# 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

1. Blind Gut 
    - no cavity between gut and body wall
    - one opening
    - primitive form
2. tube-within-a-tube 
    - flow through digestive tube
    - body cavity between git and body wall
    - separate opening (mostly)

## Digestion

- the breakdown of large molecules into smaller ones

### Digestive enzymes (hydrolases)

- carbohydrases
- proteases
- lipases
- nucleases

### Food processing in animals

Occurs in Five phases

1. Ingestion 
    - food is taken into the body and moves into a digestive
2. digestion 
    - food is broken down into smaller molecules
    - chemical and mechanical
3. transport
4. absorption 
    - ions, water, and small molecules are transported into the circulatory system
5. 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