Digestion breaks down food into smaller components for absorption and utilization. This process provides energy for daily activities, supports growth, and repairs cells and tissues. It is a continuous, multi-stage process involving various organs working in coordination to transform complex food substances into simple nutrients.
The First Bites and Swallows
Digestion begins when food enters the mouth. Teeth mechanically tear and grind food into smaller pieces, increasing its surface area. Saliva moistens the food and contains salivary amylase, initiating chemical breakdown of complex carbohydrates into simpler sugars.
The tongue mixes chewed food with saliva, forming a soft, moist bolus. This bolus is pushed to the back of the throat and swallowed. Swallowing triggers peristalsis, a wave-like muscular contraction in the esophagus, which propels the bolus towards the stomach.
Digestion in the Stomach
Upon reaching the stomach, the bolus enters a muscular, sac-like organ. Its strong muscular walls churn and mix the food, continuing mechanical digestion. This action combines the food with gastric juices secreted by the stomach lining.
Gastric juices contain hydrochloric acid (HCl) and pepsin. HCl provides an acidic environment, denaturing proteins and eliminating bacteria. Pepsin begins the chemical breakdown of proteins into smaller polypeptide chains. The combined mechanical and chemical actions transform the food into chyme, a semi-liquid mixture released into the small intestine.
Nutrient Absorption in the Small Intestine
The small intestine completes most chemical digestion and absorbs nearly all nutrients. This approximately 20-foot tube is highly adapted for absorption, featuring millions of microscopic, finger-like villi and smaller microvilli, creating an immense surface area.
As chyme enters the small intestine, it mixes with digestive enzymes from the pancreas and bile from the liver and gallbladder. Pancreatic enzymes (amylase, lipase, and proteases) break down carbohydrates, fats, and proteins into simple sugars, fatty acids and glycerol, and amino acids. Bile emulsifies fats, making them more accessible for lipase. These broken-down nutrients are then absorbed through the villi into the bloodstream or lymphatic system.
The Large Intestine’s Final Tasks
After nutrient absorption in the small intestine, indigestible food and water move into the large intestine. The primary roles of the large intestine are to absorb water and electrolytes, solidifying the waste material. Most water arriving in the colon daily is reabsorbed.
The large intestine also houses gut bacteria, known as the microbiota. These bacteria break down compounds like fibers and synthesize vitamins K and some B vitamins. This process transforms liquid waste into solid feces, stored in the rectum before elimination.