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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

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Digestive System

Function and processes of the digestive system

Anatomy of the digestive system

Motility

Secretion

Regulation of GI function

Digestion and absorption


Digestive System

The gastrointestinal tract is a long tube passing through the body that opens to the outside world on both ends. Thus, the contents can be considered to be part of the external environment.

The primary function is to move nutrients, water, and electrolytes from the external environment into the body’s internal environment

The food we eat is in the form of macromolecules that must be broken down. The digestive system secretes enzymes to break down food.


5 Basic Processes of the Digestive System1. Digestion: Mechanical and chemical breakdown of

foods into units that can be taken across the intestinal epithelium into the body.

2. Absorption: Active or passive transfer of substances from the lumen of the GI tract to the extracellular fluid.

3. Motility: Movement of material in the GI tract as a result of muscle contraction.

4. Secretion: Both the transepithelial transfer of water and ions from the ECF to the digestive tract lumen and the release of substances synthesized by the GI epithelial cells.

5. Immune: Repelling foreign invaders via gut-associated lymphoid tissue (GALT). 80% of lymphocytes found in small intestive.


Basic Processes of the Digestive System

Figure 21-1

Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 21-2a

Digestive System Anatomy

Oral cavity esophagus stomach small intestine large intestine rectum


Digestive System Anatomy Mouth and pharynx

Mechanical and chemical digestion (secretion of saliva by salivary glands) Esophagus Stomach

Fundus (top) body antrum Mechanical and chemical digestion: Mixing food and acid and enzymes to create chyme.

Pyloric valve Lets out only small amount of chyme at a time, regulated by integrated signals and

feedback loops between the intestines and stomach. Small intestine

Duodenum (25cm) jejunum ileum Most digestion and absorption takes place here via intestinal enzymes and exocrine

secretions from the liver and pancreas (which enter the duodenum via ducts) Accessory organs: pancreas and liver Large intestine: colon and rectum

About 1.5L/day of the 7L/day of chyme generated ends up entering the colon. Water and electrolytes are absorbed out of the chyme and into the ECF to make

feces. Anus


Digestive System Anatomy: 4 Layers of GI Tract Wall

1. Mucosa (Inner Layer) Epithelial cells Lamina propria (connective tissue and immune cells) Muscularis mucosae (thin layer of smooth muscle) Modifications increase surface area

2. Submucosa (connective tissue, nerves, blood and lymph vessels)

3. Muscularis externa Smooth muscle in layers Nerves

4. Serosa (Outer Layer) Connective Tissue


Digestive System Anatomy: Folds of GI Tract

Entire wall folds Rugae in the stomach

Plicae in the small intestine

Mucosal folds Intestinal Villi project into the lumen

Tubular Invaginations of the Surface Gastric Glands in the stomach

Crypts in the intestine

Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 21-2c


Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 21-2e


Animation: Digestive System: Anatomy Review: Overall Function of the GI SystemPLAY


Epithelial Cells

The type changes from section to section. 4 Epithelial Cell Types

1. Transport Cells: Move water and ions into and out of the lumen

and absorb nutrients from lumen.

2. Endocrine Release hormones

3. Secretory Cells Release enzymes, mucus, and paracrine

molecules into the lumen

4. Stem Create new cells


Motility: For Mixing and Movement of Food

Tonic contractions Sustained

Smooth muscle sphincters and stomach

Phasic contractions Last a few seconds

Peristalsis moves bolus forward

Segmentation mixes

Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 21-4a

Contractions in the GI Tract


Motility

Figure 21-4b

Animation: Digestive System: MotilityPLAY

Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 21-5

Secretion

Daily mass fluid balance in digestive system


Acid Secretion by Parietal Cells


Neutralizing Bicarbonate Secretion in Duodenum: From Pancreas and Duodenal Cells


Secretion

Cl– secretion by intestinal colonic crypt cells


Secretion

Digestive enzymes secreted into mouth, stomach and intestine

Mucous cells in stomach and goblet cells in intestine: Protective of the lining

Saliva is an exocrine secretion with digestive enzymes.

Liver secretes bile. Gallbladder stores the bile until needed for digestion: Bile salts act as detergents to solubilize fats,

facilitating enzymatic fat digestion

Animation: Digestive System: SecretionPLAY


Regulation of GI Function

Long reflexes integrated in CNS

Short reflexes integrated in Enteric Nervous System (ENS) Intrinsic neurons of the two nerve plexuses of the

digestive tract lie completely w/in the wall of the gut. They carry out reflexes that are independent of control from the CNS

Reflexes involving GI peptides (Hormones, Neuropeptides and Cytokines)


Sensoryreceptors

andneurons

Localstimulus

Neuronsof

myentericand

submucosalplexuses

Inter-neurons

Sympathetic and parasympathetic neurons

Enteric nervous system“The little brain”

(pH, stretch,osmolarity,products ofdigestion)

Thecephalic

brain

Externalstimulus

(sight, smell,etc.)

Sensoryreceptors

Smoothmuscles

orsecretory

cells

Effectors

Muscle contraction/relaxation

Exocrine secretion (enzymes, mucus, acid, bicarbonate), Paracrines

Endocrine secretions

Digestive system responses

Short reflexes

Long reflexes

KEY

Stimulus

Integratingcenter

Receptor

Efferent pathway

Tissue response

or

or


Integration of long and short reflexes in the digestive system

Figure 21-11


REGULATION OF GI FUNCTION

Cephalic Reflexes: long reflexes that originate completely outside the digestive system. The smooth muscle and glands of the GI tract are under autonomic control. In general, parasympathetic neurons to the GI tract are excitatory and sympathetic are inhibitory. Feedforward Reflexes

Begin with stimuli such as the sight, smell, sound, or thought of food.

Prepare the digestive system for food (mouth watering, movement of the stomach)

Emotional Reflexes Psychologically induced diarrhea Butterflies in your stomach



Reflexes involving GI Peptides

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