Human Anatomy: Digestive System Overview

Digestive System and Metabolism

• Readings: All of Chapter 18, with the exception of 18.6.
• Learning Outcomes:
  • Functional Anatomy of the Digestive System (mouth, esophagus, stomach, small intestine, large intestine, colon, liver, gall bladder, pancreas)
  • Accessory Organs
  • Critical thinking (time restricted feeding, microbiota)
• Gastrointestinal Gas:
  • Approximately 0.5 liters per day, spread across 14-25 incidents.
• Digestion time:
  • Different digestion times depending on food.

Digestion

• Breakdown of ingested food
• Absorption of nutrients into the blood
• Excretion of waste.
• Production of cellular energy (ATP)
• Regulation of cellular activities

Structures of the Alimentary Canal

• Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

Structure of the Digestive System

• Two Main Functional Groups of Organs:
  • Alimentary Canal
  • Accessory Digestive Organs

Functions of the Salivary Glands

• Lubrication/binding
• Solubilization of dry food
• Oral hygiene by debris flushing
• Begins starch digestion (salivary amylases)
• Alkaline buffering
• Evaporative cooling (important in dogs)

From Mouth to Stomach

• Mastication (chewing food)
• Teeth (incisors, canines, premolars, and molars)
• Deglutition (swallowing)
• 25 muscles in the mouth, pharynx, larynx, upper esophagus
• Somatic motor neurons (mouth, pharynx, upper esophagus)
• Autonomic neurons (middle and lower esophagus)
• Esophagus connects pharynx and stomach (25cm long)
• Peristalsis moves food via muscular contractions.

The Stomach

• Smooth muscle layers mix and mechanically break down food.
• Oblique direction for mixing and breakdown.
• Circular, longitudinal, and oblique muscles.
• Mucosal region contains gastric pits and gastric glands.

Secretory Components of the Stomach

• Gastric pits are the openings of the gastric glands.
• Gastric glands contain mucous cells, chief cells, and parietal cells that each produce specific secretions.
• Secretions include: -Mucous cells: secrete mucus -Parietal cells: secrete hydrochloric acid (HCl), intrinsic factor for vitamin B12 -Chief (zymogenic) cells: secrete pepsinogen

Secretory Components of the Stomach- questions

• Why doesn't the stomach digest itself?
• Effects of Helicobacter pylori on the stomach

Small Intestine

• Regions:
  • Duodenum (first 25 cm)
  • Jejunum (1 meter)
  • Ileum (2 meters)
• Numerous folds and villi for absorption
• Absorbs primarily bile salts and water, electrolytes.
• Ileocecal valve empties into the large intestine.

Small Intestine: Villi

• Covered with columnar epithelial cells.
• Goblet cells secrete mucous.
• Epithelial cells at the tip of villi are continuously sloughed and replaced from intestinal crypts.
• Paneth cells secrete antibacterial molecules for protection.

Small Intestine: Microvilli

• Enzymes are attached here for digestion: (sucrase, maltase, lactase, peptidase, aminopeptidase, enterokinase, phosphatase, Ca2+, Mg2+2+-ATPase, alkaline phosphatase)

Large Intestine

• The colon.
• Bacterial colony plays an essential role in digestive processes.
• "Good" bacteria compete with pathogenic bacteria.
• Elimination of colon content through diarrhea.
• Intestinal microbiota are about 10x more than the number of human cells in the body, and they are important for immune system development.

The Colon (Large Intestine)

• The appendix does not function in digestion, but has lymph vessels.
• Ruptured appendix can cause inflammation (Peritonitis) in the peritoneal cavity.
• Intestinal microbiota about 10x more than human cells.
• "Commensal", "mutualism".
• Intestinal Microbiota originates at birth.

Time Restricted Eating/Feeding/Intermittent Fasting

• Can changing your meal schedule effectively fight body fat?
• Eating only during a smaller window of time than typical may help with weight loss.
• Mice consuming food every 24 hours or 8 hours.

Accessory Digestive Organs: Liver

• Hepatic cells that line large capillaries known as sinusoids.
• Sinusoids also lined by endothelial cells.
• Sinusoids contain Kupffer cells (phagocytes).
• Amazing regenerative capabilities: can regenerate to original state in a week if 2/3rds of liver surgically removed.

Accessory Digestive Organs: Pancreas

• Pancreatic juice contains ~20 digestive enzymes (amylase, trypsin, lipase).
• Endocrine portion makes hormones, exocrine portion makes enzymes to digest fat, protein, and carbs.

Accessory Digestive Organs: Gallbladder

• Sac-like, attached to the liver, stores and concentrates bile from liver.
• Stores and concentrates bile.
• Common complication: gallstones (mineral deposits).
• Treatment: surgery, oral ingestion of bile acids, or fragmentation by high-energy shock waves.

Learning Outcomes

• Carbohydrate, Cholesterol, Lipid, and Protein metabolism.
• Absorption of Ions and Vitamins.
• Digestion: the need for processing, foods used by the body.
• Comparative Physiology of Digestive Systems.
• Main functions of the digestive system.

Classification of Animals Based on Food Habits (Herbivores, Carnivores, Omnivores)

• Herbivores eat plants
• Carnivores eat meat
• Omnivores eat both plants and meat

Types of Digestion in Animals

• Enzymatic
• Microbial Digestion (Fermentation)

Classes of Carbohydrates

• Polysaccharides (starches, complex carbohydrates)
• Disaccharides (sucrose, maltose, lactose)
• Monosaccharides (glucose, fructose, galactose)

Chemical digestion of Carbohydrates

• Starch (Polysaccharides): Amylase breaks it down
• Disaccharides: Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase) break it down
• Monosaccharides (Simple sugars): absorbed

Digestion of Carbs in the Mouth & Stomach

• Ptyalin (a-amylase) hydrolyzes ingested starch into maltose.
• Initial starch digestion.

Digestion of Carbs in the Small Intestine

• Pancreatic amylase hydrolyzes remaining carbohydrates into maltose and other small glucose polymers.

Absorbing the products of carbohydrate digestion

• Disaccharidases (lactase, sucrase, maltase) break down disaccharides to monosaccharides (glucose, galactose, fructose).
• Monosaccharides absorbed.

Post-absorptive events, cholesterol

• Cholesterol absorbed from food without being digested.
• Liver produces most cholesterol.
• Cholesterol transported (in the blood) with lipoproteins

Lipids - Fats

• Triglycerides
• Phospholipids
• Sterols (eg., cholesterol)

Digestion of Fats in the Stomach

• Small amount of fat digestion.
• Triglycerides are digested by lingual lipase and swallowed with saliva.
• Accounts for <10% of fat digestion.

Chemical digestion of lipids

• Lipids are not water-soluble; enzymes have limited access.
• Bile salts emulsify lipids into small droplets, increasing enzyme access.

Digestion of Triglycerides by Pancreatic Enzymes

• Pancreatic lipase digests triglycerides to free fatty acids and monoglycerides (important enzyme).
• Pancreatic juice has substantial lipase enzyme concentration; sufficient to digest Triglycerides rapidly.

Absorbing Lipid Digestion Products

• Monoglycerides and fatty acids absorbed by absorptive cells.
• Triglycerides re-synthesized; enter lacteals (lymph vessels).
• Chylomicrons enter the blood.

Proteins

• Polymers of amino acids
• Protein digestion produces large polypeptides, then smaller peptides, then amino acids.
• Peptides digested by proteases and peptidases.

Chemical Digestion of Proteins

• Proteins are broken down into smaller peptide fragments by proteases, eventually to amino acids.
• Pepsin in the stomach is a crucial protease.
• Pancreatic proteases and small intestine brush border peptidases further break down the peptides.

Digestion of Proteins by Pancreatic Secretions

• Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase, proelastase) further break down proteins to peptides.

Digestion of Proteins in Small Intestinal Villi

• Exopeptidases and endopeptidases further break down peptides to individual amino acids.

Absorbing the products of protein digestion

• Amino acids are absorbed into absorptive cells using transport proteins.
• Peptides are further digested into individual amino acids using peptidases.

Post-absorptive events, amino acids

• Amino acids absorbed into cells, cannot be stored, but used in many processes.
• Excess amino acids are metabolized by the liver.
• The nitrogen portion is removed forming urea, which is excreted in urine.

Vitamin Absorption

• Water-soluble vitamins diffuse into the bloodstream and are absorbed in the jejunum and ileum;
• Fat-soluble vitamins absorbed with dietary lipids.