Forms the lining of the stomach and small intestines

The small intestine (also referred to as the small bowel) is the specialized tubular structure between the stomach and the large intestine (also called the colon or large bowel) that absorbs the nutrition from your food. It is approximately 20-25 feet in length and is about as big around as your middle finger. It is divided into three parts: the duodenum, jejunum and ileum.

The beginning portion of the small intestine (the duodenum) begins at the exit of the stomach (pylorus) and curves around the pancreas to end in the region of the left upper part of the abdominal cavity where it joins the jejunum. The duodenum has an important anatomical feature which is the ampulla of Vater. This is the site at which the bile duct and pancreatic duct empty their contents into the small intestine which helps with digestion. The jejunum is the upper part of the small intestine and the ileum the lower part, though there is no clear delineation between the jejunum and ileum.

The lining of the small intestinal mucosa is very highly specialized for maximizing digestion and absorption of nutrients. The lining is highly folded to form microscopic finger-like projections called villi which increase the surface area to help with absorption. The lining also contains specialized groups of cells that produce chemicals which help digestion, provide immune defenses, and hormones that help to control coordination of digestive process of the intestine, gallbladder, and pancreas.

An important anatomic feature of the small intestine is also its highly integrated nervous system which lies within the wall of the intestine (this is called the enteric nervous system) The enteric nervous system plays a very important role in coordinating much of the activities of the small intestine including its muscular activity of propulsion (the moving of intestinal contents).

Function & Control

The small intestine is responsible for absorption of nutrients, salt, and water. On average, approximately nine liters of fluid enters the jejunum each day. The small intestine absorbs approximately seven liters, leaving only 1.5-2 liters to enter the large intestine. Significant abnormalities of the small intestine therefore, are manifested by malabsorption of nutrients, and diarrhea. The absorptive function of the small intestine is effected by an intricate array of cells within its lining that will absorb and secrete salts and nutrients as well as water in order to maintain normal salt and water balance within the body. The absorptive function is so efficient that in a normal adult with a normal diet, over 95% of ingested carbohydrates and proteins are absorbed.

Specific regions are adapted to perform specific functions. For example, the duodenum plays an important role in coordinating how the stomach empties as well as the rate of emptying of bile duct juices into the intestine. The duodenum is also a major site for absorption of iron. The jejunum is a major site for absorption of the vitamin folic acid and the end of the ileum is the most important site for absorption for the vitamin B12, and bile salts.

Overview

What is the small intestine?

The small intestine is part of your digestive system. It makes up part of the long pathway that food takes through your body, called the gastrointestinal (GI) tract. When food leaves your stomach, it enters the small intestine, also called the small bowel. The small bowel connects to the large bowel, also called the large intestine or colon. The intestines are responsible for breaking food down, absorbing its nutrients and solidifying the waste. The small intestine is the longest part of the GI tract, and it is where most of your digestion takes place.

What are the different parts of the small intestine?

The small intestine has a beginning section, a middle section and an end section. Although there is no real separation between the parts, they do have slightly different characteristics and roles to play.

Duodenum

The duodenum is the first part of the small intestine that the stomach feeds into. It’s a short, descending chute (about 10 inches long) that curves around the pancreas in a “C” shape before connecting to the rest of the coiled intestines.

Jejunum

The remaining small intestine lays in many coils inside the lower abdominal cavity. Its middle section, called the jejunum, makes up a little less than half of this remaining length. The jejunum is characterized by many blood vessels, which give it a deep red color.

Ileum

The ileum is the last and longest section of the small intestine. Here the walls of the small intestine begin to thin and narrow, and blood supply is reduced. Food spends the most time in the ileum, where the most water and nutrients are absorbed.

Function

What does the small intestine do?

The small intestine is where most of the long process of digestion takes place. It:

• Systematically breaks food down.
• Absorbs nutrients.
• Extracts water.
• Moves food along the gastrointestinal tract.

There’s a lot involved in all this. The process can take up to five hours.

How do the different parts of the small intestine work?

Duodenum

To help break food down, the small intestine receives digestive juices from other organs in your digestive system, including your liver, gallbladder and pancreas. Ducts from these organs feed into the duodenum. Hormone glands in the lining of the duodenum signal these organs to release their chemicals when food is present.

Jejunum

After chemical digestion in the duodenum, food moves into the jejunum, where the muscle work of digestion picks up. Nerves in the intestinal walls trigger its muscles to churn food back and forth (segmentation), mixing it with digestive juices. Other muscle movements (peristalsis) keep the food moving gradually forward.

Mucosa

The walls of the small intestine are lined with a dense mucosa with many glands that both secrete and absorb. In the jejunum and the ileum, the mucosa secretes small amounts of digestive enzymes and lubricating mucus while absorbing nutrients from your food. Each section is designed to absorb different nutrients, as well as water.

The thick mucosa has so many folds and projections that its surface area is about 100 times as broad as the surface area of your skin. This is why 95% of the carbohydrates and protein you consume are absorbed in the small intestine. It also absorbs about 90% of the water that it receives during digestion. The rest will be absorbed in your large intestine.

Ileum

In the ileum, segmentation slows down and peristalsis takes over, moving food waste gradually toward the large intestine. The ileocecal valve separates the ileum from the large intestine. Nerves and hormones signal the valve to open to let food pass through and close to keep bacteria out. Special immune cells line the ileum to protect against bacteria.

Anatomy

Where is the small intestine located?

The small intestine is coiled inside the lower abdominal cavity beneath the stomach. The large intestine surrounds it, framing the edges of the abdominal cavity.

What does the small intestine look like?

It looks like a long, pink or red tube with many folds. It’s about the width of your pointer finger. It’s the width that makes it “smaller” than the large intestine, not the length.

How long is the small intestine?

The small intestine is by far the longest section of the gastrointestinal tract, about 22 feet long. The lining also has a vast surface area that’s scrunched into folds and peaks. If you stretched out the lining, it would cover a tennis court.

What is the small intestine made of?

The tube is lined with many layers of tissues and muscles. The tissues contain nerves, blood vessels, immune cells and lymphatic glands.

Care

How can I keep my small intestine healthy?

You can help take care of your bowels by following these general health guidelines:

Eat plants

Fruits, vegetables and whole grains help provide your bowels with adequate fiber. Fiber helps feed the good bacteria in your gut and helps sweep out the residue that the less-helpful bacteria might feed on. More fiber will make you crave more water, but that's also a good thing. Both fiber and water will help keep your bowel movements regular, which helps the small intestine keep up with its housekeeping.

Most fruits and vegetables are also alkalizing, which means they help balance out the overly acidic Western diet. High acid content can erode the protective mucus in the gut. Many other Western foods and lifestyle habits are acidic, including processed foods, meat and dairy products, caffeine and alcohol. We could all benefit from more alkaline foods in our diet. Simply eating plants takes care of all of this.

Moderate smoking, drinking and medications

You probably already know that tobacco and alcohol increase acid in your stomach, which erodes the protective lining. You may not know that NSAIDs — over-the-counter pain medications such as aspirin and ibuprofen — can also erode your stomach lining. If you use more than one of these at once, it multiples the damage. It’s a good idea not to use any of these too often, and avoid using them together.

What happens in your stomach can also affect the duodenum. High stomach acid can leak into the duodenum and begin to erode the lining there. Eroded stomach lining also leaves the stomach more vulnerable to bacterial infections, such as H. pylori, which can also infect the duodenum. Overuse of NSAIDs and H. pylori infection are the two leading causes of ulcers in the stomach and duodenum.

Pay attention to symptoms

Gut health is a balancing act — balancing healthy levels of different gut bacteria, acid content, immunity and tolerance to different foods. When things start to go wrong in an otherwise healthy gut, it’s often a gradual tipping of the balance. We don’t always take our bodies seriously until they start to really act up. But you can be a good friend to your gut by listening to its quieter complaints and making small adjustments.

Notice which foods you react to, or if too many indulgences in a short period are beginning to add up. Make a point of dialing it back when you notice symptoms and treat your gut to a healthy, whole foods meal and maybe some probiotics. Drink extra water and have some natural supplements handy to treat temporary constipation before it builds up. If symptoms persist, seek professional guidance to find out what’s causing them.

A note from Cleveland Clinic

On food’s long journey through your digestive system, the small intestine is where the grunt work of digestion happens. It takes up to five hours for the small intestine to process a single meal through its 22-foot length. During that time, it systematically breaks food down and absorbs 95% of its nutrients. It coordinates with the stomach, pancreas and gallbladder to cue digestive juices when food is present. It extracts excess water and sends the remaining food waste to the large intestine to be formed into stool.

What forms the lining of stomach?

What is the lining of the small intestines called?