Small Intestine – Structure and Functions

Introduction

The longest part of the digestive tube is the intestine, which is divided into the small intestine and large intestine. Small intestine is commonly known as the small bowel and  is a tubular organ in the digestive system. In fact, at 20 feet approx. in length, it is the longest part of the digestive system. Since its lumen is smaller in diameter, about 2.5 cm or an inch, than the large intestine, it is referred to as the “small” intestine. Food must be digested, metabolized, and stored for expulsion in the intestines. Digestion of food and absorption of nutrients is the main  function of the small intestine in the digestive system.

Structure and Sections of the Small Intestine

The small intestine which  is a tube-like structure is the digestive system’s longest segment. It  functions as a main site for digestion.

The small intestine has three sections, namely the duodenum, jejunum and ileum, and it runs from the stomach (pylorus) to the large intestine (cecum).

• Duodenum: The initial part of the small intestine is called the duodenum. It begins at the stomach’s pyloric sphincter, wraps around the head of the pancreas in a C-shape, and terminates at the duodenojejunal flexure. The peritoneal fold called the suspensory muscle (ligament) of the duodenum, commonly known as the Treitz ligament connects this flexure to the posterior abdominal wall. Descending, superior (duodenal bulb/ampulla), horizontal and ascending are the four sections of the duodenum
• Jejunum: The second part of the small intestine is called the jejunum. It starts at the duodenojejunal flexure. The jejunum is intraperitoneal as the mesentery properly attaches it to the posterior abdominal wall. It is found in the upper left quadrant of the abdomen
• Ileum:  The last and longest part of the small intestine is The Ileum. It  is found in the lower right quadrant of the abdomen. It can extend into the pelvic cavity. The ileum ends at the ileal orifice (ileocecal junction), where the large intestine’s cecum begins

Size of Small Intestine :

It lengthens 20 times as a person grows, from around 200 cm in a newborn to about 6 m in an adult. The duodenum is around 25 cm (10 inches) long, the ileum is about 3.6 m (12 feet) long, and the jejunum is approximately 2.5 m (8 feet) long.

Functions of the Small Intestine 

The small intestine’s major functionality is to   break down and absorb  ingested nutrients consisting of partly digested food by saliva and gastric juices. Here are some of the critical functions of the small intestine.

Digestion in small intestine

• The Role of The Small Intestine In Digestion of Proteins –  Chemical breakdown of protein  starts in the stomach and continues until the jejunum. Enzymes such as trypsin and chymotrypsin secreted by  the pancreas act upon proteins and convert them into small  peptides. Dipeptides and tripeptides release amino acids which are the simplest form of proteins
• The Role of Small Intestine In Digestion of Lipids – bile salts released by the liver and gallbladder aid in the digestion of lipids.Enzymes lipases released by the pancreas work on fats and lipids present in the diet and convert them into  the triglycerides  then into monoglycerides and free fatty acids. Lipase is water-soluble, enzymes whereas  cholesterol is also water-soluble while  fatty triglycerides (simple components of fats)are not. The bile salts keep the triglycerides suspended in water until lipase can break them down into smaller pieces that the intestinal villi can absorb
• The Role of Small Intestine in the Digestion of Carbohydrates – Carbohydrates are broken down b y enzymes pancreatic amylases into simple sugars (monosaccharides) like glucose, fructose, etc. Some carbohydrates are also broken down by pancreatic amylase only into oligosaccharides (sucrose, lactose) 

Intestinal enzymes  like sucrase, lactase, etc. form  monosaccharides. Some carbs and fibers transit undigested to the large intestine, where they may be broken down by intestinal bacteria, depending on their nature.

• The Role of Small Intestine In Digestion of nucleic acids- DNA  and RNA are nucleic acids which are converted into simplest forms like nitrogenous bases (adenine (A), guanine (G), thymine (T)/uracil (U), and cytosine(C),pentose sugars( ribose and deoxyribose) and phosphates by the action of enzymes nucleases

Absorption of nutrients in small intestine 

• Absorption of Food Takes Place in the Small Intestine – Once the food is broken down, into the simplest form it is absorbed by the inner walls of the small intestine and goes  into the bloodstream. The nutrients are reduced in size so that they can pass through the epithelial cells of the gastrointestinal tract or “be transported.” Simple/passive diffusion, assisted diffusion, primary active transport and secondary active transport are various mechanisms to absorb nutrients. The plicae circulares, which deploy many tiny finger-like projections of tissue called villi, increase the absorptive surface. In addition, individual epithelial cells also have projections that resemble fingers known as microvilli. Because of its enormous inner surface area, the small intestine is ideal for absorption
• The Small Intestine Function In Digestive System To Absorb Electrolytes – Diffusion and active transport are used to absorb the salt. Chloride (Cl-) is either absorbed passively with sodium or transferred actively. Iodine (I-) and nitrate (NO3-) can either follow Na+ ions passively or actively absorb them. Calcium ions (Ca2+) are actively absorbed in a calcitriol-stimulated mechanism (the active form of Vitamin D). Active transport systems absorb iron ions (Fe2+ and Fe3+), potassium ions (K+), magnesium ions (Mg2+), and phosphate ions (PO43-)
• The Small Intestine Function In Digestive System To Absorb Vitamins And Minerals – Vitamins, particularly fat-soluble vitamins (Vitamins A, D, E, and K), are absorbed in the same way as dietary fats are. In addition, through diffusion water-soluble vitamins like vitamins, B and C are absorbed
• Water is One of the Other Chemicals Absorbed In The Small Intestine – A large portion of water gets absorbed into the intestine through osmosis. The small intestine absorbs around 80%, the large intestine absorbs 10%, and the feces excretes the remaining 10%

Role of Autonomic Nervous System on Small Intestine

•  Onward movement of its contents by peristalsis is increased by parasympathetic stimulation 
• Parasympathetic stimulation increases the secretion of intestinal juice

Note:  Food is transported from the stomach to the colon, where it is passed to the large intestine, which eventually transports it to the rectum and out of the body via the anus.

Conclusion

The small intestine carries food from the stomach up to the colon.Small intestine is one of the most essential parts of the digestive system where maximum food is converted into simple water soluble form. Duodenum and jejunum are the site for digestion while illum helps in maximum absorption of nutrients. The large intestine starts after illum and carries undigested food to the colon then the rectum and out of the body via the anus.