Digestive system

Definition of
• Digestion – The mechanical and chemical processes whereby ingested food is converted into substances that can be absorbed by the intestinal tract and utilized by the body
• Digestive system is a 25 to 30 foot long tube it also includes the organs that secrete digestive juices
• http://highered.mheducation.com/sites/0072495855/student_view0/chapter26/animation__ organs_of_digestion.html

Digestion of nutrients
• Mouth
– Saliva: moistens and lubricates, especially starches and fats
– Enzymes begins to digest starch & some fat
– Chewing: crushes and softens foods like plant fiber

Digestion of nutrients
• Stomach
– Carbohydrate
• Starch digestion continues, acids destroy bacteria in food, some alcohol is absorbed.
• Nothing happens to fiber
– Fat
• Only small amount is digested.
• It is last to leave the stomach.
– Protein
• Acids break apart proteins and activate stomach enzymes. The enzymes break the protein into individual or small groups of amino acids .

Digestion of nutrients
Stomach
• Stomach ejects the liquefied food into small intestine (1-2 teaspoons at a time) through the pyloric valve until all the food is in the small intestine.
• These small ejection pulses stimulate intestinal walls to contract and relax, which in turn churn and mix the food with digestive enzymes.

Digestion of nutrients
• Small intestine, pancreas, liver, gallbladder
– Carbohydrates
• Pancreatic enzymes digest starch
• Enzymes in intestinal walls convert starch fragments and sugars to simple sugars during absorption
– Fat
• Bile produced by the liver
• Bile is stored in the gallbladder
• Bile is secreted by the gallbladder into the small intestine where it mixes with fat and makes the fat ready for pancreatic enzymes

Digestion of nutrients
• Small intestine, pancreas, liver, gallbladder (cont.)
– Fiber
• Binds some chemicals
(cholesterol, some minerals) and prevents their absorption
– Protein
• Enzymes of small intestine and pancreas digest protein into smaller fragments and free amino acids .
• During absorption, enzymes in intestinal walls break protein fragments into free amino acids.

Digestion of nutrients
• Large intestine
(Colon)
– Carbohydrates – undigested carbohydrate partially broken down by intestinal bacteria in colon
– Fiber – some is digested by bacteria, mostly excreted undigested
– Fat – any undigested fat is excreted
– Protein – undigested residue is excreted

Illustration 7-8 An   example   of   a   health ‐ promoting   species   of   bacteria   (B.
infantis)   found   in   the   large   intestine.
Illustration 7-8 p9

Digestion of nutrients
Summary
Digestion is completed when macronutrients are reduced to absorbable size and micronutrients are released from the food
• Carbohydrates as simple sugars (glucose, fructose and galactose)
• Proteins as amino acids
• Fats as free fatty acids and glycerol

Digestion of nutrients
Summary
• Vitamins and minerals stay the same
• Fiber – not absorbed but provide a larger
“bulge”
• The larger the food bulge, the greater the intestinal muscular activity and therefore, the faster (shorter) the time through the intestines (transit time)

Absorption
• Most absorption takes place in small intestine
– Except for water, sodium and end products of bacterial digestion which are absorbed in the large intestine
• Transfer of nutrients from the digestive system into the lymphatic and circulatory systems for eventual distribution to the body cells.

Illustration 7-3 Scanning electron micrographs of crosssections of the small intestine (left) and the large intestine
(right). Note the high density of villi in the small intestine and the relative flatness of the lining of the large intestine.
Small intestine Large intestine
Illustration 7-3 p5

Absorption
• Lymph vessels and blood vessels infiltrate the villi which line the intestinal tract. The lymph and blood vessels transport the absorbed nutrients to major branches of the lymphatic and circulatory systems

Absorption
Fatty acids are absorbed into the lymphatic system

Absorption
Glucose and amino acids are absorbed into the circulatory system

Absorption
• Almost all absorption occurs at the intestinal villi
• The nutrients in the lymph and blood mix in the heart and are then sent to the body cells via the circulatory system.

Nutrients are absorbed as
• Carbohydrates simple sugars
– Glucose , 99% absorption
• Protein
– Amino acids , 92% absorption
• Fat
– Fatty acids , 95% absorption

Nutrients are absorbed as
• Minerals
– cations (+positive charge) and anions (-negative charge)
– Absorption efficiency varies
• Fiber
– Unabsorbed
• Water
– Absorbed in large intestine

Digestive disorders
• Incidence in US adults
– Constipation
– Hemorrhoids
– Heart burn
15%
75%
(at one point in life)
33%
– Ulcers 3.5%
– Diarrhea (chronic) 1.2%
• Constipation and hemorrhoids
– Can be caused by too little fiber intake
– 28% of school children suffer from constipation

Digestive disorders
• Ulcers and heartburn
– Ulcers occur when cells lining the stomach and/or duodenum are damaged, often by excessive production of stomach acid
– Heart burn occurs when acidic stomach content is released into the esophagus causing a burning sensation
A stomach ulcer.

Digestive disorders
• Diarrhea
– About 3.5 million deaths occur world wide/year in children under 5 years of age
– Caused by bacterial/viral contaminated food or water, lack of immunization or vitamin A, zinc and other nutrient deficiency
– Diarrhea depletes fluids and nutrients causing malnutrition. Once rehydrated, solid foods should be given.

Digestive disorders
• Lactose intolerance
– Due to a lack of the enzyme, lactase , milk and milk products create gas, cramps and diarrhea

Management of lactose intolerance
• DO NOT omit milk entirely, because you lose a variety of nutrients that are not easily replaced
• Use lactase-treated cow’s milk or lactase drops; tablets usually don’t work - they get digested
• Eat yogurt – lactose is fermented in yogurt
• On advice of a doctor, you might try soy milk that is fortified with calcium.
• Eat other fermented milk products (if tolerated)