Gastrointestinal Physiology Quiz

Gastrointestinal Physiology

• The digestive system's function is performed by four basic processes: secretion, motility, digestion, and absorption.
• Digestion breaks down complex molecules in food into simpler ones for absorption.
• Digestive juices and wall motility assist in this breakdown.
• Waste products (undigested food) are excreted.
• The gut plays a role in the immune system through lymphoid tissue.

Learning Objectives

• Students should be able to discuss the overall functions of the digestive system (processes and regulation of gut function).
• They should outline the functions of saliva.
• Students should discuss the stomach's functions, including its protective mechanisms.
• They should discuss the digestive roles of pancreatic juice and bile.
• Students should be able to describe carbohydrate, protein and fat digestion and absorption.
• Students should identify functions of the large intestine.
• Students should summarize gut hormones.

Digestive System

• The digestive system includes the alimentary canal and digestive glands.
• The alimentary canal extends from the mouth to the anus.
• Digestive glands, located outside the gastrointestinal tract, secrete their substances into the tract. Salivary glands, liver, gallbladder, and exocrine pancreas are examples.
• Other glands are located within the walls of the stomach and small intestine.

Components of the Digestive System

• The digestive system comprises the alimentary canal and accessory digestive glands.
• The alimentary canal starts in the mouth and ends at the anus.
• The accessory digestive glands include salivary glands, liver, gallbladder, and exocrine pancreas.
• Glands in the stomach and small intestine walls complete the system.

General Functions of the Gut

• Food digestion and absorption are essential functions of the gut.
• Digestion involves the breakdown of complex food molecules into simpler ones.
• Secretion of digestive juices and gut wall motility are necessary for proper digestion.
• Excretion of undigested food materials is also crucial.
• The gut's immune function protects the body via lymphoid tissue.

Structure of the Digestive Tract Wall

• The digestive tract wall has four main layers: mucosa, submucosa, muscularis externa, and serosa.
• The mucosa lines the lumen and contains exocrine and endocrine cells.
• The submucosa is composed of connective tissue, containing blood vessels, and lymph vessels.
• The muscularis externa consists of an inner circular and outer longitudinal layer with myenteric plexus in between.
• Serosa is the outer protective layer.

Basic Digestive Processes

• Secretion, motility, digestion, and absorption are the basic processes of the digestive system.

Gut Secretions

• Exocrine glands in the gut wall, plus salivary glands, pancreas, and liver secrete digestive juices containing water, electrolytes, mucus, and enzymes.
• These help in the process of digestion.

Gut Motility

• Food movement in the digestive tract is by smooth muscle contraction (involuntary).
• Types of gut motility include: propulsive and mixing.

Propulsive & Mixing Movements

• Propulsive movement (peristalsis) moves food forward in the esophagus and intestines.
• Mixing movement (segmentation) mixes food with digestive juices in the small and large intestines.

Digestion & Absorption

• Digestion uses enzymes which break down food molecules.
• Absorption happens predominantly in the small intestine.
• Undigested material is eliminated via defecation.

Control of Gut Functions

• Gut functions are controlled by neural and hormonal mechanisms.
• Enteric nervous system is also a network of neurons in the gut wall which regulates it independently of the autonomic nervous system (brain of the gut)).
• Autonomic nerves (extrinsic innervation) include sympathetic and parasympathetic nerves which either stimulate or inhibit activities in the gut.
• Hormones in the gut mucosa regulate gut motility and secretion.

Neural and Endocrine Regulation

• Endocrine cells in the gut mucosa release hormones regulating gut motility and secretions.
• These hormones travel in the bloodstream to target areas.

Salivary Glands

• Saliva is secreted by three pairs of salivary glands: parotid, submandibular, and sublingual.

Functions of Saliva

• Saliva has digestive functions by containing amylase to start carbohydrate digestion.
• It moisturizes the buccal cavity, aiding in articulation.
• Saliva has a defensive action with antibacterial enzymes and antibodies.
• Saliva maintains a near neutral pH.

Swallowing (Deglutition)

• Swallowing is a process moving food from mouth to stomach through pharynx and esophagus.
• Swallowing phases include buccal (voluntary, tongue movement), pharyngeal (involuntary, stimulated by pressure receptors), and esophageal phases (involuntary peristalsis).

The Stomach

• The stomach stores food, mixes it with gastric secretions, and produces chyme (a liquid mixture).
• Gastric secretions contain HCl and enzymes (pepsin).
• Parietal cells secrete HCl that helps protein digestion, kills microorganisms, and absorbs certain minerals.
• Chief cells secrete pepsinogen (enzyme precursor for digestion).
• The intrinsic factor assists in Vitamin B12 absorption.
• Evacuation of chyme from the stomach into the small intestine occurs gradually.

Protective Mechanisms of the Stomach

• Thick, alkaline mucus acts as a barrier against the acid of the stomach.
• Pepsinogen is secreted and activated by HCl in the stomach to facilitate protein digestion.
• The mucosal barrier has rapid cell turnover, maintaining its integrity.

The Pancreas

• Pancreas has both exocrine and endocrine components.
• Duct cells secrete bicarbonate-rich fluid to neutralize incoming acidic chyme.
• Acinar cells secrete digestive enzymes (amylase, trypsin, chymotrypsin, carboxypeptidase, lipase).
• Hormones (like insulin and glucagon) are produced in the islets of Langerhans.

Pancreatic Juice

• Pancreatic juice contains digestive enzymes necessary for carbohydrate, protein, and triglyceride digestion.
• Pancreatic juice has bicarbonate to neutralize acidic chyme protecting the intestinal mucosa.

Hormonal Regulation of Pancreatic Secretion

• Hormones like secretin and cholecystokinin regulate pancreatic secretions in response to chyme composition, fat & protein entry into the duodenum

The Liver

• The liver is involved in continuous bile production and secretion.
• Bile is stored and concentrated in the gallbladder between meals.
• Bile helps in fat digestion and absorption when food is present.

Liver Functions

• Liver has secretory functions (bile production), synthesis of plasma proteins (ex serum albumin).
• Carbohydrate metabolism: regulating blood glucose level.
• Protein metabolism: production of amino acids.
• Fat metabolism: synthesis of cholesterol & lipoproteins.
• Important storage functions and detoxification.

Bile

• Bile, produced by the liver, is essential for the digestion and absorption of fats and fat-soluble vitamins in the small intestine.
• Bile salts emulsify fat, breaking large fat globules into smaller ones.
• This improves the effectiveness of pancreatic digestion & absorption.
• This ensures adequate exposure of fats to digestive enzymes.

Functions of the Small Intestine

• Chyme from the stomach is mixed with intestinal, pancreatic juices.
• The small intestine is responsible for complete digestion of food through its secretions (digestive enzymes-e.g., peptidases).
• Absorption of nutrients occurs in this part.

Overview of Digestion in the Small Intestine

• Carbohydrates digestion is completed in the small intestine by disaccharidases in the intestinal juice.
• Protein digestion is completed by the enzyme aminopeptidase in the intestinal juice.
• Fats are digested in the small intestine lumen with the aid of pancreatic lipase, yielding fatty acids and monoglycerides.

Large Intestine

• The large intestine absorbs water and electrolytes.
• The large intestine is responsible for defecation: elimination of undigested material.
• Vitamins, like vitamin K, are produced by bacteria in the large intestine.