Chapter 45 & 46

Digestive System

Key Concepts

Animal nutrition
general principles of digestion and absorption of food
overview of vertebrate digestive systems
mechanisms of digestion and absorption in vertebrates

Intro to nutrition

nutrient
- any substance taken in by an organism that is needed for:
  - survival
  - growth
  - development
  - tissue repair
  - or reproduction

nutrition
- process of consuming and using food for nutrients

animals receive nutrients by consuming food

Dietary categories

basic similarities in organ system function lead to similarities in nutritional requirements

different animal physiologies can have different nutrient demands

Herbivores
- eat only plants
- digestive system contains micro-organisms that help digest cellulose

Carnivores
- eat only animal flesh or fluid

Omnivores
- eat both

Animals are heterotrophic

Heterotrophs
- ingest feeders

cannot manufacture more food

require already synthesized organic compounds of plants of other animals to supply materials
- survival
- maintenance
- growth
- reproduction

Gut Tracts

Two types

Digestion

the breakdown of large molecules into smaller ones

Digestive enzymes (hydrolases)

carbohydrases

proteases

lipases

nucleases

Food processing in animals

Occurs in Five phases

Ingestion
- food is taken into the body and moves into a digestive

digestion
- food is broken down into smaller molecules
- chemical and mechanical

transport

absorption
- ions, water, and small molecules are transported into the circulatory system

egestion
- undigested materials and other waste are passed from the body
- elimination or exceretion

Alimentary canal

digestive tract or tube
- Gastrointestinal tract

Five regions of food processing

Single tube with opening at each end

contains smooth muscles in walls

lined with epithelial cells
- synthesize and secrete digestive enzymes
- secrete hormones
- transport digestive materials

several specialized regions
- different structures for different processes
- storage area

Structure of GI Tract

some general structure from midpoint of esophagus, to the anus or cloaca
- lumen lined by epithelial and glandular cells
- secretory cells release a protective layer of mucus
- other cells release hormones
- glands release enzymes, acids, water, and ions

Epithelial cells linked by tight junctions and surrounded by layers of tissue made of smooth muscle, neurons, connective tissues, and blood vessels
- neurons activated by sight and smell of food and presence of food in tract

Region of Reception

Buccal cavity

mouth and accessory structures

ingestion site and digestion site

chemical and mechanical

jaws, teeth, cheek muscles, tongue, and salivary glands (saliva)

Pharynx

back of mouth cavity

point that respiratory and digestive system cross paths

Region of Conduction

Esophagus

- tube carrying materials from mouth cavity to the rest of the alimentary canal
- forces/pushes good down
- conducts food from pharynx to stomach
- Peristalsis
  - rhythmic wave-like contractions which propel food forward in the GI tract
- No new digestion here
  - only chemical continuation from buccal cavity

Region of digestion and storage

Stomach (mostly)

- saclike organ evolved for storing food
- muscular nature helps break up food
- partial protein digestion
- regulates rate of emptying into small intestine

Secretions
- hydrochloric acid
  - kills microbes
  - dissolves particulate matter
  - secreted by parietal cells
- Pepsinogen
  - converted to pepsin to begin protein digestion
  - secreted by Chief cells

Epithelium coated with an alkaline mucus

carbohydrate digestion continues from mouth

little lipid digestion happens

lumen (cavity) stomach
- pepsinogen + HCL -> pepsin (for protein breakdown)

Region of terminal digestion and absorption

Small intestine

near

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