Digestion and Human Nutrition


    Nutrition involves all the processes by which the body takes in, digests, absorbs, and uses food. The digestive system breaks down the food that is eaten into smaller particles that is usable and beneficial to us. Energy that animals and humans use to survive is provided in this manner. 


I. Digestive System

  A. General Features

       1. Digestive System: body cavity or tube in which food is reduced to particles, then to molecules small enough to be absorbed into the internal environment.

     a. Incomplete Digestive System- Among invertebrates, a saclike gut cavity that has a single opening “two way”

Figure 1: Flatworms (phylum Platyhelminthes) have incomplete digestive systems. Food enters and waste departs the body by the same path-through a muscular tube called a pharynx.


b.      Complete Digestive Systems- “One Way” has both a mouth and an anus, between these divided regions specialized for transport, processing, and storage.

       2. Regional specializations correlate with feeding behavior

a.       Ruminants (type of hoofed mammal) accept a steady flow of plant material during feeding time, then slowly liberate nutrients during resting time.

b.      Predators or scavengers gorge on their food, and may not eat again for some time. The digestive system stores food being gulped down too fast to be digested and absorbed



 B. The Human Digestive System

  1. Overall task

a.       Mechanical processing and motility- movements that break up, mix, and propel food material

b.      Secretion- release of digestive enzymes and other substances into the lumen (space inside the tube)

c.       Digestion- breakdown of food into particles, then into nutrient molecules small enough to be absorbed

d.      Absorption- passage of digested nutrients and fluid across the tube wall and into blood or lymph

e.       Elimination- expulsion of undigested and unabsorbed residues from the end of the gut

  2. Complete Digestive System

a.       would extend 6.5 to 9 meters(21-30 ft)

b.      main regions: mouth, pharynx, esophagus, and gastrointestinal tract(the gut)

c.       accessory organs- secrete substances into different regions of the system

salivary glands, gallbladder, liver, and pancreas.



Figure 2: Major components of the human digestive system.




II. Food Preparation and Storage

  A. Into the Mouth, Down the Tube

       1. The Mouth

a.       Oral Cavity- food breakdown begins

b.      Tooth- Contain enamel, dentin, and inner pulp

c.       Tongue- Organ that function in positioning food in the mouth, swallowing, and speech




Figure 3: The human tongue consists primarily of muscle. It positions food for efficient chewing and swallowing and also functions in speech. Its surface is covered with circular and filament-like papillae.


2.      The Tube

a.       Saliva- contains salivary amylase (enzyme), bicarbonate, and water

b.      Pharynx- (airway to lungs) tubular entrance to the esophagus and the trachea.

c.       Esophagus- connects the pharynx with the stomach

d.      Sphincter- ring of smooth muscles closes up or open passageway to the body surface.

  B. The Stomach

1.      stomach- muscular, stretchable sac

a.       mixes and stores ingested food, secretions dissolve and degrade the food, control passage of food into small intestine

b.      Gastric fluid- helps dissolve food into chyme, a liquid mixture



Figure 4: The stomach is lined with mucosa (glandular epithelium and underlying layer of connective tissue that faces the lumen). The mucosa secretes the substances that make up the gastric fluid.


III. Digestion in the Small Intestine

A.     pancreas- secretes enzymes and bicarbonate into the small intestine during digestion

B.     liver- stores and interconverts carbs., lipids, and proteins absorbed from the gut

C.     gallbladder- stores bile secreted from the liver

D.     The small intestine receives approximately 9 liters of fluid from the stomach, liver, and pancreas

E.      Pancreatic Secretions

1.      essential for digestion, breaks down carbs, proteins, nucleic acids, or fats

2.      secretes bicarbonate, a buffer that helps neutralize HCL arriving from stomach

F.      Bile and the Digestion of Fats

1.      Bile- fluid secreted by the liver used when stomach is empty

2.      Emulsification- process where bile salts speed up fat digestion.

- emulsion- suspension of fat droplets


IV. Absorption from the Small Intestine

  A. Structure Speaks Volumes About Function

1.      Villi- Absorptive structure

2.      Microvillus- threadlike projection of the plasma membrane, increase surface area available for absorption

Figure 5: Each villus is about a millimeter long and is covered with epithelial cells. The surface of each epithelial cell is covered with threadlike projections of the plasma membrane called microvilli. Each cell has about 1,000 microvilli,

"brush border" cell.


 B. Micelle formation- tiny droplets that are bile salts added with products of fat digestion form micelles


V. Functions of the Large Intestine

  A. Materials not absorbed in small intestine move into the large intestine, or colon

1.      colon concentrates and stores feces, a mixture of undigested and unabsorbed material, water, and bacteria

  B. As ion concentrations in lumen drop, water concentration increases. Water moves out of lumen through osmosis

  C. The appendix is a slender projection from the cecum, a cup-shaped pouch

  D. Bulk- refers to a volume of fiber and other undigested material that cannot be decreased by absorption in the colon



VI. Controls Over the System

  A. Controls

1.      Homeostatic controls respond to changes in internal environment.

2.      Controls over digestive system act before food is absorbed into internal environment

3.      Interact to exert control:

a.       nervous system

b.      nerve plexuses in gut wall

c.       endocrine system

  B. Four Gastrointestinal Hormones

1.      gastrin- secreted by endocrine cells in stomach lining when amino acids and peptides are in stomach. Stimulates secretion of acid into stomach.

2.      secretin- peptide hormone. Stimulates the pancreas to secrete bicarbonate

3.      CCK (cholecystokinin)- enhances actions of secretion and stimulates gallbladder contractions

4.      GIP (glucose insulinotropic peptide)- released in presence of glucose and fat in small intestine. Stimulates insulin secretion


VII. Human Nutritional Requirements

A.     Carbohydrates

1.      Complex carbohydrates are easily broken down to glucose units. Body's main energy source.

2.      Starch is abundant in fleshy fruits, cereal grains, and legumes including beans and peas

3.      Carbohydrate called simple sugars don't have fiber of complex carbohydrates or vitamins and minerals of whole foods

4.      Many sugars are "hidden"

-         corn syrup, corn sweeteners, dextrose



B.     Lipids

       1. Body cannot function without fats and other lipids

ex. Phospholipid lecithin: required component of cell membranes

       2. Fats

a.       serve as energy reserves

b.      cushion many organs

c.       provide insulation beneath the skin

       3. Essential fatty acids- needs to be synthesized and obtained from food  

       4. Cholesterol- used in synthesis of bile acids and steroid hormones, required component of cell membrane

a.       saturated fats, like butter, tend to raise level of cholesterol in blood

b.      too much cholesterol may damage circulatory system

C.     Proteins

1.      Essential amino acids- needs to be synthesized and obtained from food

-         methionine (cysteine), isoleucine, leucine, lysine, phenylalanine (tyrosine), threonine, tryptophan, and valine

2.      Animal proteins- most are "complete"

Plant proteins- "incomplete" (lack one or more of essential amino acids)

3.      Net protein utilization (NPU)- measure used to compare proteins from different sources

-         range from 100(all essential amino acids present in ideal proportions) or 0(one or more absent, when eaten alone, the protein is not complete)

4.      Protein deficiency- more damaging among young since brain develops and grows rapidly early in life


VIII. Vitamins and Minerals

  A. Vitamins

1.      any of more than a dozen organic substances for normal cell metabolism

a.       most plants can synthesize all of these substances

b.      animals must obtain it from food

A.     Minerals- inorganic, required for normal functioning of body cells

B.     Excess amounts of many vitamins and minerals are harmful

- they may interfere with normal metabolic function




IX. Energy and Body Weight

A.     Kilocalories- 1,000 calories of heat energy, or amount of energy needed to raise temperature of 1 kilogram of water by 1C

B.     Caloric Intake and Energy Output

1.      Output varies from one person to another because of differences in:

-         physical activity, basic rate of metabolism, age, sex, hormone activity, emotional state

C.     Amount of kilocalories to take in each day

1.      Steps to find how much kilocalories to take in

a.       multiply desired weight by 10(if not very active physically), by 15(if moderately active), and by 20(if quite active)

b.      depending on age, subtract the following amount from value obtained from first step(amounts in pg.728)

D.     Obesity

1.      obesity- excess of fat in body's adipose tissues


X. Nutrition and Organic Metabolism

A.     Breakdown

1.      Body breaks down most of its carbs., lipids, and proteins. Uses breakdown products as energy sources or building blocks

2.      Fats are broken down to glycerol and fatty acids, which are released into blood.

a.       glycerol is converted to glucose in liver

B.     The Liver

1.      helps maintain concentration in blood

2.      inactivates most hormone molecules

3.      removes worn-out blood cells

4.      inactivates many toxic compounds from blood