Accessory Digestive Organs and Tract Layers

Accessory Digestive Organs

• Accessory digestive organs assist the primary organs of the digestive system. These organs are teeth, tongue, salivary glands, liver, and pancreas.

Layers of the Digestive Tract

• The digestive tract has four major layers: mucosa, submucosa, muscularis externa, and serosa/adventitia.
• The mucosa, the innermost layer, is responsible for digestion, absorption, and secretion. It consists of:
  • Epithelium: Innermost layer that performs most of the digestive, absorptive, and secretory processes.
  • Lamina propria: A layer of loose connective tissue, containing numerous blood and lymphatic vessels.
• The submucosa, a layer of dense irregular connective tissue, binds the mucosa. It contains:
  • Blood vessels
  • Lymphatic vessels
  • Nerves
  • Exocrine glands that secrete mucus.
• The muscularis externa has two layers of smooth muscle:
  • An inner circular layer that prevents food from traveling backward.
  • A longitudinal outer muscular layer that shortens the tract.
• The serosa, a serous membrane, covers the muscularis externa in most portions of the digestive tract. However, the oral cavity, pharynx, and esophagus lack this layer.
• Instead of a serosa, the oral cavity, pharynx, and esophagus have an adventitia - a dense network of collagen fibers that firmly attaches them to surrounding structures.

Movement of Digestive Materials

• Peristalsis is the wave-like contraction of the muscularis externa that propels food along the digestive tract.
• Segmentation is the churning and fragmenting movement of chyme within the small intestine (and some portions of the large intestine) that mixes contents with intestinal secretions.

Carbohydrate Digestion

• Human diets typically include three main sources of carbohydrates:
  • Sucrose (disaccharide; cane sugar)
  • Lactose (disaccharide; milk sugar)
  • Starches (polysaccharides)
• The digestive process of carbohydrates begins in the mouth, where saliva contains ptyalin (α-amylase) that hydrolyzes starch into maltose and other smaller glucose molecules.
• In the small intestine, pancreatic amylase further digests starches into maltose.
• The small intestine also contains enzymes that break down disaccharides and small glucose polymers into monosaccharides:
  • Lactase
  • Sucrase
  • Maltase
  • α-dextrinase
• The final products of carbohydrate digestion are monosaccharides, primarily glucose, with smaller amounts of galactose and fructose.
• Absorption of carbohydrates occurs as monosaccharides via facilitated diffusion and cotransport mechanisms.

Lipid Digestion

• Triglycerides are the most common lipids in the diet.
• Limited digestion of triglycerides occurs in the stomach by lingual lipase, but most fat digestion occurs in the small intestine.
• Emulsification breaks down fat globules into smaller sizes for enzyme accessibility.
• Bile salts, especially lecithin, are crucial for fat emulsification.
• Pancreatic lipase is the most important enzyme for triglyceride digestion.
• Bile salts also form micelles with monoglycerides and free fatty acids. Micelles transport these products to the intestinal epithelium.
• Primarily absorbed as fatty acids and 2-monoglycerides, which re-form into triglycerides in the intestinal cells.
• Triglycerides, along with other absorbed lipids, combine with proteins, forming chylomicrons which are released into the lymph before entering the circulatory system.

Protein Digestion

• Dietary proteins are long chains of amino acids connected by peptide linkages.
• Protein digestion begins in the stomach, where pepsin (secreted by chief cells) breaks down proteins into smaller peptides.
• In the small intestine, pancreatic enzymes (trypsin, chymotrypsin, carboxypolypeptidase, and elastase) break down polypeptides further.
• Intestinal peptidases then complete the digestion by breaking down the remaining polypeptides into amino acids, dipeptides, and tripeptides.
• Absorption primarily occurs as amino acids through facilitated diffusion and cotransport mechanisms.

This quiz covers the accessory organs of the digestive system, including teeth, tongue, and salivary glands, as well as the four major layers of the digestive tract: mucosa, submucosa, muscularis externa, and serosa/adventitia. Test your knowledge on their functions and characteristics in this engaging quiz.