The human digestive tract begins with the oral cavity and continues with the pharynx, esophagus, stomach, small intestine, large intestine, and anus. Accessory organs, such as the salivary glands, pancreas, liver, and gallbladder, also play essential roles in digestion.
The oral cavity (the mouth) is where mechanical and chemical digestion of food begins. Mechanical digestion is the breakdown of large food particles into smaller particles through physical actions, such as the biting and chewing action of teeth (mastication) or the churning motion of the stomach. Chemical digestion refers to the enzymatic breakdown of macromolecules into smaller molecules and begins in the mouth when the salivary glands secrete saliva. Saliva lubricates food to facilitate swallowing and provides a solvent for food particles. Saliva is secreted in response to a nervous reflex triggered by the presence of food in the oral cavity. Saliva contains the enzyme salivary amylase (ptyalin), which hydrolyzes starch to maltose (a disaccharide). Food that has been sufficiently moistened and broken down to be swallowed down the esophagus is called a bolus.
The esophagus is the muscular tube leading from the mouth to the stomach. Food is moved down the esophagus by rhythmic waves of involuntary muscular contractions called peristalsis. The esophagus is closed off from the stomach by contraction of a muscular structure called the lower esophageal (cardiac) sphincter.
The body of the esophagus lies within the thoracic cavity, which is negatively pressured relative to the environment on inhalation. The abdominal cavity has a relative positive pressure. Therefore, without normal defense mechanisms, the pressure gradients favor a continual reflux of gastric (stomach) materials into the esophagus, resulting in a pathological condition known as gastroesophageal reflux disease (GERD). The pathophysiology of gastroesophageal reflux is as follows:
The stomach, a large, muscular organ located in the upper abdomen, stores and partially digests food. The walls of the stomach are lined by the gastric mucosa, a mucous membrane that contains the gastric glands. Mucous cells in gastric pits along the membrane secrete mucus to protect the stomach lining from the harshly acidic juices (pH = 2) present in the stomach. Chief cells found within the gastric glands synthesize pepsinogen, which is converted to pepsin upon contact with stomach acid and breaks down proteins. Parietal cells, also present within gastric glands, synthesize and release hydrochloric acid (HCl), which alters the pH of the stomach and kills bacteria, and intrinsic factor, which is necessary for the absorption of vitamin B12. The churning of the stomach (mechanical digestion), combined with this enzymatic activity (chemical digestion), produces an acidic, semifluid mixture of partially digested food known as chyme. The chyme passes into the first segment of the small intestine, the duodenum, through the pyloric sphincter.
Chemical digestion is completed in the small intestine. The small intestine is divided into three sections: the duodenum, the jejunum, and the ileum. The small intestine is highly adapted to absorption. To maximize the surface area available for digestion and absorption, the intestine is extremely long (greater than six meters in length) and highly coiled. In addition, numerous finger-like projections called villi extend out of the intestinal wall. Villi contain capillaries and lacteals (vessels of the lymphatic system). Amino acids and monosaccharides pass through the villi walls into the capillary system. Blood from the digestive tract enters the hepatic portal system of the liver, where it is detoxified and stripped of some of its nutrients. Large fatty acids and glycerol pass into the lacteals and are then reconverted into fats (fatty acid + glycerol). Note that some nutrients, such as glucose and amino acids, are actively absorbed (i.e., requiring energy), whereas others are passively absorbed.
Most digestion in the small intestine occurs in the duodenum, where the secretions of the intestinal glands, pancreas, liver, and gallbladder mix together with the acidic chyme entering from the stomach. The intestinal mucosa secretes lipases (for fat digestion), aminopeptidases (for polypeptide digestion), and disaccharidases (for maltose, lactose, and sucrose digestion). For example, the disaccharidase lactase breaks down lactose (milk sugar). This enzyme is most often present in infants, but many adults lack the enzyme and are lactose-intolerant. In these people, lactose in the small intestine cannot be digested and is metabolized by bacteria, producing intestinal discomfort.
The activity of the digestive system is regulated by several hormones, which allow for optimum flow of materials through the digestive tract and help regulate the hunger and satiation mechanisms. These hormones include:
The liver produces bile that is stored in the gallbladder before release into the small intestine. Bile contains no enzymes; it emulsifies fats, breaking down large globules into small droplets. Emulsification of fats exposes a greater surface area of the fat to the action of pancreatic lipase. In the absence of bile, fats cannot be digested. The liver’s functions also include storage of glycogen, conversion of ammonia to urea, protein synthesis, detoxification, and cholesterol metabolism.
The pancreas produces enzymes such as amylase for carbohydrate digestion, trypsin for protein digestion, and lipase for fat digestion. When the pancreas releases chymotrypsin and enterokinase, enterokinase cleaves trypsinogen to trypsin. Trypsin then cleaves and activates the other zymogens (enzyme precursors). The pancreas secretes a bicarbonate-rich juice that neutralizes the acidic chyme arriving from the stomach in the duodenum. The pancreatic enzymes operate optimally at this higher pH.
The large intestine is approximately 1.5 m long and absorbs salts and any water not already absorbed by the small intestine. The rectum provides for transient storage of feces before elimination through the anus.