The digestive system does not digest food in the order in which you ate it. Rather, different parts of the digestive system digest food differently.
Eating food is critical for survival. Without the nutrients that one gets from food, it’s impossible for humans and many other heterotrophs to survive. The digestive system is responsible for breaking down food and making the individual nutrients in food available for the cells of the body to use.
To answer the question in the title of this article, let’s look at the process of breaking down food into its smallest components and seeing how the digestive system actually works.
The first bite – bolus
The process of digestion starts from the very first bite that a person takes. The saliva produced in the mouth, along with the mechanical act of chewing, begins the process of digestion. Saliva has enzymes called salivary amylase that break down the starch in food. There are a few salivary lipases that also break down fat. After chewing the food, this mass of incompletely broken-down food is called a bolus.
The bolus then moves into the stomach via the esophagus.
The churner – stomach
A lot more digestion occurs in the stomach. Several enzymes are secreted in the stomach that further break down the constituents of food, such as pepsinogen, which when converted to pepsin, its active form, breaks down proteins into smaller peptides. The stomach also produces hormones like gastrin, which increases the mobility of the stomach.
Contrary to popular view, the hydrochloric acid secreted by parietal/oxyntic cells in the stomach does not have any significant digestive functions. The HCl serves two purposes. First, to kill any pathogens that might have entered along with the food. Second, to inactivate the salivary enzymes which denature in the low pH (about 2 pH units) conditions of the stomach.
The stomach’s main job is to mix everything up. If you had a donut immediately followed by a burger, the stomach would mix everything up until the burger was indistinguishable from the donut. The muscles of the stomach are strong and produce powerful contractions that churn the food into chyme.
Phases of digestion
The stomach completes its digestion in three phases–the cephalic phase, the gastric phase and the intestinal phase, in that order.
The cephalic phase begins with the thought of food, followed by its taste and feel in the mouth during chewing. These thoughts and tactile sensations of food are ultimately registered in the medulla oblongata. The medulla oblongata, through the vagus nerve, stimulates the stomach cells to secrete its enzymes, hormones and other factors.
The gastric phase begins when food enters the stomach, causing the stomach to distend. The muscle contractions that mix the food become stronger and gastric secretions increase.
The final phase is the intestinal phase. As the stomach vigorously tosses and turns the food around, the pyloric sphincter (located at the junction of the small intestine and the stomach) allows some of the food to pass through it into the small intestine. When the duodenum, the first segment of the small intestine, encounters the acidic chyme and fat droplets in the chyme, it secretes several hormones that begin the emptying of the stomach.
Taking it in – the small intestine
As the chyme enters the small intestine, several things happen. Briefly, the liver secretes bile salts via the gallbladder, while the pancreas secretes bicarbonate and various digestive enzymes into the duodenum. The bicarbonate ions neutralize the acidic chyme and make it more basic. The bile salts serve to emulsify the fat so that lipases can effectively break down fat. Most digestive enzymes are secreted by the pancreas, such as peptidases like trypsin, chymotrypsin and carboxypeptidases (which break down peptides into amino acids), lipases (which break down fat into individual triglyceride molecules), pancreatic amylases, and nucleases (which break down nucleic acids like DNA and RNA).
The small intestine also has its own digestive enzymes. These are called brush border enzymes, as they are present on the cells of the small intestine.
The small intestine is where most nutrients are absorbed. The three parts of the small intestine—the duodenum, the jejunum and the ileum—absorb different nutrients and have different capacities. Most of the nutrient absorption occurs in the duodenum and jejunum of the small intestine, but the ileum is also important for the absorption of certain nutrients.
As the food enters the large intestine, most nutrients have already been absorbed. The large intestine now absorbs the water and some of the remaining minerals that are left. The large intestine converts the chyme into feces by secreting mucus and the gut microflora (the bacteria present in the gut) changes it chemically. After all this, the end product is our fecal matter (poop) that remains to be excreted.
To answer the primary question in the title of the article, yes and no. The digestive system does not work in the order of the food you have eaten. That being said, if you eat a burger and then scarf down a donut an hour later, the digestive system will digest the burger first.
The digestive system does begin the break down of some nutrients before others. As we saw above, carbohydrates begin digestion in the mouth (salivary amylases), but enzymatic protein breakdown begins in the stomach.
So, the next time you have both a burger and a donut, remember that they’ll eventually both become one large pulp of nutrients and exit the body as brown nutrient-barren blobs.