Digestive System — Reading Comprehension

Digestion is the process by which the body changes food into substances it can use for energy, growth, and repair. When a person eats, the food must be broken down into smaller molecules that can be absorbed and delivered to cells. The digestive system is a complex network of organs and specialized chemicals that work together to accomplish this task. Understanding how the digestive system functions reveals how our bodies extract nutrients and energy from what we eat—and why this process is essential for life.

Mouth to Stomach: The Start of Digestion

The journey of food begins in the mouth, where both mechanical digestion and chemical digestion occur. Teeth break food into smaller pieces (mechanical), while saliva, containing enzymes, starts to break down carbohydrates (chemical). The chewed food, now called a bolus, moves to the esophagus. Muscular contractions called peristalsis push the food toward the stomach. In the stomach, powerful gastric acid and digestive enzymes further break down proteins, turning the mixture into a semi-liquid called chyme. The stomach’s strong muscles churn the food, increasing the efficiency of chemical breakdown. On average, food spends 2-4 hours in the stomach.

Small Intestine: Absorption of Nutrients

Chyme enters the small intestine, which is over 6 meters long in adults. Here, most nutrient absorption takes place. The small intestine is lined with millions of tiny fingerlike structures called villi, which increase the surface area for absorption. Accessory organs—the liver, pancreas, and gallbladder—release substances that help digestion: bile from the liver breaks down fats, pancreatic enzymes digest proteins, fats, and carbohydrates, and the gallbladder stores and releases bile as needed. Nutrients like glucose, amino acids, and fatty acids pass through villi into the bloodstream. About 90% of nutrient absorption occurs in the small intestine.

Large Intestine and Beyond: Water Absorption and Waste Removal

After leaving the small intestine, indigestible food material moves into the large intestine. Here, water and minerals are absorbed, and the remaining material is turned into solid waste called feces. Helpful bacteria in the large intestine also produce some vitamins. Waste is stored in the rectum and eliminated through the anus. The end result of digestion is that nutrients are delivered to cells, where they are used in cellular respiration to create energy the body needs for all activities. This connection between digestion and cellular respiration is why eating is vital for survival.

Modern science has uncovered the details of digestion through experiments, technological advances like endoscopy, and biochemical analysis. Understanding the digestive system helps doctors treat illnesses, improve nutrition, and develop new food technologies.

Interesting Fact: 
The surface area of the small intestine, thanks to villi and microvilli, is about the size of a tennis court—allowing maximum absorption of nutrients!

What is the main function of the digestive system?

Which organ releases bile to help break down fats?

Where does most nutrient absorption occur in the digestive system?

What is the main purpose of villi in the small intestine?

What happens to food in the stomach?

What is peristalsis?

Based on the passage, why is digestion important for cellular respiration?

If the villi in the small intestine were damaged, what might happen?

True or False: The large intestine is mainly responsible for absorbing most nutrients from food.

True or False: Enzymes in saliva start the chemical breakdown of food in the mouth.