Small Intestine

The small intestine is a part of the digestive system. Studying the small intestine parts and functions is crucial for understanding the process of digestion and absorption of nutrients in the digestive system. In this article, we will discuss the small intestine's location, function, and parts in detail.

Small Intestine Parts and Function

The small intestine is a part of the digestive system. The small intestine is located between the stomach and the large intestine. It is a long, coiled tube measuring about 20 feet in length in adults. Its main function is to further digest food from the stomach and absorb nutrients, including carbohydrates, proteins, fats, vitamins, and minerals, into the bloodstream for distribution throughout the body.

The small digestive tract is comprised of three parts or segments: the duodenum, the jejunum, and the ileum. On its proximal (close) end, the small digestive tract — starting with the duodenum — is associated with the stomach. On its distal (far) end, the ileum — the last portion of the small digestive system — is associated with the internal organ (colon). The jejunum lies between the duodenum and the ileum.

Now let's see each part in detail.

Parts of Small Intestine

The three parts of small intestines are:

Duodenum

The duodenum, the initial segment of the small intestine, is only 10 to 15 centimeters in length. It starts at the pylorus, marking the end of the stomach, where the valve regulates the passage of food into the small intestine. This segment then curves around the pancreas before terminating in the upper left quadrant of the abdomen, where it connects with the jejunum.

Functionally, the duodenum completes the primary phase of digestion. Here, food from the stomach undergoes mixing with enzymes from the pancreas and bile from the gallbladder. These enzymes and bile help in the breakdown of food particles.

The duodenum comprises four distinct parts:

1. Superior Part: This segment, also known as the initial segment or D1, lies intraperitoneally and extends proximally, including the duodenal bulb. It is connected to the liver by the hepatoduodenal ligament, which contains the bile duct, proper hepatic artery, and hepatic portal vein.
2. Descending Part: Continuing into the second part or D2, the duodenum moves retroperitoneally. Here, the common bile duct and pancreatic duct join to form the hepatopancreatic ampulla, which empties into the descending part of the duodenum.
3. Horizontal Part: Progressing to the third part or D3, the duodenum extends ventrally from the aorta and inferior vena cava, running from right to left across the body. The aorta, originating from the left ventricle of the heart, undergoes a distinctive arching course.
4. Ascending Part: Finally, the fourth part or D4 ascends cranially along the left side of the vertebral column, ultimately connecting with the jejunum at the duodenojejunal flexure. This junction is marked by the suspensory ligament of the duodenum, also known as the ligament of Treitz, delineating the boundary between the upper and lower gastrointestinal tract.

Jejunum

The jejunum constitutes approximately two-fifths of the small intestine. Its primary function is the absorption of essential nutrients such as carbohydrates, fats, and amino acids. Peristalsis, the rhythmic contraction of smooth muscles that propels nutrients through the digestive tract, is very vigorous and rapid in the jejunum. Nutrients absorbed by the jejunum enter the bloodstream, from where they are distributed to various organs in the body.

The mucosal surface of the small intestine features specialized structures that significantly increase the surface area available for nutrient absorption. These structures also regulate and slow down the movement of nutrients through the small intestine, facilitating efficient absorption.

Villi

Villi are finger-like projections found within the circular folds of the small intestine, measuring about 1 millimeter in length. They serve to greatly increase the surface area for nutrient absorption. Each villus contains tiny blood vessels called capillaries, allowing nutrients such as sugars and amino acids to be absorbed directly into the bloodstream.

Microvilli

Microvilli, as the name suggests, are even smaller than villi. They are microscopic hair-like projections on the surface of cells lining the mucosal surface of the small intestine. There are a vast number of microvilli—roughly 200 million per square millimeter in the small intestine. Enzymes located on the surface of microvilli help in the breakdown and absorption of sugars and amino acids.

Ileum

The ileum, the longest and final segment of the small intestine, absorbs vitamin B12 and reabsorbs conjugated bile salts. Measuring about 3.5 meters (11.5 feet) long, it extends from the jejunum to the ileocecal valve, which leads to the colon. Suspended by the mesentery, the ileum has slender smooth muscle walls and slower peristaltic contractions compared to other digestive organs. Additionally, its lining is less permeable than that of the upper small intestine. 

Functions of Small Intestine

The small intestine is the primary site for nutrient absorption in the digestive system. Here's a breakdown of its key functions:

Protein Absorption

• Proteins are broken down into smaller peptides and amino acids by enzymes like trypsin and chymotrypsin from the pancreas. This process, which begins in the stomach, continues in the small intestine.
• Peptides and amino acids are then absorbed across the intestinal lining and transported into the bloodstream.

Lipid Absorption

• Fats and lipids are broken down into smaller molecules (free fatty acids and monoglycerides) by lipases, enzymes released by the pancreas.
• Bile salts, produced by the liver and stored in the gallbladder, emulsify fats (make them water-soluble) to facilitate lipase action.
• After digestion, fatty acids and monoglycerides are absorbed by the intestinal lining and transported into the lymphatic system (eventually reaching the bloodstream).

Carbohydrate Absorption

• Starches are broken down into simple sugars (monosaccharides) by pancreatic amylase. Some complex carbohydrates may be further broken down by intestinal bacteria in the large intestine.
• Simple sugars are then absorbed across the intestinal lining and transported into the bloodstream.

Absorption Mechanisms

• The large surface area of the small intestine is crucial for efficient absorption. This area is increased by folds in the intestinal lining (plicae circularis) and finger-like projections on the epithelial cells (villi). Villi are further equipped with even smaller projections called microvilli, maximizing surface area for nutrient absorption.
• Different nutrients utilize various mechanisms for crossing the intestinal lining:
  • Simple diffusion: Small, non-polar molecules like short-chain fatty acids can passively diffuse across the cell membrane.
  • Facilitated diffusion: Some sugars and amino acids require carrier proteins to facilitate their passage across the membrane.
  • Active transport: Certain nutrients like glucose and some amino acids are actively transported against a concentration gradient using energy from ATP.

Layers of Small Intestine

There are four layers of small intestine: Mucosa, Submucosa, Muscularis Mucosa, serosa.

Mucosa

• The small intestine mucosa performs digestive and absorptive functions, with a gastrointestinal epithelium covering the lamina propria.
• Villi and microvilli significantly increase the surface area for absorption, enhancing nutrient uptake.

Submucosa 

• A thin layer of tissue supporting the mucosa and connecting it to the muscular layer.
• Contains blood vessels, lymphatic vessels, and nerves, including the submucous plexus.

Muscularis Mucosa 

• The outermost layer of the mucosa, composed of elastic fibers and smooth muscle cells.
• Smooth muscle cells extend into the villi, aiding in movement and absorption.

Lamina Propria 

• Connective tissue core of the villi, housing immune cells and nerve endings.
• Supports villi structure and function, facilitating nutrient absorption.

Serosa 

• Thin membrane lining internal body cavities and organs, providing lubrication.
• Made of mesothelium tissue and allows frictionless movement within vital organs.

Conclusion - Small Intestine

In summary, the small intestine, spanning about 20 feet in length and comprising three segments—the duodenum, jejunum, and ileum—plays a crucial role in digestion and nutrient absorption. Its specialized structures, including villi and microvilli, enhance absorption efficiency, while enzymatic actions facilitate the uptake of proteins, lipids, and sugars. Structurally, its layers ensure effective digestion and absorption processes, highlighting its essential role in nutrient uptake and overall digestive health.