- The digestive system is a complex network of organs and structures responsible for the mechanical and chemical breakdown of food, absorption of nutrients, and elimination of indigestible substances.
- Its primary function is to convert food into energy and essential molecules that the body can use for growth, repair, and maintenance. The system encompasses a continuous tube known as the alimentary canal, along with various accessory organs that support digestion through the production of enzymes, hormones, and digestive fluids.
- The alimentary canal begins at the mouth, where digestion first occurs. Mechanical digestion involves chewing (mastication), while chemical digestion begins with enzymes like amylase in saliva that start breaking down carbohydrates. The food is then formed into a bolus and travels through the pharynx and esophagus via rhythmic muscular contractions called peristalsis, ultimately reaching the stomach.
- The stomach is a muscular, sac-like organ that performs both mechanical churning and chemical digestion. Gastric glands secrete hydrochloric acid (HCl) and pepsinogen, which together create an acidic environment that breaks down proteins into peptides. The stomach also produces mucus to protect its lining from self-digestion. After processing in the stomach, the resulting semi-liquid mixture, known as chyme, is gradually released into the small intestine.
- The small intestine is the primary site for nutrient absorption and is divided into three parts: the duodenum, jejunum, and ileum. In the duodenum, chyme is mixed with bile from the liver and gallbladder as well as digestive enzymes from the pancreas. Bile emulsifies fats, making them easier to digest, while pancreatic enzymes further break down carbohydrates, proteins, and lipids. The intestinal lining, covered in villi and microvilli, maximizes surface area for efficient absorption of nutrients into the bloodstream.
- Remaining undigested material moves into the large intestine, which is composed of the cecum, colon, and rectum. Here, water and electrolytes are absorbed, and the material is compacted into feces. The large intestine also hosts a vast community of beneficial bacteria—known as the gut microbiota—which aid in fermentation of undigested carbohydrates and produce certain vitamins such as vitamin K and biotin. The feces are eventually stored in the rectum and expelled through the anus during defecation.
- Accessory organs play essential roles in digestion without being part of the alimentary canal itself. The liver produces bile, detoxifies harmful substances, and metabolizes nutrients. The gallbladder stores and concentrates bile, releasing it into the small intestine as needed. The pancreas produces both digestive enzymes and bicarbonate to neutralize stomach acid, as well as hormones like insulin and glucagon for blood sugar regulation.
- The digestive system is intricately regulated by both the nervous and endocrine systems. Local nerve networks called the enteric nervous system coordinate reflexes and muscle contractions within the gut. Hormones such as gastrin, secretin, and cholecystokinin (CCK) regulate secretions and motility to ensure digestion occurs efficiently and at the right times.
- Disorders of the digestive system are common and can range from minor issues like indigestion or constipation to serious conditions such as inflammatory bowel disease (IBD), ulcers, gallstones, or colorectal cancer. Lifestyle factors such as diet, hydration, stress, and physical activity play crucial roles in maintaining digestive health.
