Small Intestines

The small intestines perform several essential functions in the digestive process:

• Digestion: The small intestines play a key role in breaking down complex nutrients from food, including carbohydrates, proteins, and fats. Digestive enzymes from the pancreas and bile from the liver help further break down these nutrients into smaller molecules for absorption.

• Absorption: Nutrient absorption is a primary function of the small intestines. The walls of the small intestines are lined with tiny, finger-like projections called villi, which are covered in even smaller hair-like structures called microvilli. These structures increase the surface area for absorption of nutrients. Nutrients such as glucose, amino acids, fatty acids, vitamins, and minerals are absorbed into the bloodstream through the walls of the small intestines.

• Nutrient transport: The absorbed nutrients are transported through the bloodstream to various cells and tissues throughout the body, providing the energy and building blocks needed for growth, repair, and metabolism.

• Water and electrolyte absorption: The small intestines also absorb water and electrolytes, helping maintain proper fluid balance in the body.

• Immune function: The small intestines have immune functions, with immune cells distributed throughout the intestinal lining to protect against harmful pathogens that may enter the digestive tract.

• Hormone Secretion: Specialised cells in the small intestines produce hormones that regulate digestive processes. For example, cells in the duodenum (the first section of the small intestines) release hormones such as cholecystokinin (CCK) and secretin, which help regulate the release of digestive enzymes and bile.

The small intestines consist of several components and structures:

• Duodenum: The duodenum is the first and shortest section of the small intestines. It receives partially digested food from the stomach and mixes it with bile from the liver and pancreatic enzymes. This mixing and digestion process begins here.

• Jejunum: The jejunum is the middle section of the small intestines. It continues the digestion and absorption of nutrients.

• Ileum: The ileum is the last and longest section of the small intestines. It further absorbs nutrients and is the site where vitamin B12 and bile salts are absorbed. The ileum connects to the large intestines at the ileocecal valve.

• Villi and microvilli: Villi are finger-like projections that line the inner surface of the small intestines, while microvilli are tiny hair-like structures on the surface of the villi. These structures increase the surface area for nutrient absorption and are covered with specialised cells responsible for nutrient absorption.

• Muscular layers: The walls of the small intestines have smooth muscle layers that contract to mix food with digestive enzymes and propel it through the intestines in a process called peristalsis.

• Blood vessels and lymphatics: The small intestines have a rich blood supply to transport absorbed nutrients to the rest of the body. Lymphatic vessels, called lacteals, are also present to absorb and transport dietary fats.

• Intestinal glands: Intestinal glands (crypts of Lieberkühn) are located between the villi and secrete mucus, digestive enzymes, and hormones.

In summary, the small intestines are a crucial part of the digestive system responsible for digestion, absorption, and nutrient transport. They consist of three sections (duodenum, jejunum, and ileum), are lined with villi and microvilli for nutrient absorption, and have a rich blood and lymphatic supply. Their functions are essential for providing the body with nutrients and energy.