cómo funciona el intestino delgado en la digestión y la absorción

Title: How the Small Intestine Works in Digestion and Absorption

Profile: The small intestine is key in the digestion and absorption of nutrients, highlighting the duodenum and its function.

Keyword: ['Digestive system', 'Health']

The small intestine, this seemingly common digestive organ, actually plays a crucial role in our lives. Every time we enjoy a delicious meal, the small intestine works silently, ensuring that the valuable nutrients from food are fully absorbed, providing our body with a constant supply of energy and nutrients. So how does this extraordinary organ function?

First, let us marvel at the vast surface area of the small intestine. Did you know? If the small intestine were completely unfolded, its area would be enough to cover an entire tennis court! This design is not coincidental, but rather to ensure that every bit of nutrition from food can be effectively captured and absorbed.

The small intestine is not only an important part of the digestive system, but it also serves as a guardian of our health. Its average length is approximately 6 meters, and its diameter ranges between 2.5 and 3 centimeters. It consists of three parts: the duodenum, the jejunum, and the ileum, each of which serves a unique function.

The duodenum, as the initial part of the small intestine, connects the stomach with the small intestine. Once food has undergone initial digestion in the stomach, it enters the duodenum, where it experiences a key process of enzymatic hydrolysis, being transformed into easily absorbable amino acids. Although the duodenum is the shortest section of the small intestine, averaging only 30 centimeters in length, its function is crucial. It utilizes bile and digestive enzymes from the gallbladder, liver, and pancreas to completely break down food, thereby preparing for the subsequent absorption process.

Next, the jejunum begins to perform its role. Situated after the duodenum, it is primarily responsible for transporting food molecules that have already been broken down. The inner surface of the jejunum is covered with circular folds and intestinal villi, structures that significantly increase the surface area of the small intestine, allowing for more complete contact of food molecules with the intestine, which facilitates the absorption of carbohydrates and proteins. Once absorbed, these nutrients are transported through the bloodstream to various parts of the body.

Finally, the ileum, as the last part of the small intestine, serves a 'closing' function. It captures nutrients that may have been lost in the earlier sections and performs the final absorption. In particular, vitamin B12 and bile acids, which are two substances crucial for the human body, are absorbed and utilized in the ileum.

The small intestine pushes food into the large intestine through rhythmic contractions. This process is carried out jointly by the smooth muscles that make up the intestinal wall, which, through rhythmic contractions and relaxations, allow food to progress through the intestine. Finally, waste is temporarily stored in the large intestine and expelled from the body through the rectum.

Let us delve into the structure of the small intestine! The lumen of the small intestine is the main site for the digestion and absorption of food; the circular folds increase the surface area, forming valve-like structures to prevent food reflux; the mucosa is found in the inner wall of the small intestine, covered with intestinal villi; the submucosa serves a supportive and connective function; the serosa acts as a protective layer, preventing damage to the small intestine from other organs; and the blood vessels are densely distributed around the small intestine, ensuring that nutrients can smoothly enter the bloodstream and disperse throughout the body.

So, what exactly are the nutrients? They mainly include three categories: lipids (fats), carbohydrates, and proteins. These macromolecules are further broken down in the small intestine into smaller molecules, such as monosaccharides, disaccharides, and fatty acids, to facilitate absorption through the intestinal wall and into the bloodstream. Additionally, we need to obtain some vitamins and minerals from food that the body cannot synthesize on its own, such as vitamin B12.

It is worth mentioning the villous structures in the small intestine. These tiny structures, like fingers, cover the mucosa of the small intestine, and, along with the circular folds, increase the surface area of the small intestine, making the absorption process more efficient. The epithelial cells on the villi extend numerous microvilli, further enlarging the absorption area; while the central lacteal is responsible for absorbing nutrients that cannot directly enter the bloodstream; the capillary network is responsible for absorbing substances such as monosaccharides and amino acids through the villous epithelial tissue.

The small intestine can be considered a miracle of the human digestive system. Its large surface area, precise organizational structure, and efficient absorption function perfectly achieve the transformation and supply of nutrients. This ingenious physiological system continuously provides the necessary energy and nutrients for the organism.

The images in the text come from "How it Works" magazine.

Author: Scientific Popularization Team of "How It Works"

Son Yi Fei, Director of the Department of History of Medical Education at Hebei Medical University, member of the Medical History Society of the Chinese Medical Association.