Human Digestion

• Small intestine is the primary site for digestion and absorption.

Accessory Digestive Organs

• Includes teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Oral Cavity and Salivary Glands

• Mechanical digestion starts here.
• Salivary glands produce saliva in response to food.
• Saliva contains salivary amylase, initiating starch digestion.
• A bolus forms when ingested materials mix with saliva.

Pharynx

• During swallowing, the bolus moves to the pharynx.
• Mucus is secreted to aid swallowing.

Esophagus

• Transports the bolus from the pharynx to the stomach.
• Mucus secretions lubricate it.

Stomach

• Bolus mixes with gastric secretions through smooth muscle contractions.
• Epithelial cells in the stomach produce secretions.
• Chyme forms from this mixing process.

Duodenum

• The first segment of the small intestine, part of the upper GI tract, originating at the pyloric sphincter in a C-shape around the pancreas head.
• Continuous with the jejunum at the duodenojejunal flexure.
• Primarily retroperitoneal, with the initial portion being intraperitoneal.
• It receives chyme from the stomach and secretions from the liver, gallbladder, and pancreas.

Small Intestine

• Digestion occurs, and absorption mainly happens here via there segments= duodenum, jejunum, ileum.

Jejunum

• The middle segment of the small intestine.
• It constitutes two-fifths of the small intestine's total length.
• The primary location for chemical digestion and nutrient absorption.
• Intraperitoneal, suspended by the mesentery proper.

Ileum

• The last region of the small intestine, making up three-fifths of its length.
• The distal end terminates at the ileocecal valve, which has a sphincter controlling the entry of materials into the large intestine.
• Intraperitoneal, suspended by the mesentery proper.
• It continues the absorption of digested material.

Large Intestines

• Absorbs water and electrolytes from remaining digested material, compacting watery chyme into feces.
• Stores feces until eliminated through defecation
• Contains resident flora

Anal Canal

• The rectum stores fecal matter before defecation.
• Rectal valves retain fecal matter during gas passage.
• The anal canal comprises the terminal centimeters of the large intestine.

Liver

• The liver is the largest internal accessory digestive organ.
• Located in the right upper quadrant of the abdomen, inferior to the diaphragm.
• Bile production is its main function.

Enzymes and Digestion

• Proteins get digested with pepsin in the stomach, and pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) and brush border enzymes (aminopeptidases, carboxypeptidases, dipeptidases) in the small intestine.
• Carbohydrates are mainly digested in the oral cavity and small intestine, utilizing salivary amylase, pancreatic amylase, and brush border enzymes; co-transport with Na and facilitated diffusion are involved.
• Fats are digested

Lipid Breakdown in Stomach

• Lingual lipase: Component of saliva, activated in the stomach.
• Gastric lipase: Produced by chief cells, digests 30% of triglycerides into diglyceride and fatty acid; does not need bile salts.

Lipid Breakdown in Small Intestine

• Pancreatic lipase: Produced by pancreas, released into the duodenum.
• It digests triglycerides into monoglycerides and two fatty acids.
• Emulsification via bile salts is required to separate large lipid droplets into smaller droplets.

Fat-Soluble Vitamins

• Vitamins A, D, E, and K dissolve in fat (not water).
• Absorption occurs with lipid micelles in the digestive tract.
• Excess is stored in fat cells.

Vitamin Functions

• Vitamin A: Precursor for retinal, the visual pigment.
• Vitamin E: Stabilizes and prevents damage to cell membranes.
• Vitamin K: Needed for synthesizing specific blood-clotting proteins.
• Vitamin D: Enhances calcium absorption from the GI tract.

Glycolysis

• An anaerobic process in the cytoplasm.
• Glucose is oxidized to form two pyruvate molecules.
• Results in -2 ATP and 2 NADH from NAD+.
• Pyruvate converts to lactate if oxygen is sufficient.

Intermediate Stage

• Pyruvate converts to acetyl CoA.
• CO2 is formed and NADH is produced.

Citric Acid Cycle

• Acetyl CoA forms citric acid.
• CO2, ATP, FADH2, and NADH are produced in this cycle.

Electron Transport System

• Involves the transfer of hydrogen and electrons from NADH and FADH2.
• ATP is formed via oxidative phosphorylation.
• 30-32 ATP produced per glucose.

Energy Source Preference

• The body prefers to use glucose for energy, but lipids and proteins can be broken down to make AATP if glucose is depleted.
• The body prioritizes proteins as the last energy source.

Role of Insulin and Glucagon

• Insulin promotes glucose uptake and storage in the absorptive state.
• Glucagon promotes nutrient release into the bloodstream in the post-absorptive state.
• Glucagon also breaks down fats and stimulates gluconeogenesis to maintain blood glucose levels.

Lacteals and Lipid Absorption

• Lacteals are lymphatic vessels in the small intestine, absorbing digested fats without processing in the liver.
• Triglyceride digestion produces glycerol and fatty acids.
• Glycerol can convert to glucose for ATP production; fatty acids can oxidize and enter the Krebs cycle.