Consequences of Fluid loss from the GI Tract Flashcards
(28 cards)
What is the average volume intake by adults?
- Liquids - 1200ml
- Food - 1000ml
- Metabolically produced - 350ml
- E.g Oxidation of carbs, free FA, glycogen
- Total - 2550ml
What is the average volume output by adults?
- ‘Insensible water loss’ (skin and lungs) - 900ml
- This water loss refers to loss of H2O via transepidermal diffusion e.g evaporation via skin and resp tracts
- Sweat - 50ml
- Faeces - 100ml
- Urine - 1500ml
- Total - 2550ml
What are the sites of water loss?
- Skin
- Resp passageways
- GIT
- Urinary tract
- Menstrual flow
- Some trauma can cause fluid loss
What is the average intake and output of NaCl?
Intake:
- Food - 10.5
Output:
- Sweat - 0.25g
- Faeces - 0.25g
- Urine - 10g - varies considerably
- Total output - 10.5g
Kidneys can alter fluid excretion to match gain with loss
Why can diarrhoea, excessive sweating, vomiting and haemorrhage be so problematic?
Loss of water and salts, will lead to imbalance of electrolytes, lower blood volume/ BP etc.
In what ways can fluid be lost?
- Defecation; diarrhoea
- Vomiting
- Urination
- Ventilation
- Sweating
- Menstruation
- Untreated or uncontrolled diabetes
- Kidney disease
Cause dehydration
In what ways can fluid be gained?
- Drinking
- Metabolic
Describe the causes of diarrhoea
Diarrhoea - Increased freq, with increased volume and fluidity of faeces. >3 unformed stools per day. Change in bowel movement
Occurs are result of:
- Falilure of water absorption
- Increased secretion of water
Causes of decreased absorption:
- Increased number of osmotic particles (osmotic diarrhoea)
- Abnormal increase in secretions of GIT (secretory diarrhoea)
- Increased rate of flow of intestine contents (deranged motility diarrhoea)
Describe osmotic diarrhoea
Decreased absorption of electrolytes and nutrients:
- Disaccharidase deficiency (disaccharide deficiency/malabsorption), drug induced, malabsorption of galactose; bloating, nausea, water diarrhoea
- Too much water is draw into the bowels
Describe secretory diarrhoea
Ach, Sub P act via increase Ca2+ to increase rate of intestinal secretions (increase peristalsis)
- Excessive laxative use, defects in digestion and absorption; infections e.g . cholera (vibrio cholerae)
- Survives acidic conditions of gut
- Enterotoxin stimulates adenylate cyclase - increase cAMP - Na+, Cl- and H20 loss
- Treatment - Replace water, electrolytes, glucose
Describe deranged motility diarrhoea
Increased rate of flow of faeces through bowels
Lack of absorption of nutients; some agents may promote secretion as well as motility:
- GI stasis may promote diarrhoea by stimulating bacterial overgrowth
Describe the parasitic causes of diarrhoea
Entamoeba histolytica: (EH)
- Anaerboic parasitic protozoan
- Asymtompatic or amoebic dysentery (dysentry - painful, bloody, low volume diarrheoa)
- Gradual onset → Systemic symptoms (anorexia, headache)
- Metronidazole can be used to treat
Giardia lambilia: (GL)
- Microscopic parasite
- Steatorrhoea and abdominal pain
- Maldigestion and malabsorption of lipids, carbs, vit A, B12, folic acid
- Metronidazole or tinidazole to treat
What are the causes of bloody diarrhoea?
- Chronic disease
- IBD - Ulcerative colitis, Crohn’s
- Neoplasm
What are the major consequences of severe diarrhoea?
- Decreased blood volume (hypovolemia)
- Metabolic acidosis - Due to increased HCO3- loss as result of diarrhoea
- Loss of water, nutrients, Na+, K+, HCO3-
- Volume depletion could lead to acidosis/alkalosis
What are the consequences of excessive vomiting?
- Increased salt and water loss
- Severe dehydration
- Circulatory problems
- Metabolic alkalosis - due to loss gastric acid (HCl)
- Death 💀😭
Lost in vomit:
- Food
- Gastric acid
- Mucus with Na+, K+, Cl-, HCO3-
- Upper intestinal contents (including bile)
- Blood
What are the wider consequences of fluid loss from the GI tract?
- Hypovolemia
- Haemo-concentration/polycythaemia (dehydration) = Increased RBC
- Dehydration
- Ionic imbalances; poor perfusion of tissues
- Malnutrition and increased mortality
What are the consequences of hypovolemia?
- Decreased venous return, arterial hypotension, myocardial dysfunction due to increased myocardial oxygen demand, but tissue perfusion reduced
- Increased anaerobic metabolism - Acidosis (lactate)
- Acidosis and myocardial dysfunction → Multi-organ failure
How does the body respond to water loss?
Compensatory mechanisms:
- CVS adaptation
- Renal adaptation
- Behavioural adaptation which stimulates fluid intake when possible to do so
Describe the physiological adaptations to hypovolemia as a result of diarrhoea
Diarrhoea causes:
- Increased Na+ and H2O loss
- Decreased plasma volume
- Decreased venous pressure
Cardiovascular adapations:
- A decrease in venous pressure is mediated by baroreceptors, as is atrial pressure and arterial blood pressure. Baroreceptors detect the level of distension of blood vessels/ the heart, so will detect a decrease in the case of hypovolemia
- Decrease venous return, decrease atrial pressure, decreased EDV (amount of blood at end of filling of right and/or left ventricle), decrease SV, decrease CO, decrease arterial BP
- These will send afferent fibres to the brain, activating baroreceptor-reflex, efferent fibres act on kidneys to induce release of renin from JG cells etc, activate RAAS
Renal adaptations:
- Increased constriction of renal arterioles
- Decreased net glomerular filtration pressure - This can also occur as a direct result of decreased arterial blood pressure, as there would already be less blood volume in the kidneys
- Decrease GFR
- Decrease Na+ and H20 secretion
Describe the phsyological adaptations to water loss by sweating
Severe sweating leads to a loss of hypo-osmotic salt solution:
- Decreased plasma volume:
- Decrease GFR
- Increased plasma aldosterone
- Both of the above lead to decreased sodium excretion
- Also leads to increased plasma ADH
- Increased Plasma osmolality (decreased H2O secretion):
- Increased plasma vasopressin
- Decrease H2O excretion
Describe the role of RAAS in response to hypovolaemia
Hypovolaemia - Decreased plasma volume:
- Increased activity of renal sympathetic nerves (B1), decreased arterial pressure, decreased GFR resulting in decreased flow to macular densa
- Intrarenal baroreceptors detect distension of arterioles, decreased = renin release, also caused by decreased arterial pressure, and decreased NaCl load in macula densa
- Macula densa senses NaCl in fluid so decreased NaCl causes:
- Increase renin release
- Decreased resistance to blood flow in afferent arterioles via vasodilation → Normal GFR
- Macula densa senses NaCl in fluid so decreased NaCl causes:
- Increase plasma renin, reacts with angiotensiogen from liver to form Ang I. Ang I to Ang II via ACE. Ang II acts on AT1 receptors at adrenal cortex to induce aldosterone release.
- Increased aldosterone release. Aldosterone acts on ENaC channels of principal cells of collecting duct, increase Na+ reabsorption
- Decreased Na+ excretion
Describe the physiological response to hypovolaemia with increased potassium concentration
Decreased plasma volume:
- Increased plasma Ang II
- Leads to increased aldosterone secretion by adrenal cortex
- Increase plasma aldosterone
- Increase Na reabsorption, increased potassium secretion via cortical collecting ducts
- Decreased Na excretion, increased potassium excretion
Increased plasma potassium:
- Increases aldosterone secretion by adrenal cortex
- Increase plasma aldosterone
- Increase Na reabsorption, increased potassium secretion via cortical collecting ducts
- Decreased Na excretion, increased potassium excretion
Describe the physiological response to hypovolemia due to thirst
- Decreased plasma volume - Detected by baroreceptors, stimulates thirst centre in hypothalamus, increases production of Ang II, which also stimulates thirst cente of hypothalamus
- Increased plasma osmolarity - Detected by osmoreceptors, stimulate thirst centre in hypothalamus
- Dry mouth, throat - Stimulates thirst centre in hypothalamus
- Metering of water intake by GIT - Avoids over-rehydration, also stimulates thirst cente in hypothalamus
Describe the role of vasopressin in response to hypovolemia
- Decreased plasma volume
- Decreased venous, atrial, arterial pressures (mediated by baroreceptors)
- Increase vasopressin secretion via posterior pituitary gland
- Increased plasma vasopressin
- Acts on collecting ducts, increases tubular permeability to H2O, increases H2O reabsorption
- Decreased H2O excretion
