1. How does the food change in consistency and form?
Food undergoes significant changes in consistency and form as it travels through the digestive system. Below is a complex process involving both mechanical and chemical digestion:
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Mouth:
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Mechanical Digestion: The process begins with chewing, where the teeth break food into smaller pieces, increasing its surface area. The tongue shapes the food into a bolus for easier swallowing (Digestive System (for Teens), n.d.).
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Chemical Digestion: Saliva, produced by salivary glands, contains enzymes like amylase that start breaking down carbohydrates. This moistens the food, facilitating its passage down the esophagus (Digestive System (for Teens), n.d.).
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Esophagus:
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Food is moved through the esophagus via peristalsis, a series of coordinated muscle contractions. This stage does not involve significant changes in food consistency or composition (Your Digestive System &Amp;Amp; How It Works, 2024).
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Stomach:
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Mechanical Digestion: The stomach's muscular walls churn food, mixing it with gastric juices to enhance breakdown (How Food Moves Through Your Body, n.d.).
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Chemical Digestion: Gastric acid and enzymes further digest proteins into smaller peptides. By the time food leaves the stomach, it has transformed into a semi-liquid substance called chyme (How Food Moves Through Your Body, n.d.).
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Small Intestine:
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Chemical Digestion: Chyme enters the small intestine, where it is mixed with bile (from the liver) and pancreatic juices that contain enzymes for digesting fats, carbohydrates, and proteins. This stage is crucial for nutrient absorption as food is broken down into its simplest forms: amino acids from proteins, fatty acids from fats, and simple sugars from carbohydrates (Professional, 2024).
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Absorption: The walls of the small intestine are lined with microvilli that absorb these nutrients into the bloodstream for use by the body.
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Large Intestine:
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As undigested food moves into the large intestine, water is absorbed from the waste material, transforming it from a liquid state to solid stool. This process also involves some fermentation of undigested materials by gut bacteria (Your Digestive System &Amp;Amp; How It Works, 2024).
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2. How could the body absorb the nutrients from the foods we eat?
Nutrient absorption starts in the mouth, where the act of chewing breaks food into smaller pieces. This mechanical breakdown increases the surface area of the food, making it easier for enzymes to act upon it. Saliva, which contains enzymes like amylase, begins the chemical digestion of carbohydrates, converting them into simpler sugars (Libretexts, 2022).
Once swallowed, the food travels down the esophagus to the stomach, where it undergoes further processing. The stomach's muscular walls churn and mix the food with gastric juices, which are composed of hydrochloric acid and digestive enzymes (Your Digestive System &Amp;Amp; How It Works, 2024). These substances work together to break down proteins into smaller peptides, transforming the food into a semi-liquid substance known as chyme.
As chyme enters the small intestine, it encounters bile produced by the liver and pancreatic juices secreted by the pancreas. These fluids contain a variety of enzymes that facilitate the digestion of carbohydrates, proteins, and fats. Carbohydrates are broken down into simple sugars such as glucose, proteins are reduced to amino acids, and fats are emulsified into fatty acids and glycerol. The small intestine is uniquely structured to maximize nutrient absorption; its lining is covered with millions of tiny finger-like projections called villi and microvilli (Body, n.d.). These structures significantly increase the surface area available for absorption.
Nutrient absorption occurs through various mechanisms as digested food passes through the epithelial cells lining the villi. Water-soluble nutrients, including glucose and amino acids, enter capillaries within these villi through processes such as diffusion and active transport. In contrast, fat-soluble nutrients like fatty acids and vitamins A, D, E, and K are absorbed into lymphatic vessels known as lacteals.
Once absorbed, these nutrients enter the bloodstream through capillaries in the villi. They are transported to various cells throughout the body where they serve critical roles in energy production, growth, and repair. Before being distributed to other parts of the body, most nutrients first travel to the liver via the hepatic portal vein for processing. The liver plays a vital role in regulating nutrient levels in the blood and converting them into forms that can be used by cells.
References:
Body, V. (n.d.). Absorption and Elimination | Digestive Anatomy. https://www.visiblebody.com/learn/digestive/digestive-absorption-and-elimination
Digestive system (for teens). (n.d.). https://kidshealth.org/en/teens/digestive-system.html
How food moves through your body. (n.d.). Saint Luke’s Health System. https://www.saintlukeskc.org/health-library/how-food-moves-through-your-body
Libretexts. (2022, June 8). 18.3: Digestion and absorption. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18:_Digestive_System/18.3:_Digestion_and_Absorption
Professional, C. C. M. (2024, October 29). Digestive system. Cleveland Clinic. https://my.clevelandclinic.org/health/body/7041-digestive-system
Your Digestive System & How it Works. (2024, October 7). National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works