Consequences of Fluid Loss from the GI Tract Flashcards
what are the ways in which fluids are lost and gained?
Water loss: Defecation; diarrhoea Vomiting: what is lost? Urination Ventilation Sweating Menstruation What is lost in all of above? Water gain: Drinking Metabolic
What causes diarrhoea?
↑ frequency, with ↑ volume and fluidity of faeces
> 3 unformed stools per 24hrs
Change in bowel movement
Occurs as a result of:
Failure of water absorption
↑ secretion of water
or both of above
Causes of decreased absorption:
↑ number of osmotic particles (osmotic diarrhoea)
↑ rate of flow of intestinal contents (deranged motility diarrhoea)
Abnormal increase in secretions into the GIT (secretory diarrhoea)
What is osmotic diarrhoea?
Osmotic diarrhoea: Decreased absorption of electrolytes and nutrients Disaccharidase deficiency (disaccharide deficiency/malabsorption), drug-induced, malabsorption of galactose); bloating, nausea, watery diarrhoea
What is secretory diarrhoea?
ACh, substance P, 5-HT and neurotensin act via ↑[Ca2+] to increase rate of intestinal secretion
Excessive laxative use, defects in digestion and absorption; infections [e.g. cholera (Vibrio cholerae)]
Survives acidic conditions of the gut
Enterotoxin stimulates adenylate cyclase → ↑ cAMP → Na+, Cl- and water loss
Replace water, electrolytes and glucose (treatment)
Traveller’s diarrhoea: bacteria
Inflammatory bowel disease → chronic exudative diarrhoea
What is deranged motility diarrhoea?
Deranged motility diarrhoea: lack of absorption; some agents may promote secretion as well as motility
GI stasis may promote diarrhoea by stimulating bacterial overgrowth
Parasitic causes of diarrhoea: Entamoeba histolytica and Giardia lamblia
Entamoeba histolytica → asymptomatic or amoebic dysentery
Gradual in onset → systemic symptoms (anorexia, headache)
Metronidazole is the main treatment
Giardia → steatorrhea and abdominal pain; maldigestion & malabsorption of lipids, CHOs, vit A, vit B12, folic acid
Metronidazole or tinidazole
Dysentery: painful, bloody, low volume diarrhoea
What causes bloody diarrhoea?
Chronic disease
Ulcerative colitis
Neoplasm
Major consequences of severe diarrhoea:
Consequences of excessive loss of water, nutrients, Na+, K+ and HCO3- :
↓ blood volume (hypovolaemia)
Metabolic acidosis – due to loss of HCO3- e.g., in diarrhoea
Volume depletion could lead to acidosis/alkalosis
What are the consequences of excessive vomiting?
↑ salt and water loss Severe dehydration Circulatory problems Metabolic alkalosis – due to loss of gastric acid (HCl) Death What is lost in vomit? Food Mucus with Na+, K+, Cl-, HCO3- Gastric acid Upper intestinal contents (incl. bile) Blood
What are the consequences of fluid loss from GI tract?
Hypovolaemia
Haemoconcentration/polycythaemia (dehydration) =↑[RBC]
Dehydration
Ionic imbalances; poor perfusion of tissues
Malnutrition and ↑ mortality
Consequences of hypovolaemia:
↓ venous return
Arterial hypotension
Myocardial dysfunction due to ↑ myocardial oxygen demand, but tissue perfusion is reduced
↑ anaerobic metabolism → acidosis
Acidosis and myocardial dysfunction → multi-organ failure
What is the body’s response to water loss?
Cardiovascular adaptation
Renal adaptation
Behavioural, and this stimulates fluid intake when possible to do so
What is the physiological response to the consequences of hypovolaemia (diarrhoea)?
We are losing a lot of sodium and water due to diarrhoea so therefore the volume of blood will be decreased and venous pressure will be decreased
That has effect on the sympathetic nerves, specifically the ones that innervate the renal system, so the activity of renal sympathetic nerves increases
That has an effect on the kidneys, specifically constriction of renal arterioles goes up, this is all trying to ensure that there is decreased glomerular filtration rate, to try to conserve the body’s water
The decrease in venous pressure will decrease the venous return and the arterial pressure will go down
With this the stroke volume goes down, cardiac output decreases as well and then the arterial blood pressure decreases
The decrease in arterial blood pressure here will have an effect on the renal sympathetic nerves
The decrease in arterial pressure will also have an effect on the renal nerves leading to an increase in activity
These are all reflexes mediated by venous, atrial and arterial baroreceptors
What are the physiological adaptations to water loss by sweating?
Severe sweating leads to hypovolaemia (blood volume goes down)
Blood volume goes down
Ultimately you get an increase in plasma aldosterone which tries to increase the blood volume by decreasing sodium excretion
Loss of hypoosmotic salt solution will increase the plasma osmolarity
Therefore that will increase the plasma vasopressin (ADH) and if you increase that you insert water channels into the collecting duct so less water is excreted in urine
What is the body’s response to the consequences of hypovolaemia (RAAS cascade)?
So as we have mentioned with decreased blood volume there is an increase in activity of renal sympathetic nerves
This has an effect on the renal juxtaglomerular cells which increases renin secretion
One thing we need to be aware of is that when you decrease the GFR the flow of plasma to macula densa is decreased
And that has an effect on sensing the Na concentration is low so therefore that stimulates the renal angiotensin cascade so renin goes up
There is a signal cascade which leads to stimulation of the adrenal cortex causing increased secretion of aldosterone
When plasma aldosterone goes up it has an effect of increasing sodium reabsorption
Figure showing the effects of decreased plasma volume and how it causes increased sodium reabsorption and decreased sodium loss; reabsorption of water is increased →↑blood volume and thus BP
What is the physiologic response to the consequences of hypovolaemia with increased potassium concentration?
Normally we tend to get increased potassium concentrations when sodium drops
This is important for increasing aldosterone secretion which has an effect on sodium reabsorption and therefore water retention
Once aldosterone effect kick in you are going to secrete potassium
What are some other physiological responses to the consequences of hypovolaemia?
Decreased plasma volume decreases all the pressures
Aldosterone goes up increasing tubular permeability to water which increases water reabsorption
When there is thirst, hypovolaemia has set in, there is an increase of plasma osmolarity
This activates osmoreceptors (that’s where the sensation of thirst comes from)
When you are thirst your mouth is dry so that has an effect
Decrease in plasma volume activates baroreceptors which increases angiotensin II leading to thirst
The metering of water intake by the GI tract tells your brain when to stop drinking before the water is even absorbed (so you don’t over drink)
What regulates water loss?
Vasopressin (antidiuretic hormone, ADH)
ADH stimulates thirst mechanism
Increased osmolarity of body fluids → ADH release
How does ADH work?
Inserts water channels (aquaporin-2) in collecting duct of kidney
This increases the permeability of the collecting ducts to H2O and results in concentrated urine production
Intake of plain H2O → a ↓ in osmolarity of blood and interstitial fluid
This decreases ADH secretion and the removal of the water channels
If there are no water channels, this causes decreased permeability of collecting ducts to water and increased water loss
