The small intestine is a vital organ in our digestive system responsible for absorbing nutrients from our food to nourish our bodies. It is a long, narrow tube measuring about 20 ft in length in adults and is divided into three parts: the duodenum, jejunum, and ileum. The small intestine has a remarkable ability to absorb nutrients, thanks to its unique structure and the diverse functions performed by its various cell types. In this article, we’ll take a closer look at the small intestine and explore how a physiologist would describe its functions and mechanisms of action.
The Anatomy of the Small Intestine
The small intestine is located in the lower part of the digestive tract, connecting the stomach to the large intestine. It is a narrow, muscular tube that is coiled and suspended in the abdominal cavity by the mesentery. The small intestine is divided into three parts, each with unique functions:
The duodenum is the first portion of the small intestine and is the shortest part. It is C-shaped and sits immediately after the stomach, wrapping around the head of the pancreas. It receives chyme from the stomach and digestive enzymes from the pancreas and bile from the liver and gallbladder via the common bile duct. The presence of these digestive juices helps break down the chyme and neutralize the acidic gastric juices of the stomach.
The jejunum is the middle portion of the small intestine and is about 8 ft in length. It is located in the upper part of the abdomen and is supported by a double layer of the mesentery. This layer is rich in blood vessels and lymphatics, and it helps support and attach the jejunum to the posterior abdominal wall. The jejunum is responsible for the majority of nutrient absorption, including carbohydrates, proteins, and fats.
The ileum is the last portion of the small intestine, extending to the large intestine. It is about 12 ft long and is located in the lower part of the abdomen. The ileum has a thinner wall and fewer blood vessels than the jejunum. The ileum absorbs the remaining nutrients from the chyme, including vitamins, minerals, and water.
The Mechanism of Nutrient Absorption
The small intestine is well-designed for absorbing nutrients from food. Its inner surface is covered with millions of finger-like projections called villi, which increase the surface area available for nutrient absorption. Each villus is lined with even smaller protrusions called microvilli, which in turn are covered with tiny hair-like structures called cilia. These structures all work together to propel and absorb nutrients rapidly through the intestinal wall.
The cells lining the small intestine are equipped with transport proteins that help them absorb specific nutrients. For example, cells in the duodenum and jejunum secrete enzymes that break down proteins, carbohydrates, and lipids. Certain cells also absorb specific vitamins and minerals, such as iron and calcium.
The absorbed nutrients then pass into the bloodstream and are transported to the liver, where they are further processed and either stored or distributed to other parts of the body where they are needed.
The Role of Hormones in Small Intestine Function
Several hormones produced in the small intestine play a critical role in regulating digestive processes. These hormones are secreted in response to specific stimuli, such as the presence of food in the gut.
Cholecystokinin (CCK) is one of the most important hormones produced in the small intestine. It is secreted by cells in the duodenum and jejunum in response to the presence of fat or protein in the gut. CCK stimulates the pancreas to secrete digestive enzymes and tells the gallbladder to release bile, which helps break down lipids. CCK also acts on the brain to reduce food intake, helping to regulate appetite.
Another important hormone produced in the small intestine is secretin. Secretin is secreted by cells in the duodenum in response to the presence of acidic chyme. It stimulates the pancreas to secrete bicarbonate, which neutralizes the acidic chyme as it enters the small intestine.
Diseases and Disorders of the Small Intestine
The small intestine can be affected by a variety of diseases and disorders that can impair its ability to absorb nutrients. These can include:
- Celiac disease: a condition in which the small intestine is damaged by exposure to gluten, a protein found in wheat, barley, and rye, leading to malabsorption of nutrients.
- Inflammatory bowel disease (IBD): chronic inflammation of the small intestine or colon, including Crohn’s disease and ulcerative colitis, which can cause diarrhea, abdominal pain, and malabsorption of nutrients.
- Short bowel syndrome: a condition that occurs when the small intestine is shortened due to surgical removal or congenital defect, leading to malabsorption of nutrients.
- Bacterial overgrowth: a condition in which the normal balance of bacteria in the small intestine is disrupted, leading to malabsorption of nutrients.
The small intestine is a vital organ in the digestive system, responsible for absorbing nutrients from our food to nourish our bodies. Its unique structure and diverse functions make it a master of absorption. Understanding how the small intestine works can help us better care for ourselves and avoid common digestive disorders.
- What role does the small intestine play in digestion? The small intestine is responsible for the majority of nutrient absorption, including carbohydrates, proteins, fats, vitamins, minerals, and water.
- How does the small intestine absorb nutrients? The small intestine absorbs nutrients through its villi, small finger-like projections covered with microvilli and cilia. These structures increase the surface area for nutrient absorption, while cells lining the small intestine secrete digestive enzymes and have transport proteins that help them absorb specific nutrients.
- What are some common diseases and disorders of the small intestine? Common diseases and disorders of the small intestine include celiac disease, inflammatory bowel disease, short bowel syndrome, and bacterial overgrowth.
- Marieb, E. (2013). Human Anatomy and Physiology. Pearson.
- Eisenberg, M. (2016). The Handy Physiology Answer Book. Visible Ink Press.
- Silverthorn, D. (2013). Human Physiology: An Integrated Approach. Pearson.