The Human Digestive System

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The Human Digestive System
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The Human Digestive System

• The human digestive tract is similar to that of
  the earthworm, in terms of both the anatomy and
  physiology.
• The digestive tract is actually a tube, with a
  number of specialized organs along its path.

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Digestive System continued

• If the tube was stretched out, it would be about
  9 m in length.

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Path of Ingested Food

• Oral Cavity (mouth)
• Pharynx (throat)
• Esophagus (gullet)
• Stomach
• Small intestine
• Large intestine
• Rectum
• Anus

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Digestive Glands

• Along the digestive path, there are several
  digestive glands that include the salivary
  glands, the liver and the pancreas.
• These glands produce secretions that enter the
  tube via small tubes called ducts.

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The Mouth and Pharynx

• When food enters the mouth there are two
  important processes that occur
• 1. Mechanical breakdown- grinding action of
  teeth.
• 2. Chemical digestion-action of saliva. Refer to
  Handout-Saliva and Salivation.

Bolus
Bolus
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Mouth and Pharynx continued.

• When the food has been chewed, moistened, and
  turned into the bolus, it is pushed to the back
  of the pharynx by the tongue.
• When the bolus arrives at the back of the throat,
  the swallowing reflex is activated.
• Food is forced into the esophagus.

Bolus
Bolus
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The Esophagus

• Food is pushed along the esophagus to the
  stomach, by means of wave-like contractions
  called peristalsis.
• In peristalsis, muscles in front of the bolus
  relax, and those behind the bolus contract. This
  sends food ahead to the stomach. See handout
  called Peristalsis and Segmentation.

Esophagus
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The Stomach

• When the bolus reaches the stomach it encounters
  a ring of muscle tissue called a sphincter.
• Because of peristalsis, the sphincter relaxes and
  food enters the stomach.
• The stomach is a thick-walled muscular sac and
  can hold more than 2 L of material.
• Refer to handout- Stomach Structure and
  Functions.

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Chemical Digestion of Food

• Chemical breakdown of food begins in the stomach.
  
• Two types of glands are involved here
• 1. Pyloric Glands - These glands secrete mucus
  that protects the stomach from enzymes and acids
  i.e. prevents self-digestion.

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Chemical Digestion of Food continued

• 2. Gastric Glands - produce gastric juice which
  contains
• (a) Hydrochloric Acid - pH 1.5-2.5
• (b) Pepsin- works best in an acid environment.
  Breaks down large proteins into smaller chains of
  amino acids called polypeptides.

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Release of Gastric Juice

• 1. Sight, thought, smell - stimulation of the
  brain causes gastric glands to release gastric
  juice (GJ).
• 2. Food touching stomach lining results in GJ
  release.
• 3. Food stretching stomach or certain substances
  (protein, caffeine, alcohol), results in release
  of the hormone gastrin, from the stomach lining.
  Gastrin in turn stimulates release of GJ.

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From Stomach to Intestine !

• In the stomach, food is turned into a soupy
  material called chyme.
• Chyme passes through the pyloric sphincter and
  into the duodenum (the first part of the small
  intestine).
• This takes about 2-6 hours.

Duodenum
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The Small Intestine (S.I.)

• The S.I. Is about 6.5 m long and 2.5 cm in
  diameter.
• S.I environment is alkaline.
• Most chemical digestion occurs in the S.I.
• Nearly all absorption occurs in the S.I.
• Peristalsis and segmentation move food and
  digestive juices through the S.I.(see handout).

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The Small Intestine (S.I.) cont.

• In the S.I., chyme is mixed with the following
• 1. Bile from the liver
• 2. Pancreatic juice from the pancreas.
• 3. Intestinal juice from the cells in the
  intestinal wall.
• The above secretions contain enzymes that
  complete digestion.

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Digestive Juices in the S.I.
Chyme from stomach
Pancreatic juice and enzymes
Secretin and cholecystokinin
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Pancreas Secretions

• Two products
• 1. Pancreatic Juice- contains sodium bicarbonate,
  that neutralizes the acid chyme.
• 2. Pancreatic Enzymes- Three enzymes.

Pancreas
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Pancreas Enzymes

• 1. Amylase -breaks down starch into maltose.
• 2. Proteases - two types trypsin and
  chymotrypsin. Both act on large protein
  molecules.
• 3. Lipases - break down fats.

Pancreas
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Bile

• Produced in liver and stored in the gall bladder.
• Bile release is stimulated by cholecystokinin.
• Bile travels to S.I. via bile duct, where it
  breaks up fats and oils into small droplets. This
  process is called emulsification.

Gall bladder
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Intestinal Juice

• Intestinal glands produce this juice.
• Contains several enzymes
• Fats- fatty acids and glycerol.
• Carbohydrates-simple sugars.
• Protein-amino acids.

Intestinal glands
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Absorption in the S.I.

• S.I. is the site of absorption.
• Simple sugars, amino acids, vitamins and minerals
  are absorbed through the wall of the S.I.
• Fatty acids and glycerol are absorbed by small
  lymphatic vessels called lacteals.

S.I.
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Absorption in the S.I. Cont.

• S.I. is an excellent absorption site
• 1. Very long(6.5m)-allows time for absorption to
  occur.
• 2. Highly folded lining.
• 3. Lining is covered in villi.
• 4. Epithelial cells of the intestinal lining have
  microvilli..

Villus
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The Villus
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The Large Intestine

• 1.5 m in length, 6 cm in diameter.
• No digestion here !
• Reabsorption of water.
• Absorption of vitamins produced by bacteria (Vit.
  K Vit. B).
• Gets rid of indigestible material.
• Storage of feces.

Large Intestine