The Digestive System

Biology 260

M. Iyengar

Embryonic Development of the Digestive System

• The embryo folds at the lateral edges.

– Endoderm becomes the gut tube• Foregut: pharynx to duodenum

• Midgut: duodenum to transverse colon

• Hindgut: transverse colon to anal canal

– Gut tube connects to the developing mouth and anus.

– Double-layered membrane around the gut tube• Dorsal anchors to the posterior body wall

• Ventral anchors to the anterior body wall

The Digestive System Anatomy

1. Gastrointestinal (GI) tract/ alimentary canal

– A hollow muscular tube extending from the mouth to the anus

– Performs all the digestive functions

– Mouth, Pharynx, Esophagus, Stomach, Small, and Large Intestine

2. Accessory digestive organs

– Assists with some of the digestive functions

– Teeth, Tongue, Salivary Glands, Pancreas, Liver, and Gallbladder

The Digestive System Anatomy

Digestive System Functions

1. Ingestion- Taking in food and liquid into the mouth

2. Propulsion – mixing and movement thru the GI tract

3. Digestion – Break down food into nutrients

A. Mechanical – tearing or grinding food

B. Chemical – large molecules → small molecules

4. Absorption - nutrients move into blood or lymph

5. Defecation – Removal of waste and indigestible substances as feces

Digestive System Functions

The Peritoneum

• Back to back layers of serous membrane

– Parietal peritoneum – lines the body wall

– Visceral peritoneum –surrounds digestive organs

– Peritoneal cavity – fluid filled space between parietal and visceral peritoneum

The Peritoneum

Parietalperitoneum

Wall ofbody trunk

Kidney(retroperitoneal)

Peritonealcavity (withserous fluid)

Stomach

Posterior

AnteriorVisceral peritoneum

Liver

Falciformligament

Intraperitoneal organs• Liver, stomach, majority of

SI (ileum, jejunum), Cecum, Appendix, Transverse colon, and Sigmoid colon

Retroperitoneal organs:• Organs located behind the

peritoneum • Kidneys, Pancreas, Duodenum,

ascending and descending colon, rectum

Mesenteries

1. Lesser omentum: connects liver to stomach & duodenum

2. Mesentery proper: Supports the long coils of the jejunum and ileum

3. Mesocolon: anchors the transverse & sigmoid colon to the posterior abdominal wall

4. Greater Omentum:

• Ventral: covers transverse colon and small intestine

• Dorsal: Wraps spleen and transverse colon to become the mesocolon

• Back to back layers of peritoneum that extend from the organs to the body wall• Suspends organs in the space and stores fat• Provides a way for blood & lymphatic vessels + nerves to reach

digestive organs

Mesenteries

General Histology of GI Organs

1. The mucosa – inner epithelial layer

• Lamina propria: areolar CT

• Muscularis mucosae: small smooth muscle layer• creates mucosal folds in some parts of the GI tract

2. The submucosa

– areolar CT w/ blood & lymphatic vessels + nerves

3. The muscularis externa

– Smooth muscle layers external to the submucosa• Circular, Longitudinal, and Oblique (optional)

4. The serosa

– the outermost layer = visceral peritoneum

Histology of the GI tract

Histology of the GI tract

Mucosa changes along the GI tract

• Mouth, pharynx, esophagus, & anus → stratified squamous

• Stomach & Large intestine → simple columnar

• Small intestine → simple columnar with microvilli

Muscularis externa changes along the GI tract

• Mouth, pharynx, & esophagus → Skeletal muscles

• Posterior esophagus, Small intestine, & Large intestine → Smooth muscle (Circular & Longitudinal layers)

• Stomach → Smooth muscle (Oblique, Circular, & Longitudinal layers)

The Mouth

• Anatomy

– Lips - anterior boundary

• Labial frenulum - connects lips to gums

• Lingual frenulum – connects tongue to gums

– Checks – lateral boundaries

– Oropharynx – posterior boundary

– Mucosa = Stratified squamous epithelium + Lamina propria

• Ingestion

• Propulsion

• Digestion

– Mechanical

– Chemical (carbohydrates)

Anatomy of the Mouth

• Palate: the superior boundary, forms the roof of the mouth

• Hard: anterior region, tongue forces food against its surface during chewing

• Soft: Posterior region, moves during swallowing to close off nasopharynx

– Palatoglossal arches: anchoring of soft palate to tongue (laterally)

– Palatopharyngeal arches: anchoring of soft palate to oropharynx (posteriorly)

– Uvula: free edge of the soft palate

Teeth

Anatomy

• Crown: visible portion of teeth above gum line

– Dentin: bonelike and collagen components, outer layer of tooth

– Enamel: covering, protects dentin from wear and tear of chewing

• Neck: area where the crown joins the roots at the gums

• Root: between 1-3/ teeth

– Pulp cavity: internal connective tissue, blood vessels and nerves

• Positioned in alveoli of the mandible and maxilla.• Mechanical digestion

The Tongue

• Inferior boundary of the mouth

• Interlacing fascicles of skeletal muscle

• Propulsion and helps form some consonants

Taste buds:

• Vallate papillae: V-shaped row, posterior surface

• Foliate papillae: pleat-like on the lateral, posterior aspect of the tongue

• Fungiform papillae: resembles tiny mushrooms on tongue surface

• Filiform papillae: rough surface of tongue, grasp & manipulate food during chewing

Anatomy of the Mouth

The Salivary Glands

• Chemical digestion (carbohydrates)

– Produce saliva that moistens mouth and breaks down starch via amylase

Anatomy

– Parotid – anterior to the ears, w/ duct parallel to zygomatic arch

– Submandibular – medial aspect of the mandibular body

– Sublingual – under the tongue, anterior to submandibular gland

The Pharynx• Passageway for air and food

• Propulsion

– Nasopharynx is closed by the uvula during swallowing

– Oropharynx and laryngopharynx

• Lined with stratified squamous epithelium

• Skeletal muscles for swallowing

• At the epiglottis the larynx is closed and food moves to the esophagus

The Esophagus

• Propulsion = deglutition (swallowing)

Anatomy

• Muscular tube posterior to the trachea– Continuation of the pharynx

– Joins the stomach inferior to the diaphragm

• Gastroesophageal sphincter (cardiac sphincter) - entry to stomach

• Histology:– Stratified squamous epithelium

– Muscularis externa:

1. Skeletal muscle first third of length

2. Skeletal and smooth second third

3. Smooth muscle at last third

The Esophagus

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Deglutition (Swallowing)

Peristalsis

• Major means of propulsion

• Adjacent segments of the alimentary canal muscles relax and contract

Segmentation

• Nonadjacent segments of the alimentary canal muscles relax and contract

• Mixes food with digestive juices

Movement Through the Digestive Tube

The Stomach

• Mechanical digestion

– Churn, mix, and pummel food.

• Chemical digestion (proteins)

– Chief cells – make pepsinogen

HCl + pepsinogen → Pepsin, which break proteins

– After food is processed in the stomach it is called chyme (creamy paste)

• Temp. storage of food

– Full → hold 1 gallon

– Empty → folds into rugae

– No absorption in stomach except alcohol & some drugs

• Propulsion

The Stomach

• On the left side of the abdominal cavity, hidden by the liver and diaphragm

– Cardiac – superior portion

• Cardioesophageal sphincter is the junction between the esophagus and the stomach

– Fundus - dome-shaped, superior portion

– Body – main portion

– Pyloric regions – inferior portion that connects to the SI

• Pyloric valve (pylorus) is the junction between the stomach and small intestine

The Stomach

Stomach Histology

• Mucosa

– Gastric pit – simple columnar ET

– Gastric glands – several specialized cells

• Mucous neck cells – make & secrete mucus

• Parietal cells – make hydrochloric acid (HCl) + intrinsic factor

• Chief cells – make pepsinogen

HCl + pepsinogen → Pepsin, which break proteins

• Enteroendocrine cells – make the hormone gastrin

– Rugae: Folds of mucosa & submucosa

• Muscularis externa: – Longitudinal– Circular– Oblique

Stomach Histology

Emptying the Stomach

The Small Intestine – Gross Anatomy

• Propulsion

• Mechanical digestion

• Chemical digestion (all 4 biomolecules)

– Bile from the Liver → emulsifies fats

– Pancreatic enzymes

• Absorption of nutrients

1. Building blocks to make more complex molecules, such as muscle proteins, hormones, and enzymes.

2. To supply energy for sustaining life processes.

3. Storage for future use.

Digestive System EnzymesEnzyme Produced by Released into Digestion of Final product

Amylase Salivary glandspancreas

Oral cavityDuodenum

carbohydrates Simple sugars

Pepsin

Trypsin

Peptidase

Stomach

Pancreas

pancreas

Stomach

Duodenum

Duodenum

proteins Amino acids

lipase pancreas Duodenum Lipid/fat Glycerol and fatty acid

nuclease pancreas Duodenum Nucleic acid nucleotides

The Small Intestine – Gross Anatomy

1. Duodenum (~25cm = 10 in)

– Hepatopancreatic duct –

» delivers bile from liver

» delivers pancreatic juices from the pancreas

– Duodenal glands, creates mucus to neutralize chyme

2. Jejunum: (~3ft) - absorption

3. Ileum (~6ft) – absorption

• Muscular tube extending from the pyloric sphincter to the ileocecal sphincter

– Pyloric sphincter - controls how much chyme enters SI

– ileocecal sphincter – controls how much chyme enters the LI

The Small Intestine

The Small Intestine – Histology

•Mucosa – Modifications to increase surface area –Circular folds - Ridges of mucosa & submucosa,

• Contains capillary and lymphatic vessels

• Slow the movement of chyme through SI

• Chyme remains in SI longer, higher chance of absorption

– Villi - finger-like projections of the mucosa

• Covered with simple columnar epithelium -> absorptive cells

– Microvilli – projects of the cell plasma membrane

• Increase surface area for absorption

The Small Intestine—Structural Features

Disorder of the Small Intestine

• NEW Celiac disease research

– Gluten proteins may cause an autoimmune response in some individuals

– WBC release chemicals to trigger inflammation and antibodies

– Overtime the lining of the intestine atrophies or dies

• Nutrient absorption is difficult

Infographic: Immune Irritation in the Gut

http://www.the-scientist.com/?articles.view/articleNo/49506/title/Infographic--Immune-Irritation-in-the-Gut/http://www.the-scientist.com/?articles.view/articleNo/49506/title/Infographic--Immune-Irritation-in-the-Gut/

The Pancreas

• Retroperitoneal, deep to stomach greater curvature

• Tadpole-shape - Head, body and tail

• Exocrine function (90%)– Acinar cells make, store, and secrete pancreatic enzymes

• Enzymes are activated in the duodenum

• Secrete bicarbonate rich fluid to neutralize chyme

• Endocrine function: – Pancreatic islets

• Insulin (β-cells): Uptake of glucose by cells

• Glucagon (α-cells): Conversion of glycogen to glucose

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The Pancreas

The Liver

1. Bile production

– emulsifies fats so they can be broken down easily

2. Metabolic functions:

– Stores vitamins

– Processes fats and amino acids

– Detoxifies blood of poisons and drugs

– Makes blood proteins.

• Large wedge-shaped organ on the right side of the body• Right and left lobe; separated by falciform ligament

• Quadrate and Caudate lobe (part of left lobe)

• Hepatocytes - functional cells of the liver

Gross Anatomy of the Liver

Gross Anatomy of the Liver

– Hepatic artery (R/L) - brings O2 rich blood into the lobules

– Hepatic portal vein (R/L) - blood vessel that bring nutrients from digestive system to liver to be filtered

– Common hepatic duct→ R+L Hepatic duct – delivers bile to the duodenum

• Porta Hepatis: Area near the center of inferior surface,

• Where blood vessels and nerves enter/exit the liver

Microscopic Anatomy of Liver

• Lobules = Hexagonal shape divisions of the liver

• Hepatic triad at each corner

• Hepatic portal venule + Hepatic arteriole →

– Hepatic sinusoids: permeable blood capillaries

→ central vein → hepatic vein → inferior vena cava

• Bile ductule: collect bile from smaller canaliculi

– Bile is directed to R & L Hepatic ducts

→ common hepatic duct → bile duct → duodenum

Microscopic Anatomy of Liver

The Gallbladder

• Muscular sac in a shallow fossa on the inferior surface of the liver

– Fundus protrudes from the liver

• Stores and concentrates bile

– Expels bile into the cystic duct → bile duct → duodenum

• Gallstones- too much cholesterol or bile salts can lead to crystallization of cholesterol and can plug cystic duct

The Gallbladder

The Large Intestine

• Propulsion

• Absorption

– Water, electrolytes, & vitamins

• Defecation

• Muscular tube that frames the small intestine on three sides.

– Extends from ileocecal valve to the anus

– Mucosa

• Simple columnar & goblet cells

Gross Anatomy of Large Intestine

• Cecum: Small sac at the entrance to the LI, below the iliocecal valve

– Vermiform appendix: Contains lymphoid tissue to neutralize pathogens

• Colon – divided into distinct segments

– Ascending, transverse, descending, and sigmoid colon

– Teniea coli: longitudinal strips of smooth muscle that cause puckering of large intestine

– Haustra: Pouches of the colon, produces by adaptation of its length with the teniea coli

Gross Anatomy of Large Intestine

• Rectum – descends along the inferior half of the sacrum

• Anal Canal - the last subdivision of the large intestine

– Internal anal sphincter: smooth muscle

– External anal sphincter: skeletal muscle

Gross Anatomy of the Large Intestine