Digestion September 2014. REMINDER These slides are supplementary notes to help you visualize important parts of the chalkboard lectures. They do NOT.

Digestion

September 2014

REMINDER

These slides are supplementary notes to help you visualize important parts of the chalkboard lectures. They do NOT replace attendance in class. The key skills lists are not included, and less visual portions of the lecture may be skipped altogether!

Roles of the Digestive SystemIngestion: 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

ChewingChewing 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)

The TeethThe 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 FormationCavities 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!

Peristalsis

The EsophagusLike 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

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 pepsinogen Active pepsin

Example: Pepsin, a protein that breaks down other proteins, is made when pepsinogen is activated by stomach acid’s low pH.

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.

• 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

The Large Intestine

• The large intestine, or colon, reabsorbs water and some minerals. Do not worry about learning its regions in this class, but

DiarrheaWhen 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 lambliaRotaviruses (MUCH

smaller!)