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Chapter 14 : Digestive system and Nutrition
The digestive system brings nutrients to the body
Digestive system =
the (hollow) organs that get nutrients in the body = GASTROINTESTINAL TRACT with LUMEN inside.
+ 4 accessory organs : salivary glands // liver // galbladder // pancreas.
The walls of the GI tracts are composed of 4 layers
Mucosa : innermost = a mucous membrane in contact with lumen => Nutrients must cross to enter blood.
Submucosa : CT with blood vessels, lymph vessels & nerves. Food absorbed by mucosa enter blood & lymph vessels.
Muscularis : 2 or 3 layers of smooth muscle for movement. Inner layer fibers oriented circular around lumen, outer
ones parallel to the long axis of the digestive tube (Exception = stomach has diagonal layer of inside the other two)
Serosa : thin CT, surrounds and protects other layers AND attaches digestive system to walls of the body cavities.
Sphincters : Some GIT organs are separated by thick rings circular smooth M =>contract => close passage betw organs.
Digestion = 5 BASIC PROCESSES
1. Mechanical processing and movement : Chewing breaks food into smaller pieces, and two types of movement
(motility) mix the content of the lumen and propel it forward.
2. Secretion : Fluid, digestive enzymes, acid, alkali, bile, and mucus are all secreted into the GI tract at various places.
In addition, several hormones that regulate digestion are secreted into the bloodstream.
3. Digestion : The contents of the lumen are broken down both mechanically and chemically into smaller and smaller
particles, culminating in nutrient molecules.
4. Absorption : Nutrient molecules pass across the mucosal layer of the GI tract and into the blood or lymph.
5. Elimination : Undigested material is eliminated from the body via the anus.
TWO TYPES OF MOTILITY aid digestive processes (smooth muscle)
Peristalsis : “BOLUS” (food lump)stretches GIT => smooth muscle contracts BEHIND it and relaxes IN FRONT OF IT.
Occurs in all parts of tGIT but especially in ESOPHAGUS (transports food rapidly to stomach)
Segmentation :sections of smooth muscle contract/relax randomly = mixing of content lumen. Food particles are
pressed against mucosa => enables body to absorb nutrients. Primarily in small intestines as food is digested/absorbed.
The mouth (ORAL CAVITY) is the entrance to GIT & processes food for swallowing
Teeth bite and chew food
Chidren have only 20 teeth. Most adults 32 teeth.
CROWN = covered by ENAMEL (calcium and phosphate), inside DENTIN (soft cavity with blood vessels to supply living
dentin and nerves) and a ROOT (below the gum line) => The entire tooth is lined with PERIODONTAL MEMBRANE.
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