1. How does the food change in consistency and form?
The digestive process of our body starts as the food enters the mouth. The teeth break down the food into smaller pieces. The salivary glands simultaneously release saliva which contains amylase, a starch-digesting enzyme. Tongue movements manipulate the mixture of saliva and food, helping shape it into a ball called a bolus, and pushing it to the back of the mouth and into the pharynx. From the pharynx, the bolus moves into the esophagus. Within the esophagus, food is pushed along by peristalsis. At the end of the esophagus, there's a muscle called a sphincter that controls the passage of food into the stomach. In the stomach, gastric juice mixes with the food, creating a mixture called chyme. After 2-6 hours, peristaltic contractions move the contents from the stomach into the small intestine. At the small intestine, the chyme from the stomach mixes with pancreatic juice from the pancreas and bile from the liver. The nutrients of the chyme get absorbed in the small intestine as it passes through its three segments (duodenum, jejunum, and ileum) via peristalsis. Then, the leftover mixture enters the large intestine, where water, ions, some vitamins, and minerals are absorbed. Lastly, the waste matter called feces leaves the body through the anus.
2. How could the body absorb the nutrients from the foods we eat?
After most of the digestion is completed in the duodenum, the chyme moves through the peristaltic action into the jejunum and ileum. There, nutrient absorption occurs across the lining of the intestine. These sections have an extensive surface area created by its folds, villi, and microvilli, which greatly increases the rate of nutrient absorption.
The absorption process can be either passive or active, depending on the nutrient. For instance, fructose moves down its concentration gradient via facilitated diffusion from the lumen of the small intestine into epithelial cells. After entering these cells, fructose exits through the basal surface and is absorbed into tiny blood vessels at the core of each villus. Other nutrients, including amino acids, small peptides, vitamins, and most glucose molecules, are actively transported against concentration gradients into the epithelial cells of the villus. Active transport allows for much greater absorption of these nutrients compared to passive diffusion alone.
The capillaries and veins that carry nutrient-rich blood away from the villi converge into the hepatic portal vein, a blood vessel that leads directly to the liver. From the liver, blood then travels to the heart and onward to other tissues and organs, thereby regulating the distribution of nutrients throughout the body.
Additionally, the small intestine absorbs approximately 8.3 liters of fluid, with the remaining portion passing into the large intestine. The absorption of water in the gastrointestinal tract primarily occurs through osmosis, moving from the intestines' lumen through absorptive cells and into blood capillaries.
REFERENCES:
Tortora, G. J., & Derrickson, B. (2017). Principles of anatomy & physiology. Fifteenth edition; Wiley Loose-Leaf Print Companion. Hoboken, New Jersey, John Wiley & Sons, Inc.
Urry, L. A., Cain, M. L. 1., Wasserman, S. A., Minorsky, P. V., Reece, J. B., & Campbell, N. A. (2017). Campbell biology. Eleventh edition. New York, NY, Pearson Education, Inc.