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The Digestive System. The Digestive System. 4 steps for processing nutrients from food Ingestion Digestion Absorption Elimination. Ingestion & Digestion. Ingestion: Eating or drinking Digestion: the breaking down of food into molecules that are small enough to be absorbed by the body

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The Digestive System

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The digestive system

The Digestive System

The digestive system1

The Digestive System

4 steps for processing nutrients from food

  • Ingestion

  • Digestion

  • Absorption

  • Elimination

Ingestion digestion

Ingestion & Digestion

Ingestion: Eating or drinking

Digestion: the breaking down of food into molecules that are small enough to be absorbed by the body

Mechanical digestion: Chewing and grinding food into smaller pieces, increasing the surface area

Chemical digestion: Breaking chemical bonds of macromolecules (carbs, proteins and lipids) into monomers (sugars, amino acids, glycerol & fatty acids)

  • Hydrolysis reactions that are carried out by enzymes

Absorption elimination

Absorption & Elimination


  • Cells absorb small molecules (monomers) allowing them to enter into the circulatory system for immediate use or be transported to storage organs for later use


  • Removing undigested waste (feces)

Digestive system

Digestive System

Digestive Tract

  • A.k.a. an alimentary canal (2 openings)

  • The mouth, pharynx, esophagus, stomach, small intestine, and large intestine

    Accessory Glands & Organs

  • Aid in digestion, but not a part of the digestive tract

  • Salivary glands, pancreas, liver, gallbladder.

  • Secrete digestive juices and enzymes into the digestive tract

What s your size

What’s Your Size?



  • Plays a role in ingestion and digestion

  • Teeth, tongue, hard palate aid in mechanical digestion

  • Tongue moves food down to the esophagus

  • Salivary glands aid in chemical digestion

    • Release saliva (~1L/day), a watery liquid containing digestive enzymes

    • Salivary amylase begins to break down starch into smaller carbohydrates

    • Lubrication

Chewed up food is now called a bolus



  • Located behind the trachea

  • As you swallow, the epiglottis closes over the tracheaand the food bolus passes through pharynx into the esophagus

  • The bolus is pushed through the esophagus by a series of muscle contractions called peristalsis

Heart burn

Heart Burn

  • After food passes into the stomach, the cardiac sphincter closes the esophagus

  • If the sphincter opens at an inappropriate time, stomach acid can flow back into the esophagus

  • Esophagus does not have the same protective lining as the stomach



I ate too much…I’m STUFFED!

  • Elastic, muscular sac with a protective lining that contains millions of gastric glands

  • Can ~ 2 L

  • Mechanical and chemical digestion



Chemical Digestion:

  • The hormone, gastrin, stimulates gastric glands to release gastric juice

    • Hydrochloric acid breaks apart cells and kills bacteria

    • Enzymes e.g. pepsin

    • Mucus lubrication and protection

      Mechanical Digestion:

  • Stomach muscles churn

  • Bolus becomes an acidic, nutrient-rich liquid called chyme

Small intestine

Small Intestine

  • Chyme passes through the pyloric sphincter and enters the small intestine (peristalsis)

  • Long, narrow tube where digestion is completed and absorption begins

  • 3 parts:

    • Duodenum

    • Jejunum

    • Ileum



  • Bile from the liver and gallbladder are secreted into the duodenum to help with lipid digestion

  • Pancreas secretes pancreatic juice into the duodenum

    • Neutralizes acidic chyme

    • Pancreatic amylase

    • Trypsin & chymotrypsin (protein digestion)

    • Pancreatic lipase

  • Small intestine releases enzymes too

Jejunum ileum

Jejunum & Ileum


  • Contains more villi, therefore more absorption

  • Breakdown of remaining polypeptides and carbohydrates


  • Absorption

  • Push remaining undigested material into the large intestine

Large intestine

Large Intestine

  • Undigested material passes through the sphincter between the ileum and the cecum

  • Absorption of water and dissolved minerals

  • Intestinal bacteria produce vitamins

  • Indigestible material (feces) passes through the rectum to the anus

    • Anal sphincters help control timing of elimination

Increased surface area

Increased Surface Area

  • The small and large intestine are folded to  surface area and therefore  absorption

  • Within the folds, there are finger-like projections called villi

    • Lacteal absorbs fats and fat-soluble vitamins

    • Capillaries absorb glucose, amino acids, minerals and water-soluble vitamins

Regulation of digestive processes

Regulation of Digestive Processes

The digestive system





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