Digestive System Overview

Digestive System Overview

• The digestive system is a disassembly line, breaking down nutrients into usable forms for the body and absorbing them.
• Most food cannot be directly used, needing to be broken down into smaller components like amino acids and monosaccharides.
• These components are universal to all species.

Digestive System Functions

• Ingestion: Selective intake of food.
• Digestion: Mechanical and chemical breakdown of food into usable forms.
• Absorption: Uptake of nutrients into the blood and lymph.
• Compaction: Absorbing water and consolidating indigestible residue into feces.
• Defecation: Elimination of feces.

Digestive Tract and Accessory Organs

• The digestive tract is a muscular tube extending from the mouth to the anus.
• It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• The gastrointestinal (GI) tract includes the stomach and intestines.
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

The Mouth (Oral Cavity)

• Also known as the buccal cavity.
• Functions include ingestion, taste, mastication (chewing), sensory responses, speech, and respiration.
• The mouth is enclosed by the cheeks, lips, palate, and tongue.
• Mechanical digestion occurs through teeth and tongue action.

Salivary Glands

• Three pairs of larger salivary glands (parotid, submandibular, and sublingual) release saliva.
• Intrinsic salivary glands are numerous, dispersed in the oral tissues (e.g., lingual glands).
• Saliva keeps the mouth moist and inhibits bacterial growth.

The Pharynx

• A muscular funnel connecting the oral cavity/nasal cavity to the esophagus/larynx.
• A point where digestive and respiratory tracts intersect.
• The pharynx facilitates swallowing using the tongue and palate.

The Esophagus

• A muscular tube (25-30cm long), posterior to the trachea.
• Connects the pharynx to the stomach.
• Its superior opening lies between vertebra C6 and the cricoid cartilage.

The Stomach

• A muscular sac (in the upper left abdominal cavity).
• Primarily for food storage and mechanical breakdown.
• Mechanically breaks down food, liquefies it, and begins protein and fat chemical digestion.
• Chyme is a semidigested, soupy/pasty food mixture.
• Most digestion occurs after the chyme passes to the small intestine (2-3 hours).
• Pylorus: The end of the stomach, with the pyloric sphincter regulating chyme passage to the duodenum.

The Small Intestine

• A coiled tube in the upper-right abdomen.
• Regions: duodenum, jejunum, ileum.
• Crucial for nutrient absorption (especially in the duodenum and jejunum).
• Nearly all chemical digestion and nutrient absorption occurs here.
• Large surface area from folds, villi, and microvilli.
• Extensive folding allows for maximized absorption.

The Ileocecal Valve

• Thick muscular sphincter at the junction of the ileum (small intestine) and cecum (large intestine).
• Regulates food residue flow and prevents feces from backing into the ileum.

The Large Intestine

• Receives about 500 mL of indigestible food residue daily.
• Reduces it to about 150 mL of feces through water and salt absorption.
• Portions: cecum, colon, rectum, anal canal.
• Named for its relative diameter, not length.
• Absorbs significant amounts of water and salts.
• Defecation is the process of eliminating feces.

The Liver

• Large reddish-brown gland, under the diaphragm.
• The body's largest gland.
• Responsible for bile secretion to aid fat digestion.
• Bile acids emulsify fat molecules for better enzyme action.
• Also performs detoxification processes and other metabolic functions.

The Pancreas

• Spongy digestive gland, posterior to the greater curvature of the stomach.
• Endocrine (hormone secretion) and exocrine (secretion of pancreatic juice) functions.
• Endocrine: Pancreatic islets secrete hormones like insulin and glucagon.
• Exocrine: Secretes 1200-1500 mL of pancreatic juice daily for digestion/neutrality.