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Anatomy

Abdomen

Question 1 of 180

A 65 year old lady complains of faecal incontinence. Dysfunction of which of the following structures is most likely a contributing factor to her symptoms:

Answer:

The levator ani muscle consists of two major portions , the pubococcygeus and iliococcygeus, which help support the pelvic viscera and resist increases in intra- abdominal pressure. The puborectalis is the most medial and inferior portion of the pubococcygeus which forms a loop around the anorectal junction which helps maintain an anorectal angle at around 90 degrees. To facilitate defaecation, the puborectalis muscle relaxes, decreasing the angle between the ampulla of the rectum and the upper portion of the anal canal.

The anal canal is the last 4 cm of the adult gastrointestinal tract and begins at the terminal end of the rectal ampulla, where it narrows at the pelvic floor. The anal canal is divided into the upper two-thirds and the lower one-third by the pectinate (dentate) line.

Anal Canal. (Image by Cancer Research UK [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons)

Anatomical Position

The anorectal junction is pulled forward by the action of the puborectalis muscle (part of the levator ani muscle) and so the anal canal moves in a posterior direction as it passes inferiorly through the pelvic floor.

As it passes through the pelvic floor, it is surrounded along its entire length by the internal anal sphincter (involuntary) and external anal sphincter (voluntary), which normally keep it closed.

The anal canal terminates as the anus after passing through the perineum.

Modified by FRCEM Success. Original by or CC BY-SA 3.0 , via Wikimedia Commons

Relations of the Anal Canal. (Image modified by FRCEM Success. Original by Unknown [CC BY-SA 3.0 , via Wikimedia Commons)

Control of Defecation

Defecation is initiated by distension of the rectal ampulla which activates visceral afferent impulses transmitted to the spinal cord by the pelvic splanchnic nerve. Parasympathetic stimulation increases peristalsis and relaxes the internal anal sphincter, facilitating defecation. Sympathetic stimulation causes a decrease in peristalsis, and maintains tone in the internal anal sphincter, inhibiting defecation.

To facilitate defecation:

  • Intra-abdominal pressure is increased by breath-holding and contracting the diaphragm and the abdominal muscles
  • The puborectalis muscle relaxes, decreasing the angle between the ampulla of the rectum and the upper portion of the anal canal
  • The smooth muscle in the wall of the rectum contracts, the internal anal sphincter relaxes, and the external anal sphincter relaxes

Lymphatic Drainage

Above the pectinate line, the anal canal drains to the internal iliac lymph nodes which subsequently drain to the lumbar (para-aortic) nodes.

Below the pectinate line, the anal canal drains to the superficial inguinal nodes. These nodes have efferent vessels that drain primarily into the external iliac nodes and ultimately to the lumbar (para-aortic) nodes.

Innervation

Above the pectinate line, the anal canal receives innervation from the autonomic nervous system via the inferior hypogastric plexus.

Below the pectinate line it is innervated by the somatic fibres of the pudendal nerve (S2 - S4) which also innervates the external anal sphincter and most of the skin over the perineum.

The anococcygeal nerves originate from the coccygeal plexus (S4 - C0) and innervate skin in the anal triangle of the perineum.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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