Few symptoms unsettle patients more than a change in bowel habit or the sight of blood where none should be. Stools, after all, are deeply personal. People know their own rhythms, patterns, and tolerances, and when those patterns change, instinct tells them something is wrong. More often than not, that instinct is correct.
For the clinician, these symptoms demand respect rather than reflex reassurance. Sometimes they signal benign dysfunction. Sometimes they whisper of inflammation. And sometimes, quietly and without drama, they announce cancer.
A brief word on blood supply, because anatomy always matters
The gastrointestinal tract is not fed randomly. Its blood supply reflects its embryological origins and explains many disease patterns.
The coeliac trunk supplies the foregut, nourishing the oesophagus, stomach, and proximal duodenum. The superior mesenteric artery takes over for the midgut, perfusing the small bowel and colon up to the proximal two-thirds of the transverse colon. The inferior mesenteric artery supplies the hindgut, from the distal transverse colon to the upper rectum. Below this, the rectum enjoys a dual supply from systemic and portal circulations, which is why bleeding patterns and collateral flow behave the way they do.
This vascular map explains why some diseases cluster where they do, why ischaemia strikes watershed areas, and why rectal bleeding can be so dramatic yet rarely catastrophic.
Change in bowel habit: what does it really mean?
A change in bowel habit is not simply diarrhoea or constipation. It is any persistent alteration from a patient’s baseline: frequency, consistency, urgency, calibre, or ease of passage. A person who has passed one formed stool daily for thirty years knows when something has shifted.
The causes range from functional disorders to structural disease, and the challenge is telling one from the other.
The common culprits behind altered bowel habit
Sometimes the bowel misbehaves without structural disease. Irritable bowel syndrome is the archetype. It is driven by altered gut motility, visceral hypersensitivity, and brain–gut axis dysfunction. Symptoms fluctuate, pain is relieved by defecation, and there is no weight loss, anaemia, or nocturnal disturbance. The bowel is irritated, not injured.
Coeliac disease, by contrast, is immune-mediated. Gluten exposure triggers villous atrophy in the small intestine, impairing absorption and leading to diarrhoea, bloating, weight loss, and fatigue. Iron deficiency is often the clue that unmasks it.
Then there are the inflammatory disorders. Inflammatory bowel disease, whether ulcerative colitis or Crohn’s disease, alters bowel habit through mucosal inflammation. Diarrhoea becomes frequent and urgent, often bloody, and accompanied by systemic illness. The bowel is inflamed, ulcerated, and unpredictable.
Thyroid disease reminds us that the gut listens to hormones. Hyperthyroidism accelerates transit, producing diarrhoea. Hypothyroidism slows everything down, leading to constipation that no amount of fibre will fix.
Diverticular disease alters bowel habit through disordered colonic motility and structural change. Constipation predominates, punctuated by pain or inflammation, and occasionally bleeding.
And looming quietly behind all of these is colorectal cancer, the most important diagnosis not to miss. Tumours narrow the lumen, disrupt motility, and bleed insidiously. A change in bowel habit in an older adult, particularly toward looser stools or alternating diarrhoea and constipation, is cancer until proven otherwise.
Finally, bowel obstruction, partial or complete, forces change through mechanical failure. Stools narrow, frequency drops, distension builds, and eventually nothing passes at all.
Rectal bleeding: colour tells a story
Blood from the gastrointestinal tract behaves according to gravity, time, and chemistry.
Bright red blood suggests a distal source. Darker blood implies higher origins or slower transit. Mixed blood and stool hint at intraluminal disease; blood coating the stool points toward the anal canal.
Haemorrhoids are the commonest cause of rectal bleeding. They bleed painlessly, often splashing the bowl with bright red blood, and rarely cause systemic illness.
Anal fissures bleed too, but they announce themselves with pain, sharp, tearing pain during defecation that patients never forget.
Diverticular bleeding is dramatic and sudden, often painless, and surprisingly brisk. It can fill the toilet bowl and yet stop as abruptly as it began.
Inflammatory bowel disease bleeds through ulcerated mucosa. Blood is mixed with mucus and stool, accompanied by urgency and systemic symptoms.
And then there is colorectal cancer again. Its bleeding is often occult, chronic, and insidious. Patients may not notice blood at all, only fatigue, anaemia, or weight loss.
Diarrhoea and constipation: patterns, not problems
Diarrhoea arises from excess secretion, inflammation, malabsorption, or rapid transit. Infection, IBD, coeliac disease, endocrine disorders, and medication are frequent causes.
Constipation reflects slowed transit, outlet obstruction, dehydration, or neurological dysfunction. Lifestyle factors dominate early; malignancy and strictures must be excluded later.
Both are symptoms, not diagnoses, and both demand context.
Colorectal cancer: the disease we must not miss
Colorectal cancer develops slowly. Normal mucosa becomes dysplastic, then adenomatous, then malignant. This transformation takes years, which is why screening works and why delay kills.
Patients present with a constellation of clues rather than a single symptom: change in bowel habit, rectal bleeding, iron deficiency anaemia, unexplained weight loss, abdominal mass, or fatigue. Right-sided cancers bleed quietly; left-sided cancers obstruct noisily.
The bowel often adapts until it no longer can.
Investigating rectal bleeding and bowel change
Investigation begins with listening and examining. A digital rectal examination remains indispensable. Blood tests reveal anaemia or inflammation. Stool tests detect occult blood.
But the definitive investigation is endoscopy. Flexible sigmoidoscopy visualises the distal colon; colonoscopy examines it all, allows biopsy, and saves lives. CT scanning complements this when obstruction, advanced disease, or complications are suspected.
Young patients with clear anorectal causes may not need full investigation. Older patients, or anyone with red flag features, always do.
Management: from reassurance to resection
Benign causes are treated conservatively: dietary modification, fibre, topical therapy, treatment of inflammation or hormonal imbalance.
Inflammatory disease requires long-term medical management. Coeliac disease responds to strict dietary exclusion. IBS improves with reassurance and symptom control.
Cancer demands a multidisciplinary approach. Surgery remains the cornerstone, supported by chemotherapy and radiotherapy depending on stage. Early disease is curable. Late disease is manageable, but at a cost.
Surgery is also indicated for uncontrolled bleeding, obstruction, perforation, or refractory disease.
Closing thoughts
A change in bowel habit is the gut adjusting, or failing to. Rectal bleeding is the bowel speaking plainly. Both deserve attention, neither should be dismissed.
Most causes are benign. Some are life-changing. A few are life-ending if ignored.
The clinician’s role is not to panic, but to recognise patterns, respect warning signs, and investigate decisively.
Because when the bowel changes its mind, it is rarely doing so without a reason.
