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Digestive System: Physiology

I). Process of Digestion

Process of Digestion

A). Ingestion

B). Mechanical

C). Chemical Ingestion

D). Absorption

E). Defecation

F). Propulsion


II).  Site of digestion

Carbohydrate digestion

Protein digestion

Fat digestion

Nucleic acid


III).  Smooth Muscle in the Digestive Tract

A).  Involuntary control


B).  Smooth muscle is arranged in 2 layers at right angles to each other:

circular layer contracts: P Q

longitudinal layer === contracts:

When the organ contracts the circular muscles the lumen constricts P Q and the organ lengthens.

When organ contracts longitudinal muscles it shortens and dilates.


This results in a rhythmic motion called peristalsis.

inch worm

This is also  how earthworms crawl



IV). Function of Mouth


A). Ingestion

B). Mechanical

1).  Teeth

2).  Cheek

3). Tongue




C). Chemical Digestion

1). Saliva release


2). Composition of salvia

i). Water

ii). Salivary amylase

iii). Mucin

iv). Lysozyme and Immunoglobulin A (IgA)

iv). Electrolytes

D). Propulsion

Swallowing (Deglutition)


1). Buccal Phase


2). Pharyngeal-Esophageal Phase



*problem: passage of food could follow to any opening

(larynx, nasopharynx, or back out the mouth) why doesn't it?


IV). Function of the Esophagus

Propulsion only


A). Peristalsis

1). circular muscles

2). longitudinal muscles

Gastroesophegeal reflex

B). Gastroesophegeal sphincter relaxes food enter the stomach


V). Function of the Stomach

A). Chemical Digestion

Chemical digestion in the stomach

1). Goblet cells


2). Gastric glands

gastric pits

gastric juice

a). Mucous Neck Cells


b). Parietal cells or Oxyntic cells

  • Intrinsic Factor
  • Hydrochloric Acid (HCl)

mucosal barrier

(stomach contents have a pH of 1.5-3.0)

  • bicarbonate mucous.

  • tight junctions between cells

  • damaged cells are shed


c). Chief Cells or Zymogenic Cells

pepsinogen pepsin


Chief of Pepsi-Cola Chief cells of stomach produce Pepsin

Parietal cells Produce Acid & Release Intrinsic factor


d). Enteroendocrine Cells

  • gastrin

  • histamine

  • endorphins

  • serotonin

  • cholecystokinin

  • somatostatin


3). Control of Gastric Secretion

Gastric Secretion

a). Cephalic Phase or Reflex (brain)

b). Gastric Phase

Stomach distension-> stretch receptors -> sympathetic nervous system


Proteins -> raise pH (lower acidity) -> gastrin


c) Intestinal Phase


 chyme -> enterogastric reflex -> inhibits stomach secretion


B). Mechanical Digestion

Mechanical digestion


1). Gastric filling

a). Reflexive relaxation

b). Plasticity


2). Gastric Mixing

Constant rhythm is set by pacemaker cells


C). Propulsion or Gastric Emptying

1). Peristaltic waves push the food

2). Vomiting or Emesis


VIII). Functions of the Small Intestine

A). Chemical Digestion

1). Requirements

a). Materials

b). pH

c). Chyme delivery


2). Site of:

a). Complex Carbohydrate Digestion:



Chemical digestion



Salivary amylase in mouth

Pancreatic amylase

Brush border enzymes



b). Protein Digestion


Pepsin in the stomach

trypsin & chymotrypsin

in pancreatic juice

Other pancreatic enzymes &

Brush border enzymes on the microvilli

break peptides into single amino acids.


c). Fat Digestion

Chemical digestion

Bile salts from liver

 emulsify the fats

In a larger fat globule only the

surface lipids are exposed

to the enzymes.

Lipase from the

pancreatic juice

cleave off fatty acids


Fat digestion


d). Nucleic Acid Digestion

Pancreatic nucleases break DNA & RNA into nucleotides

Nucleotides broken down by

enzymes on the brush border


B). Absorption

1). Intestinal Juice

2). What is absorbed:

a).  Virtually all of the
Nucleic Acids
b).   electrolytes
c).  Most of the water

C). Mechanical Digestion & Propulsion

1). Mixes

2). Moves


IX). Functions of the Large Intestine

Large Intestine


A). Chemical Digestion


B). Absorption


C). Propulsion

Distension results in the haustra contracting  moving material to the next haustra


D). Defecation


Defecation Reflex

initiated when rectal walls stretch

parasympathetic reflex

walls of the sigmoid colon and the rectum to contract & relaxation of the anal sphincter

External sphincter control is voluntary control

If defecation is delayed: the reflex stops

until the next mass movement


If defecation is not delayed

voluntary actions include



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