Part 2: Physiology of Digestion

In this section, we are going to review material you have already taken in the Anatomy and Physiology course. This section is more based on the concept of scientific physiology. If any of the material doesn’t seem familiar, please refer to your Anatomy and Physiology course or review it, in your textbook for that course.

The digestive system is comprised of a series of organs that participate in the breakdown and absorption of nutrients and accessory nutrients from our diet. These nutrients are used to supply the body with both its energy and most of the nutrient molecules used to maintain the structure of the body. The digestive tract or gastrointestinal tract (GIT) is a tube measuring up to nine meters (30 feet) in length, that is continuous with the integumentary system, and thus is not only comprised of similar tissues but maintains similar functions to protect us from the plethora of microorganisms that are contained within it. Associated with this tract are a number of accessory digestive organs which secrete various substances into the lumen or opening of the

digestive tract. Here, they denature and break down the food we consume into tiny particles that can be absorbed across the lining of the digestive tract, into the blood and lymphatic systems, for further processing. Overall, the digestive tract participates in a variety of activities that include: mastication or chewing by the mouth; the secretion of digestive substances; the mixing and propulsion of the digesting food through the tract; the absorption of the nutrient particles into the body; and the elimination of substances that cannot be digested or are regarded as waste products. The process of digestion is comprised of two primary components: mechanical digestion, in which the food is broken down by mechanical activities such as chewing by the mouth and the churning of organs like the stomach; and chemical digestion, in which the food is denatured and broken down into smaller and smaller particles by chemical means.

Histology and structure of the digestive tract

There are several layers of tissue that make up the digestive tract, some of which vary depending upon what area of the tract we are talking about. The deepest layer of the GIT is the mucosa, a lining of epithelial tissue, areolar connective tissue and a layer of smooth

muscle fibers. Just above the mucosal epithelium is the lamina propria, a layer of areolar connective tissue that contains blood vessels as well as specialized lymphatic vessels called mucosa-associated lymphatic tissues (MALT) that play a key role in maintaining the integrity of the digestive tract against microbes that try to penetrate into the body. Laying above this is the muscularis mucosae, a thin layer of smooth muscle cells that cause the digestive tract to be arranged in a series of tiny folds, gently moving to increase the absorptive surface of the GIT. Superficial to the mucosa is the submucosa, a layer of areolar connective tissue that binds the mucosa to the underlying tissue, houses blood and lymphatic vessels that converge from the lamina propria, and contains an extensive network of neurons called the submucosal plexus. This plexus contains both sensory and motor neurons from the ANS that form what is called the enteric nervous system (ENS), sometimes referred to as the “second brain,” or the “brain of the gut”. The ENS contains enteric neurons, sympathetic and parasympathetic postganglionic neurons, and parasympathetic ganglia. Lying superior to the submucosa is the muscularis, which regulates the movement of ingested food through the GIT. In certain areas such as the mouth, throat, and anus muscularis this area contains skeletal muscle fibers, which allow these areas to be under our (mostly) conscious control. The other areas of the muscularis however are comprised of smooth muscle fibers arranged in two layers: an inner layer of circular fibers, and an outer layer of longitudinal fibers. Lying between these layers in the mysenteric plexus, is another branch of the ENS that functions to regulate muscular contraction. The most external layer of the GIT is the serosa, a serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium).

Words to remember or review *To allow for consistency, many of the following images are taken from your Anatomy and Physiology textbook.

• Mouth or buccal cavity: cheeks, lips or labia, orbicularis oris muscle, hard palate, soft palate, oropharynx , nasopharynx, uvula, palatopharangeal arch, fauces, pharynx, palantine tonsils, lingual tonsils

• Salivary glands, parotid glands, masseter muscle, parotid ducts, submandibular glands, lingual frenulum. Sublingual glands are located above the submandibular glands, their sublingual ducts opening into the anterior floor of the oral cavity. Associated with these major salivary glands are a number of smaller minor salivary glands that line the cheek, lips, palate, and tongue.

• Saliva, 99.5% water, slightly acidic ions, mucus, lysozymes, immunoglobulin A, salivary amylase, lingual lipase, 1000-1500 mL of saliva on a daily basis.

• Tongue: papillae, fungiform papillae, circumvallate papillae, foliate papillae, lingual glands, lingual lipase

• Teeth: alveolar processes of the mandible and maxillae, gingivae, periodontal ligaments, crown, neck, roots, enamel, dentin, pulp, mastication, bolus

• Pharynx: internal nares, larynx anteriorly, swallowing or deglutition,

epiglottis, trachea • Esophagus: posterior to the trachea, esophageal hiatus, upper

esophageal sphincter (UES), lower esophageal sphincter (LES)

• Stomach: J-shaped enlargement, four parts: the cardia, the fundus, the body, the pylorus, pyloric sphincter. The inside of the stomach has rugae, gastric pits, gastric glands, mucous cells, parietal cells secrete intrinsic factor and also hydrochloric acid, chief cells secrete pepsinogen and gastric lipase, gastric juice secreted is about 2000-3000 mL daily, G cells that release hormones, chyme, peristaltic motions called mixing waves

• Gastric digestion: three phases: cephalic phase, gastric phase, intestinal phase. Secretin acts to decrease gastric secretions, whereas cholecystokinin inhibits gastric emptying and protein denaturation

• Pancreas: head, body, tail, epithelial cells called acini, pancreatic juice, pancreatic islets (islets of Langerhans), pancreatic duct, hepatopancreatic ampulla (ampulla of Vater), accessory duct, pancreatic juice secreted is 1200 - 1500 mL daily (water, salts, sodium bicarbonate, and several enzymes), pancreatic amylase (polysaccharides into di/trisaccharides), trypsinogen (which is converted into trypsin by enterokinase for proteins), chymotypsinogen (catalyzed into chymotrypsin by trypsin, breaking down proteins), procarboxypeptidase into carboxypeptidase by trypsin, pancreatic lipase (triglycerides that have been emulsified), ribonuclease (breaks down RNA), deoxyribonuclease (breaks down DNA), trypsin inhibitor (deactivates trypsin if accidentally released)

• Liver: 1.4 kg in the average adult, inferior to the diaphragm, occupying most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity, gallbladder, two lobes (a larger

right lobe and a smaller left lobe), divided by the falciform ligament, lobules (six-sided consists of specialized hepatocytes), arranged

• around a central vein, sinusoids, stellate reticuloendothelial (Kupffer) cells, bile canaliculi, hepatocytes, bile ducts, portal triad, left and right hepatic ducts, common bile duct

• Bile: yellowish-brown to greenish-brown liquid manufactured by the hepatocytes, produce about 800-1000 mL on a daily basis, its pH is about 7.6-8.6, acts in the emulsification of fats, and consists of: water, bile acids, bile salts, cholesterol, lecithin, bile pigments and several ions, and bile salts - which are comprised mostly of sodium and potassium salts, of cholic and chenodeooxycholic acid, bilirubin (heme portion of hemoglobin), and stercobilin -characteristic brown color of the feces

• Small intestine: primary absorptive surface for digested food particles. Measures approximately 6 meters in length, comprised of villi and microvilli, duodenum (25 cm, receives the acidic chyme, secretions of the liver pancreas and gallbladder), jejunum (1 m long), ileum (2 m long), ileocecal sphincter, villi (lamina propria, a venule, a capillary bed, and a lymphatic lacteal), Paneth cells (lysozymes), Peyer’s patches, duodenal (Brunner’s) glands, up to 90% of the nutrients, are absorbed within the small intestine

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• Colon or large intestine: measures approx. 1.5 meters in length, and is comprised of: the ileocecal sphincter, cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, anus,

tenia coli, haustra, mass peristalsis, bacteria (ferment indigestible materials such as fibers), flatulence, produce vitamin K and some B vitamins, water in the chyme, feces (indigestible fibers, inorganic salts, sloughed off epithelial cells, undigested chyme, and water), internal anal sphincter, and external anal sphincter.