Digestive System | Summary

The main organs of the digestive system include the mouth, the esophagus, the stomach, the small intestine, and the large intestine. The mouth softens the food into a bolus, which is moved, via peristalsis, down the esophagus and into the stomach. Three layers of muscle churn the food into a uniform mixture called chyme. Parietal cells form HCl, which activates the enzyme pepsin and begins protein digestion.

The small intestine is very long and has many folds called villi that increase surface area to increase efficiency of digestion and nutrient absorption). The three parts of the small intestine are the duodenum, the jejunum, and the ileum. The duodenum receives digestive enzymes from the pancreas and bile from the gall bladder. Most chemical digestion occurs in the duodenum, and much of the absorption as well. The jejunum and the ileum complete nutrient absorption.
The large intestine reabsorbs water from the forming feces and is home to thousands of species of microbes (that contribute to our health in a number of ways). The appendix is an immune organ that holds a reservoir of bacteria to repopulate the colon after diarrheal diseases (and probably has other functions as well). The large intestine also includes the ascending, transverse, descending, and sigmoid portions of the colon, and ends with the straight rectum.

Accessory organs include the salivary glands (parotid, sublingual, and submandibular), the liver (makes bile, stores glucose as glycogen, along with about 200 other functions!), the gall bladder (stores and concentrates bile), and the pancreas (makes digestive enzymes such as amylase, proteases, and lipases).

The intestinal wall has four layers: mucosa, submucosa, smooth muscle, and peritoneum.
Sugars and amino acids enter the hepatic portal vein and go to the liver for processing. Fats directly enter the lymphatic vessels and are dumped into the vena cava and go straight to the heart, which relies almost exclusively on fat for fuel.

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