Download File - SMIC Nutrition Science

Name: Babe Lin <3
Ch 3 The Digestive System – Chemical Breakdown
Regulatory Control Mechanisms
Involuntary activities of the digestive system (mobility of the GI tract and secretion of enzymes and
hormones) are regulated by the central nervous system and endocrine system systems via neural and
hormonal signals.

Enteric Nervous System - although the term “enteric” typically pertains to the intestine, this “local”
network of nerves spans the entire length of the GI tract. These neural networks are embedded in the
submucosa and muscularis tissue layers, and receive information in the form of electrical impulses
from sensory receptors.
o Chemoreceptor
detect changes in the chemical composition of the luminal contents
o Mechanical receptor
detect stretching in the walls of the GI tract (rugae, plica circulare)
o Photoreceptors
o Pain receptors

not involved in the digestive system
Central Nervous System - consists of the brain and the spinal cord can also initiate neural
communication with the GI tract, sensory stimuli (sight, smell and thought of food) and emotional
stimuli (sadness, anger, anxiety, etc) affect GI function as well.

Endocrine System - chemical messengers released by the
the GI tract into the
blood stream
endocrine
cells of the mucosal lining of
in response to the chemical and physical changes. Like neural
signals, some hormones influence the transit time and the release of GI secretions, and others
communicate the appetite centers in the brain.
Salivary Glands
Functions of saliva include:

Tasting
– dissolves food chemicals for taste cells

Lubricate
– mucus facilitates swallowing, makes food squishy

Protect
– antibacterial lysozyme (not as effective as stomach acid)

Digestion of carbohydrates via amylase (~10% of amylose - carbohydrate)
The salivary glands include the paired parotid, submandibular and sublingual glands.
Sublingual glands
Parotid glands
Submandibular glands

Salivation is controlled by the CNS, specifically the autonomic nervous system responsible for
involuntary actions such as breathing, blinking, sweating, etc.
o Parasympathetic nerves are stimulated by the sight, smell and thought of food to secrete
watery, enzyme-rich saliva from the parotid glands for digestion purpose. (acidic substances,
chewing, nausea, intestinal irritation, etc.)
o Sympathetic nerves are responsible for the fight-or-flight response and inhibit digestive
activities by producing small amount of viscous saliva from the sublingual and
submandibular glands. (fear, fatigue, sleep, dehydration, etc.).
Esophagus produces only mucus for protection (peristalsis), lubrication. How?
Stomach
Cephalic phase – gastric juice secretion
Gastric phase – distention of the stomach triggers the endocrine and exocrine secretion
The stretching of stomach rugae triggers a neural response via mechano-receptors signaling the brain that
the stomach is full. Shortly thereafter, hunger diminishes, causing a person to stop eating.
The stomach lining is covered with numerous small holes known as the gastric pits connected to the gastric
glands that penetrate deep into the mucosal layer. The gastric glands are made up of four types of cells:

Enteroendocrine cells – release the hormone gastrin into the blood stream to stimulate the exocrine
cells (parietal cells) and the churning of the stomach

Mucous cells – release mucus that forms a protective layer, gastric mucosal barrier, to neutralize
(balance) stomach acid and prevent self-digestion.
o Only aspirin and alcohol pass through the gastric mucosal barrier, they destroy cells leaving
the stomach wall susceptible to peptic ulcer development

Chief cells (exocrine cells) – secrete pepsinogen (enzyme precursor) that needs to be activated

Parietal cells (exocrine cells) – secrete intrinsic factor (IF) for the absorption of vitamin B12 and
hydrochloric acid (HCl) that...
o Act as antibacterial (pH of HCl is about 2)
o Breakdown plant cell walls (cellulose)
o denature (uncoil) proteins
o Activates pepsinogen into pepsin, a prote(protein)ase(enzyme) or
proteo(protein)lytic(cutting) enzyme that breaks down proteins.
The rate of gastric emptying (time it takes for chyme to leave the stomach) is influenced by several factors

Volume – increased volume = increased rate of peristalsis = increased rate of gastric emptying

Consistency of the chyme – increased chewing time = greater consistency of chyme = increased rate
of gastric emptying

Composition –carbohydrates are easier to digest than fat and proteins, so a lot of fat = decreased rate
of gastric emptying

Release of the hormone CCK by the small intestine – CCK slows down gastric empyting & triggers
the release of pancreatic juice
Small Intestine
The acidic chyme in the duodenum also triggers the release of another hormone called secretin. Together
with CCK, the two hormones regulate the release of enzymes from the pancreas (CCK) and gallbladder
(secretin), the relaxation of sphincters, and GI mobility.
Brush border enzymes also facilitate digestions

Disaccharidase – breaks down disaccharides into monosaccharides

Peptidase – breaks down oligopeptides into amino acids
Pancreas
The pancreatic juice is an alkaline (basic) solution consists of water, sodium bicarbonate, and various
enzymes that neutralize the acidic chyme. Pancreatic secretion is released into the duodenum.

protease – trypsinogen can be activated into trypsin (also chymotrypsinogen and
procarboxypeptidase)

Pancreatic amylase - ~90% of polysaccharides

lipase – for the digestion of lipids
Endocrine cells also release insulin and glucagon in the regulation of blood glucose level.
Liver and Gallbladder
Fats are water- insoluble and are therefore more difficult to digest and absorb than other nutrients. To
counter this, fats in the duodenum trigger the release of bile synthesized by the liver, stored and released
from the gallbladder. Bile consists of

Bicarbonate solution – neutralizes acidic chyme from the stomach

Organic compounds are
(pigment) – eliminated in the feces
o
o Bile acid and cholesterol - acts as detergents to emulsify fat globules into micelle, to increase
surface area for subsequent digestion with lipase (enzyme). About 95% of bile salt is
recycled in the ileum while cholesterol is eliminated with bilirubin.
Fats  CCK  digestive enzymes
Acids  secretin  bicarbonate
Distention of duodenum  CCK and secretin  intestinal secretion
Large Intestine

Mucus – lubricates feces and protects the intestinal wall

Bicarbonate – protection from bacterial acid