The small intestine is the longest segment of the digestive system. In adults, it is 6 meters on average. It is responsible for absorbing nutrients from food into the bloodstream, distributing them throughout the body and using them in the processes necessary for life. How exactly is the small intestine built? What happens at its various stages?
The small intestineis located between the stomach and the large intestine. It is the longest part of the digestive system. In an adult human, it reaches 6-8 m, although it happens that people who have had part of this organ removed from the so-called short bowel syndrome, they digest properly even if the intestine is 1.5-2 meters long. For this to happen, however, the following condition must be met: a properly functioning ileocecal valve (Bauhin valve) that separates the small intestine from the large intestine. Importantly, the intestine can regenerate itself. Children who have had a part of their intestine removed, but 50-60 cm have been kept, have a good chance that the intestine will become long enough to be able to fulfill its function to the fullest extent.
The small intestine starts right behind the stomach. Its first fragment can be defined as the pylorus, through which food from the stomach enters the duodenum. The duodenum passes into the jejunum, which in turn passes into the ileum. The small intestine ends with the mentioned ileocecal valve, which prevents the bacterial flora of the large intestine from entering the small intestine. If the valve is working properly, the small intestine is absolutely clean in this regard. If it fails, we are at risk of developing a bacterial overgrowth.
The walls of the intestine are strongly muscled, and thanks to perist altic movements (rhythmic, wave-like contractions), they move the chyme (chyme) down. The intestine is lined with a well-vascularized mucosa with numerous mucous glands. The intestine (lumen) is approximately 4 cm wide at the beginning, but narrows and reaches 2.5 cm at the end.
Functions of the small intestine
- production of intestinal juice, or digestive juice consisting of exfoliated cells in the intestinal epithelium and a solution containing digestive enzymes such as proteases that break down proteins and oligopeptides into amino acids; intestinal juice secretion is hormonally stimulated under the influence of chyme
- digesting food - proteins are digested by trypsin andchymotrypsin, fats are broken down by lipase, and carbohydrates - amylase; as a result of these processes, macronutrients are broken down into component parts and can be absorbed into the blood, and with it transported to every cell of the body
- nutrient absorption
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What's happening in the duodenum?
The duodenum is located at the back of the abdominal cavity and has the shape of an oblong sac 25-30 cm long. It is the only part of the small intestine that can be clearly separated from the rest (the jejunum and the ileum that follow, do not have a noticeable border between them). In the construction of the duodenum, we distinguish:
- upper part (pad), which starts at the level of the 1st lumbar vertebra at the pylorus and ends at the neck of the gallbladder
- descending (transverse) part that runs vertically down from the neck of the gallbladder to the level of the 3rd lumbar vertebra
- ascending (lower), which initially runs horizontally, then obliquely upwards, at the level of the 2nd lumbar vertebra, with the duodenocular flexion, it passes into the jejunum.
The partially digested chyme reaches the duodenum and mixes with pancreatic juice (alkaline pancreatic secretion), bile (liver secretion), digestive juices from Lieberküh's glands and Brunner's gland secretions (highly alkaline secretions neutralizing the acidic food passing from the stomach).
Here, proteins, carbohydrates and fats are broken down into component parts suitable for absorption and use by the body. For example, most of the iron is absorbed in the duodenum.
What's going on in the jejunum?
The length of the jejunum is approximately 2/5 of the upper part of the small intestine (excluding the duodenum).
The most characteristic feature of the structure of the intestine are the intestinal villi, as well as the microvilli which are located on the villi. These are the protrusions of the mucosa which increase the absorption area of the intestine up to 250m2 .
Through the intestinal villi, the absorbable particles get into the blood, and with it into the cells of the entire body. The jejunum is essential for the absorption of nutrients from food.
What is happening in the ileum?
The ileum is approximately 3 meters long and makes up about 3/5 of the lower part of the small intestine. The final processes of food absorption take place in it.
In the ileocecal area, i.e. where the small intestine meets the large intestine, vitamin B12 is absorbedand bile acid s alts. Here, too, further water resorption takes place, although in the ileum "residual" food is still in liquid form.
It is worth adding that the jejunum and the ileum are intraperitoneally attached to the back wall of the abdominal cavity with the mesentery, which allows them to be very mobile. Thanks to the mesentery, the intestine forms loops that lie between the individual sections of the large intestine.
As mentioned, it is difficult to draw a clear line between the jejunum and the ileum. What distinguishes these two parts of the digestive system is certainly the location (the winding one is closer to the fat one) and the diameter (the winding one is narrower than the empty one). In addition, in the ileum there are many more lymph nodes found in the mucosa that contribute to building the body's immunity. They are part of the lymphatic system and resemble small lymph nodes. They contain lymphocytes responsible for the humoral immune response. Lymph nodes are found throughout the body, but the ileum is the most abundant.
Diseases of the small intestine
- duodenal ulcer
- celiakia
- Whipple's disease
- bacterial outgrowth (blind loop syndrome)
- short bowel syndrome
- intussusception
- leaky gut syndrome
- Protein Loss Enteropathy
- cancer