1. How does the food change in consistency and form?
Digestion is the process of breaking down ingested food into small molecules that can be used by body cells. Mechanical digestion breaks down food through cutting, grinding, or churning. In chemical digestion, digestive enzymes or mucus secretion facilitate catabolic reactions.
Mechanical digestion
- Mastication or chewing in the oral cavity reduces food into a soft, flexible, easily swallowed mass called a bolus.
- Propulsion and retropulsion in the stomach mixes gastric contents with gastric juices, turning them into a soupy liquid called chyme.
- Segmentations are localized, mixing contractions in the small intestine that mix chyme with digestive juices. When segmentation stops, a type of peristalsis called migrating motility complex (MMC) begins.
- The large intestine has haustral churning that involves a series of contractions. A slower rate of peristalsis occurs before a final, strong wave of mass peristalsis at the middle of the transverse colon which drives the contents into the rectum.
Chemical digestion
- In the mouth, salivary amylase initiates the breakdown of starch and lingual lipase is secreted but is inactive (breaks down triglycerides into fatty acids and diglycerides once activated in the stomach).
- G cells in the stomach secrete gastrin which stimulates parietal and chief cells to secrete enzymes. Parietal cells denature proteins via secretion of hydrochloric acid (HCl). Chief cells secrete pepsinogen and gastric lipase. When pepsinogen makes contact with HCl, it is activated into pepsin which breaks down proteins into peptides. Gastric lipase splits triglycerides into fatty acids and monoglycerides.
- The completion of the digestion of carbohydrates, proteins, and lipids, is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine.
- Bacteria in the large intestine initiates the final stage of digestion through fermentation of carbohydrates and release of gases (hydrogen, carbon, and methane). Mucus is secreted and chyme is prepared for elimination.
2. How could the body absorb the nutrients from the foods we eat?
All the chemical and mechanical phases of digestion from the mouth through the intestines are directed toward changing food into forms that can pass through the absorptive epithelial cells lining the mucosa and into the underlying blood and lymphatic vessels.
These forms are monosaccharides (glucose, fructose, and galactose) from carbohydrates; single amino acids, dipeptides, and tripeptides from proteins; and fatty acids, glycerol, and monoglycerides from triglycerides.
Absorption is the movement of products of digestion from the lumen of the GI tract into blood or lymph. Once absorbed, these substances circulate to cells throughout the body. A few substances can be absorbed without undergoing digestion, including vitamins, ions, cholesterol, and water.
Small intestine performs about 90% of all nutrient absorption.
- Glucose and galactose are transported via secondary active transport with Na+, while fructose is transported via facilitated diffusion. These monosaccharides enter the capillaries via facilitated diffusion.
- Single amino acids are transported either through active transport or secondary active transport with Na+. Dipeptides and tripeptides are transported via secondary active transport with H+. These amino acids enter the blood capillary by diffusion.
- Triglycerides are transported through simple diffusion. Small short-chain fatty acids go to blood capillaries by diffusion. On the other hand,large short-chain fatty acids, long-chain fatty acids, and monoglycerides form larger structures called chylomicrons. These exit the cells via exocytosis and enter lacteals which travel through lymphatic vessels, the thoracic duct, and enter the bloodstream at the left subclavian vein.
Large intestine absorbs water, ions, and vitamins.
- Of the 0.5–1.0 liter of water that enters the large intestine, all but about 100–200 mL is normally absorbed via osmosis. The large intestine also absorbs ions, including sodium and chloride, and some vitamins.
References:
- Tortora, G. J., & Derrickson, B. H. (2012). Principles of anatomy and physiology (13th ed., pp. 1001-1010). Wiley.
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of anatomy and physiology (15th ed., pp. 900-941). Wiley.