Digestive system

Ingestion

The process of ingestion begins with saliva, which dissolves food and contains mucin for lubrication, amylase for breaking down polysaccharides, and antimicrobial components like antibodies and lysozyme. As swallowing commences, the epiglottis closes off the airway to prevent choking, while the pharynx directs the food bolus to the esophagus, a muscular tube that moves food toward the stomach via peristalsis (rhythmic muscle contractions).

Once in the stomach, food undergoes both mechanical and chemical digestion. The stomach’s acidic environment is created by parietal cells secreting hydrochloric acid; this low pH activates pepsin, a protease specializing in protein breakdown. Meanwhile, goblet cells secrete a protective mucus layer that guards the stomach lining from self-digestion. The stomach’s elastic, banana‐shaped structure can stretch considerably and is sealed by the cardiac sphincter at the top and the pyloric sphincter at the bottom.

The liver plays multiple roles in digestion and metabolism. It synthesizes bile from cholesterol, which is stored in the gall bladder and released to emulsify fats into smaller droplets known as micelles, increasing surface area for lipase to act. The liver also makes and stores glycogen, carries out gluconeogenesis, and performs detoxification (including ammonia removal). Additionally, it stores certain vitamins and iron.

Finally, bile may be held in the gall bladder until needed, at which point it is secreted into the duodenum to assist fat digestion.

Pancreas

The pancreas is a tadpole-shaped gland that produces a wide range of digestive enzymes and bicarbonate. These enzymes include Amylase (for starch digestion), various Proteases (for protein digestion), Lipase (for fats), and Ribonuclease (for nucleic acids).

The pancreas also synthesizes $HC{O}_3^{-}$ to neutralize acidic chyme from the stomach, ensuring an optimal pH for enzymes once they enter the small intestine. Because this production is exocrine, the pancreatic secretions flow via a duct into the duodenum, the first section of the small intestine.

Small intestine

The small intestine—subdivided into the duodenum, jejunum, and ileum—is the principal site of digestion and absorption. Its inner lining features folds, villi, and microvilli, all of which greatly increase surface area, maximizing nutrient uptake.

Within each villus, blood capillaries absorb most nutrients, while lacteals (specialized lymphatic capillaries) take up digested fats. The enterocytes (intestinal absorptive cells) move molecules into circulation through both active transport and facilitated diffusion, depending on concentration gradients.

Although the pancreas supplies most of the enzymes needed, the small intestine itself produces some additional enzymes (e.g., certain proteases and amylases). The combined effect of pancreatic secretions and intestinal enzymes, paired with bicarbonate’s neutralizing action, allows the small intestine to efficiently break down and absorb the nutrients necessary for bodily functions.

Large Intestine

The large intestine is divided into several regions:

• The cecum, which is a blind pouch containing the appendix
• The ascending colon, transverse colon, descending colon, and sigmoid colon
• The rectum, which stores feces.

Unlike the small intestine, the large intestine has no folds or villi. Its primary function is to absorb any remaining water not taken up in the small intestine.

Within it, bacterial flora ferment undigested nutrients (producing gas) and synthesize vitamin K, necessary for blood clotting. The rectum connects to the outside through the anal sphincter, which opens during defecation to release waste.

Muscular control

Muscular control throughout the digestive tract relies on strategic valves and coordinated motion. The cardiac sphincter (or gastroesophageal sphincter) at the junction of the esophagus and stomach prevents food from traveling backward, while the pyloric sphincter at the stomach’s exit moderates food flow into the small intestine.

The rhythmic contractions known as peristalsis move food through each segment of the gastrointestinal tract, ensuring that digestion and absorption proceed efficiently.

Endocrine control

The endocrine system regulates the digestive process by releasing hormones that act on specific target tissues throughout the gastrointestinal tract:

• gastrin is secreted by G cells in the stomach, stimulating parietal cells to produce gastric acid that aids in digestion.
• In response to the acidity of chyme entering the small intestine, secretin is released from duodenal S cells, prompting the pancreas and liver to secrete bicarbonate and bile, respectively.
• Similarly, cholecystokinin (CCK) is produced by I cells in the small intestine, which induces the gallbladder to contract and the pancreas to release digestive enzymes.
• Additionally, hormones such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) help regulate satiety and slow gastric emptying, fine-tuning the overall digestive process.

Nervous control

Concurrently, the enteric nervous system (ENS), often referred to as the “second brain,” provides autonomous control over digestive functions by coordinating local reflexes. This network of neurons, embedded within the walls of the gastrointestinal tract, manages motility, secretion, and blood flow.

Although the ENS operates independently, it communicates with the central nervous system via the vagus nerve to adjust digestive activity in response to food intake and other stimuli.