Digestion

and Human Nutrition

Chapter 37

Fat Storage

37.1 Functions of Digestive Systems

Digestive system

• Breaks food down into molecules small enough to

be absorbed into the internal environment

• Stores and eliminates unabsorbed materials

• Promotes homeostasis by its interactions with

other organ systems

The Digestive System

Incomplete and Complete Systems

Some invertebrates have an incomplete

digestive system: a saclike gut with a single

opening

Most animals, and all vertebrates, have a

complete digestive system: a tube with two

openings (mouth and anus) and specialized

areas between them

Incomplete and Complete Systems

Five Tasks of the Digestive System

1. Mechanical processing

2. Secretion

3. Digestion

4. Absorption

5. Elimination

Dietary Adaptations

Key Concepts:

OVERVIEW OF DIGESTIVE SYSTEMS

Some animal digestive systems are saclike, but

most are a tube that extends through the body

In complex animals, a digestive system interacts

with other organ systems in the distribution of

nutrients and water, disposal of residues and

wastes, and the maintenance of the internal

environment

37.2 The Human Digestive System

The Human Digestive System

Fig. 37.6, p.620

GALLBLADDER

Accessory Organs

SALIVARY

GLANDS

PANCREAS

LIVER

MOUTH (ORAL CAVITY)

PHARYNX

ESOPHAGUS

STOMACH

LARGE INTESTINE

(COLON)

RECTUM

ANUS

SMALL INTESTINE

Major Organs

37.3 Food in the Mouth

Carbohydrate

digestion begins in the

mouth

• Chewing mixes food

with enzyme-rich

saliva

Fig. 37.7a, p.621

Fig. 37.7b, p.621

37.4 Breakdown in the Stomach

Protein digestion begins in the stomach

• A muscular sac with a glandular lining (mucosa)

that secretes gastric fluid (producing chyme)

Fig. 37.8, p.622

duodenum mucosa

submucosa

oblique

muscle

circular

muscle

longitudinal

muscle

serosa esophagus

pyloric sphincter

Digestion in the Small Intestine

The small intestine completes most digestion

Ducts from the pancreas and gallbladder empty

into the small intestine

• Pancreas secretes digestive enzymes

• Bile assists in fat digestion (emulsification), is

made in the liver and stored in the gallbladder

Structure of Small Intestine

Fig. 37.9a, p.623

Fig. 37.9b, p.623

Chemical Digestion

Controls Over Digestion

Local controls, the nervous system, and the

endocrine system respond to the volume and

composition of food in the gut

Cause changes in muscle activity and in

secretion rates for hormones and enzymes

Hormonal Controls of Digestion

37.5 Absorption

From the Small Intestine

Three features of the small intestine lining

increase absorption

• Lining (mucosa) is highly folded

• Many villi (multicelled absorptive structures)

extend from each of the folds

• Brush border cells with microvilli project into the

lumen; function in digestion and in absorption

Structure of Small Intestine

Fig. 37.10a, p.624

Fig. 37.10b, p.624

Fig. 37.10c, p.624

Solute Absorption

Membrane proteins in brush border cells

transport salts, simple sugars, and amino acids

from the intestinal lumen into the villus interior

A blood vessel inside each villus takes up

absorbed sugars and amino acids

Fat Absorption

In the lumen, bile salts combine with fatty acids

(micelle formation)

Monoglycerides and fatty acids diffuse into brush

border cells, recombine as triglycerides, and

move into the interstitial fluid by exocytosis

From interstitial fluid, triglycerides enter lymph

vessels that deliver them to blood

Digestion and Absorption

in the Small Intestine

37.6 The Large Intestine

Water and mineral ions are absorbed in the

large intestine (colon) which compacts

undigested solid wastes as feces

Feces are stored in the rectum, the stretchable

tubular region just before the anus

Colon Structure

Fig. 37.12b, p.626

transverse colon

descending colon

colon polyp

Key Concepts:

HUMAN DIGESTIVE SYSTEM

In humans, digestion starts in the mouth,

continues in the stomach, and is finished in the

small intestine

Secretions from salivary glands, the pancreas,

and the liver function in digestion

Key Concepts:

HUMAN DIGESTIVE SYSTEM (cont.)

Most nutrients are absorbed in the small

intestine

The large intestine (colon) absorbs water and

concentrates and stores wastes

37.7 Organic Metabolism

Small organic compounds absorbed from the gut

are stored, used in biosynthesis or as energy

sources, or excreted by other organ systems

Blood flows from the small intestine to the liver,

which eliminates ingested toxins and stores

excess glucose as glycogen

Pathways of Organic Metabolism

Fig. 37.13a, p.27

excreted

in urine

urea

ammonia

Cytoplasmic Pool

of Amino Acids

Cytoplasmic Pool

of Carbohydrates, Fats

(interconvertible forms)

dietary proteins, amino acids dietary carbohydrates, lipids

FOOD INTAKE

instant

energy

sources

for cells

specialized

derivatives

(e.g., steroids,

acetylcholine)

building

blocks for

cell

structures

storage

forms

(e.g.,

glycogen)

nitrogen-

containing

derivatives

(e.g., hormones,

nucleotides)

building

blocks for

structural

proteins,

enzymes

37.8 Human Nutritional Guidelines

Food must provide energy and raw materials

• Essential amino acids

• Essential fatty acids

Most nutritionists agree that you should minimize

intake of refined carbohydrates and saturated fat

• Glycemic index (GI)

2006 Nutritional Guidelines

37.9 Vitamins and Minerals

Food must also provide two additional types of

compounds needed for metabolism: vitamins,

(organic) and minerals (inorganic)

Vegetarian diets can meet all these needs only if

foods are carefully combined

Major Vitamins

Major Vitamins

Major Minerals

Major Minerals

Key Concepts: ORGANIC

METABOLISM AND NUTRITION

Nutrients absorbed from the gut are raw

materials in the synthesis of the body’s complex

carbohydrates, lipids, proteins, and nucleic acids

A healthy diet normally provides all nutrients,

vitamins, and minerals necessary to support

metabolism

37.10 Body Weight

Obesity increases the risk of health problems

and shortens life expectancy

• Body mass index (BMI) of 30 or more

To maintain body weight, energy (caloric) intake

must balance with energy (caloric) output

“Ideal” Adult Weights

Genes, Hormones, and Obesity

Genetic factors can make it difficult for people to

reach and maintain a healthy weight

Hormones affect appetite and metabolic rate

• Examples: leptin, PYY

Hormonal Control of Appetite

Fig. 37.16, p.633

protein

product

(leptin)

ob gene

d 1995. Three different

research teams develop

and use genetically

engineered bacteria to

produce leptin, which,

when injected in obese

and normal mice, triggers

significant weight loss,

apparently without

harmful side effects.

c 1994. Late in the year,

Jeffrey Friedman of

Rockefeller University

discovers a mutated form

of what is now called the

ob gene in obese mice.

Through DNA cloning

and gene sequencing,

he defines the protein

that the mutated gene

encodes. The protein,

now called leptin, is a

hormone that influences

the brain’s commands to

suppress appetite and

increase metabolic rates.

b Late 1960s. Douglas

Coleman of the Jackson

Laboratories surgically

joins the bloodstreams of

an obese mouse and a

normal one. The obese

mouse now loses weight.

Coleman hypothesizes

that a factor circulating in

blood may be influencing

its appetite, but he is not

able to isolate it.

a 1950. Researchers at

the Jackson Laboratories

in Maine notice that one

of their laboratory mice is

extremely obese, with an

uncontrollable appetite.

Through cross-breeding

of this apparent mutant

individual with a normal

mouse, they produce a

strain of obese mice.

Key Concepts: BALANCING

CALORIC INPUTS AND OUTPUTS

Maintaining body weight requires balancing

calories taken in with calories burned in

metabolism and physical activity