Digestive System - Anatomy & Physiology

Function of digestive system

Breakdown of food and absorption of nutrients

Length of digestive system

• 186 m^₂ surface area

• 8m long

  • 0.5m from tongue to duodenum

  • 5.5-6m small intestine

  • 1.5m large intestine

How many teeth?

32 adult teeth

Mechanical digestion in mouth

Mastication (chewing) increases surface area of food

Chemical digestion in mouth

• Salivary amylase breaks starch (polysaccharide) down to disaccharides

  • Saliva secreted by the 3 pairs of salivary glands:

    • Parotid gland

    • Submandibular gland

    • Sublingual gland

Pharynx

Where swallowing occurs

Esophagus

• Muscular tube

• 25cm long

• Peristalsis

2 sphincters in stomach

• Cardiac sphincter: between esophagus and stomach

• Pyloric sphincter: between stomach and small intestine

Layers of the stomach (from superficial to deep)

• Serosa

• Muscular layer: muscle fibers run in 3 directions

• Submucosa

• Mucosa

Mucosa

• Lining of the stomach

• Simple columnar epithelium (produces mucus) with gastric pits containing gastric glands

• Gastric glands secrete gastric juice

  • Chief cells produce pepsinogen

  • Parietal cells produce HCl (hydrochloric acid) and intrinsic factor (aids in B12 absorption in small intestine)

Mechanical digestion in stomach

Muscles contract and mix food with gastric juice to form chyme (semi-fluid paste)

• Gastric juice = pepsinogen, rennin (infants only), HCl, mucus

Chemical digestion in the stomach

• HCl converts pepsinogen to pepsin → breaks proteins into large polypeptides

• Rennin: digests milk protein (makes it look curdled), is present only in infants

• Minimal/no nutrient absorption here (small amount of water, some salts, some fat-soluble drugs like aspirin, and 20% of alcohol)

Structure of small intestine

• 3 sections: duodenum, jejunum, ileum

• Lots of goblet cells to secrete mucus

• Features to increase surface area

  • Villi

    • Villus contains 1 lacteal (modified lymphatic vessel) and capillary bed for absorption of nutrients

  • Microvilli

  • Circular folds

Function of small intestine

• Primary site of digestion and absorption

• Enzymes from pancreas enter via the pancreatic duct

• Additional enzymes embedded in the membranes of the microvilli

  • Peptidases

  • Sucrase

  • Maltase

  • Lactase

  • Lipase

Accessory Organs

• Pancreas: lies under the curve of the stomach

• Liver

• Gallbladder

Pancreas

• Produces pancreatic juice composed of:

  • Bicarbonate (makes the pH = 8)

  • Pancreatic enzymes

• Pancreatic juice is emptied into the duodenum via the pancreatic duct

Pancreatic amylase

• Finished starch (carbohydrate) breakdown

• Monosaccharides move by facilitated diffusion and active transport out of intestinal lumen into capillaries of villi

Trypsin, Chymotrypsin, Carboxypeptidase

• Break polypeptides into smaller peptides

Aminopeptidase and dipeptidase

• Break peptides down to amino acids

  • Amino acids move by active transport out of intestinal lumen into capillaries of villi

Function of liver

Makes bile which enters the small intestine

Other functions:

• Detoxifies drugs and alcohol

• Stores glycogen when glucose is in excess

• Filters old RBCs

• And more!!

Functions of bile

• Emulsifies fat which increases surface area for pancreatic lipase to break triglycerides down into monoglycerides of glycerol and fatty acids

• Increases absorption of fatty acids, cholesterol, and vitamins A, D, E, and K

Gallbladder function

• Stores bile

• Gallstones form when cholesterol in bile precipitates out and forms crystals