Expansion
About 9 litres enters the gut daily and only 100 ml is lost in stool
Expansion
Daily inputs to the gut lumen:
| Source | Volume |
|---|---|
| Diet | 2000 ml |
| Saliva | 1500 ml |
| Gastric juice | 2000 ml |
| Bile | 500 ml |
| Pancreatic juice | 1500 ml |
| Intestinal secretions | 1500 ml |
| Total | about 9 litres |
Absorption:
- Small intestine: about 7.5 litres
- Colon: about 1.4 litres, with a reserve capacity up to about 5 litres
- Stool: about 100 ml
The key insight is that most of the fluid is endogenous. A high-output fistula, prolonged vomiting, ileus with sequestration, or a secretory diarrhoea therefore deplete the extracellular compartment far faster than reduced intake alone ever could.
Electrolyte content differs by site, which guides replacement: gastric losses are rich in hydrogen, chloride and potassium (causing hypochloraemic alkalosis), while small bowel, biliary and pancreatic losses are rich in sodium and bicarbonate (causing acidosis).