Overview of Digestion and Metabolism Processes

Secretion

Release of substances (enzymes, mucus, acid, bicarbonate, bile) to aid digestion.

Saliva

Contains amylase and mucus.

Gastric juice

Contains HCl, pepsinogen, and mucus.

Pancreatic juice

Contains enzymes and bicarbonate.

Bile

Emulsifies fats.

Digestion

Mechanical: Chewing (mouth), churning (stomach), segmentation (small intestine). Chemical: Enzymatic breakdown of carbs, proteins, fats into absorbable units.

Absorption

Nutrient transport into blood or lymph.

Motility

Muscular contractions to move and mix food.

Peristalsis

Waves of contraction pushing food forward.

Segmentation

Back-and-forth mixing (especially in small intestine).

GI Tract Anatomy

Mouth → Esophagus → Stomach → Small Intestine (Duodenum → Jejunum → Ileum) → Large Intestine → Rectum → Anus.

Upper GI

Mouth, pharynx, esophagus, stomach.

Stomach Regions

Cardia, Fundus, Body, Pylorus (with pyloric sphincter).

Mucosa

Contains gastric glands.

Submucosa

Connective tissue, blood vessels.

Muscularis externa

Oblique, circular, longitudinal muscle.

Serosa

Outer membrane.

Parietal Cells

Secrete HCl (acid) and intrinsic factor necessary for vitamin B12 absorption.

Chief Cells

Secrete pepsinogen and gastric lipase.

G Cells

Secrete gastrin to stimulate acid secretion and gastric motility.

Mucous Cells

Secrete mucus and bicarbonate to protect stomach lining.

Salivary Amylase

Breaks down starch into maltose.

Lingual Lipase

Begins fat digestion.

Pepsin

Digests proteins into peptides.

Pancreatic Amylase

Continues starch digestion.

Pancreatic Lipase

Digests fats into fatty acids and monoglycerides.

Trypsin

Digests proteins; activates other enzymes.

Chymotrypsin

Digests proteins.

Carboxypeptidase

Cleaves amino acids off peptides.

Brush Border Enzymes

Final digestion of carbs and proteins.

Lactase

Converts lactose into glucose and galactose.

Sucrase

Converts sucrose into glucose and fructose.

Maltase

Converts maltose into 2 glucose.

Aminopeptidase

Removes single amino acids from peptides.

Dipeptidase

Splits dipeptides into individual amino acids.

Enteropeptidase

Activates trypsinogen into trypsin.

Bile

Produced by liver, stored in gallbladder.

Bile

Produced by liver, stored in gallbladder. Released into duodenum in response to CCK. Function: Emulsifies fats (breaks large fat globules into tiny droplets) → increases surface area for lipase to act.

Fatty acids and monoglycerides

Absorbed into enterocytes and packaged into chylomicrons. Transported via lymphatic system to circulation.

Gastrin

Triggered by stomach stretch and protein presence. Increases acid secretion and increases gastric motility.

Secretin

Triggered by acidic chyme in duodenum. Stimulates pancreas to secrete bicarbonate.

CCK (Cholecystokinin)

Triggered by fats and proteins in duodenum. Stimulates pancreatic enzyme release and gallbladder contraction.

GIP (Gastric Inhibitory Peptide)

Triggered by presence of nutrients. Stimulates insulin release and inhibits gastric emptying.

Esophagus Motility

Peristalsis moves bolus to stomach.

Stomach Motility

Churning, propulsion, and retropulsion for mechanical digestion.

Small Intestine Motility

Segmentation and peristalsis for mixing and nutrient absorption.

Large Intestine Motility

Haustral churning and mass movement for water absorption and feces propulsion.

Haustral Churning

Local contraction in individual haustra to mix contents and enhance water/electrolyte absorption.

Mass Movements

Large peristaltic waves moving feces toward rectum, triggered mainly by gastrocolic reflex.

Acinar cells

Produce digestive enzymes (exocrine) in the pancreas.

Duct cells

Secrete bicarbonate to neutralize stomach acid in the pancreas.

Islets of Langerhans

Produce hormones (endocrine) in the pancreas.

Pancreatic amylase

Breaks down carbohydrates.

Pancreatic lipase

Breaks down fats.

Trypsinogen

Activated to trypsin by enteropeptidase, activates protein digestion enzymes.

Chymotrypsinogen

Activated to chymotrypsin for protein digestion.

Procarboxypeptidase

Activated to carboxypeptidase for protein digestion.

Insulin

Produced by beta cells, lowers blood glucose.

Glucagon

Produced by alpha cells, raises blood glucose.

Somatostatin

Produced by delta cells, inhibits both insulin and glucagon.

Liver Functions

Detoxification (e.g., drugs, alcohol), metabolism (carbs, proteins, fats), storage (glycogen, vitamins, iron), bile production (for fat digestion), plasma protein production (albumin, clotting factors).

Bile salts

Emulsify fats.

Cholesterol

A component of bile.

Bilirubin

Waste from RBC breakdown.

Cephalic Phase

Initiated by sight, smell, taste, or thought of food; stimulates gastric secretions via vagus nerve (parasympathetic).

Gastric Phase

Triggered by stomach stretch (mechanoreceptors) and food proteins (chemoreceptors); increases gastric secretions (acid, enzymes) and motility.

Intestinal Phase

Triggered by chyme entering duodenum; inhibits gastric emptying and stimulates secretion of secretin and CCK.

Carbohydrate Digestion

Begins with salivary amylase (mouth), continues with pancreatic amylase (small intestine), and ends with brush border enzymes (lactase, sucrase, maltase).

Carbohydrate Absorption

Glucose and galactose → secondary active transport with Na+ (SGLT1); Fructose → facilitated diffusion (GLUT5); all enter blood via GLUT2 transporters.

Protein Digestion

Begins in stomach with pepsin, continued by pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase), completed by brush border peptidases.

Protein Absorption

Amino acids absorbed by Na+-linked secondary active transport.

Fat Digestion

Lingual lipase (minor, in mouth); pancreatic lipase (major digestion in small intestine, needs bile emulsification first).

Fat Absorption

Bile salts emulsify fat droplets; pancreatic lipase breaks triglycerides → monoglycerides + fatty acids; absorbed into enterocytes → reassembled into triglycerides → packed into chylomicrons → enter lymphatic lacteals.

Water-soluble vs Fat-soluble Vitamin Absorption

Water-soluble vitamins (B, C): absorbed directly into blood; Fat-soluble vitamins (A, D, E, K): absorbed along with dietary fat into lymph.

Daily Water Handling

Total water entering GI tract ≈ 7-8 liters/day; most absorbed in small intestine; remaining absorbed in large intestine.

Cellular Respiration 4 Reactions

Glycolysis (cytoplasm, anaerobic), Pyruvate oxidation (mitochondria), Citric Acid Cycle (mitochondria), Oxidative phosphorylation (ETC in mitochondria).

Fates of Glucose

ATP production (immediate energy), Glycogenesis (storage as glycogen), Lipogenesis (storage as fat if excess).

Glucose Uptake

Insulin-dependent (muscle, adipose); Insulin-independent (brain, liver, RBCs).

Glycogenesis

Glucose → glycogen (storage, liver + muscle).

Glycogenolysis

Glycogen → glucose (fasted state, liver primarily).

Gluconeogenesis

Making new glucose from non-carbs (amino acids, lactate).

Fates of Lipids

Energy production via beta-oxidation, storage as triglycerides, used to build membranes.

Lipolysis

Triglycerides → glycerol + fatty acids (mobilize energy during fasting).

Lipogenesis

Excess carbs/proteins → converted into fat.

Types of Proteins

Structural (e.g., collagen), Enzymatic (e.g., digestive enzymes), Transport (e.g., hemoglobin).

Absorptive (Fed) vs Post-absorptive (Fasted) State

Fed: insulin high → storage mode (glycogenesis, lipogenesis); Fasted: glucagon high → breakdown mode (glycogenolysis, lipolysis, gluconeogenesis).