Digestion February 2015

Key Skills Identify the major parts of the gastrointestinal tract, its accessory organs, and their functions. Parts of tract: Mouth, esophagus, stomach, small intestine, large intestine Accessory organs: Tongue, teeth, salivary glands, pancreas, gallbladder, liver Explain where and how different nutrients are broken down and absorbed. Describe causes and effects of several common digestive system ailments, such as stomach ulcers and diarrhea.

Where does digestion begin? A) In the mouth B) In the stomach C) In the small intestine D) In the large intestine E) Somewhere else

Digestion: Physical and Chemical Breakdown of Food Physically separating food into pieces (Examples: Chewing; emulsifying lipids into smaller droplets with bile) Chemically breaking down food macromolecules into pieces (Examples: Enzymes digesting starch in the mouth and protein in the stomach.)

An Unconventional Answer Most textbooks agree that we begin digestion in our mouths, with chewing… But it might be fair to argue that, in a sense, humans have learned how to start digesting food outside of their bodies! For example, cooking can unfold proteins and make them more digestible.

Roles of the Digestive System Ingestion: Taking food into the body. Digestion: Breaking down food mechanically and chemically. Absorption: Getting nutrients from food (small intestine). Excretion (defectation): Releasing solid waste.

The Digestive System Consists of… GI/digestive tract: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus Accesory organs: Teeth, tongue, salivary glands, pancreas, liver, gallbladder

How Does Chewing Work? Take a moment to reflect on how you chew and swallow food. What, exactly, happens when you chew a piece of food and swallow it? Write down as many details as you can think of (limit one paragraph).

Chewing Chewing involves: Grinding and cutting by the teeth Chemical breakdown by salivary enzymes, such as amylase, which breaks starch. Shaping of food into a bolus by the tongue Tasting by the tongue (to detect poisons and nutrients)

Salivary Amylase The enzyme salivary amylase (a carbohydrase) is a protein that is shaped to stick to starch and help it break apart! Electrolytes, or charged particles, in the saliva help the amylase function properly. (Chloride and calcium)

The Teeth The structure of teeth suits their function. Hard enamel makes them resistant to wear and tear. Molars and premolars near the back of the mouth grind. Incisors and canine teeth pinch and cut.

Cavity Formation Cavities form when bacteria growing on the teeth secrete acidic waste that slowly breaks through the enamel and dentin to reach the pulp. Sugary and starchy foods are a big energy source for these bacteria. Bacteria at the gumline may also get into the bloodstream and increase the risk of heart disease. It’s important to floss and brush regularly to protect your teeth and heart!

Swallowing (real-time MRI source: Biomedizinische NMR Forschungs GmbH)

Peristalsis The Esophagus Like the rest of the digestive tract, the esophagus is lined with muscles and a protective layer called the mucosa. These muscles squeeze food through the esophagus like toothpaste through a tube. This movement is called peristalsis, and is also used to move food through the intestines. The opening between the stomach and esophagus is the cardioesophageal sphincter. Acid reflux, or heartburn, occurs when stomach acid flows back up through this opening, and can damage the mucosa.

Emesis (Vomiting) In vomiting, food and stomach acid are moved upwards through the esophagus (reverse peristalsis.) This allows the body to eliminate poisonous substances. However, it also depletes the body of water and electrolytes. People who throw up frequently, such as bulimics who purge by vomiting, risk damaging their esophageal mucosa, teeth, and even their heart (which needs those electrolytes!) Tooth erosion from bulimia:

The Stomach Stores and churns food, then gradually releases it into the small intestine. Secretes acids and enzymes (pepsin and trypsin) that digest proteins. Mucosa protects stomach. Absorbs alcohol, some medications

Digestion in the Stomach Protein digestion begins in the stomach, and starch/carbohydrate digestion continues there! Fats still need to be separated into droplets before they can be digested properly.

Peptic (Stomach) Ulcers Holes in the stomach’s mucosa (lining). Primary cause is usually the bacterium Helicobacter pylori. Painkiller overdose and alcohol use can also contribute. Taking painkillers without water is not recommended. Factors that increase stomach acid secretion, such as stress, might contribute to the worsening of ulcers… …but the usual treatment now is antibiotics to get rid of H. pylori.

What Happens to the Stomach Acid and Enzymes? When the stomach releases its acid and enzymes into the duodenum… why doesn’t that mix digest our intestines? The stomach lining is thick and protected, but the intestinal lining has to be thinner to absorb nutrients…

Safety Mechanisms Why don’t we digest our own bodies? 1)Stomach enzymes are inactive without stomach acid. Low pH activates them. 2)Stomach acid is neutralized when it reaches the small intestine by pancreatic bicarbonate. (gets cut off) IInactive pepsinogenActive pepsin Example: Pepsin, a protein that breaks down other proteins, is made when pepsinogen is activated by stomach acid’s low pH.

The small intestine absorbs most nutrients. Continues digestion. Bile from liver, stored in gallbladder, breaks fat to droplets. Enzymes from pancreas and intestine lining break down starch (amylase), protein (chymotrypsin), and lipids (lipases). Small Intestine + Accessory Organs

The Folds of the Small Intestine Maximize Surface Area for Absorption Fingerlike projections = villi (singular: villus). Microscopic fingerlike projections on individual cells: microvilli

Absoption in the Small Intestine Lipids Amino acids Sugars

The Large Intestine The large intestine, or colon, reabsorbs water and some minerals. Bacteria naturally present in the colon help us with this. Do not worry about learning its regions in this class.

“Colon Cleaning” Some people claim that the colon accumulates toxins and that people can benefit from washing out this waste with a “colon cleaning.” For a healthy person (not a person with Hirschsprung’s disease or fecal impaction), are enemas or colonic irrigation – attempts to wash the waste out of the colon – a good idea?

Gut Flora The normal bacteria of our guts are beneficial, provided they don’t escape. They can produce vitamins we need, such as vitamin K and some B vitamins. They also outcompete harmful bacteria, such as Clostridium difficile, that might otherwise settle in our guts. Excessive antibiotic treatment or colon cleaning can lead to infection by killing normal flora.

Diarrhea When the large intestine does not reabsorb water and electrolytes properly, diarrhea can occur. In a healthy adult, diarrhea is not usually a medical emergency. However, severe diarrhea can remove water and electrolytes from the body, leading to dehydration and even death. In infants, young children, and the elderly, both of whom have more difficulty controlling water balance, diarrhea can be fatal. Worldwide death toll from diarrhea in infants and small children is over 1 million deaths/year. Oral rehydration therapy could prevent these deaths!

Causes of Diarrhea Many viruses, bacteria, and parasites that spread bacteria are spread by the fecal-oral route via contaminated drinking water. Example of bacterial cause: Cholera, Vibro cholerae. Responsible for recent outbreak in Haiti following earthquake that damaged water/sewage infrastructure. Example of viral causes: Noroviruses and rotaviruses. Rotavirus is the major cause of infant diarrhea. Example of parasitic agent: Giardia Vibrio cholerae Giardia lamblia Rotaviruses (MUCH smaller!)