Just by smelling that homemade apple pie or thinking about how delicious that ripe tomato is going to be, you start salivating — and the digestive process begins in preparation for that first bite. Along the way, food is broken down into tiny molecules so that the body can absorb nutrients it needs:. The digestive system is made up of the alimentary canal also called the digestive tract and other organs, such as the liver and pancreas.
The alimentary canal is the long tube of organs — including the esophagus, stomach, and intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet about 9 meters long. Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty meal, our salivary glands in front of the ear, under the tongue, and near the lower jaw begin making saliva spit.
As the teeth tear and chop the food, spit moistens it for easy swallowing. A digestive enzyme in saliva called amylase pronounced: AH-meh-lace starts to break down some of the carbohydrates starches and sugars in the food even before it leaves the mouth. Swallowing, done by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx pronounced: FAIR-inks. The pharynx is a passageway for food and air.
A soft flap of tissue called the epiglottis pronounced: ep-ih-GLAH-tus closes over the windpipe when we swallow to prevent choking. From the throat, food travels down a muscular tube in the chest called the esophagus pronounced: ih-SAH-fuh-gus. Waves of muscle contractions called peristalsis pronounced: per-uh-STALL-sus force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.
At the end of the esophagus, a muscular ring or valve called a sphincter pronounced: SFINK-ter allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus.
The stomach muscles churn and mix the food with digestive juices that have acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. The cephalic phase reflex phase of gastric secretion, which is relatively brief, takes place before food enters the stomach.
The smell, taste, sight, or thought of food triggers this phase. For example, when you bring a piece of sushi to your lips, impulses from receptors in your taste buds or the nose are relayed to your brain, which returns signals that increase gastric secretion to prepare your stomach for digestion. This enhanced secretion is a conditioned reflex, meaning it occurs only if you like or want a particular food. Depression and loss of appetite can suppress the cephalic reflex.
The gastric phase of secretion lasts 3 to 4 hours, and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach.
For example, when your sushi reaches the stomach, it creates distention that activates the stretch receptors. This stimulates parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice. Partially digested proteins, caffeine, and rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn induces parietal cells to increase their production of HCl, which is needed to create an acidic environment for the conversion of pepsinogen to pepsin, and protein digestion.
Additionally, the release of gastrin activates vigorous smooth muscle contractions. However, it should be noted that the stomach does have a natural means of avoiding excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells in the stomach react by suspending HCl secretion and increasing mucous secretions. The intestinal phase of gastric secretion has both excitatory and inhibitory elements.
The duodenum has a major role in regulating the stomach and its emptying. When partially digested food fills the duodenum, intestinal mucosal cells release a hormone called intestinal enteric gastrin, which further excites gastric juice secretion.
This stimulatory activity is brief, however, because when the intestine distends with chyme, the enterogastric reflex inhibits secretion. One of the effects of this reflex is to close the pyloric sphincter, which blocks additional chyme from entering the duodenum. The mucosa of the stomach is exposed to the highly corrosive acidity of gastric juice.
Gastric enzymes that can digest protein can also digest the stomach itself. The stomach is protected from self-digestion by the mucosal barrier. This barrier has several components.
First, the stomach wall is covered by a thick coating of bicarbonate-rich mucus. This mucus forms a physical barrier, and its bicarbonate ions neutralize acid. Finally, stem cells located where gastric glands join the gastric pits quickly replace damaged epithelial mucosal cells, when the epithelial cells are shed. In fact, the surface epithelium of the stomach is completely replaced every 3 to 6 days.
The stomach participates in virtually all the digestive activities with the exception of ingestion and defecation. Although almost all absorption takes place in the small intestine, the stomach does absorb some nonpolar substances, such as alcohol and aspirin. Within a few moments after food after enters your stomach, mixing waves begin to occur at intervals of approximately 20 seconds.
A mixing wave is a unique type of peristalsis that mixes and softens the food with gastric juices to create chyme. When you swallow a small ball of mushed-up food or liquids, a special flap called the epiglottis say: ep-ih-GLOT-iss flops down over the opening of your windpipe to make sure the food enters the esophagus and not the windpipe.
If you've ever drunk something too fast, started to cough, and heard someone say that your drink "went down the wrong way," the person meant that it went down your windpipe by mistake. This happens when the epiglottis doesn't have enough time to flop down, and you cough involuntarily without thinking about it to clear your windpipe.
Once food has entered the esophagus, it doesn't just drop right into your stomach. Instead, muscles in the walls of the esophagus move in a wavy way to slowly squeeze the food through the esophagus.
This takes about 2 or 3 seconds. Your stomach, which is attached to the end of the esophagus, is a stretchy sack shaped like the letter J. It has three important jobs:. The stomach is like a mixer, churning and mashing together all the small balls of food that came down the esophagus into smaller and smaller pieces. It does this with help from the strong muscles in the walls of the stomach and gastric say: GAS-trik juices that also come from the stomach's walls.
In addition to breaking down food, gastric juices also help kill bacteria that might be in the eaten food. If you stretched out an adult's small intestine, it would be about 22 feet long 6. The small intestine breaks down the food mixture even more so your body can absorb all the vitamins, minerals, proteins, carbohydrates , and fats. The grilled chicken on your pizza is full of proteins — and a little fat — and the small intestine can help extract them with a little help from three friends: the pancreas say: PAN-kree-uss , liver, and gallbladder.
Those organs send different juices to the first part of the small intestine. These juices help to digest food and allow the body to absorb nutrients. The pancreas makes juices that help the body digest fats and protein. A juice from the liver called bile helps to absorb fats into the bloodstream. And the gallbladder serves as a warehouse for bile, storing it until the body needs it. Your food may spend as long as 4 hours in the small intestine and will become a very thin, watery mixture.
It's time well spent because, at the end of the journey, the nutrients from your pizza, orange, and milk can pass from the intestine into the blood. Once in the blood, your body is closer to benefiting from the complex carbohydrates in the pizza crust, the vitamin C in your orange, the protein in the chicken, and the calcium in your milk.
Next stop for these nutrients: the liver! And the leftover waste — parts of the food that your body can't use — goes on to the large intestine. The nutrient-rich blood comes directly to the liver for processing. The liver filters out harmful substances or wastes, turning some of the waste into more bile.
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