Gastrointestinal tract
The small intestine begins at the duodenum and is a tubular structure, usually between 6 and 7 m long. Its mucosal area in an adult human is about 30 m. Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:
Large intestine
As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium. Since the average length of the human colon is 160.5 cm and the average inner circumference of the colon is 6.2 cm, the inner surface epithelial area of the human colon has an average area of about 995 sq cm, which includes 9,950,000 (close to 10 million) crypts.