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Digestive Juices and Their Functions
Salivary Glands

Bicarbonate fluid Lubricates food during chewing.
Salivary Lipase Prepares fat for digestion.

Gastric Secretions

Acid Initiates digestion of protein and kills bacteria.
Pepsin Aids in initiation of protein digestion.
Gastric Lipase Initiates digestion of dietary fat.
Mucous Lubricates and protects lining of stomach.
Intrinsic Factor Aids in absorption of Vitamin B12 by small intestine.

Liver Secretions

Bile Acids Detergent that helps dissolve dietary fat.
Phospholipids Promotes absorption of fats.
Cholesterol Excreted in bile.
Immunoglobulins Protect from bacteria and other harmful organisms.
Mucus Protects from bacteria.

Pancreatic Secretions

Bicarbonate Neutralizes acid and protects digestive enzymes.
Water and electrolytes Fluid delivery system for digestive enzymes.
Amylase Digests dietary starch and glycogen.
Lipases Digest dietary fat.
Proteases Digest dietary protein.

Small Intestine

Substance Absorption.
Iron In Duodenum.
Vitamin B12 In Ileum.
Bile Acids Ileum absorbs and recycles to liver.
Water and Electrolytes 90% absorbed in small intestine.
Carbohydrates Enzymes along the tips of the intestinal cell digest prior to absorption, i.e. lactase digests milk, sugar.
Protein After digestion by gastric & pancreatic enzymes.
Fat After digestion by pancreatic enzymes & help of bile.

Hormonal Regulation and Coordination

Substance Absorption.
Endocrine Hormones arrive by blood stream, secreted from a distant site.
Insulin Controls glucose metabolism.
Gastrin Stimulates gastric acid/juice secretion and promotes growth of gastric and intestinal cells.
Secretin Stimulates pancreatic secretion of fluid and bicarbonate, the liver to produce bile, and the stomach to produce pepsin.
Motilin Regulates peristalsis and secretion between meals.
Paracrine Hormones released locally.
Substance P Influences secretion, absorption, blood flow, motility, and immunology.

Hormones that can act as endocrine or Paracrine substances

CCK Signal for gallbladder contraction, pancreatic secretion (and growth), & satiety.
Somatostatin Inhibits secretion by gut cells, nerves, and hormone secreting cells.
Neurotensin Increases blood flow & stimulates secretion.

Nerve Regulation and Coordination

Extrinsic Nerves Come from the unconscious part of brain or from the spinal cord.
Acetylcholine Nerve transmitter stimulates peristalsis and secretion.
Adrenaline Nerve transmitter that relaxes gastric & intestinal muscle & decreases blood flow.
Intrinsic Nerves Are within the muscle walls of the digestive system hollow organs. Intrinsic enteric nervous system acts like a local computer, Initiates patterned program responses reflexively when stimulated, and control & coordinate peristalsis.