Human Digestive System: Functions and Processes
Mouth
Digestive Functions
- Mechanical Decomposition: Chewing
- Chemical Digestion: Salivary glands secrete enzymes.
- Serous (watery): Contains ptyalin or amylase for starch digestion.
- Mucosa: Lubricates food, protects oral mucosa, and facilitates swallowing.
Saliva Functions
- Protects dental mucosa
- Cleans teeth
- Starts carbohydrate digestion
- Helps form the bolus
- Lubricates the oral cavity and pharynx
- Dissolves chemical substances for taste perception
- Immune function
Salivary Glands
- Major Glands: Parotid, submaxillary, sublingual
- Minor Glands: Located on the palate, tongue, and lips
Pharynx
Common passage for respiratory and digestive systems. Contains tonsils and is divided into nasopharynx, oropharynx, and laryngopharynx.
Esophagus
Tube connecting the pharynx and stomach (cardia). Muscle layers perform peristaltic movements to facilitate food passage. Esophageal glands secrete mucus for lubrication.
Swallowing Process
- Voluntary Stage: Food passage from mouth to pharynx; bolus formation.
- Reflex Stage: Food passage from pharynx to esophagus. Epiglottis blocks the larynx to prevent food from entering the respiratory tract.
- Reflex Stage: Bolus moves through the esophagus to the stomach via involuntary muscle contractions (peristaltic waves).
Stomach
Divided into cardia, fundus, body, antrum, pylorus, lesser curvature, and greater curvature. Mucosa contains glands that secrete gastric juice.
Gastric Juice Components
- Hydrochloric acid: Softens food and eliminates microorganisms.
- Mucin: Protects the gastric mucosa.
- Water: Solvent.
- Enzymes:
- Pepsin: Digests proteins into peptides.
- Gastric Lipase: Digests fats, emulsifying them into fatty acids and glycerol.
- Intrinsic Factor: Aids in vitamin B12 absorption.
Stomach Functions
- Stores food for gradual release to the esophagus.
- Mixes food with gastric juice to form chyme.
- Gradually empties chyme into the small intestine.
Stomach Emptying
- Hormonal Control: Enterogastrone, released by the intestinal mucosa, inhibits stomach muscle contractions, slowing chyme passage. Gastrin stimulates gastric acid and pepsin secretion.
- Nervous Control: Regulated by the autonomic nervous system, particularly in response to duodenal distension and acid presence.
Digestion in the Small Intestine
Chyme mixes with pancreatic juice, bile from the gallbladder, and intestinal juice in the duodenum. Pyloric sphincter controls chyme release. Absorption occurs in the intestinal villi.
Intestinal Absorption
- Monosaccharides and Simple Sugars: Absorbed through the mucosa into capillaries.
- Amino Acids and Vitamins: Absorbed through capillaries in the villi.
- Fatty Acids: Absorbed by lymphatic capillaries, then enter the bloodstream via the portal vein; some fat enters the lymph tissue directly.
Large Intestine
Last portion of the digestive tract, larger in diameter than the small intestine. Divided into cecum (with appendix), colon (ascending, transverse, descending, sigmoid), and rectum. Ends with the anus, which has internal (involuntary) and external (voluntary) sphincters.
Large Intestine Functions
- Relaxes the ileocecal valve and allows cecum filling.
- Moves chyme into the colon.
- No digestive enzymes are produced; mucus lubricates and protects the mucosa.
- Absorbs water from chyme.
- Forms and eliminates solid waste (feces), composed of undigested material, cellular debris, and bacteria.
- Intestinal flora (bacteria) digest cellulose, protein fragments, and produce vitamin K, vitamin B12, thiamine, riboflavin, and gases.
Defecation
Rectal distension triggers a reflex that relaxes the anal sphincters, allowing for bowel evacuation. This process is aided by contractions of abdominal, pelvic, and diaphragmatic muscles. Complications include constipation and diarrhea.