How does fluid homeostasis go wrong?
There are some conditions and situations when fluid homeostasis can go wrong. The effects can be for osmolality to go too high (hyperosmolality) or too low (hypo-osmolality).
What causes osmolality to go too high?
Not enough antidiuretic hormone (or it has lost its effect)
- Diabetes insipidus:
- This is not to be confused with diabetes mellitus, which is much more common.
- Diabetes insipidus is due either to the brain not being able to make antidiuretic hormone (ADH) anymore (cranial diabetes insipidus) or the kidneys losing their ability to react to it (nephrogenic diabetes insipidus).
- It can cause severe lack of fluid in the body (dehydration).
Loss of body fluid
- Some medicines such as diuretics - for example, thiazide diuretics or loop diuretics - can overwhelm the kidneys' ability to hold back water.
- Severe diarrhoea.
- Severe sickness (vomiting).
- Excessive sweating.
What causes osmolality to go too low?
Too much antidiuretic hormone
- Neurological causes: brain cancer and brain tumours, brain injury, multiple sclerosis, systemic lupus erythematosus, bleeding into the brain.
- Lung disorders: lung cancer, pneumonia, asthma, tuberculosis.
- Other cancers: stomach cancer, pancreatic cancer, leukaemia and lymphoma.
- Some medicines:
- Non-steroidal anti-inflammatory drugs (NSAIDs) - for example, ibuprofen.
- Selective serotonin reuptake inhibitors (SSRIs) - for example, fluoxetine.
- Proton pump inhibitors - for example, omeprazole.
- Angiotensin-converting enzyme (ACE) inhibitors - for example, lisinopril.
- Ecstasy (also called MDMA): increases ADH but also users tend to drink excessively, possibly to counter the other effect of ecstasy, which is to increase body temperature.
- Exercise - see below.
Drinking too much water
Drinking an excessive amount of water can overwhelm the body's fluid homeostasis, leading to water intoxication.
- It has occurred in people who are drinking to excess in an effort to feel full when trying to lose weight.
- Water intoxication has also resulted from water drinking competitions and initiation ceremonies.
- It can occur with exercise, in which case it is called exercise-associated hyponatraemia (EAH):
- This has been a particular problem with marathon runners, who worry so much about not drinking enough that they drink too much.
- Exercise is a trigger for ADH release and this will worsen the effects of drinking too much.
- Some people unwisely use an anti-inflammatory painkiller before any sporting activity, which will make EAH even more likely.
- It has recently been reported after less demanding exercise, including yoga.
The effect of these problems is due to the very low osmolality of sodium (Na+). This is called hyponatraemia and when severe, can lead headache, dizziness, disorientation and confusion. Ultimately it can lead to coma and death.
Further reading and references
McCartney M; Waterlogged? BMJ. 2011 Jul 12343:d4280. doi: 10.1136/bmj.d4280.
Osmolality; Lab Tests Online
Hyponatraemia; NICE CKS, March 2015 (UK access only)
Hew-Butler T, Loi V, Pani A, et al; Exercise-Associated Hyponatremia: 2017 Update. Front Med (Lausanne). 2017 Mar 34:21. doi: 10.3389/fmed.2017.00021. eCollection 2017.
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