Consequences of Fluid Loss from the GI System Flashcards
State the sources of water intake and output in the body and average values for them (ml)
- Intake of water -
- Liquids - 1200ml
- Food - 1000ml
- Metabolically produced - 350 ml
Total - 2550ml - Output of water -
- Insensible water loss (lungs and skin - 900ml)
- Sweat - 50ml
- Faeces - 100ml
- Urine - 1500ml
Total - 2550ml
State the sites of water loss in the body
Skin - sweating
Respiratory passageways - ventilation
GI tract - vomiting/defecation
Urinary tract - urination
Menstrual flow
Trauma
When is excretion of Na+ and Cl- high?
In diarrhoea, sweating, vomiting and haemorrhage - loss of water and salts
State the sources of NaCl intake and output daily including average values (g)
Daily NaCl intake -
- Food - 10.5g
NaCl output -
- Sweat - 0.25g
- Faeces - 0.25g
- Urine - 10g - varies a lot
Total output - 10.5g
What is diarrhoea?
- Increased frequency with increased volume and fluidity of faeces
- More than 3 unformed stools per 24hrs
- Change in bowel movement
What does diarrhoea result from?
- Failure of water absorption
- Increased secretion of water
State the causes of decreased absorption
- Increased number of osmotic particles
- Increased rate of flow of intestinal contents
- Abnormal increase in secretions of GI tract
Describe the different types of diarrhoea
- Osmotic diarrhoea -
- Decreased absorption of electrolytes and nutrients
- Disaccharidase deficiency (disaccharide deficiency/malabsorption)
- Drug induced
- Malabsorption of galactose
- Bloating, nausea, watery diarrhoea
- Secretory diarrhoea -
- Acetylcholine, substance P, 5-HT and neurotensin act via increasing the [Ca2+] to increase the rate of intestinal secretion
- Excessive laxative use, defects in digestion and absorption and infections like cholera
- Cholera survives the acidic conditions of the gut and the enterotoxins stimulates adenylate cyclase which increases cAMP which then causes Na+, Cl- and water loss
- The water, electrolytes and glucose can be replaced as a treatment
- Deranged motility diarrhoea -
- Lack of absorption, some agents may promote secretion and motility
- GI stasis may promote diarrhoea by stimulating bacterial overgrowth
Describe the parasitic causes of diarrhoea
- Parasitic causes of diarrhoea -
- Entamoeba histolytica and giardia lamblia
- Entamoeba histolytica -
- Asymptomatic or amoebic dysentery (painful blood low volume diarrhoea) - gradual onset with systemic symptoms like anorexia and headaches
- Treated with metronidazole
- Giardia -
- Steatorrhoea and abdominal pain - maldigestion and malabsorption of lipids, CHO, vit A, vit B12 and folic acid
- Treated with metronidazole or tinidazole
State the causes of bloody diarrhoea
- Chronic disease
- Ulcerative colitis
- Neoplasm
What are the major consequences of severe diarrhoea
- Loss of water, nutrients, Na+, K+ and HCO3-
- Decreased blood volume (hypovolaemia)
- Metabolic acidosis - due to loss of HCO3- e.g. in diarrhoea
What are the consequences of excessive vomiting
- Increased salt and water loss
- Severe dehydration
- Circulatory problems
- Metabolic alkalosis - due to a loss of gastric acid
- Death
What is lost in vomit?
- Food
- Mucus containing Na+, K+, Cl- and HCO3-
- Gastric acid
- Upper intestinal contents including bile
- Blood
State the consequences of fluid loss from the GI tract
- Hypovolaemia
- Haemoconcentration/polycythaemia (dehydration) - increases [RBC]
- Dehydration
- Ionic imbalances - poor perfusion of tissues
- Malnutrition and increased mortality
State the consequences of hypovolaemia
- Decreased venous return
- Arterial hypotension
- Myocardial dysfunction due to increased myocardial oxygen demand , tissue perfusion is reduced
- Increased anabolic metabolism leads to acidosis
- Acidosis and myocardial dysfunction causes multi organ failure
In what ways does the body respond to water loss?
- CVS adaptation
- Renal adaptation
- Behavioural
Describe the renal and cardiovascular responses to water loss
- There is increased Na+ and H2O is lost due to diarrhoea
- This then decreases the plasma volume and venous pressure of the blood
- Renal -
- This then increases the activity of the renal sympathetic nerves
- In the kidneys there is increased constriction of the renal arterioles, which decreases the net glomerular filtration pressure and glomerular filtration rate
- This leads to decreased Na+ and water secretion and more reabsorption
- CVS -
- The decreased venous pressure means there is decreased venous return
- Hence atrial pressure decreases and the ventricular end diastolic volume decreases
- Stroke volume also decreases which reduces cardiac output
- The decreased arterial blood pressure then causes increased activity of renal sympathetic nerves and decreases net glomerular filtration pressure
Describe how water loss by sweating is responded to
- Severe sweating leads to a loss of hypoosmotic salt solution
- This decreases the plasma volume which then causes decreased glomerular filtration rate and increased plasma aldosterone which decrease sodium excretion as well as increase plasma vasopressin which reduced water excretion
- It also decreases plasma osmolarity as the water concentration has decreased which increases plasma vasopressin which then reduces water excretion
Describe how the body responds to hypovolaemia with the RAAS cascade
- Decreased plasma volume leads to increased activity of renal sympathetic nerves, decreased arterial pressure and decreased glomerular filtration rate (reduced flow to macula densa
- The increased activity of renal sympathetic nerves which occurs due to stimulation of intrarenal baroreceptors increases renin secretion from renal juxtaglomerular cells
- The decreased glomerular filtration rate has decreased flow to macula densa which then decreases the NaCl concentration in macula densa - the macula densa sense [NaCl] passing through it - decreased NaCl causes increased renin release and decreased resistance to blood flow in the afferent arterioles via vasodilation - this increases renin secretion from renal juxtaglomerular cells
- The reduced arterial pressure has a direct effect as there is less stretch so renin secretion from renal juxtaglomerular cells increases
- This increases the plasma renin which increases plasma angiotensin II
- This then increases aldosterone secretion from the adrenal cortex which increases plasma aldosterone
- This then leads to increased Na+ reabsorption from cortical collecting ducts which decreases Na+ excretion
Describe how hypovolaemia is responded to in terms of potassium concentration
- There is a decreased plasma volume which leads to increased plasma angiotensin II
- There is also increased plasma potassium, both of which lead to an increase in aldosterone secretion from the adrenal cortex
- This increases the plasma aldosterone which leads to increased sodium and potassium reabsorption from the cortical collecting ducts
- This decreases sodium excretion and increases potassium excretion
How does the body respond to the consequences of hypovolaemia
- Decreased plasma volume leads to reduced venous, atrial and arterial pressure
- There is then a reflex mediated by cardiovascular baroreceptors which leads to increased vasopressin secretion from the posterior pituitary
- There is an increased plasma vasopressin which leads to increased tubular permeability to H2O and increased H2O reabsorption in the collecting ducts
- This leads to reduced H2O in the urine
Describe what causes thirst
- There is decreased plasma volume which stimulates baroreceptors and increases angiotensin II which leads to thirst
- There is also increased plasma osmolarity which stimulates osmoreceptors which causes thirst
- Dry mouth, throat and metering of water intake by GI tract both cause thirst
State what regulates water loss and how it works
- Vasopressin (antidiuretic hormone) - ADH stimulates thirst and ADH release is caused by increased osmolarity of body fluids
How does ADH work?
- ADH activates the V2 receptor on the renal collecting ducts
Inserts water channels (aquaporins 2) in the collecting duct of the kidney
- This increases the permeability of the collecting ducts to water and results in concentrated urine production
- Intake of water causes a decrease in osmolarity of the blood and interstitial fluid which decreases ADH secretion and the removal of water channels
- If there are no water channels this causes decreased permeability of collecting ducts to water and increased water loss in the urine
State the factors that regulate ADH release
- Large decrease in blood volume (detected by baroreceptors)
- Severe dehydration - GFR decreases causing less water in urine
- Hyperventilation results in increased fluid loss
- Vomiting/diarrhoea - causes increase fluid loss of GI tract
- Fever, heavy sweating and burns cause large water loss
- These lead to the generation of hyperosmolar renal medulla
- ADH secretion in all these cases leads to conservation of body water
- An intake of copious amounts of water results in high blood pressure which causes increased GFR and increased H2O in urine
State the consequences of dehydration
- Water controls your temperature so dehydration can lead to heat shock, headache, fainting and death
- The heart has to work harder - there is higher blood viscosity, higher blood pressure and blood clotting
- Brain has priority - takes water from the rest of the body
- Kidneys get overloaded - toxins accumulate, cannot be flushed away and so kidneys are damaged
- Digestive problems - constipation, acid reflux, indigestion, toxins accumulate, reduced nutrient absorption
How does dehydration lead to hypercalcaemia
- Dehydration causes an imbalance of electrolytes in the bloodstream as NaCl and K+ decrease and Ca2+ increase
- Increased Ca2+ is hypercalcaemia which causes an increased risk of kidney stones, kidney failure and arrhythmia
State the symptoms of hypercalcaemia
- Nausea
- Vomiting
- Loss of appetite
- Constipation
- Abdominal pain
- Excessive thirst
- Fatigue, lethargy, muscle weakness
- Joint pain, confusion
