Digestion, Absorption, and Transport

Digestion, Absorption, and Transport

• Digestion is the breakdown of food into usable components. It requires teeth, tongue strength, sufficient moisture/fluid, and healthy upper GI tract function (including stomach acid and digestive enzymes).
• The small and large intestines also play roles, moving food and absorbing nutrients and water.
• Absorption is the transfer of digested food parts into the body. The small intestine absorbs nutrients, while the large intestine absorbs water and bile acids.
• Digestion involves many muscles: lips, tongue, jaw, epiglottis, and the circular and longitudinal muscles in the GI tract.
• Peristalsis is the wave-like tightening and relaxing of the muscles to move food.
• Segmentation is where the circular muscles contract and squeeze to further break down food.
• The stomach also uses diagonal muscles for churning action.
• Sphincters control the flow of digestive contents.
• Digestive enzymes are released by various organs to aid food breakdown (hydrolysis).

Secretion of Digestive Agents

• Five organs secrete digestive agents: salivary glands, stomach, pancreas, liver (and gallbladder), and small intestine.
• Salivary glands produce saliva (pH ~7), containing amylase and other enzymes to liquefy food, protect teeth/mouth/esophagus.
• Stomach releases gastric juice (pH < 2), containing acid to break down protein, kill bacteria and contains enzymes, mucus to protect the stomach lining.
• Pancreas produces pancreatic juice (pH 8), containing enzymes for all macronutrients, and bicarbonate to neutralize stomach acid.
• Liver (gallbladder) produces bile, vital for digesting dietary fat.
• Small intestine crypt cells produce enzymes for macronutrient digestion.

Absorption

• Absorption occurs in the small intestine's villi.
• Absorbed nutrients use three primary methods: simple diffusion, facilitated diffusion, and active transport.
• Crypt cells produce enzymes, while goblet cells produce mucus.

Transport

• Water-soluble nutrients and small fats enter the vascular (blood) system and travel to the liver.
• Larger fats and fat-soluble vitamins form chylomicrons, entering the lymphatic system and then the bloodstream near the heart.

Circulatory System

• The vascular system is a closed system of vessels, with the heart acting as a pump to circulate blood.
• Blood delivers oxygen and nutrients to tissues, while removing carbon dioxide and waste.

Lymphatic System

• The lymphatic system is a one-way route, circulating by body movement.
• It collects fluid from between cells into tiny vessels, and lymph collects in the thoracic duct, entering the bloodstream near the left subclavian vein.

Gut Microbiome

• The gut microbiome is important, as indicated in various studies including Marco et al (2021).

Gastrointestinal Hormones and Nerve Pathways

• Hormones and nerves regulate digestion.
• More than 50 hormones involved, reacting to changes in stomach pH, food presence, and chyme content.
• Stomach pH outside the ideal range (1.5-1.7) triggers changes in hormonal control mechanisms, and nervous responses.
• Pyloric sphincter stimulation is controlled by acidic chyme.
• Pancreas secretions are regulated by substances like secretin, reacting to acidic chyme and indicating the type of enzymes to release based on macronutrient composition.
• Gallbladder activation is regulated by the presence of fat and the release of cholecystokinin (CCK).
• Intestinal motility is influenced by CCK (delaying digestion).