Warm-Up Write down in correct sequence all the organs (at least 5) through which their food passes as it travels along the digestive tract. Then try to list any glands or organs that are found along the digestive tract, but through which food does not pass.

Human Digestive System

Human Digestive System Digestion is the ability to process food in the body into a form that can be absorbed and used or excreted. Digestion involves three principle processes: Mechanical digestion: takes place in the mouth, your teeth chew the food Chemical digestion: using chemicals to digest/ break down food, this takes place in your mouth and stomach where acid and enzymes mix with the food. Absorption: pulling nutrients out of the food, occurs in the small intestine Human Digestive System The 19th century German philosopher, Ludwig Feuerbach, said, “A man is what he eats.” Food fuels the body’s cells and is used as building material for repair and in some cases, is stored for future use. The purpose of digestion is to break food down to molecules that can be used by cells. Digestion involves three principle processes: mechanical digestion, chemical digestion and absorption. The process begins in the mouth. Chewing initiates the mechanical breakdown of food and is followed by secretion of saliva, which moistens and lubricates food for swallowing. Saliva also contains amylases (enzymes), which start the chemical breakdown of carbohydrates. The swallowing reflex begins in the pharynx and initiates rhythmic waves of smooth muscle contractions called peristalsis. Peristaltic contractions transport food to the stomach and allow a person to swallow even if he/she are upside down. The stomach contains an extra layer of muscle that aids in mechanically mixing and churning food into a semiliquid form called “chyme.” Chemical digestion begins with proteins through the action of hydrochloric acid and the enzyme, pepsin. Only water and a few substances, such as aspirin and alcohol, are absorbed by the lining of the stomach. As food enters the small intestine (so named because of diameter, not the length), secretions from the liver, gall bladder and pancreas are added. The small intestine completes digestion of food materials and the nutrients are absorbed into the blood. Fingerlike projections called villi (covered with microvilli) line the interior of the small intestine. Villi increase the surface area for absorption and secretion. The large intestine is much shorter than the small intestine but is greater in diameter. The large intestine does not contain villi, which greatly reduces its available surface area for absorption. It has no role in digestion. Only water and vitamin K, synthesized by bacteria living in the the colon are absorbed into the bloodstream from the large intestine. Undigested or unabsorbed food is eliminated. A nutrient is a substance the body needs for growth, repair and maintenance. Nutrients include carbohydrates, proteins, lipids, vitamins, minerals and water. Carbohydrates must be broken down into monosaccharides and are the body’s main source of energy. Proteins are broken down to amino acids and supply the raw materials for growth and repair. The body requires 20 amino acids, 10 of which it cannot make and must obtain from outside sources. Lipids are reduced to fatty acids and glycerin. They are used to make steroid hormones, cell membranes and also store energy. Vitamins are organic molecules that aid in the regulation of body processes. Finally, water is required for metabolism and chemical reactions within the body, for transport of substances around the body, and for regulation of body temperature. Approximately two-thirds of the body weight is water. References: Campbell, N.E. & Reece, J.B. (2002). Biology,(6th ed.). San Francisco: Benjamin Cummings. Clark, J.O.E. (Ed.). (1989). A Visual Guide to the Human Body. Barnes & Noble Books. Raven, P.H. & Johnson, G.B. (2002). Biology, (6th ed.). McGraw-Hill. Image References: LifeART. (1998). Super Anatomy Collection 1-9. Baltimore, MD: Williams & Wilkins, All rights reserved.

Structures The GastrointestinaI tract (GI), also called the alimentary canal is the system of organs that take in food, digest it to extract nutrients and expels the waste. These organs are the mouth, pharynx, esophagus, stomach, small intestine and large intestine. Major Functions: Ingestion Digestion Absorption Defecation or Excretion

Accessory organs: Organs that help with digestion but are not part of the digestive tract. These organs are the tongue, salivary glands, liver, gall bladder, and pancreas.

Human Digestive System Diagram Villi Mouth Liver Pharynx Stomach Large Intestine Small Intestine Esophagus Diagram of Human Digestive System The digestive system includes the gastrointestinal tract (mouth, pharynx, esophagus, stomach, small and large intestines) along with accessory structures (teeth, tongue, salivary glands, liver, gall bladder and pancreas). Food materials are broken down to usable nutrients and absorbed into the bloodstream. They are used by the body for metabolism, building and repair. Some nutrients are stored within the body. The lining of the small intestine consists of tiny folds, called villi, which, in turn, are covered by microvilli. Microvilli have brush border enzymes to hydrolyze lactose and sucrose. The villi contain capillaries and lymphatic vessels for the absorption of nutrients. References: Campbell, N.E. & Reece, J.B. (2002). Biology,(6th ed.). San Francisco: Benjamin Cummings. Raven, P.H. & Johnson, G.B. (2002). Biology, (6th ed.). McGraw-Hill. Image References: LifeART. (1998). Super Anatomy Collection 1-9. Baltimore, MD: Williams & Wilkins, All rights reserved.

Following the Trail The process begins in the mouth. Chewing initiates mechanical breakdown of food and is followed by secretion of saliva, which moistens and lubricates food for swallowing. Saliva also contains amylases (enzymes), which start the chemical breakdown of carbohydrates. The swallowing reflex begins in the pharynx and initiates rhythmic waves of smooth muscle contractions called peristalsis. Peristaltic contractions transport food to the stomach and allow a person to swallow even if he/she are upside down.

Following the Trail II The stomach contains an extra layer of muscle that aids in mechanically mixing and churning food into a semiliquid form called “chyme.” Chemical digestion begins with proteins through the action of hydrochloric acid (HCl) and the enzyme, pepsin. Only water and a few substances, such as aspirin and alcohol, are absorbed by the lining of the stomach.

Following the Trail III As food enters the small intestine secretions from the liver, gall bladder and pancreas are added .

The small intestine completes digestion of food materials by absorbing nutrients into the blood stream The lining of the small intestine consists of tiny folds or fingerlike projections, called villi, which, in turn, are covered by microvilli which increase surface area The villi contain capillaries and lymphatic vessels for the absorption of nutrients Microvilli have brush border enzymes to hydrolyze lactose and sucrose.

Cross-Section of small intestine Villi Microvilli

The large intestine does not contain villi and it plays no role in digestion Only water and vitamin K are absorbed from the large intestine Undigested or unabsorbed food is eliminated through the rectum and then anus.

Nutrition Food materials are broken down to usable nutrients and absorbed into the bloodstream. They are used by the body for metabolism, building and repair Some nutrients are stored within the body Nutrients include carbohydrates, proteins, lipids, vitamins, minerals and water. Carbohydrates Must be broken down into monosaccharides Body’s main source of energy.

Nutrition II Proteins Lipids are reduced to fatty acids and glycerol Broken down to amino acids Supply the raw materials for growth and repair. The body requires 20 amino acids, 10 of which it cannot make and must obtain Lipids are reduced to fatty acids and glycerol They are used to make steroid hormones, cell membranes Store energy

Nucleic acids are reduced to nucleotides. DNA and RNA The genetic material of all cells.

Where does each nutrient get broken down?

Carb- Digestion Protein Digestion Nucleic Acid Digestion Fat Digestion Mouth, Throat, Esophagus Polysacch. Into Disacch. Stomach Polypepties into smaller proteins DNA, RNA into nucleotides Fat into glycerol, fatty acids and glycerides Small Intestine Disacch. Into Monosacch. Small proteins into amino acids Nucleotides into nitrogen base, sugar and phospate

What enzymes break down each nutrient? Carb Digestion Protein Digestion Nucleic Acid Digestion Fat Digestion Salivary amylase (amylose is starch, a polysacch) Peptidases (amino acids are connected by “peptide” bonds) Nucleotidases (nucleotides are building block of DNA and RNA) Bile salts and Lipase (fats are made of lipids)

Nutrition III Vitamins are organic molecules that aid in the regulation of body processes Vitamin C: healthy teeth, gums and blood vessels; improves iron absorption and resistance to infection Vitamin K: for normal blood clotting and synthesis of proteins found in plasma, bone, and kidneys

Water Required for metabolism and chemical reactions within the body Transport of substances around the body Regulation of body temperature. Approximately two-thirds of the body weight is water.

Cool Facts Your intestines will grow to at least 25 feet as an adult. Be glad you're not a full-grown horse their coiled-up intestines are 89 feet long! Food sloshing in the stomach can last 3-4 hours It takes 3 hours for food to move through the intestine Food drying up and hanging out in the large intestine can last 18 hours to 2 days! Americans eat over 2 billion pounds of chocolate a year. In your lifetime, your digestive system may handle about 50 tons!!