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 The digestive system processes food, extracts nutrients, and eliminates the waste. It performs these tasks through these five stages: 

  1. Ingestion: intake of food

  2. Digestion: mechanical (physical) and chemical (break macromolecules into monomers) breakdown of food

  3. Absorption: the uptake of nutrients from food

  4. Compaction: absorbing water and turning the food into feces

  5. Defecation: elimination of feces

The system has two main subdivisions: the gastrointestinal (GI) tract and the accessory organs. The GI tract includes all organs and parts that food goes through. The accessory organs include any organs that are not a part of the GI tract, but plays a role in digestion.

The GI tract is made of four distinct tissue layers: the mucosa, submucosa, muscular externa, and serosa. All layers play a role in protecting the tract from friction or infection, as well as moving food down the tract.

 Four membranes help structure the intestines and keep them in place. The mesentery helps keep the small intestine in place, as well as providing a link to the body through blood vessels. The greater omentum is a flap over the large and small intestines that loosely protects them. The lesser omentum helps keep the GI tract in place by binding to the stomach and liver.

 

 Food movement down the GI Tract

Mouth:

Oral Cavity

Fauces

Oropharynx 

Laryngopharynx

The parts of the mouth function primarily to mechanically digest food by mastication, or chewing. Chemical digestion begins for carbohydrates, as salivary amylase is secreted. Teeth are used to crush food into smaller bits. 

 

Esophagus:

Upper esophageal sphincter

Esophagus

Cardiac sphincter AKA cardioesophageal sphincter

The esophagus is a straight tube. Peristalsis in the esophagus helps move food via wavelike contractions down to the stomach. 

 

Stomach:

Fundus

Body of stomach

Pyloris

Pyloric sphincter

The stomach is a muscular sac that serves as a rest stop for chyme before entering the small intestine. Aside from the typical tissue layers of the GI tract, the stomach also has gastric pits holding chief cells and pareital cells, which secrete pepsinogen and hydrochloric acid, respectively. This aids in chemical digestion. They are introduced into the stomach as a combination called gastric juice.

 

Small Intestine:

Duodenum

Jejunum

Ileum

Ileocecal sphincter

The small intestine is the primary location of nutrient absorption and chemical digestion. Anatomically, the small intestine has circular folds, villi, and microvilli, which all aid in increasing surface area to maximize absorption. Segmentation is also used to churn the chyme. The liver, gallbladder, and pancreas are accessory organs that play key roles in the small intestine. The liver, made up of hepatic lobules, produces bile which is stored in the gallbladder. Bile aids in the chemical digestion of fats, as it emulsifies them. Bile is moved from the gallbladder to the pancreas through the bile duct, and combines with the pancreatic duct to go through the hepatopancreatic sphincter into the duodenum. 

 

Large Intestine:

Cecum

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

Rectum

The large intestine plays a large role in water reabsorption and eliminating feces out the anus. Moving amounts of feces through the colon is achieved through haustral contractions. The large intestine also contains gas, which is excreted as flatus. Defecation is achieved through both voluntary and involuntary contractions. 

 

 Is Your Poop Good to Go?

You can tell a lot from your poop! Looking at the Bristol Stool Chart, typical feces should be type 4: shaped like a sausage, smooth, soft, and easy to pass! If you are lower than a type 4, your feces is too hard. Consider adding more fiber in your diet to soften and ease your stool. If your stool is higher than a type 4, your feces is too soft and watery (think diarrhea). This may mean that your large intestine is having difficulty reabsorbing water. 

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