-
How does the food change in consistency and form?
When food is ingested into the mouth, the teeth and tongue work together in chewing or mastication. This process – physical breakdown or mechanical digestion – results in the tearing, grinding, and breaking down of food into smaller pieces which prepares them for degradation with the aid of enzymes. Simultaneously, chemical breakdown takes place as the food is mixed with saliva, which process particularly happens with starch. Consequently, the food mass known as bolus will then be pushed to the pharynx by the tongue through the deglutition process. Through the esophagus, the food mass will now reach the stomach and will undergo churning as the stomach walls become activated wherein it is exposed to enzyme-containing gastric juice which helps in processing food, turning it into chyme with an appearance similar to that of soupy liquid. This will afterwards enter the small intestine where it will be further digested and the nutrients associated will be absorbed by the body. As the mass reaches the large intestine, absorption of most of the remaining water and limited vitamins and ions will occur and the indigestible residues are now in the form of feces.
-
How could the body absorb the nutrients from the foods we eat?
In general, the body absorbs the nutrients contained in foods we eat as they reach the stomach where the enzyme-containing gastric juice with secretions of mucous cells, parietal cells, and chief cells enable limited absorption such as some water, ions, short-fatty acids, alcohol, drugs and vitamins. Subsequently, the site where many nutrients are absorbed is in the small intestines. This is due to the presence of brush border enzymes and pancreatic juice that is rich in enzymes. The large intestines also play a role in absorbing limited amounts of nutrients. As they are being absorbed, the nutrients will now enter the bloodstream and be transported to various body parts.
Transport processes also play a role in nutrient absorption wherein most are absorbed via active transport. On the other hand, water is absorbed through osmosis; some lipids by diffusion; and large molecules by pinocytosis.
Moreover, it is also important to note that macromolecules have varying processes of how the body will absorb them. For one, the carbohydrates group will be absorbed in the form of monosaccharides in the small intestine. Two of the most common monosaccharides, glucose and galactose, will be transported into epithelial cells through protein carriers under the secondary active transport process. Meanwhile, fructose is absorbed only by facilitated diffusion.
Secondly, protein digestion and absorption are done through an active transport mechanism in the small intestine which is made easier as they are initially broken down into smaller peptides from the stomach with the help of hydrochloric acid and pepsin. In the small intestine, aminopeptidase and dipeptidase as secreted by the brush border cells further break down the peptides which enable them to be easily transported into the bloodstream.
In the case of lipid digestion, the pancreatic lipase in the small intestine breaks triglycerides into monoglyceride and 2 free fatty acids, which involves short and long-chain fatty acids. The short-chain fatty acids can easily be absorbed by simple diffusion while the latter involves the help of bile salts and lecithin to transform it to micelle so it can be absorbed via simple diffusion. Concurrently, the free fatty acid and monoacylglycerides will be reincorporated into triglycerides and be converted into a water-soluble chylomicron and then released into the lymphatic vessels and empty into the bloodstream where lipoprotein lipase enzyme will break them down to be available for the body cells to use or be stored as fat.
Reference:
Tortora G.J. & Derrickson B. (2017). Chapter 24-25 An Introduction to the Human Body. Principles of Anatomy and Physiology. John Wiley & Sons, Inc. 15th edition. Pp 898-992.