Introduction to Digestive System

Introduction to the Digestive System

• The digestive system provides the body with a continuous supply of water, electrolytes, and nutrients.
• The system requires movement of food, secretion of digestive juices, absorption of nutrients, circulation of blood, and control of these functions.

Functions of the Digestive System

• Motility: movement of food through the digestive tract, including ingestion, mastication, deglutition, and peristalsis.
• Secretion: includes exocrine and endocrine secretions, such as gastric juice, digestive enzymes, and hormones.
• Digestion: breakdown of food molecules into smaller subunits.
• Absorption: passage of digested end products into the blood or lymph.
• Storage and elimination: temporary storage and elimination of indigestible food molecules.

Intestinal Wall Structure

• The intestinal wall consists of five layers: serosa, longitudinal muscle layer, circular muscle layer, submucosa, and mucosa.
• The mucosa contains sparse bundles of smooth muscle fibers called mucosal muscle.

Neural Control

• The enteric nervous system (ENS) controls gut motility and secretion.
• The autonomic nervous system (ANS) also plays a role in neural control.
• The ENS consists of two major networks: myenteric plexus (Auerbach's plexus) and submucosal plexus (Meissner's plexus).

Neurotransmitters Secreted by ENS

• Acetylcholine (ACH): excites GI activity.
• Norepinephrine: inhibits GI activity.
• ATP: neurotransmitter.
• Serotonin: neurotransmitter.
• Dopamine: neurotransmitter.
• Cholecystokinin (CCK): neurotransmitter.
• Somatostatin: neurotransmitter.
• Nitric oxide (NO) and carbon monoxide (CO): gases.
• Vasoactive intestinal peptide (VIP): neurotransmitter.

Salivary Glands and Saliva

• Three main salivary glands: parotid, submandibular, and sublingual.
• Saliva contains enzymes, such as amylase.

The Liver and Bile

• The liver is the largest organ in the body, located in the upper right and central portions of the abdominal cavity.
• Functions of the liver include:
  • Formation and secretion of bile.
  • Metabolic functions, such as carbohydrate, protein, and fat metabolism.
  • Vascular function for storage and filtration of blood.
  • Immunity functions, including the presence of Kupffer's cells.

The Pancreas

• The pancreas is a complex organ composed of endocrine and exocrine portions.
• The endocrine portion secretes insulin and glucagon.
• The exocrine portion secretes digestive enzymes.

Phases of Secretion

• Cephalic phase: begins before food reaches the stomach and possibly even before eating.
• Gastric phase: begins when food enters the stomach.
• Intestinal phase: begins after food enters the small intestine.

The Small Intestine

• The small intestine is divided into duodenum, jejunum, and ileum.
• The intestinal contents are mixed with mucosal secretion, pancreatic juice, and bile, and digestion is completed in the small intestine.

The Colon

• The principal functions of the colon are:
  • Absorption of water and electrolytes from the chyme to form solid feces.
  • Storage of fecal matter until it can be expelled.