1. How does the food change in consistency and form?
- Through the power of digestion, a process of mechanically and chemically breaking down foods into tiny molecules, the nutrients are able to be absorbed and moved into the bloodstream. Mechanical digestion, like chewing and churning, refers to the physical breakdown of food without changing the food's composition. On the other hand, when enzymes and chemicals (juices) chemically breakdown the food into its building blocks (monomers), this refers to the chemical digestion.
- We can see the changes in the consistency and form of the foods by observing the activities occurring in the gastrointestinal tract, particularly in the alimentary canal. When food is placed into the mouth, it's physically broken down by mastication and, when mixed with saliva, will make a ball-like mixture called a bolus. This bolus will be propelled by deglutition (swallowing) and peristalsis in the pharynx and esophagus. When the bolus is moved to the stomach, it will be mixed with gastric juices and reduced in size, which will form the chyme. It will be propelled and undergo segmentation for it to be mixed with enzymes. The chyme is further broken down and absorbed in the small intestines. When nutrients have been absorbed, the undigested materials are propelled into the large intestine. These food residues, mixed with mucus, bacteria, and water, are called feces. Feces are stored in the rectum until they leave the GI tract through the anus by means of defecation.
2. How could the body absorb the nutrients from the foods we eat?
- Absorption is the movement of nutrients into the bloodstream as the end products of digestion are absorbed via the blood or lymph vessels. Aside from being the body's major digestive organ, the small intestine is also the site of nutrient absorption.This is made possible by structural modifications like increasing the surface area (like the villi and circular folds, "plicae circulares") of the small intestines, thereby increasing metabolic rate. The finger-like projections formed by the mucosa are called villi. This houses a capillary bed (transporting nutrients into the bloodstream) and lacteals (absorbing monoglycerides and acids). The microvilli, tiny projections of the plasma membrane, secrete the brush border enzymes needed for the final digestion. Generally, water is absorbed along the length of the small intestines. Each major food group uses different enzymes to be digested and then absorbed. While water and food nutrients are absorbed in the small intestines, alcohol and aspirin are virtually the only items that are absorbed in the stomach. In addition, the large intestine is where water and electrolytes (salts) are absorbed from materials that weren't digested as food.
- The major food groups have different mechanisms for them to be digested and, later on, absorbed by the body. Here are the various processes:
CARBOHYDRATES
- Initial Digestion: At the mouth, starch and/or disaccharides are digested initially by the salivary amylase.
- Intermediate Digestion: In the small intestine, the pancreas releases pancreatic amylase, which converts them to oligosaccharides and dissacharides.
- Final Digestion: Still in the small intestine, lactose, maltose, and sucrose (forms of disaccharides) are broken down to simple sugars (monosaccharides): galactose, glucose, and fructose due to the release of brush border enzymes.
- Absorption: The monosaccharides will enter the blood capillaries in the villi and then be transported to the liver via the hepatic portal vein (HPV).
PROTEINS
- Initial Digestion: Proteins are digested initially in the stomach due to the presence of pepsin (pepsinogen + hydrochloric acid, HCl), which turns them into large polypeptides.
- Intermediate Digestion: These will be reduced to small polypeptides due to the pancreatic enzymes (ex. trypsin) released in the small intestines.
- Final Digestion: The brush-border enzymes (ex. aminopeptidase) released in the small intestines will convert them to monomers: amino acids.
- Absorption: Amino acids, similar to the monosaccharides, will enter the capillary blood found in villi and be transported to the liver via the hepatic portal vein.
LIPIDS
- Initial Digestion: Some references mention that digestion begins when lipids are exposed to lingual lipase. While some say that the detergent action of bile salts from the liver will emulsify the unemulsified fats, which take place in the small intestine.
- Intermediate and Final Digestion: The emulsification will be followed by exposure to pancreatic lipase in the small intestines, which will convert them to monoglycerides and fatty acids, or glycerol and fatty acids.
- Absorption: The former ones (fatty acids and monoglycerides) will enter the lacteal of the villi. They will be transported to the systemic circulation via the lymph in the thoracic duct. The latter ones (glycerol and fatty acids) will have the same mechanisms as carbohydrate and protein monomer absorption (blood capillaries > HPV > liver).
REFERENCES
- Marieb, E. N., &; Keller, S. M. (2021). Chapter 14:The Digestive System and Body Metabolism. In Essentials of human anatomy & physiology (Thirteenth, pp. 482–507). essay, Pearson Education Limited.
- Tortora, G. J., &; Derrickson, B. (2017). Chapter 24: The Digestive System. In Principles of anatomy and physiology (Fifteenth, pp. 898–945). essay, Wiley Custom Learning Solutions.