Causes of constipation. Artwork of four causes of constipation. Constipation is a condition where it is difficult to evacuate faeces (undigested food) from the bowels. Spastic constipation (upper left) is where the intestinal muscles are too active and have contracted, trapping faeces in the last part of the colon (large intestine). Hard faeces can become impacted (lower left) and block the rectum. Irritable bowel syndrome affects the entire bowel (upper right) and also causes constipation. Bowel, or colon, cancer can block the bowel (lower right, cancer on left wall). Also in the cancerous bowel is an abscess (pink) and inflamed veins (piles or haemorrhoids, blue). Nerves are shown in green.