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The nuts and bolts of digestion In the beginning
But you're not really done yet — what happens to that bite? Once the food enters your mouth, the many organs of your digestive tract jump into action. Here's a look at how your digestive system works, from top to bottom.
Mouth and salivary glands
Not all of the work is chemical, though. As you savor the sweet apple tang, your teeth work to grind the pie while your tongue mixes it with saliva. This combination transforms it into a bolus — a soft, moist, rounded mass suitable for swallowing.
Esophagus
When the bolus reaches the lower end of your esophagus, pressure from the food signals a muscular valve — the lower esophageal sphincter — to open and let it enter your stomach.
Stomach
Once the concoction is well mixed, waves of muscle contractions propel it through the pyloric valve and into the first section of your small intestine (duodenum). The pyloric valve releases less than an eighth of an ounce of chyme at a time. The rest is held back for more mixing.
Pancreas, liver and gallbladder
Pancreas. The pancreas produces the hormones insulin and glucagon, which help regulate the level of sugar (glucose) in your blood. It also produces digestive enzymes that help break down proteins, carbohydrates and fats. Liver. The liver performs more than 500 functions, including storing nutrients, filtering and processing chemicals in food, and producing bile, a solution that helps digest fats and eliminate waste products. Gallbladder. The gallbladder stores and concentrates bile. As fatty food enters the duodenum, the gallbladder contracts and releases bile into this portion of your small intestine.
Small intestine
What remain at the end of the ileum are mostly water, electrolytes — such as sodium and chloride — and waste products, such as plant fiber and dead cells shed from the lining of your digestive tract.
Large intestine
When the sphincter muscles in your anus relax, the rectal walls contract to increase pressure. Sometimes you have to use your abdominal muscles, which press on the outside of the colon and rectum. These coordinated muscle contractions expel the stool.
Healthy digestionThe types of food you eat, the amount of exercise you get, the pace of your day and your level of stress all can affect the health of your digestive system. Good lifestyle habits can go a long way toward keeping your digestive system on track:
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The sweet aroma of fresh-baked apple pie hangs in the air, teasing your taste buds, almost begging you to take another bite. You spear the still-steaming, last morsel with your fork and place it in your mouth. The last bite was as delicious as the first.
Once that first bite hits your mouth, salivary glands kick into high gear, pumping out digestive juices (saliva) that begin to break down your food chemically. You have smaller salivary glands in the lining of your mouth and three pairs of larger salivary glands — the parotid glands, sublingual glands and the submandibular glands. Together they produce 1 to 3 pints of saliva a day.
As you swallow that last bite of pie, muscles in your mouth and throat propel it to your upper esophagus, the tube that connects your throat to your stomach. In the wall of your esophagus, muscles create synchronized waves — one after another — that propel the pie into your stomach. In this process, called peristalsis, muscles behind the bolus of pie contract, squeezing it forward, while muscles ahead of it relax, allowing it to advance without resistance.
After sliding into your stomach, the pie soon becomes unrecognizable. The stomach's wall, lined with three layers of powerful muscles, begins churning and mixing it into smaller and smaller pieces. Gastric juices, rich in acid and enzymes, pour out of glands that line your stomach. The acid and enzymes help break down food into a thick, creamy fluid called chyme.
Once in your duodenum, the food receives a variety of digestive juices from your pancreas, liver and gallbladder:
When bile and pancreatic digestive juices converge with other juices secreted by the wall of your small intestine, digestion shifts into high gear. What was once apple pie is propelled into the second portion of your small intestine, the jejunum. Here it's further broken down into smaller molecules of nutrients that continue to be absorbed. Then it slides into the final and longest portion of your small intestine — the ileum — where virtually all of the remaining nutrients are absorbed through the lining of the ileum's walls.
As this residue passes through the colon, your body absorbs nearly all of the water, leaving a usually soft but formed substance called stool. Muscles in the wall of your colon separate the waste into small segments that are pushed into your lower colon and rectum. As the rectal walls are stretched, they signal the need for a bowel movement.
Practice good eating habits.
