Digestion, Absorption, and Metabolism of the Macronutrients

Humans digest the food they eat without thinking due to our autonomic nervous system (Crash Course, 2012).  In humans, digestion starts in the mouth (Crash Course, 2012). When we take a bite of food and chew it with our teeth it breaks food down into smaller pieces (Crash Course, 2012).  An enzyme in our saliva breaks down starch and another type of enzyme begins the digestion of fat (Sizer & Whitney, 2014). Food then passes through the esophagus into the stomach (Crash Course, 2012).  The stomach churns food and mixes gastric juices with the food to further breakdown food, turning food into a soupy mixture called chyme (Crash Course, 2012).  The cells within the stomach release gastric juice, which is a strong acid required to stimulate a protein-digesting enzyme and to begin digestion of protein (Sizer & Whitney, 2014). Only a small amount of fat is absorbed in the stomach and it’s the last to leave the stomach (Sizer & Whitney, 2014).  The liver makes bile which aids in the digestion of fats in our diets and the gallbladder stores bile until the body needs it (Sizer & Whitney, 2014).  Food then enters the small intestines where enzymes help to digest carbohydrates, protein, and fat (Sizer & Whitney, 2014). Cells within the small intestines have enzymes on the surface that breaks sugars and starch remains into simple sugars and then they are absorbed (Sizer & Whitney, 2014). Bile breaks down the fat and prepares it for enzyme action (Sizer & Whitney, 2014). The pancreas makes enzymes that break down fat and expels them into the small intestine where fats are split into components and they are then absorbed (Sizer & Whitney, 2014). Enzymes within the small intestines and pancrease break protein into smaller pieces called free amino acids (Sizer & Whitney, 2014).  Only some proteins are absorbed in the small intestines (Sizer & Whitney, 2014). The large intestine takes the undigested proteins, fat, and carbohydrates and expels it in the form of feces through the rectum (Sizer & Whitney, 2014).

The Digestive System: CrashCourse Biology #28