Chapter 23A Digestive System

Slides by Barbara Heard and W. Rose.figures from Marieb & Hoehn 9th ed.

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Ch. 23: Digestive System

Introduction/Overview

Functional Anatomy

Physiology of Digestion & Absorption

Digestive SystemIntroduction/Overview

Digestive System Organs

Digestive Processes

Basic Concepts

Relations & Common Features of Digestive Organs

Functional Anatomy

Physiology of Digestion & Absorption

Digestive System Organs (Fig 23.1)Alimentary Canal = path through

Mouth to anusDigests food and absorbs fragmentsMouth, pharynx, esophagus, stomach, small intestine, and large intestine

Accessory organs = "off-to-the-side" helpersTeeth, tongue, gallbladderDigestive glandsSalivary glandsLiverPancreas

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Figure 23.1 Alimentary canal and related accessory digestive organs.

Mouth (oral cavity)Tongue*

Esophagus

Liver*

Gallbladder*

Smallintestine

Salivaryglands*

Pharynx

StomachPancreas*

Largeintestine

(Spleen)

Parotid glandSublingual glandSubmandibular gland

DuodenumJejunum

Ileum

Anus

Transverse colon

Descending colon

Ascending colonCecumSigmoid colonRectumAppendix

Anal canal

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Digestive Processes

• Six essential activities1. Ingestion

2. Propulsion

3. Mechanical breakdown

4. Digestion

5. Absorption

6. Defecation

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Figure 23.2 Gastrointestinal tract activities.

Ingestion

Mechanicalbreakdown

Digestion

Propulsion

Absorption

Defecation

Food

PharynxEsophagus• Chewing (mouth)

• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)

Stomach

Lymphvessel

Small intestineLargeintestine

Bloodvessel

Mainly H2OFeces

Anus

• Churning (stomach)• Segmentation (small intestine)

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Figure 23.3 Peristalsis and segmentation.

Frommouth

Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally.

Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward then backward.Food mixing and slow food propulsion occur.

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Basic Concepts•Gut lumen is "outside"•Digestion control mechanisms try to control chemical environment in lumen to optimize absorption•Digestion is provoked by mechanical & chemical stimuli (sensors in walls): stretch, pH, osmolarity, which can cause secretion and propulsion via reflexes (Fig. 23.4)•Control by local as well as remote reflexes (Fig 23.4)

• Short reflexes: via enteric nerve plexus• Long: via CNS

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GI Tract Regulatory Mechanisms

1. Mechanoreceptors and chemoreceptors – Respond to stretch, changes in osmolarity

and pH, and presence of substrate and end products of digestion

– Initiate reflexes that• Activate or inhibit digestive glands • Stimulate smooth muscle to mix and move lumen

contents

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GI Tract Regulatory Mechanisms

2. Intrinsic and extrinsic controls– Short reflexes - enteric nerve plexuses (gut

brain) respond to stimuli in GI tract– Long reflexes respond to stimuli inside or

outside GI tract; involve CNS centers and autonomic nerves

– Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract

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Figure 23.4 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract.

External stimuli(sight, smell, taste,

thought of food)

Visceral afferents

Internal(GI tract)stimuli

Chemoreceptors,osmoreceptors, ormechanoreceptors

Long reflexes

Central nervous system

Local (intrinsic)nerve plexus("gut brain")

Effectors:Smooth muscle

or glands

Extrinsic visceral (autonomic)efferents

Short reflexes

Lumen of thealimentary canal

Gastrointestinalwall (site of shortreflexes)

Response:Change in

contractile orsecretory activity

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Peritoneum and Peritoneal Cavity

• Peritoneum - serous membrane of abdominal cavity– Visceral peritoneum on external surface of

most digestive organs– Parietal peritoneum lines body wall

• Peritoneal cavity– Between two peritoneums– Fluid lubricates mobile organs

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Figure 23.5a The peritoneum and the peritoneal cavity.

Abdominopelviccavity

Vertebra

Peritonealcavity

Alimentarycanal organ

Liver

Two schematic cross sections of abdominal cavity illustratethe peritoneums and mesenteries.

Ventralmesentery

Parietalperitoneum

Visceralperitoneum

Dorsalmesentery

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Peritoneum and Peritoneal Cavity

• Mesentery - double layer of peritoneum– Routes for blood vessels, lymphatics, and

nerves– Holds organs in place; stores fat

• Retroperitoneal organs posterior to peritoneum

• Intraperitoneal (peritoneal) organs surrounded by peritoneum

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Figure 23.5b The peritoneum and the peritoneal cavity.

Alimentarycanal organ

Alimentary canal organ ina retroperitoneal position

Some organs lose their mesentery and move,becoming retroperitoneal, during development.

Mesenteryresorbedand lost

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Homeostatic Imbalance

• Peritonitis– Inflammation of peritoneum– Causes by e.g., piercing abdominal wound,

perforating ulcer, ruptured appendix– Peritoneal coverings stick together, localizing

infection– Dangerous and lethal if widespread– Treated with debris removal and antibiotics

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Blood Supply: Splanchnic Circulation

• Branches of aorta serving digestive organs– Hepatic, splenic, and left gastric arteries– Inferior and superior mesenteric arteries

• Hepatic portal circulation– Drains nutrient-rich blood from digestive

organs– Delivers it to the liver for processing

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Figure 23.6 Basic structure of the alimentary canal.

Intrinsic nerve plexuses

Mucosa

Submucosa

Muscularis externa

Glands insubmucosa

Serosa

LumenMucosa-associatedlymphoid tissue

Duct of gland outside alimentary canal

Gland in mucosa

Lymphatic vesselVein

ArteryNerve

Mesentery

• Myenteric nerve plexus• Submucosal nerve plexus

• Epithelium• Lamina propria• Muscularis mucosae

• Longitudinal muscle• Circular muscle

• Connective tissue

• Epithelium (mesothelium)

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Histology: all alimentary canal organs have same basic 4 layersFrom lumen out (Fig 23.6):•Mucosa = mucous membrane = epithelial layer of cells

Secrete: mucus, digestive enz, hormonesAbsorb broken down compounds into bloodProtect against infection

•SubmucosaConnective tissie with blood vessels & lymphatics for absorption, lymph nodes, elastic fibers (shape retention)

•Muscularis externaInner circular musclesOuter longitudinal muscles

•SerosaThe peritoneum already mentioned

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Figure 23.6 Basic structure of the alimentary canal.

Intrinsic nerve plexuses

Mucosa

Submucosa

Muscularis externa

Glands insubmucosa

Serosa

LumenMucosa-associatedlymphoid tissue

Duct of gland outside alimentary canal

Gland in mucosa

Lymphatic vesselVein

ArteryNerve

Mesentery

• Myenteric nerve plexus• Submucosal nerve plexus

• Epithelium• Lamina propria• Muscularis mucosae

• Longitudinal muscle• Circular muscle

• Connective tissue

• Epithelium (mesothelium)

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Enteric nervous system•Submucosal plexus

In submucosaControls glandular secretions & thin submucosal muscular layer

•Myenteric nerve plexusBetween circular & longitudinal muscle layersControls GI tract motility

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Enteric Nervous System•Linked to CNS via afferent visceral fibers

•Long ANS fibers synapse with enteric plexusesSympathetic impulses inhibit digestive activities

Parasympathetic impulses stimulate digestive activities