Gastrointestinal System

Question 1

CONTRIBUTION OF THE GIS TO HOMEOSTASIS

Answer 1

• GIS helps deliver nutrients to circulatory system (humans get water & nutrients from external environment)
• Serves as excretory functions

Question 2

DIGESTION

Answer 2

Breaking down food into molecules that can be absorbed (2 types)

• Mechanical→ apply physical force to food (chomping, mixing)
• Chemical→ Converts complex large molecules into simpler ones (through enzymes and chemicals)

Question 3

DIGESTION STARTS AT ORAL CAVITY

Answer 3

• Mechanical→ teeth + tongue + saliva→ forms bolus

- Chemical→ Starch breakdown by salivary amylase (enzyme) into simpler molecules

Question 4

OESOPHAGUS

Answer 4

• Food swallowed enters oesophagus→ bolus transported to stomach by peristalsis
• Peristalsis→ muscle contraction and relaxation (GI contents move along tract this way)

Question 5

FROM OESOPHAGUS TO STOMACH

Answer 5

• Bolus goes inferiorly along oesophagus (throat→ neck→ thoracic cavity→ abdominal cavity)
• Sphincter (smooth muscle) and diaphragm contact and close of oesophagus at this point

Question 6

THE STOMACH

Answer 6

• Mechanical→ churning action combines bolus with gastric acids (forms chyme)
• Chemical→ Gastric juice contains hydrochloric acid and pepsin (enzyme) → starts protein digestion
• Gastric juice contains intrinsic factor→ essential for B12 absorption

Question 7

LEAVING THE STOMACH

Answer 7

• Approx 3ml chyme released into small intestine with each wave of peristalsis
• Only small amounts of chyme delivered to SI→ food needs to be completely mixed with digestive enzymes

Question 8

UNIQUE STRUCTURAL FEATURES OF THE LINING OF THE SMALL INTESTINE

Answer 8

• Mucosa (made of villi; finger-like projections) and submucosa are in circular folds
• Mucosa cells’ plasma membrane in contact with chyme→ made of microvilli
• Microvilli→ primary surface of nutrient absorption in GI tract; membrane packed with enzymes (helps break down complex nutrients)

Question 9

LIVER AND GALL BLADDER: BILE

Answer 9

• Bile essential for fat digestion
• Produced in liver; travels to duodenum via bile duct (enters duodenum via Sphincter of Oddi)
• Between meals sphincter closed and bile backs up; drains to gall bladder where it’s stored
• Arrival of chyme in duodenum→ sphincter opens and gall bladder contracts (forces bile out)

Question 10

PANCREATIC JUICE SECRETION

Answer 10

• Pancreatic juice reached duodenum via same sphincter of Oddi
• Juice rich in enzymes; degrades all types of nutrient molecules

Question 11

DIGESTION IN SMALL INTESTINE

Answer 11

• Involves bile, pancreatic juice and digestive enzymes attached to surface of mucosal cells of SI
• Net result→ soup of smaller and simpler molecules

Question 12

ABSORPTION

Answer 12

• Smaller and simpler molecules produced by digestion, transported across mucosa cells and into submucosa
• Most nutrients absorbed into submucosal capillaries; blood in them drains to hepatic portal vein
• Exception; fats (these are transported from GIT via lymphatic vessels of tract)

Question 13

ABSORPTION OF WATER

Answer 13

• In addition to what is ingested; 9L of water secreted into GIT
• 80% of osmosis (water reabsorption) occurs in SI
• Nearly all water entering LI is reabsorbed→ only 100-150mL lost in faeces

Question 14

PROCESSING CHYME IN LARGE INTESTINE

Answer 14

• Every day approx 1L of chyme delivered to LI
• Absorption of nutrients in LI restricted to ions and water
• Resident bacteria of LI; important source of vit B and vit K
• Reabsorption of water renders liquid chyme to semisolid feces

Question 15

DEFECATION

Answer 15

• Local reflex triggered by bloating of rectum by feces→ initiates colon contractions
• Relaxation of internal smooth muscle sphincter occurs
• External anal sphincter must be relaxed for feces to be expelled; under voluntary control