase its surface area. The large surface area is essential because the duodenum contains a rich supply of blood vessels and lymph vessels. The vessels are needed for the absorption of vitamins, minerals, and other nutrients.

These finger-like structures help absorb nutrients such as iron, calcium, zinc, and vitamin B12. They also contain special cells that are needed for the absorption of fat.

The many capillaries in the villi bring the nutrients from the lumen of the duodenum to the blood. Vitamin B12, for example, is too big to be absorbed through a capillary, so the molecules are cleaved by protease enzymes before they can be absorbed.

The villi also contain lysosomes that produce hydrochloric acid that helps break down food into the blood.

The Jejunum

Figure: Jejunum flap

The jejunum is located between the duodenum and the ileum. It is the second part of the small intestine. It has its own set of small muscles that help move food along its length and into the ileum. Its wall contains many folds called plicae circulares or circular folds.

The circular folds help increase the jejunum’s surface area, which increases its effectiveness in absorbing nutrients from food. The small intestine is the site of nutrient absorption for many nutrients, including iron and calcium.

The walls of the jejunum are lined with Peyer’s patches. They contain special cells that help to produce factors for the immune system. These include antibody-producing plasma cells, dendritic cells, and macrophages.

The jejunum also contains mucus that helps to lubricate the food passing through it, making it easier to move it along into the ileum.

The Ileum

Figure: Parts of the ileum

The final part of the small intestine is the ileum. It is the last section in the small intestine, and it joins into the beginnings of the large intestine. It is shorter than the duodenum and has a rich blood supply. This means it can provide the body with adequate amounts of essential nutrients and vitamins.

The walls of the ileum contain villi and microvilli. These are tiny hair-like structures that increase the ileum’s surface area. The large surface area makes it very effective at absorbing nutrients. As soon as the food enters the ileum, its job is done, and so the walls of the ileum contain very few digestive enzymes.

The glands in the ileum wall produce mucus that is needed to lubricate the food that is passing through it. The ileum will absorb water from the food as well as its nutrients.

The villi in the ileum contain enterocytes, which are the cells that absorb nutrients. They also produce enzymes for digesting food, as well as proteins and antibodies.

The Layers of The Small Intestine

Figure: Small intestine layers

The small intestine consists of four main tissue layers: mucosa, submucosa, serosa, and muscularis.

The Mucosa

The mucosa is the innermost layer of the small intestine. It consists of specialized epithelial and connective tissue that protects it from acid and digestive enzymes produced in the gastric region.

The mucosal layer of the small intestine is divided into three parts-the epithelial linings, the lamina propria, and the muscularis mucosa.

The epithelial lining (mucosa) covers the small intestine entirely from the duodenum to the ileum. It is made of squamous epithelial cells with microvilli (small hair-like processes) projecting into the intestinal lumen.

The mucosa contains goblet cells that produce protective mucus and glands from which digestive juices are secreted.

The lamina propria is the layer of loose connective tissue beneath the mucosa, containing blood vessels and nerves.

The Submucosa

The submucosa is located between the mucosal and muscularis layers. It contains blood vessels, lymphatic vessels, and nerves. It is also made of loose connective tissue and is very thin.

This layer is rich in blood vessels that bring nutrition to the other layers and ensure an adequate supply of oxygen and nutrients. It also has numerous lymphatic vessels. These are responsible for draining excess fluids that enter the small intestine and absorbing lipids.

The Muscularis

The muscularis layer comprises smooth muscle fibers arranged in circular and longitudinal layers—these muscles contract to push the food through the small intestine.

The circular layer extends from the duodenum to the ileum, while the longitudinal layer runs parallel to it.

The small intestine has three layers of muscles-

The inner circular layer, the outer longitudinal layer, and a middle layer are made up of both circular and longitudinal muscle fibers.  

The inner circular muscle layer applies more pressure than the outer longitudinal muscle layer, pushing food through the small intestine faster. The middle muscle layer works with both layers.

The inner circular muscle layer squeezes the food together, allowing it to move through the small intestine faster. The outer longitudinal muscle layer opens wide to allow the food through, then contracts and pushes it along.

The Serosa Layer

The outermost layer of the small intestine is its serosa layer. The serosa is made up of connective tissue, which is the type of tissue that holds the small intestine in place and protects it from harm.

The serosa layer is firmly attached to the underlying fat and other structures, which act as a cushion not to rip or tear. It is also connected to the peritoneum, a membrane layer that lines the abdominal cavity and helps protect it from any infection or irritation.

The serosa contains very little blood and nerve supply, so it does not play a role in the small intestine’s function. It is pretty thin but strong and durable.

The Lymph Nodules

Figure: Lymph nodes in the small intestine

Lymph nodules are groups of lymph tissue located in the small intestinal wall. They trap bacteria, viruses, and toxins taken into the body to prevent them from spreading into the bloodstream.

The lymph nodes help to protect the body against infections. They store antigens (substances that cause the body’s immune system to produce antibodies). If these antigens enter the body, they are trapped in lymph nodes where an immune response is mounted against them.

These nodules are linked with the lymphatic vessels, which carry white blood cells and nutrients to them from other parts of the body. They are made up of white blood cells, lymph vessels, and connective tissue.  

  • The primary function of the lymph nodes is to keep the body free from infection and disease by trapping wastes and dead cells.  
  • The nodules trap bacteria, viruses, harmful substances taken into the body, and certain cancer cells. In each nodule, these toxic substances are attacked by white blood cells.  
  • White blood cells then release chemicals that help to destroy the harmful substances, making them harmless.
  • The lymph nodes also release chemicals that activate other white blood cells in an immune response. These white blood cells produce antibodies that can attack harmful substances.
  • These nodes also identify any damaged cells as part of the body’s natural repair process. They release chemicals to these cells, so they are destroyed and replaced by healthy ones.

The Duodenal/Brunner’s Glands

The duodenal/Brunner’s glands are tiny sacs found in the mucosal layer of the small intestine. They produce an alkaline liquid called Brunner’s fluid. This fluid keeps the intestine acidic and also contains digestive enzymes.

The Brunner’s glands secrete their contents into the small intestine and then mix with food that enters the duodenum. The substances in Brunner’s fluid digest food proteins, which begin to be absorbed.

These glands also secrete some contents after the stomach has released hydrochloric acid (HCl). Food in the small intestine should be maintained at a pH level of 6 or above for the enzyme trypsin to function optimally.

If the small intestine were acidic, proteins would not be digested quickly and could cause damage. However, if the pH level is too high, then HCl can denature proteins in food.

The Small Intestine’s Blood Supply

The small intestine has an extensive blood supply. It receives its blood from the following arteries:

The celiac trunk and the superior mesenteric artery are branches of the aorta, the largest blood vessel in your body. They feed most of the small intestine.

The inferior mesenteric artery branches off the internal iliac artery, one of two arteries that branch off the aorta. It supplies most of the lower jejunum and the upper part of the duodenum.

The superior mesenteric artery branches off the aorta and supplies the upper jejunum. The inferior mesenteric artery takes care of the lower third of the duodenum and the rest of the small intestine.

The Small Intestine Digestive Processes

The small intestine is also responsible for absorbing most nutrients, water, vitamins, and minerals from food. This part of the digestive system breaks down food to be absorbed in the small intestine.

The masticated food is pushed through the small intestine by tiny muscle contractions called peristalsis. During this process, essential nutrients are extracted from the food as it passes through the small intestine.

This part of the small intestine also contains a wide range of digestive enzymes and hydrochloric acid that helps to break down food. This secretion is found in the duodenal (also known as Brunner’s) glands.

The three digestion processes that the Small intestine applies

  1. It begins by mixing the food with liquids from the gallbladder and pancreas to loosen it up.
  2. Then, the liver and pancreas release enzymes that break down the food. The Brunner’s glands secrete digestive juices to help digest proteins.
  3. The small intestine secretes enzymes that break down carbohydrates, fats, and proteins. Some of these enzymes include proteases, amylases (which are responsible for breaking down carbohydrates), lipases, nucleotides, and nucleoside phosphates.

These enzymes help break the large food molecules into smaller fragments that are then absorbed into the intestinal wall.

The small intestine completes this process by absorbing nutrients from food so the body can use them.

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Bottom Line

The small intestine is an organ that absorbs water and nutrients from food. It also serves as a long tube for the digestive system, where chemicals break down carbohydrates to produce glucose.

In addition, it regulates hormones in the body by secreting stomach acid into the duodenum of your intestines which triggers signals to release insulin or gastric juices when you are hungry. Overall, this vital part of our digestion process has many functions.

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