Unit 2: Digestion and Nutrition

WHAT YOU WILL LEARN IN THIS UNIT:Identify major structures and functions

of the human digestive system from a diagram, model, or specimen.

 Describe the processes of mechanical

digestion that take place at various sites along the alimentary canal

Identify functions of secretions along the digestive tract.

Identify sites of chemical digestion along the alimentary canal, as well as the type of nutrient being digested.

Explain the role of enzymes in the chemical digestion of nutrients and identify factors that influence their action.

 Describe the processes of absorption

that take place at various sites along the alimentary canal.

Describe the homeostatic role of the liver with respect to the regulation of nutrient levels in the blood and nutrient storage.

Describe the functions of each of the six basic types of nutrients — carbohydrates, lipids, proteins, vitamins, minerals, and water.

Identify dietary sources for each of the six basic types of nutrients — carbohydrates, lipids, proteins, vitamins, minerals, and water.

Evaluate personal food intake and related food decisions.

Investigate and describe conditions/disorders that affect the digestive process.

Use the decision-making process to investigate an issue related to digestion and nutrition.

Introduction to DigestionThe main function of the digestive

system is to disassemble food you eat into molecules so it can be used by your body for energy.

The pathway that food takes as it moves through the body during digestion is known as the digestive tract or the alimentary canal.

Five Stages of the Digestive Process

1. Motility – movement of food through the digestive system.

2. Secretion – release of digestive juices in response to a specific stimulus.

3. Digestion – the physical and chemical breakdown of food into small particles.

4. Absorption – passage of the molecules into the bloodstream.

5. Elimination – removal of undigested food and waste.

Structures Involved in DigestionMouthFood enters the alimentary canal

through the mouth. Inside the mouth there are two types

of digestion:A) Mechanical (physical) Digestion Teeth break up food into smaller

piecesTongue guides food to the back of the

mouth

B) Chemical DigestionAs food is chewed, salivary glands in mouth

secrete saliva4 – 8 cups of saliva are produced by salivary

glands each daySaliva is composed of water, mucous and

amylaseSalivary Amylase (enzyme) breaks down starch

into smaller moleculesSaliva provides lubrication for chewing and

swallowingSaliva helps maintain pH in the mouth and helps

regulate plaque.

There are three glands:1. Parotid gland located just in front of and

slightly below the ear largest of the glands

secretes salts, an enzyme (Salivary Amylase), and a watery fluid into the mouth

2. Submaxillary gland located below the parotid gland

near the angle of the lower jawproduces watery fluid and some

mucous into the mouth

3.Sublingual glandlocated under the tongue and

secretes mostly thick, stringy mucus, salts, and salivary amylase

PharnyxMost commonly known as the

throatServes two purposes:

◦moves the air into your lungs through trachea

◦moves food into your stomach through esophagus

Uvulaa small mass of tissue that hangs

down in the back of the mouth, and moves up to close off the openings to the nasal cavity when food is swallowed into the esophagus.

Soft Palatethe roof of the pharynx in the

back of the mouththis soft tissue functions in

closing off the openings to the nose and ears as food is swallowed.

EpiglottisSmall flap of cartilage located behind

the tonguePrevents food from entering the

respiratory tract by closing over the air passage each time you swallow

When you talk or laugh as you swallow, the epiglottis may open, causing food to enter the upper portion of respiratory tract. Your response, a reflex, is to choke and cough, forcing the food out of the respiratory tract.

EsophagusCarries food from mouth to stomachFood is passed through the esophagus

through:A. Secretions of mucusCells of inner lining secrete mucusB. PeristalsisMuscle layers in the esophagus

alternatively relax and contract to push food ahead (involuntary movement)

Peristalsis Along Esophagus

Peristalsis VideoNOT FOR THOSE WITH A WEAK S

TOMACH

PERISTALISIS, STORAGE AND EXCRETION

Review of SwallowingVideo

Demos

Cardiac SphincterSeparates the esophagus from

the stomach Opens and closes as food passes

throughPrevents stomach acid from

entering the esophagus

StomachStomachMain function is food storage It changes size and shape according to the

position of the body and the amount of food inside

Holds about 2 litres of foodEnzymes, water, and hydrochloric acid combine to

form gastric juicepH of stomach is about 2Food remains in stomach for 2-4 hours and is the

consistency of tomato soup when it is ready to leave

Chyme is a mixture of food and gastric juice

Both physical and chemical digestion take place in the stomach:Physical digestion

Peristalsis Three layers of muscles located within

the wall of the stomach contract to physically break down food into smaller pieces

Chemical digestionInner lining is lined with a layer of cells

Three types of cells: one type secretes pepsin (enzyme)

breaks down proteins in food one type secretes hydrochloric acid and water

kills bacteria and parasites ingested in food provides the optimal pH for pepsin to

function one type secretes mucous

protects cells from gastric juices by forming a mucous layer between the lining and contents of stomach

ulcers are a result of mucus lining wearing too thin and cells are attacked by gastric juice

Pyloric SphincterTwo muscles keep food within

stomach Muscles relax to allow partially

digested food move into the small intestine

Small IntestineSmall diameter (2-3 cm) but long length

(about 6 m)Digestion of food is completed in small

intestineFood is moved along by muscle

contraction to further mechanical breakdown of food

Chemical digestion of carbohydrates and proteins occurs through enzymes produced and secreted by the pancreas and liver

Chemical Digestion Occurs in duodenum (first 25 cm of

small intestine) Most of the enzymes and chemicals that

function in the duodenum enter through ducts that collect secretions (juices) from pancreas, liver, and gallbladder

Secretions Found in Small Intestine (Duodenum)A) Secretions of Pancreassecretes a mixture of enzymes

that break down carbohydrates, proteins, and fats.

B) Secretions of Liverproduces bilebile is stored in gallbladder and is

then passed to the duodenumbreaks down fats

Absorption of Food As digested food moves through small intestine,

it passes over thousands of tiny fingerlike structures called villi

Mainly occurs in next two sections of small intestine – jejunum and ileum

The function of villi is to absorb digested food Food molecules diffuse or are actively

transported through connective tissue cells into bloodstream

Blood travels to liver to remove any excess glucose or amino acids from blood

 

Large IntestineAlso called the colon1.5 m long, about 6.5 cm

diameterThere are three portions of the

large intestine. These three sections create an upside-down U-shape and surround the small intestine.

 

Direction of food

Two main functions:1. Intestinal walls absorb

water and salts 2. Vitamin B and Vitamin K

synthesis, which the body absorbs as needed

Food stays in large intestine for 18 to 24 hours

Where the large and small intestine join, there is a small projection called the appendix◦Bacteria may grow and secrete toxic waste

in appendix, resulting in painful inflammation called appendicitis

◦More common in children than adults because the opening to the appendix is larger in children

 The only residue of food at the end of the colon is solid, indigestible waste called feces which travels to the rectum

RectumFeces are eliminated through the

rectumFeces are 75% water and 25%

solid matter 30% dead bacteria 10-20% inorganic matter 2-3% protein 30% undigested fiber of the food

Overall journey of food through digestive tract lasts 24-33 hours

AnimationsDigestionFor Lunch Part 1For Lunch Part 2