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Functions of the Digestive System

  • As soon as food enters your mouth, or is ingested, breakdown begins.

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  • Food is processed in your body in four

stages—ingestion, digestion, absorption, and elimination.

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Functions of the Digestive System

  • Digestion is the process that breaks down food into small molecules so that they can be absorbed and moved into the blood.
  • From the blood, food molecules are transported across the cell membrane to be used by the cell.

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  • Unused molecules pass out of your body as wastes.
  • Digestion is mechanical and chemical.

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Functions of the Digestive System

  • Mechanical digestion takes place when food is chewed, mixed, and churned.
  • Chemical digestion occurs when chemical reactions occur that break down large molecules of food into smaller ones.

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Enzymes

  • An enzyme is a type of protein that speeds up the rate of a chemical reaction in your body.
  • One way this happens is by reducing the amount of energy necessary for a chemical reaction to begin.

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  • Enzymes work without being changed or used up.

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Enzymes

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Click image to view movie.

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Enzymes in Digestion

  • Many enzymes help you digest carbohydrates, proteins, and fats.
  • Amylase (AM uh lays) is an enzyme produced by glands near the mouth.

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  • This enzyme helps speed up the breakdown of complex carbohydrates, such as starch, into simpler carbohydrates—sugars.

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Enzymes in Digestion

  • In your stomach, the enzyme pepsin aids the chemical reactions that break down complex proteins into less complex proteins.
  • In your small intestine, a number of other enzymes continue to speed up the breakdown of proteins into amino acids.

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Enzymes in Digestion

  • The pancreas, an organ on the back side of the stomach releases several enzymes through a tube into the small intestine.
  • The resulting sugars are turned into glucose and are used by your body’s cells.

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  • Different enzymes from the pancreas are involved in the breakdown of fats into fatty acids.

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Organs of the Digestive System

  • Your digestive system has two parts—the digestive tract and the accessory organs.
  • The major organs of your digestive tract are the mouth, esophagus (is SAH fuh guhs), stomach, small intestine, large intestine, rectum, and anus.

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Organs of the Digestive System

  • The tongue, teeth, salivary glands, liver, gallbladder, and pancreas are the accessory organs.
  • They are important in mechanical and chemical digestion.

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Organs of the Digestive System

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  • Your liver, gallbladder, and pancreas produce or store enzymes and chemicals that help break down food.

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The Mouth

  • Mechanical and chemical digestion begin in your mouth.
  • Mechanical digestion happens when you chew your food with your teeth and mix it with your tongue.

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  • Chemical digestion begins with the addition of a watery substance called saliva (suh LI vuh).

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The Mouth

  • As you chew, your tongue moves food around and mixes it with saliva.
  • Saliva is produced by three sets of glands near your mouth.

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The Mouth

  • Although saliva is mostly water, it also contains mucus and an enzyme that aids in the breakdown of starch into sugar.
  • Food mixed with saliva becomes a soft mass and is moved to the back of your mouth by your tongue.

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  • It is swallowed and passes into your esophagus.

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The Esophagus

  • Food moving into the esophagus passes over the epiglottis (ep uh GLAH tus).
  • This structure automatically covers the opening to the windpipe to prevent food from entering it, otherwise you would choke.

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  • Your esophagus is a muscular tube about 25 cm long.
  • It takes about 4 s to 10 s for food to move down the esophagus to the stomach.

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The Esophagus

  • No digestion takes place in the esophagus.
  • Mucous glands in the wall of the esophagus keep the food moist.

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  • Smooth muscles in the wall move food downward with a squeezing action.
  • These waves of muscle contractions, called peristalsis (per uh STAHL sus), move food through the entire digestive tract.

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The Stomach

  • The stomach is a muscular bag.
  • When empty, it is somewhat sausage shaped with folds on the inside.

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  • As food enters from the esophagus, the stomach expands and the folds smooth out.

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The Stomach

  • Mechanical and chemical digestion take place in the stomach.
  • Mechanically, food is mixed in the stomach by peristalsis.

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  • Chemically, food is mixed with enzymes and strong digestive solutions, such as hydrochloric acid solution, to help break it down.

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The Stomach

  • Specialized cells in the walls of the stomach release about 2 L of hydrochloric acid solution each day.

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  • The acidic solution works with the enzyme pepsin to digest protein.

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The Stomach

  • It also destroys bacteria that are present in the food.

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  • The stomach also produces mucus, which makes food more slippery and protects the stomach from the strong, digestive solutions.
  • Food moves through your stomach in 2 hours to 4 hours and is changed into a thin, watery liquid called chime (KIME).
  • Little by little, chime moves out of your stomach and into your small intestine.

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The Small Intestine

  • Your small intestine is small in diameter, but it measures 4 m to 7 m in length.

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  • As chime leaves your stomach, it enters the first part of your small intestine, called the duodenum (doo AH duh num) where most digestion takes place.
  • Here, a greenish fluid from the liver, called bile, is added to break up large fat particles.

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The Small Intestine

  • Chemical digestion of carbohydrates, proteins, and fats occurs when a digestive solution of bicarbonate ions and enzymes from the pancreas is mixed in.

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  • The solution helps neutralize the stomach acid.
  • Your pancreas also makes insulin, a hormone that allows glucose to pass from the bloodstream into your cells.

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The Small Intestine

  • Absorption of food takes place in the small intestine.

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covered with fingerlike projections called villi (VIH li).

  • The wall has many ridges and folds that are

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The Small Intestine

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  • Villi increase the surface area of the small intestine so that nutrients in the chime have more places to be absorbed.

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The Small Intestine

  • Peristalsis continues to move and mix the chime.

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  • The villi move and are bathed in the soupy liquid.
  • Nutrients move into blood vessels within the villi.

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The Small Intestine

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  • From here, blood transports the nutrients to all cells of your body.
  • Peristalsis continues to force the remaining undigested and unabsorbed materials slowly into the large intestine.

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The Large Intestine

  • When the chime enters the large intestine, it is still a thin, watery mixture.

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  • The main job of the large intestine is to absorb water from the undigested mass.

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The Large Intestine

  • After the excess water is absorbed, the remaining undigested materials become more solid.

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  • Muscles in the rectum, which is the last section of the large intestine, and the anus control the release of semisolid wastes from the body in the form of feces (FEE seez).

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Bacteria Are Important

  • Bacteria live in many of the organs of your digestive tract including your mouth and large intestine.

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  • The bacteria in tour large intestine feed on undigested material like cellulose. In turn, bacteria make vitamins you need—vitamin K and two B vitamins.
  • Bacterial action also converts bile pigments into new compounds.

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Section Check

Question 1

This illustration represents _______.

A. absorption

B. digestion

C. elimination

D. ingestion

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Section Check

Answer

The answer is D. Ingestion occurs as food enters your mouth.

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Section Check

Question 2

Answer

_______ are a type of protein that speeds up the rate of a chemical reaction in your body.

The answer is enzymes. Enzymes reduce the amount of energy necessary for a chemical reaction to begin.

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Section Check

Question 3

Which is a major organ in your digestive tract?

A. esophagus

B. gallbladder

C. pancreas

D. salivary glands

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Section Check

Answer

The answer is A. The major organs of the digestive tract are mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. Food passes through all of these organs.