The Digestive System

Functions of the Digestive System

1. Breaks down food into molecules the body can use.

2. Absorbs nutrient molecules into the blood and carries them throughout the body.

3. Eliminates wastes from the body.

Digestion

• The process where food molecules are broken down into nutrients that are then carried by the blood to the body’s cells where they are used to produce energy.

Forms of Digestion

• Mechanical– Foods are physically broken down into

smaller pieces• Food is broken down by moving it.

• Chemical– Chemicals produced by the body break foods

down into their smaller building blocks.• For example, proteins are broken down into amino

acids; large sugars are broken down to glucose

Absorption

• The process by which nutrient molecules pass through the wall of the digestive system into the blood.

Elimination

• Materials that are not absorbed are eliminated (removed) from the body as wastes.

The Mouth

• Both mechanical and chemical digestion begin in the mouth.

• Mechanical Digestion– Teeth cut, tear, grind,

and crush food into smaller pieces.

– Saliva moistens the pieces of food into a slippery mass.

The Mouth

• Chemical Digestion– Salivary glands in the

mouth produce saliva, a mixture of water and enzymes.

• Enzymes in saliva digests starch, changing it into simpler sugars.

– Enzymes are proteins that speed up chemical reactions in the body.

» The shape of an enzyme allows it to bind only to certain types of substances.

The Esophagus

• When you swallow, food travels from your mouth into your esophagus.

• The esophagus connects the mouth to the stomach.– Lined with mucus, which

makes food easier to swallow and move along.

– Food only stays in the esophagus for 10 seconds.

Peristalsis

• After food enters the esophagus, contractions of smooth muscle push food toward the stomach.– Involuntary muscle contractions.

• Squeezes food through the digestive system

– Also occurs in the stomach and farther down the digestive system.

• Keeps food moving in one direction

The Stomach

• Most mechanical digestion and some chemical digestion take place in the stomach.

• The stomach is a J-shaped muscular pouch.– Also used to store

food temporarily.

The Stomach

• Mechanical Digestion– Three layers of smooth muscles produce a churning motion.– Mixes food with fluids like clothes move in a washing machine

• Chemical Digestion– As the food churns in the stomach it mixes with chemicals which

breaks food molecules into its building blocks.• Pepsin is an enzyme that changes proteins into short chains of

amino acids.• Hydrochloric acid helps the pepsin to work; it also kills bacteria that

enter your stomach– The acid does not burn a hole in your stomach because your stomach

is lined with mucus.

Ulcers

• When the lining of the stomach is damaged, the stomach acid can burn holes in the stomach.– When this happens,

an ulcer forms.

Chyme

• The mixture of partially digested food and digestive juices that is formed in the stomach.

Small Intestine

• Chyme leaves the stomach and enters into the small intestine.

• Most chemical digestion takes place here.

• Makes up 2/3 of the digestive tract.– About 6 meters long, but

is only 2-3 centimeters wide.

Small Intestine• By the time chyme

reaches the small intestine, starches and proteins have been partially digested. Fats have not been digested at all.– Chyme mixes with

chemicals produced by the accessory organs which will do most of the rest of the chemical digestion of the food molecules.

• Almost all absorptions of nutrients takes place in the small intestine.

The Liver• Breaks down medicines

and helps eliminate nitrogen from the body

• The role of the liver is to produce bile.– Bile breaks up fat

molecules• It is not an enzyme; it acts

more like a soap.• Mixes with the fats in the

food to form small fat droplets.

– Fat droplets are chemically broken down by enzymes from the pancreas.

Cirrhosis

• Cirrhosis of the liver can be caused by the over-consumption of alcohol over long periods of time.

Gall Bladder

• Bile flows from the liver into the gall bladder.– The job of the gall

bladder is to store bile.

• After eating, bile passes through a tube from the gall bladder into the small intestine.

Gall Stones• Whenever the bile

hardens inside the gall bladder, gall stones are formed.

• If the stones clog the bile duct into the small intestine, it becomes very painful.– The gall bladder can be

safely removed to prevent further complications.

• Bile will flow directly from the liver into the small intestine.

– No bile will be stored.

Pancreas

• The pancreas produces enzymes that flow into the small intestine and help break down starches, proteins, and fats.– The pancreas also

produces insulin, a hormone that controls the level of sugar (glucose) in the blood.

Absorption• After chemical digestion takes

place, the small nutrient molecules are ready to be sent to the body’s cells by the blood.– The small intestine is lined

with finger-like projections that absorb the nutrient molecules.

• These structures are called villi.

– The villi contain capillaries that allow the nutrients to pass through them.

» Villi greatly increase the surface area available for absorption.

Large Intestine

• By the time material reaches the end of the small intestine, most of the nutrients have been absorbed.

• The remaining material moves into the large intestine.– About 1.5 meters long.

• Contains bacteria that feed on the material passing through it.

– Helpful bacteria because they produce vitamins.

Large Intestine

• Also called the colon.• The material entering the

large intestine contains water and undigested food, such as fiber.– As the material moves

through the large intestine, water is absorbed into the bloodstream.

• The remaining material is readied for elimination from the body.

Rectum

• The large intestine ends in a short tube called the rectum, which compresses waste materials into a solid form.

• Waste material is eliminated from the body through the anus, a muscular opening at the end of the rectum.