What Is Colorectal Cancer? Colorectal cancer starts in the colon or the rectum. These cancers can also be called colon cancer or rectal cancer, depending on ...Whatpatientsandcaregiversneedtoknowabout cancer,coronavirus,andCOVID-19.CloseCriticalAlertBannerSearchVerenespañol|Downloadthistopic[PDF]AboutColorectalCancerWhatIsColorectalCancer?KeyStatisticsforColorectalCancerWhat’sNewinColorectalCancerResearch?CancerA-ZColorectalCancerAboutColorectalCancerWhatIsColorectalCancer?Colorectalcancerstartsinthecolonortherectum.Thesecancerscanalsobecalledcoloncancerorrectalcancer,dependingonwheretheystart.Coloncancerandrectalcancerareoftengroupedtogetherbecausetheyhavemanyfeaturesincommon.Cancerstartswhencellsinthebodystarttogrowoutofcontrol.Tolearnmoreabouthowcancersstartandspread,seeWhatIsCancer?ThecolonandrectumTounderstandcolorectalcancer,ithelpstoknowaboutthenormalstructureandfunctionofthecolonandrectum.Thecolonandrectummakeupthelargeintestine(orlargebowel),whichis partofthedigestivesystem,alsocalledthegastrointestinal(GI)system(seeillustrationbelow).Mostofthelargeintestineismadeupofthecolon,amusculartubeabout5feet(1.5meters)long.Thepartsofthecolonarenamedbywhichwaythefoodistravelingthroughthem.  Thefirstsectioniscalledtheascendingcolon.Itstartswithapouchcalledthececum,whereundigestedfoodiscomesinfromthesmallintestine.Itcontinuesupwardontherightsideoftheabdomen(belly).Thesecondsectioniscalledthetransversecolon.Itgoesacrossthebodyfromtherighttotheleftside.Thethirdsectioniscalledthedescendingcolonbecauseitdescends(travelsdown)ontheleftside.Thefourthsectioniscalledthesigmoidcolonbecauseofits“S”shape.Thesigmoidcolonjoinstherectum,whichthenconnectstotheanus. Theascendingandtransversesectionstogetherarecalledtheproximalcolon.Thedescendingandsigmoidcolonarecalledthedistalcolon.Howdothecolonandrectumwork?Thecolonabsorbswaterandsaltfromtheremainingfoodmatterafteritgoesthroughthesmallintestine(smallbowel). Thewastematterthat'sleftaftergoingthroughthecolongoesintotherectum,thefinal6i