The duodenum is the upper portion of the small intestines in mammals. The duodenum is the section of the intestines that connects to the stomach and pancreas. It's the location where digestion takes place.
The duodenum's function is to accept chyme from the stomach and complete the digestion of food. Chyme is the mixture of stomach acid and swallowed food. Bile released from the gallbladder is also delivered to the duodenum for fat digestion.
The inner lining of the duodenum is made of crypts. These crypts increase the surface area of the intestinal membrane, making digestion more efficient.
Smooth muscle is the type of tissue that aligns the intestinal tract. The smooth muscle is what pushes the food from the esophagus down to the rectum. The duodenum also contains smooth muscle to push waste products down to the large intestine.
Protection from microbes is accomplished using the paneth cells. Paneth cells secrete bactericidal enzymes that destroy microbes. This helps inhibit the overgrowth of intestinal flora, which are the bacterial cells that naturally inhabit the gut.
Once digestion and breakdown of nutrients such as fats, carbohydrates, and proteins have occurred, the duodenum absorbs the tiny molecules and allows it to enter the bloodstream.