Digestive System and Metabolism Chapter 18

Digestive System and Metabolism

• Readings include all of Chapter 18, excluding 18.6.
• Learning outcomes include functional anatomy of the digestive system, accessory organs, and critical thinking about time-restricted feeding and microbiota.
• Gastrointestinal gas production averages about 0.5 liters per day, spread across 14-25 incidents.
• The time it takes to digest food from consumption to excretion is not specified in the provided text.
• A reference to an online source for the answer to a question is provided, but the specific answer is not.

Digestion and Metabolism

• Digestion includes breakdown of ingested food, absorption of nutrients, and removal of waste products.
• Metabolism involves production of cellular energy (ATP), and regulation of cellular activities.

Structures of the Alimentary Canal

• Structures include mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.

Functional Anatomy of the Digestive Tract

• The digestive tract includes the oral cavity, teeth, tongue, salivary glands, pharynx, esophagus, stomach, small intestine, large intestine, liver, gall bladder, pancreas, rectum, and anus.

Functions of the Salivary Glands

• Functions include lubrication/binding, solubilization of dry food, oral hygiene, initiating starch digestion, and evaporative cooling (important in dogs).

From Mouth to Stomach

• Mastication (chewing) involving teeth (incisors, canines, premolars, molars).
• Deglutition (swallowing) process involves 25 pairs of muscles in the mouth, pharynx, larynx, and upper esophagus; it transitions to autonomic neurons to control middle and lower esophagus.

From Mouth to Stomach- The Teeth

• Teeth are the hardest structures, with 20 primary and 32 secondary (adult) teeth.

From Mouth to Stomach - Esophagus

• Esophagus is ~25 cm long; movement of food assisted by peristalsis.

From Mouth to Stomach - Peristalsis

• Peristaltic contraction moves food.

The Stomach

• Smooth muscle layers (circular, longitudinal, and oblique) mix and break down food.

Gross Anatomy of the Stomach

• Stomach contains circular, longitudinal, and oblique muscles, and the mucosal region includes gastric pits, and gastric glands.

Secretory Components of the Stomach

• Gastric pits, opening the gastric glands (mucous cells, chief cells, and parietal cells).
• Each cell type produces a specific secretion.
• The stomach's lining does not digest itself due to mucous cells and pepsin production/inactivation by pH levels.
• Helicobacter pylori can cause peptic ulcers.

Small Intestine

• Regions include duodenum, jejunum, and ileum.
• Large number of folds and villi to increase surface area for absorption.
• Important for absorption of nutrients, bile salts, water, and electrolytes.
• Continues from the ileocecal valve to the large intestine.

Small Intestine: Villi

• Columnar epithelial cells
• Goblet cells
• Paneth cells secrete antibacterial molecules.
• Sloughed-off epithelial cells from tips of villi are replaced by new cells from intestinal crypts.

Small Intestine: Microvilli

• Microscopically small projections
• Brush border enzymes are contained within the microvilli membrane, assisting in disaccharide digestion.
• Enzymes such as sucrase, maltase, lactase, and others are within the microvilli to aid in disaccharide digestion.

Critical Thinking - Time Restricted Eating

• Studies suggest that changing eating times to restrict eating hours can help with weight loss and impact body composition.
• This type of feeding/fasting can have an effect by lessening daily hunger swings.

Large Intestine

• Regions include ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.
• Colon (large intestine) contains bacteria that perform important digestive processes.
• "Good" bacteria outcompete pathogenic bacteria. When pathogenic bacteria take over the colon reacts by eliminating colon content - diarrhea is the result.
• Intestinal microbiota are about 10x more numerous than human cells.
• The appendix is present; function is unknown; likely contains "good bacteria" reservoir, potentially recolonizing the colon for diarrhea recovery. Inflammation or rupture in the appendix can cause peritonitis.

Accessory Digestive Organs: Liver

• Hepatic cells line large capillaries called sinusoids, also lining these are endothelial cells.
• Sinusoids have Kupffer cells, phagocytes.
• Liver has regenerative capabilities, able to regenerate to original mass within a week with 2/3rds of the liver removed surgically.

Accessory Digestive Organs: Pancreas

• Pancreatic juice contains ~20 digestive enzymes (amylase, trypsin, lipase).
• Endocrine portion secretes hormones (insulin, glucagon, somatostatin), and exocrine portion secretes digestive enzymes.

Learning Outcomes - Carbohydrate Metabolism

• Critical thinking on factors like sugar, insulin, genetics, and diet.

Learning Outcomes - Lipid, Protein Metabolism

• Critical thinking on factors like dietary fats, protein sources, and other relevant factors.

Learning Outcomes - Absorption of Vitamins

• Understanding water soluble vs. fat soluble vitamins.

Main Functions of the Digestive System

• Motility, secretion, digestion, and absorption are the major functions.

Classification of Animals Based on Food Habits

• Herbivores, carnivores, omnivores, and other dietary classifications.

Types of Digestion in Animals

• Enzymatic digestion and microbial or fermentation.

Classes of Carbohydrates

• Polysaccharides, disaccharides, and monosaccharides are main carbohydrate groups which are included in a diet.

Chemical digestion of Carbohydrates

• The process of breaking down complex carbohydrates into simpler forms.

Digestion of Carbs in the Mouth & Stomach

• Chewing mixes food with saliva containing ptyalin (a-amylase), hydrolysis occurs and starches convert to maltose etc.

Digestion of Carbs in the Small Intestine

• Pancreatic amylase is potent than salivary amylase and converts carbs to maltose. Almost all carbs are hydrolyzed within 30 minutes.

Absorbing the products of carbohydrate digestion

• Diffusion of monosaccharides (glucose, galactose, and fructose) into capillaries.

Post-absorptive events: simple sugars

• Glucose converted to glycogen for energy production and storage.

Insulin and Glucose: Post-absorptive events

• Insulin helps bring glucose into cells

Simple Sugars and Insulin

• Simple sugars like glucose, are taken into cells alongside insulin.

Glycemic Index Chart

• Illustrates the glycemic index of foods.

Glycemic Index & Insulin

• Influence of Glycemic Index on blood glucose levels.

Cholesterol Absorption

• Cholesterol is primarily absorbed without digestion.

Post-absorptive events: cholesterol

• Cholesterol absorption from food without digestion.

Lipids - Fats

• Triglycerides, phospholipids, sterols are major groups of dietary fats.

Digestion of Fats in the Stomach

• A minimal amount of fat digestion occurs in the stomach.

Chemical Digestion of Lipids

• Lipids are not water soluble, thus having problems with digestion.

Digestion of Triglycerides by Pancreatic Enzymes

• Triglycerides are broken down.

Chemical digestion of lipids

• Steps including emulsification and hydrolysis are included.

Absorbing lipid digestion products

• Monoglycerides and fatty acids are absorbed into cells.

Digestion of proteins

• Protein polymers consist of amino acid chains and are digested to produce amino acids to be absorbed.

Chemical digestion of proteins

• Protein polymers are broken into peptides then amino acids that pass into blood stream.

Digestion of Proteins in the Stomach

• Pepsin digests proteins.

Digestion of Proteins by Pancreatic Secretions

• Pancreatic secretions include trypsin, chymotrypsin, carboxypeptidase, and proelastase.

Absorbing the products of protein digestion

• Amino acids are transported into absorptive cells by transport proteins.

Post-absorptive events: amino acids

• Amino acids cannot be stored directly, and are metabolized by the liver.

Absorption of Vitamins

• Water-soluble and fat-soluble vitamins are absorbed via diffusion and endocytosis methods.

This quiz covers Chapter 18 of the digestive system and metabolism, focusing on the functional anatomy of the digestive tract, accessory organs, and key concepts like time-restricted feeding and microbiota. Participants will explore the breakdown of food, absorption of nutrients, and the metabolic processes involved in energy production.