Digestive System

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Digestive System

• Miss Hillemann
• Human Anatomy Physiology
• Neshaminy High School

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Organs of the Digestive System

• Gastrointestinal (GI) tract
• Continuous tube that extends from mouth to anus
• Mouth, pharynx, esophagus, stomach, small
  intestine, and large intestine
• Accessory digestive organs
• Teeth, tongue, salivary glands, liver,
  gallbladder, and pancreas.
• Teeth- mastication (physical breakdown of food)
• Tongue- assists in chewing and swallowing
• Others- produce or store secretions to aid in
  chemical breakdown of food.

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Functions of the Digestive System

• Ingestion- eating and drinking.
• Secretion- secretes 7 L of H2O, acid, buffers,
  and enzymes into lumen of GI tract
• Mixing and propulsion- alternating contraction
  and relaxation of smooth muscle in walls of GI
  tract
• Mix food and secretions
• Propel them toward anus.

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Functions of the Digestive System (contd)

• Digestion
• Mechanical digestion- teeth cut and grind food.
  Smooth muscles of stomach and small intestine
  churn food.
• Chemical Digestion- enzymes break down food into
  smaller molecules.
• Absorption of fluids, ions, and small molecules
• Defacation- waste products leave through the anus.

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Layers of the GI tract

1. Mucosa- inner lining. Mucous membrane.
2. Submucosa- areolar connective tissue that binds
   mucosa to muscularis. Contains many blood and
   lymphatic vessels that receive absorbed food
   molecules.
3. Muscularis- thick layer of muscle.
4. Serosa and peritoneum- outermost layer. Composed
   of simple squamous epithelium and areolar
   connective tissue.

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Teeth

• Humans have different teeth for different
  functions.
• Incisors- closest to midline. Chisel-shaped. Cut
  into food.
• Cuspids- next to the incisors. One pointed
  surface (cusp) to tear and shred food.
• Premolars- have two cusps to crush and grind
  food.
• Molars- have 3 blunt cusps to crush and grind
  food.

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Digestion in the Mouth

• Mastication (chewing)- food is manipulated by the
  tongue, ground by the teeth, and mixed with
  saliva.
• Food is reduced to a soft, flexible, easily
  swallowed mass called a bolus.
• Salivary amylase- begins breakdown of starch.

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Pharynx Esophagus

• When food is swallowed, it passes from the mouth
  into the pharynx.
• Funnel-shaped tube composed of skeletal muscle
  and lined by mucous membrane.
• Food then passes into the esophagus.
• Muscular tube lined with stratified squamous
  epithelium.
• Transports food to the stomach and secretes mucus.

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The Esophagus

• From the throat, the chewed food passes through
  the esophagus, or food tube, into the stomach.
• Food is moved along by contractions of smooth
  muscle.
• These contractions, known as peristalsis, squeeze
  the food through the esophagus into the stomach.
• The cardiac sphincter closes the esophagus after
  food has passed into the stomach.

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The Esophagus

• Peristalsis

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Stomach

• Chief cells secrete an inactive gastric enzyme
  called pepsinogen.
• Parietal cells produce hydrochloric acid (HCl),
  which kills microbes in food and converts
  pespinogen to pepsin.
• Pepsin begins the digestion of proteins.
• Parietal cells secrete intrinsic factor for the
  absorption of vitamin B12.
• G cells secrete gastrin.

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Stomach
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The Stomach

• Pepsin and hydrochloric acid begin protein
  digestion.
• Pepsin breaks proteins into smaller polypeptide
  fragments.
• Other enzymes are denatured by stomach acid.
• Mechanical Digestion 
• The stomach contracts to churn fluids and food,
  gradually producing a mixture known as chyme.
• After 12 hours, the pyloric valve between the
  stomach and small intestine opens and chyme flows
  into the small intestine.

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Stomach Regulation

• The secretion of gastric juices into the stomach
  is regulated by
• Peptic gastrin
• Secretin
• Thought, smell, or presence of food in the mouth

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The Stomach
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The Small Intestine

• As chyme is pushed through the pyloric valve, it
  enters the duodenum.
• The duodenum is the first of three parts of the
  small intestine.
• It is where most digestive enzymes enter the
  intestine.
• Most chemical digestion and absorption of food
  occurs in the small intestine.

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Regions of the Small Intestine

• Duodenum- C-shaped tube. 25 cm long.
• Jejunum- 1 m long and extends from duodenum to
  ileum.
• Ileum- 2 m long.
• Following digestion, most nutrient absorption
  occurs in the jejunum and ileum of the small
  intestine.

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The Small Intestine

• Accessory Structures of Digestion

Liver
Bile duct
Pancreas
Gallbladder
Pancreatic duct
Duodenum
To rest of small intestine
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Pancreas

• Just behind the stomach is the pancreas. During
  digestion, the pancreas
• Produces enzymes that break down carbohydrates,
  proteins, lipids, and nucleic acids.
• Produces sodium bicarbonate
• A base that neutralizes stomach acid so that
  these enzymes can be effective.

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Liver

• Assisting the pancreas is the liver, which
  produces bile.
• Bile emulsifies (dissolves and disperses)
  droplets of fat in fatty foods.
• Enzymes break down smaller fat molecules.
• Bile is stored in the gallbladder.

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Absorption in the Small Intestine

• The folded surfaces of the small intestine are
  covered with fingerlike projections called villi.
  
• Within each villus there is a network of blood
  capillaries and lymph vessels that absorb and
  carry away nutrients. 
• Cell surfaces of villi have more projections
  called microvilli.
• These provide an enormous surface area for the
  absorption of nutrient molecules.
• Slow, wavelike contractions of smooth muscles
  move the chyme along this surface.

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Absorption in the Small Intestine

• The Small Intestine

Villus
Small intestine
Circular folds
Epithelial cells
Villi
Capillaries
Lymph vessel
Vein
Artery
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Absorption in the Small Intestine

• Nutrient molecules are absorbed into the cells
  lining the small intestine.
• Most products of carbohydrate and protein
  digestion are absorbed into the capillaries in
  the villi.
• Molecules of undigested fat are absorbed by lymph
  vessels.

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The Large Intestine

• When the chyme leaves the small intestine, it
  enters the large intestine, or colon.
• The large intestine removes water from the chyme.
  
• Water is absorbed quickly, leaving undigested
  materials behind.
• Concentrated waste material passes through the
  rectum and is eliminated from the body.

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Digestive System Control

• Parasympathetic Nervous System (rest-and-digest
• Hormones
• Gastrin- secreted by stomach. stimulates parietal
  cells to secrete HCl and chief cells to secrete
  pepsinogen.
• Secretin- secreted in duodenum. stimulates
  production of bicarbonate to neutralize acidic
  chyme.
• Cholecystokinin- secreted by small intestine.
  Stimulates contraction of gallbladder, secretion
  of pancreatic juice enzymes
• Gastric inhibitory peptide- secreted by small
  intestine. inhibits gastric acid secretion,
  stimulates secretion of insulin

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• Food is moved through the esophagus into the
  stomach by
• air pressure.
• muscle contractions.
• gravity.
• swallowing.

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• A gland that has both endocrine and exocrine
  functions is the
• liver.
• spleen.
• pancreas.
• gallbladder.

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• The enzyme in saliva that begins the digestion of
  starch is
• amylase.
• pepsin.
• lysozyme.
• peptidase.

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• Stomach muscles contract to churn and mix stomach
  fluids and food, producing a mixture known as
• chyme.
• amylase.
• bile.
• acid.

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• Absorption of vitamins, minerals, and digested
  food molecules takes place in the
• stomach.
• small intestine.
• large intestine.
• duodenum.