Consequences of fluid loss from the GI tract Flashcards
(21 cards)
Explain total body water balance: average water gain and loss in adults
Intake:
-liquids- 1200ml
-food- 1000ml
-metabolically produced- 350ml
-total- 2550ml
Output:
-insensible water loss
-sweat
-faeces
-urine
Sites of water loss:
-skin
-respiratory passageways
-GIT
-Urinary tract
-Trauma
-Menstrual flow
Explain the total balance for sodium and chloride
*Excretion is usually small but large in diarrhoea, sweating, vomiting, haemorrhage ——> loss of water and salts
Daily NaCl intake and loss:
Intake= food: 10.5g
Output (grams):
-sweat
-faeces
-urine
Kidneys can alter excretion to match gain with loss
Explain water gain and water loss
Water loss:
-defecation; diarrhoea
-vomiting
-urination
-ventilation
-sweating
-menstruation
Water gain:
-drinking
-metabolic
Explain characteristics of diarrhoea
-increase of frequency with increase in volume and fluidity of faeces
->3 unformed stools per 24hrs
-change in bowel movement
Occurs as a result of:
-failure of water absorption
-increase secretion of water
Causes of decreased absorption:
-increased number of osmotic particles
-increased rate of flow of intestinal contents
-abnormal increase in secretions of GIT
Explain different types of diarrhoea and inflammatory bowel disease
1) Osmotic diarrhoea:
-decreased absorption of electrolytes and nutrients
-disaccharide deficiency- drug induced, malabsorption;
Bloating, nausea, watery diarrhoea
2) 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
Explain deranged motility diarrhoea and parasitic causes of 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 - Giardia → steatorrhoea and abdominal pain; maldigestion & malabsorption of lipids, CHOs, vit A, vit B12, folic acid
– Metronidazole or tinidazole
Dysentery: painful, bloody, low volume diarrhoea
Explain causes of bloody diarrhoea and major consequences of severe 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
What are 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
Explain 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; 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
Explain the physiologic response to the consequences of diarrhoea
1) Losing Water and Salt: When you have diarrhea, you lose both water and salt.
2) Less Fluid in Your Blood: This loss means you have less fluid in your blood. The diagram calls this a decrease in “plasma volume”.
3) Lower Blood Pressure: Less blood volume leads to lower “venous pressure” and eventually lower “arterial blood pressure” (the pressure in your main arteries).
4) Your Body Tries to Help: Your body senses this lower blood pressure through sensors called “baroreceptors”. This triggers a few things to get your blood pressure back up:
5) Faster Heart: Although not explicitly shown as a direct arrow from baroreceptors, lower venous return leads to lower atrial pressure and ventricular end-diastolic volume, which in turn reduces stroke volume and cardiac output, potentially leading to a faster heart rate as a compensatory mechanism (part of the “Cardiovascular” response on the right).
Kidneys Get Involved:
The signals also tell your kidneys to become more active (“increased activity of renal sympathetic nerves”).
6) Kidneys Save Water and Salt: The activated kidneys then do two main things:
7) They cause the blood vessels in the kidneys to narrow (“constriction of renal arterioles”), which reduces how much fluid is filtered out of your blood (“net glomerular filtration pressure” goes down, leading to lower “GFR” - Glomerular Filtration Rate).
8) They hold onto more salt and water instead of letting you pee them out (“decreased Na+ and water secretion”). This helps to increase your blood volume again.
What happens if there is water loss by sweating
1) Losing Water and Some Salt: When you sweat heavily, you lose water and some salt from your body. The diagram calls this “loss of hypo-osmotic salt solution”.
2) Less Fluid in Your Blood: This loss of fluid means you have less liquid in your blood (“↓ plasma volume”).
3) Blood Becomes More Concentrated: Because you’re losing more water than salt, the concentration of stuff in your blood goes up (“↑ plasma osmolarity”, which also means “↓ H2O concentration” - less water compared to other things in your blood).
4) Your Body Tries to Save Water: Your body senses this higher concentration and lower blood volume. It releases a hormone called “plasma vasopressin” (also known as antidiuretic hormone or ADH).
5) Kidneys Hold onto Water: Vasopressin tells your kidneys to hold onto more water and not let you pee it out as much (“↓ H2O excretion”). This helps to bring the concentration of your blood back to normal and increase your blood volume.
6) Kidneys Also Save Salt (a bit indirectly): The decrease in blood volume also triggers another process (not directly from the increased osmolarity shown here, but linked to the overall fluid loss) that increases “plasma aldosterone”. Aldosterone tells your kidneys to hold onto more sodium (“↓ sodium excretion”), which also helps to retain water.
7) Less Fluid Filtered: The lower blood volume also leads to less fluid being filtered by your kidneys initially (“↓ GFR” - Glomerular Filtration Rate). This also contributes to less water and sodium being lost in your urine.
Explain the RAAS cascade as a response of the body to the consequences of hypovolaemia
1) Lower Blood Pressure: Less blood means lower “arterial pressure”.
2) Kidneys Sense This: Your kidneys have special cells (“renal juxtaglomerular cells”) that sense this lower pressure. They also sense less flow reaching a certain part of the kidney (“↓ GFR → ↓ flow to macula densa”), which leads to less salt (“↓ NaCl concentration”) there.
3) Kidneys Release Renin: Because of the low pressure and low salt, the kidneys release a substance called “↑ plasma renin”.
4) Renin Starts a Chain Reaction: Renin turns another substance in your blood into “↑ plasma angiotensin I”. Then, angiotensin I gets turned into “↑ plasma angiotensin II”.
Angiotensin II Does Several Things:
-It makes your blood vessels squeeze tighter, which increases blood pressure.
It tells your adrenal glands to release “↑ plasma aldosterone”.
Aldosterone Saves Salt: Aldosterone goes to your kidneys and tells them to hold onto more sodium (“↑ Na+ reabsorption”) and get rid of less sodium in your urine (“↓ Na+ excretion”).
-Saving Salt Saves Water: When you hold onto more sodium, water follows it back into your body, which helps to increase your blood volume and therefore your blood pressure.
What happens if you have hypovolaemia with increased potassium concentration
-Low Blood Volume AND High Potassium: You have less fluid in your blood (“↓ plasma volume”) AND too much potassium (“↑ plasma potassium”).
-Both Tell the Adrenal Glands: Both the low blood volume (which leads to increased “plasma angiotensin II” as we saw before) and the high potassium level signal your adrenal glands.
-Adrenal Glands Release Aldosterone: The adrenal glands then release more “↑ aldosterone secretion”, leading to “↑ plasma aldosterone”.
-Aldosterone Balances Sodium and Potassium: Aldosterone goes to your kidneys and does two main things in the “cortical collecting ducts”:
-It tells the kidneys to hold onto more sodium (“↑ sodium reabsorption”), so you pee out less sodium (“↓ sodium excretion”). This helps to increase your blood volume (because water follows sodium).
-Crucially in this case, it also tells the kidneys to get rid of more potassium (“↑ potassium secretion”), so you pee out more potassium (“↑ potassium excretion”). This helps to lower the high potassium levels in your blood.
Explain physiological response to the consequences of hypovolaemia
1) Lower Blood Pressure is Sensed: This lower blood volume leads to lower “venous, atrial and arterial pressures”.
2) Signals to the Brain: These lower pressures are sensed by special sensors called “cardiovascular baroreceptors”. They send signals to a part of your brain called the “posterior pituitary”.
3) Posterior Pituitary Releases Vasopressin: The posterior pituitary then releases more “↑ vasopressin secretion”, which leads to more “↑ plasma vasopressin” (vasopressin in your blood).
4) Vasopressin Helps Kidneys Save Water: Vasopressin travels to your kidneys and acts on the “collecting ducts”. It makes the collecting ducts more permeable to water (“↑ tubular permeability to H2O”).
5) More Water is Reabsorbed: This increased permeability means more water can move back from the kidneys into your blood (“↑ H2O reabsorption”).
6) Less Water in Urine: As a result, you pee out less water (“↓ H2O excretion in the urine”). This helps to increase your blood volume back to normal.
What makes you feel thirsty when you have low blood volume (hypovolaemia)
1) Low Blood Volume and Concentrated Blood: When you lose fluid, your “↓ plasma volume” goes down. This also makes your blood more concentrated (“↑ plasma osmolarity” - meaning there’s less water compared to other stuff in your blood).
Sensors Detect This:
Baroreceptors sense the lower blood volume. This leads to an increase in “↑ angiotensin II” (which we talked about before).
Osmoreceptors sense the increased concentration of your blood.
Signals to Your Brain: Both the angiotensin II and the signals from the osmoreceptors tell your brain that something is not right. You might also experience a dry mouth and throat.
You Feel Thirsty: These signals all contribute to the feeling of THIRST.
You Drink Water: When you drink water, your gastrointestinal (GI) tract helps to measure how much you’re drinking (“Metering of water intake by GI tract”). This helps you avoid drinking too much too quickly.
What regulates water loss
Vasopressin (antidiuretic hormone, ADH)
* ADH stimulates thirst mechanism
* Increased osmolarity of body fluids → ADH release
How does ADH work?
ADH activates the V2 receptor on the renal collecting ducts
* 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 in the urine
Explain factors that regulate ADH release
1) Large decrease in blood volume
2) Severe dehydration: GFR decreases causing less H20 in urine
3) Hyperventilation: results in increased fluid loss
4) Vomiting/diarrhoea: causes the increased fluid loss from GIT
5) Fever, heavy sweating and burns cause large H20 loss
*ADH secretion in all these cases leads to conservation of body H20
-this is through the sequence of events
-intake of copious amounts of water results in high BP which causes increased GFR and increased H20 in urine
Give a summary of factors that maintain H20 body balance
1) Thirst centre in hypothalamus:
-stimulates desire to drink
-water gain if thirst is quenched
2) ATH:
-stimulates secretion of aldosterone
-decreases loss of H20 in urine and Na+
3) Aldosterone:
-promotes reabsorption of Na+, Cl-, and H20
-decreased water loss in urine
3) ANP:
-stimulates natriuresis; increased excretion of Na+, Cl- and H20
-increased loss of water in urine
4) ADH (vasopressin):
-inserts water channels in collecting ducts and thus increases the permeability of collecting duct to water and improves its reabsorption
-decreased loss of water in urine
Explain consequences of dehydration
-dehydration can lead to heat shock, headache, fainting and even death
-brain takes water from rest of the body when thirsty
-higher blood viscosity (reduced circulation)
-higher blood pressure
-blood clotting
-toxins accumulate
-cannot be flushed
-kidneys can be damaged
-nausea, headache, irrationality, cramps, increased temperature, dizziness
Explain dehydration and its effects
Dehydration →imbalance of electrolytes in the bloodstream (↓ NaCl
and K+), but it also ↑[Ca2+]
* ↑ [Ca2+] → hypercalcaemia
– Hypercalcaemia → ↑ risk of kidney stones, kidney failure, and
arrhythmia.
Symptoms of hypercalcaemia:
If mild= asymptomatic
* Nausea
* Vomiting
* Loss of appetite
* Constipation
* Abdominal pain
* Excessive thirst
* Fatigue, lethargy
