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Chapter 3

BPK 110 Chapter Notes - Chapter 3: Pancreatic Juice, Gastric Glands, Stomach


Department
Biomedical Physio & Kines
Course Code
BPK 110
Professor
Diana Bedoya
Chapter
3

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Chapter 3: Digestion
Part of nervous/cardiovascular/endocrine system
Atoms: smallest units of an element that retains the properties of the element
Molecules: units of two or more atoms of the same or different elements bonded together
Cells: the basic structural and functional units of living things
Organs: discrete structures composed of more than one tissue that performs a specialized function
Hormones: chemical messengers produced in one part of body, released into blood, travel to other
part of body where they elicit responses
Digestion: Process of food broken down into components small enough to e absorbed into blood
Absorption: Process of taking substances from GI tract into interior of the body
Feces: body waste  unabsorbed food residue, bacteria, mucus, dead cells
Organs of the Digestive System
Gastrointestinal tract + 4 accessory organs (salivary glands, pancreas, liver, gall bladder)
GI tract- 9m (30ft long) from mouth to anus
- food in lumen not inside body because not absorbed yet
Mucosa/mucosa cells: line the lumen of gut, contributes to secretion and absorption
- live only 2-5 days (in contact with acid). Sloughed off and eliminated, new mucosal cells
generated
Transit time: time food takes to travel length of GI tract (mouth to anus)
- 24 – 72 hours (1-3 days), depending on diet, physical activity, emotional state, health status,
medication
**4 types of tissues**
Epithelium: lines cavities and surfaces, forms glands
Connective tissue: connects, supports, or separates different types of tissue and organs
Muscle tissue: smooth, skeletal, cardiac
Nervous tissue: made up of different kinds of nerve cells. Function: sensory input, control of
muscles and glands, homeostasis, mental activity
Digestive System Secretions
Mucus: a viscous fluid secreted by digestive glands. Lubricates, moistens, nod protects cells of GI
tract from harsh environment.
Enzymes: Proteins that accelerate rate of specific chemical reactions without being changed
themselves
-Hydrolysis: reaction using input of water to break down larger molecules into their structural
units

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-Condensation: reaction in which two structural units combine to create a larger molecule,
resulting in loss of a water molecule
Digestion and Absorption of Nutrients
Sensory input alone (smell/thought of food) activates nervous system to secrete saliva and stomach
digestive enzymes
Mouth
- beginning of chemical and mechanical digestion
Saliva: watery fluid produced and secreted into mouth by salivary glands; contains lubricants,
enzymes, other substances (mostly water)
Salivary amylase: breaks starch molecules into shorter carbohydrate chains
Lingual lipase: initiates digestion of fat
= chemical digestion
- Saliva protects against tooth decay by washing away food particles
- Saliva contains antibacterial enzymes, such as lysozyme, which helps prevent tooth decay
Chewing = mechanical digestion
- 32 teeth (adults)
- breaking food into smaller pieces to be swallowed; increasing surface area to come in contact
with digestive juices
- tongue mixes food and saliva and aids in chewing
Pharynx
(throat)
- responsible for swallowing
- also part of respiratory tract and brings air into lungs
- Swallowing: bolus (chewed food + saliva) directed to back of mouth by tongue, air passage
blocked by epiglottis, food goes down esophagus
Epiglottis: piece of elastic connective tissue that covers opening to lungs during swallowing
Esophagus
- no digestion here; just passageway
- bolus moved down by peristalsis
Peristalsis: coordinated muscular contraction that move material through GI tract
- food leaves esophagus and passes through lower esophageal sphincter to enter stomach
Sphincter: layer of muscle that encircles the tube of the digestive tract and act as a valve
- Sphincter contracted – valve closed. Sphincter relax – valve open
- sphincter prevents contents of stomach from going up esophagus, otherwise, heartburn
Stomach
- temporary storage site for food
- Beginning of protein digestion
- bolus mashes and mixed with acidic stomach secretions to form chyme (semiliquid food
mass)
- Some digestion occurs, but very little absorption (with exception of water, alcohol, some drugs
like Aspirin and Tylenol)
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- 3 layers of smooth muscle: horizontal, longitudinal, diagonal, for powerful contraction that
churns and mixes food
Gastric juice
- water, mucus, HCl, pepsinogen (inactive to not damage gastric glands that produce it)
- released by gastric glands in pits that are in stomach lining
- HCl begins protein digestion by unfolding proteins and converting pepsinogen  pepsin
- HCl also kills bacteria in food and stops salivary amylase
- Mucus lines stomach wall so acidic gastric juices (HCl and pepsin) do not digest the proteins
found in cells of stomach wall
Regulation of stomach activity
- Signals from nerves and hormones coming from brain, stomach, and small intestine control
stomach activity; how much stomach churns, how much gastric juices released how fast
materials empty out of the stomach
- Site, smell, thought of food stimulates brain, tell stomach to increase gastric secretions
- Stomach distension  brain stimulates gastrin secretion from stomach  increases gastric
secretion and stomach mixing
- Food in small intestine causes SI to release hormones, which increases secretion of enzymes
and substances into SI
- Chyme in SI also signals to stomach to delay its emptying so SI has time to digest that chyme
before receiving more
- Chyme in stomach takes 2-6 hours to empty into SI, but rate depends on size and composition
of meal; large meal takes longer to leave stomach; liquids leave quickly, solids take longer b/c
need to become liquefied via mixing with gastric juices
- Meal rich in starches and sugar << fibre, protein << high-fat meals stay in stomach longest,
feel full
Small Intestine
- 6m (20 feet)
- Most digestion in duodenum
- SI huge surface area via long lengths and folds
- Peristalsis propels chyme forward
- Segmentation divides up chyme, mix it with enzymes, helps chyme encounter walls of SI so
nutrients absorbed into blood
Segmentation: Coordinated, periodic muscular contractions that aid in digestion and absorption, but
do not significantly propel chyme forward
Secretions
(secretions from pancreas, gallbladder, and SI itself aid in digestion in SI)
Pancreas  pancreatic juice (digestive enzymes + bicarbonate)
- SI requires neutral environment
- Acidic chyme in SI causes intestinal cells to release a hormone secretin, which stimulates
pancreas to secrete bicarbonate into SI
- Chyme with amino acids + fats in SI  CCK (from SI) 
1. delays stomach emptying
2. Stimulates gall bladder to contract and release bile
3. Stimulates pancreas to release pancreatic amylase, pancreatic proteases, and lipases
Pancreatic amylase (starch  double sugars)
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