The Intestine – Anatomy, Diseases, and How the Intestine Works (Functions of the Intestine)

The small intestine connects to the outlet of the stomach and is divided into the following parts:

• Duodenum
• Jejunum
• Ileum

Next, the large intestine is located in the abdomen and consists of the following sections:

• Cecum with the appendix (Appendix vermiformis)
• Colon
• Rectum

The small intestine begins as a curved, C-shaped tube directly behind the outlet of the stomach. Along its approximately 30-centimeter-long course, it absorbs digestive secretions from the gallbladder and the pancreas. Ulcers can develop at its upper end. The action of acidic gastric secretions promotes this process.

The jejunum and ileum follow the duodenum

The small intestine is highly flexible and is suspended from the back wall of the abdominal cavity by a band called the mesentery. The mesentery consists of fat and connective tissue. It holds the coiled jejunum and the ileum in the lower abdomen and supplies both with nerves, blood vessels, and lymphatic vessels. After digestive secretions have mixed with the chyme from the stomach in the duodenum, the actual absorption of nutrients into the body begins. The small intestine breaks down the pre-digested food into molecules. This produces, for example, amino acids and free fatty acids. The human body then absorbs these smallest food components into the bloodstream through the intestinal mucosa. Doctors say it “absorbs” them. Indigestible substances and water remain in the small intestine.

Diseases that occur in the small intestine

• Crohn’s disease
• Intestinal obstruction (ileus), mechanical or due to muscle paralysis
• Mesenteric infarction
• Meckel’s diverticulum
• Superior mesenteric artery syndrome

Once the chyme has passed through the small intestine, there are hardly any nutrients left. Next comes the large intestine, which is approximately 150 centimeters long. The intricate mucosal surface of the small intestine gives way to a relief pattern featuring so-called crypts—small indentations that significantly increase the surface area of the intestine. Mucous glands are a key feature of this section of the intestine. The mucus they produce lubricates the feces, which are now formed from the former chyme. Even without the intake of food, feces are formed from mucus and shed cells of the intestinal mucosa. In the large intestine, water and salt are absorbed into the body. This is an important regulatory process for fluid balance. This section of the intestine is also home to the healthy bacteria of the gut flora. They ward off invading pathogens.

The cecum is the shortest part of the large intestine

The ileocecal valve separates the small intestine from the large intestine and protects both sections from the exchange of secretions and bacteria from the gut flora. Attached to the side of the cecum is the appendix (Appendix vermiformis), which is commonly referred to as the “appendix.” This smallest section of the intestine is a vestigial structure and works in conjunction with the immune system. It is notorious for its inflammations (appendicitis), which can sometimes be life-threatening; learn more in the video: 

The colon frames

the abdominal organs Next comes the main part of the large intestine, which doctors call the colon. It lies like a frame within the abdominal cavity and measures up to one meter. Inside, it condenses stool daily to approximately 200 milliliters. If the large intestine’s function is impaired due to infections or inflammation, diarrhea, constipation, and bloating can occur. The body loses too much water and salt and risks dehydration, or it thickens the stool too much.

The long journey ends in the rectum

The stool travels the final 20 centimeters of its journey through the rectum. Here, it is also stored until a sufficient amount triggers the reflex that leads to regular bowel movements. Three different layers of muscle seal off the anal canal, through which the stool finally leaves the intestine.

• Diseases of the Large Intestine

  • Appendicitis (commonly known as “inflammation of the appendix”)
  • Ulcerative colitis
  • Diverticulosis
  • Diverticulitis
  • Infectious Diarrheal Diseases
  • Polyp formation
  • Colon cancer
  • Hemorrhoids
  • Irritable bowel syndrome

The specialist for the colon and stomach is the gastroenterologist. They use diagnostic methods such as ultrasound and colonoscopy to investigate symptoms like diarrhea, chronic constipation, abdominal pain, and blood in the stool.

During a colonoscopy, the doctor uses a small, lighted camera attached to a long, flexible rubber tube to look directly into the patient’s colon. The doctor takes mucosal biopsies and performs minor surgical procedures right there during the exam. To do this, the doctor inserts the necessary instruments through a working channel on the endoscope. You usually have the option of taking a sedative to make the exam more comfortable.

The following videos show what intestinal diseases look like during a colonoscopy: 

The intestine is a complex muscular tube whose main function is to digest food and absorb important nutrients. How the intestine works is particularly evident in the interaction between the small and large intestines, with the first section of the small intestine—the duodenum—playing a central role. The gut bacteria and the microbiome consist of about 100 trillion bacteria and microorganisms that are essential for gut health. The functions of the intestine are further supported by enzymes, peristalsis, and the musculature, while the mucous membrane lines the interior and villi enhance absorption.

The intestine is part of the digestive tract and works closely with organs such as the liver, while the cecum and appendix are located in the lower right abdomen. The various parts of the intestine—such as the ascending colon, transverse colon, descending colon, and sigmoid colon—facilitate transport to the anus. Disorders such as constipation or diarrhea can occur, as can chronic inflammatory bowel diseases like Crohn’s disease and ulcerative colitis. Symptoms such as blood in the stool should be taken seriously, as they may indicate underlying conditions. Over the course of a lifetime, the intestine processes approximately 30 metric tons of food and contains millions of nerve cells connected to the nervous system.

How does the intestine work?

Its main function is to digest food and absorb nutrients. Digestion begins in the mouth before the chyme travels through the esophagus into the stomach and then into the intestines. In the small intestine, enzymes break down the food components.

What role do gut bacteria play?

Gut bacteria are part of the microbiome and support digestion as well as the immune system. About 100 trillion bacteria and other microorganisms live in the gut, fighting off pathogens and promoting gut health.

What conditions can affect the gut?

The most common include diarrhea, constipation, and inflammation of the intestinal lining. Chronic inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis can cause permanent damage to the intestines. Colorectal cancer or a tumor may also develop.

What are the sections of the gut?

The gut is divided into the small intestine and the large intestine. The small intestine includes the duodenum, jejunum, and ileum, while the large intestine consists of the colon, cecum, and rectum.

When should a colonoscopy be performed?

A colonoscopy is used for the early detection of colorectal cancer and other intestinal diseases. It is particularly important for diagnosis when symptoms such as blood in the stool or persistent discomfort are present.