36-2 The Digestive System

Digestive Tract

• Alimentary canal– one way passage through the body

• Function: – to convert food into simple molecules that

can be absorbed and used by the cells of the body

The Mouth

• Start of digestion

• Chewing of teeth tear/ crush moistened food into a fine paste

• Teeth – anchored in the bones of jaw and connected

to a network of blood vessels and nerves– Coated with mineralized enamel

The Mouth• Types of teeth

– Incisors – sharp to cut thru meat– Cuspids and bicuspids – grasp and

tear food– Molars – large flat surface for

grinding food

• Herbivores – molars are most common

• Carnivores – incisors are common

• Our teeth reflect a mixed diet of meats and plants

The MouthSalivary glands• 3 pairs –moistens food and make it easier to chew• controlled by nervous system, triggered by smell• Amylase

– enzyme to break down starches releasing sugars– Results in sweet taste

• Lysozyme– fights infection by digesting the cell walls of many bacteria

• result: chewed clump of food called the bolus

• Last – food is swallowed and safely passes the epiglottis closed over the trachea

The Esophagus

• 25 centimeters long

• Receive bolus from the mouth after swallowed and carries it to the stomach.

• Peristalsis – the contractions of smooth muscle that occur

throughout the alimentary canal the squeezes the food through the esophagus

The Stomach• Cardiac sphincter

– thick ring of muscles the close the esophagus after the food has passed into the stomach preventing back flow.

• Functions – has the ability to store food– Produce enzymes and strong acids, hydrochloric acid– Churn the food through muscle contractions

• Gastric glands – line the stomach– Produce mucus – lubricates and protects stomach wall– Produce pepsin – digests proteins

• End product of stomach – chyme

The Small Intestine

• The pyloric valve opens allowing chyme to enter the duodenum

• Duodenum– 1st part of S.I., site where digestive fluids are added and

most of the chemical digestion occurs

• Pancreas – located just below the stomach– Produces hormones that regulate blood sugar– Produces enzymes that break down carbohydrates,

proteins, lipids and nucleic acids.– Releases sodium bicarbonate – neutralizes stomach

acid and allows enzymes to function

Liver• large gland just above the

stomach• Produces bile – fluid loaded

with lipids and salts• Dissolves and disperses

droplets of fat • Regulates the levels of nutrients

in the blood• When food reaches the

remaining portion of the S.I. it is ready to be absorbed

The Small Intestine

• Jejunum and ileum – covered with villi - specialized for absorption of food

• villi are covered w/ tiny projections called microvilli

• Absorbed sugars, amino acids and other nutrients are passed directly into the bloodstream, Fats enter lymph then bloodstream

• The blood passes to the liver where excess sugar stored as glycogen, until levels return to normal regulating the blood sugar level.

The Large Intestine

• All nutrients have been removed • Also known as the colon• Function

– remove water from undigested material producing feces

• Waste is then eliminated through the rectum• Bacteria live in intestine receiving nutrients

from the feces and providing the body with vitamin K and other compounds.

Regulating Nutrient levels - SugarToo little sugar – organs suffer from loss of energy

Islets of Langerhans – (pancreas) – produce insulin• High levels of sugar – insulin is released and causes liver,

muscles and fatty tissue to remove sugar form the blood stream and store it as glycogen and fat.

• Levels of sugar drop – glucagen is released and stimulates the liver, muscles and fatty tissues to break down glycogen and fats to release sugar back into the blood.

Diabetes mellitus • person is unable to produce enough insulin and requires a

special diet or injections