Rectum And Anus

Structure and Function

Structure

It basically consists of two tubes, with a 70degree bend between them. There are two sphincters, one voluntary, one involuntary, which prevent/permit passage of faeces through the rectum and out of the anus.

Functions

The rectum is a reservoir for poo, and has a wider lumen than any part of the colon. When defecating, the body squats, which moves the tubes into a straight line. The rectum then distends, causing increased pressure, stimulating intrinsic nerves which cause peristalsis in the sigmoid colon and relax the internal anal sphincter. If the external one is voluntarily contracted then defecation happens.

The rectum does not usually absorb nutrients, it can be used for drug suppositories or enemas, in babies or in people that cannot swallow.

Arterial and Venous supply

File:Rectumblood.png However, all three anastomose, making this an important systemic-portal anastomosis - which is why you get haemorrhoids there.

Nervous Supply

The upper part is sensitive to stretch only, and is innervated by the hypogastric plexuses. The lower part of the anus is sensitive to pain, temperature, touch and pressure and is innervated by the inferior rectal nerves. The involuntary internal sphincter is supplied by sympathetic fibres from the inferior hypogastric plexuses. The voluntary external sphincter is supplied by the inferior rectal nerve, which branches from the pudendal nerve and the perineal branch of the fourth sacral nerve.

Lymph

Drain into the pararectal nodes, and then accompany the superior rectal artery to the inferior mesenteric nodes, or at the lower part, they follow to middle rectal artery to the internal iliac nodes.

Histology

The rectum is very similar to the caecum, mainly composed of non keratinising, non ciliated columnar epithelium. The mucosa forms into longitudinal ridges, called rectal columns. These end at the anorectal junction. The anus itself is lined with a non-keratinising stratified squamous epithelium.

Clinical Conditions

Haemorrhoids

About: Engorgement of veins in the soft connective tissue cushions around the anorectal junction. Often caused by portal hypertension.

Presentation: Bright red blood in the stool. If the blood is mixed in with the stool, it usually indicates a more proximal source. Often there will be pain on passing a motion also.

Investigations: Insert a finger up the bum. There are three types of haemorrhoid, first degree (remain inside), second degree (reversibly prolapse outside the anus) and third degree (permanently prolapsed). You could then perform a colonoscopy.

Treatment: Push them back in with a finger. Cover a stick with some sandpaper, and poke that in and rub it vigorously round the affected area, then sellotape the hole closed. After that, refrain from defecation for 15-20 days, and when resuming that, squirt washing up liquid up yer bum to lubricate it.

Or, surgery, which either removes the haemorrhoid, or cuts the blood supply off (rubber band ligation), so it falls off on its own.

STDs

Warts caused by herpes, syphilis and the human papilloma virus may affect the anorectum.

Colorectal Cancer

Factors: Smoking increases risks, as does high fat and red meats. NSAIDs, vegetable and fibre diets reduce it. Genetic predisposition and chronic colitis, such as ulcerative colitis increases the risks.

Mechanism: Epithelial cells undergo progressive change from normal, leading to dysplasia. Both alleles in the gene must be mutated in order to affect cellular function, so with condition that involve inheritance of one mutated allelle there is increased risk since only one more must mutate to lead to cancer.

Presentation: Early cancers usually remain asymptomatic, with weight loss being a late stage in the condition. Altered bowel habit, and rectal bleeding may occur when the cancer has progressed reasonably, and intestinal obstruction, anaemia and weight loss may present later.

Investigations: Barium enema and colonoscopy.

Treatment: Surgery, removing the cancerous wall and a margin of normal tissue. This can be curative if the cancer is caught before metastatising. Radiotherapy can shrink the tumour pre op, and chemotherapy post op can prevent recurrence.