3.5: Fluid and the Bowel

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The fluid in the bowel is generally considered as part of the transcellular fluid compartment. Turnover of fluid in the bowel is large; about 9 to 10 liters of fluid enter the gut each day:

Water Turnover in the Bowel
Water from diet	2000-3000 mls/day
Saliva	1000-2000
Gastric juice	1000-2000
Bile	500-1000
Pancreatic juice	1000-2000
Intestinal secretions	1000-2000

About 98% of this fluid is reabsorbed resulting in a faecal water loss of only 200 mls/day

This reabsorption occurs predominantly in the jejunum and ileum. About 1500 mls/day enter the colon from the ileum. This means that over a litre per day is absorbed in the colon.

The intestinal contents are essentially isotonic by the time the jejunum is reached because water can move into or out of the intestine in response to any osmotic gradient. Excess loss of intestinal content does not directly cause changes in osmolality of body fluids. As absorption of substances occurs, water moves along passively because of the osmotic gradient that is created. The colon is involved in reabsorbing water and electrolytes. Na⁺ is actively reabsorbed and water again is reabsorbed passively down its osmotic gradient. Faecal loss of sodium is only about 5 mmol/day.

It is more accurate to consider the net fluid movements in the bowel as a cycling of fluid rather than a turnover of fluid. This cycling of fluid into the gut and back to the circulation each day has been called the enterosystemic circulation.

Bowel fluid loss may be internal or external. External losses include vomiting, diarrhoea and fistulae losses. Internal losses refer to sequestration of fluid into the bowel as part of the non-functional ECF or 'third space'. The direct result of these fluid losses is an isotonic contraction of the ECF. Electrolyte disturbances are common but vary depending on the condition and renal effects. Renal retention of water occurs with hypovolaemia and tends to cause hyponatraemia.

In small bowel obstruction, about 1500 mls of fluid is rapidly pooled in the bowel. By the time vomiting occurs, about 3000 mls of fluid is in the bowel. If the patient is hypotensive, then about 6000 mls is pooled in the intestines. Significant intravenous fluid resuscitation is usually required before operation in patients with bowel obstruction.

Apart from the gastric juice, all the other secretions into the bowel are alkaline with high [HCO₃^-]

Abnormal fluid losses from the bowel cause acid-base disturbances and these can be quite severe. The typical situation is:

vomiting causes metabolic alkalosis (gastric alkalosis) with associated hyponatraemia, hypochloraemia & hypokalaemia
acute diarrhoea (esp infective) causes a hyperchloraemic normal anion gap metabolic acidosis
chronic diarrhoea (esp non-infective) can cause a metabolic alkalosis.

In summary:

A large amount of fluid is cycled through the bowel each day. (net reabsorption = 98%) Intestinal fluids are isotonic
Water is reabsorbed passively down its osmotic gradient created by the active reabsorption of nutrients and electrolytes. Electrolyte & acid-base disturbances can occur with abnormal bowel fluid losses.