Digestion in the Gastrointestinal Tract

• Digestion in the gastrointestinal tract breaks down food components into simpler subunits for efficient absorption and distribution.
• Many foods consist of polymers that must be broken down into simpler forms, the simplest of which is glucose

Physiology / Mechanisms

• Mechanical digestion is a physical change, like chewing.
• Chemical digestion involves chemical changes, like starch being converted to maltose by salivary amylase in saliva.
• Absorption mainly occurs in the small intestine.

Gastrointestinal Tract

• It consists of the mouth, esophagus, stomach, small intestine, and large intestine.
• The structures of the tract are made of smooth muscle.
• Swallowed food (bolus) enters the pharynx, which leads to either the respiratory or digestive system.
• The epiglottis directs food into the esophagus by covering the larynx during swallowing.
• Peristalsis in the esophagus facilitates minimal mechanical digestion.
• Sphincters regulate food passage in the stomach.
• The cardiac sphincter prevents food from returning to the esophagus.
• The pyloric sphincter prevents premature entry of food into the small intestine.
• The stomach empties every three hours.

Stomach

• The stomach mechanically digests food through movement, liquefying it.
• The stomach secretes hydrochloric acid, pepsinogen, and mucus.
• Pepsinogen is activated into pepsin in the stomach's acidic environment, which digests proteins.
• Hydrochloric acid maintains the acidic environment in the stomach.
• Mucus protects the stomach lining.
• The stomach absorbs some medications and alcohol.
• After digestion, the acidic, liquefied food (chyme) enters the duodenum.

Small Intestine

• The liver and pancreas connect to the duodenum via ducts.
• The liver releases bile which assists in the breakdown of fats.

Small vs. Large Intestine

• The small intestine (duodenum, jejunum, ileum) is surrounded by the large intestine in the abdomen.
• The caecum, connected to the appendix, is where the small and large intestines meet.

• In the large intestine, undigested food becomes feces or stool.
• The sections of the colon: ascending, transverse, and descending relate to the movement of food.
• The sigmoid colon connects the colon and rectum.
• The rectum stores feces and signals the need to defecate.
• The internal anal sphincter (involuntary) and external anal sphincter (voluntary) control defecation.
• Villi in the small intestine increase surface area for absorption, making it the most important organ for absorption.
• Fat-soluble vitamins (A, D, K) are absorbed in the small intestine.
• Water-soluble vitamins (B, C) are absorbed in the large intestine.

Accessory Organs and Glands

• These organs and glands are not part of the gastrointestinal tract.
• There are three salivary glands in the mouth that produce saliva, containing salivary amylase.

1. The sublingual gland is under the tongue.
2. The submandibular gland (or submaxillary gland) is under the jaw.
3. The parotid gland, the largest, is in front of the ears; it is associated with mumps, which can also affect the reproductive system and reduce sperm count.

Liver and Gallbladder

• Both are connected to the duodenum.
• The liver produces bile, which is stored in the gallbladder and helps in fat digestion and neutralizes chyme.
• The liver stores glycogen made from glucose in the bloodstream.
• The liver detoxifies harmful chemicals such as alcohol and medications.

Pancreas

• The pancreas functions as both a digestive and an endocrine organ.

Endocrine functions

• When the body's beta cells of the pancreatic islets of Langerhans detect high blood sugar (hyperglycemia) the hormone insulin is created.
• The insulin in released into the bloodstream will signal the liver and skeletal muscles to absorb excess glucose lowering blood sugar back to normal levels.
• With low blood sugar (hypoglycemia) the pancreas will create the hormone glucagon
• The glucagon in released into the bloodstream will signal the liver and skeletal muscles to break down glycogen and release it in the form of glucose raising blood sugar back to normal levels.

Digestive functions

• The pancreas releases sodium bicarbonate to neutralize chyme.
• The pancreas releases lipase, protease, and amylase to digest lipids, proteins, and carbohydrates, respectively.
• The liver and pancreas release their secretions into the duodenum.

Enzymes

• Enzymes are produced by digestive structures.
• Enzymes are proteins that speed up chemical reactions in the digestive system.

Mesentery

• It keeps digestive organs in place.
• Scientists hypothesize that it plays a role in immunity.