Location: In the abdomen, connects the stomach to the large intestine.

Function: At 20 to 23 feet long, the small intestine is the longest part of the digestive system. The long narrow tube, which hangs in sausage-like coils, is where food is digested and nutrients absorbed into the blood. It’s called the small intestine because it has a smaller diameter than the large intestine.

As the stomach releases food in small amounts into the top of the intestines, known as the duodenum, the pancreas and gall bladder pump in digestive enzymes and other fluids that help break down food. The intestines push food along through a wave-like movement of muscles called peristalsis. The lining of the small intestine is covered in tiny microvilli, microscopic, fingerlike protrusions that give the lining of the small intestine a massive surface area for the absorption of nutrients. The microvilli give the inside of the intestine the look and feel of velvet. Indigestible food passes into the large intestine.

Potential problems: When people with celiac disease eat foods containing gluten, their immune system responds by damaging the villi. That can lead to malnutrition. Cancer of the small intestine is rare and much harder to find than in the colon, which is the lower part of the large intestine leading to the rectum. Unexplained abdominal pain, unexplained weight gain or a lump in the abdomen may prompt doctors to look for it. The small intestine can also be affected by ulcers and Crohn’s disease, a disorder that causes inflammation of the digestive tract.

— Markian Hawryluk