Digestive System Anatomy, Nutrient Absorption, and Common Disorders

Key Structures of the Digestive System

Small Intestine Structures

The villi of the small intestine project into the intestinal cavity, greatly increasing the surface area for food absorption and aiding digestive secretions.

Microvilli: Are microscopic cellular membrane protrusions that increase the surface area of cells and minimize any increase in volume. They are involved in a wide variety of functions, including absorption, secretion, cellular adhesion, and mechanotransduction.

Ileocecal Valve: Is a sphincter muscle valve that separates the small intestine and the large intestine. Its critical function is to limit the reflux of colonic contents into the ileum.

The Large Intestine

The large intestine, also called the colon or the large bowel, is the last part of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces before being removed by defecation.

Accessory Digestive Organs

• Liver: A large lobed glandular organ in the abdomen of vertebrates, involved in many metabolic processes.
• Gallbladder: The small sac-shaped organ beneath the liver, in which bile is stored after secretion by the liver and before release into the intestine.
• Pancreas: A large gland behind the stomach that secretes digestive enzymes into the duodenum. Embedded in the pancreas are the islets of Langerhans, which secrete into the blood the hormones insulin and glucagon.

Essential Digestive Substances

• Enzyme: A substance produced by a living organism that acts as a catalyst to bring about a specific biochemical reaction.
• Bile: A bitter greenish-brown alkaline fluid that aids digestion and is secreted by the liver and stored in the gallbladder.

Mechanisms of Nutrient Absorption

Absorption: The process by which digested nutrients move into the tissues where they can be transported and used by the body cells.

Methods of Transport Across the Intestinal Cell Membrane

1. Passive Diffusion: The process of absorbing nutrients across the intestinal cell membrane without requiring energy or a carrier molecule.
2. Facilitated Diffusion: The process of absorbing nutrients across the intestinal cell membrane with the help of a carrier molecule.
3. Active Transport: The process of absorbing nutrients across the intestinal cell membrane with the help of a carrier molecule and energy (ATP).

Common Digestive Disorders and Conditions

• Heartburn: A form of indigestion felt as a burning sensation in the chest, caused by acid regurgitation into the esophagus.
• Gastroesophageal Reflux (GERD): The return of stomach contents back up into the esophagus. This frequently causes heartburn because of irritation of the esophagus by stomach acid.
• Gastroenteritis: Inflammation of the stomach and intestines, typically resulting from bacterial toxins or viral infection and causing vomiting and diarrhea.
• Peptic Ulcer: A lesion in the lining (mucosa) of the digestive tract, typically in the stomach or duodenum, caused by the digestive action of pepsin and stomach acid.
• Gallstone: A small, hard crystalline mass formed abnormally in the gallbladder or bile ducts from bile pigments, cholesterol, and calcium salts. Gallstones can cause severe pain and blockage of the bile duct.