Regulation of fluids Flashcards
What is the % of water neonates have compared to adults?
75-80% water with proportionately more extracellular fluid (ECF) than adults
How much larger is interstitial fluid proportion at birth as compared to an adult?
three times larger
What is the % of decrease in TBW (total body water) at 60 years of age in males?
50%
What is the reason for the decrease in TBW in males at 60yo mostly due to?
increase in adipose tissue
How many litres of fluids are in intracellular (ICF) and extracellular fluid (ECF)?
intra = 23 L extra = 19 L
Describe fluid balance in the ICF:
- cytosol of cells
- makes up two-thirds of total body water
Describe fluid balance in the ECF:
- major components includes interstitial fluid and plasma
- minor components include all other extracellular fluids
ICF and ECF are separated by?
cell membranes
What are the levels of ions in intracellular fluids?
potassium/magnesium = high sodium/chloride = low
Is there a single united compartment of intracellular fluid?
no. this concept is artificial
Is the ECF divided into several smaller compartments?
Yes
How are the compartments in ECF distinguished?
by different locations and kinetic characteristics
What are the levels of ions in extracellular fluids?
potassium/magnesium = low sodium/chloride = high
What does the interstitial fluid contain (ISF)?
all fluids which lie in the interstices of all body tissues
What are the ‘functions/purposes’ of the ISF?
- bathes all cells in body
- link between ICF and intravascular compartment
- oxygen, nutrients, wastes, chemical messengers all pass through
- has compositional characteristics of ECF
How is the ISF distinguished?
by its usually low protein concentration
Is the lymph considered part of ISF?
Yes
What is the relation of the lymphatic system with ISF?
- lymphatic system returns protein and excess ISF to circulation
- lymph more easily obtained for analysis than other ISF parts
What can we say about the plasma compartment?
the only major fluid compartment that exists as real fluid collection all in one location
What is the difference of plasma and ISF?
plasma much higher protein content and its high bulk flow (transport function)
What are the normal blood levels of sodium?
135-145 mEq/L
What are the normal blood levels of potassium?
3.5-5.5 mmoles/L
What are the normal blood levels of chloride?
96-106 mEq/L
What makes water move?
osmosis
Explain how the osmotic pressure works, making water move:
- it’s the hydrostatic (hydraulic) pressure that opposes movement of water through semi-permeable membrane
- in response to osmotic gradient
What is serum osmolality measured by?
- use of an osmometer
OR - calculated as sum of concentration of solutes present in solution
How many mL is the insensible loss of water from the body?
800mL
How many mL is the minimal sweat loss of water from the body?
100mL
How many mL is the faecal loss of water from the body?
200mL
How many mL is minimal urine volume to excrete solute load, of water from the body?
500mL
So, the daily obligatory water loss in mL is?
1,600mL
What are the external fluid inputs?
- oral intake of fluid and foods
- IV fluids
What are the internal fluid inputs?
metabolic water production
When is metabolic water produced?
during oxidation of food
- carbs are completely metabolised to carbon dioxide and water
How much per day of metabolic water is produced?
350-400mls/day
Give some examples of fluid loss:
- pure gastric vomiting
- bilious vomiting
- panting
- free water gain
- diarrhea
What happens in pure gastric vomiting?
- loss of HCl
- volume causes hypochloremic metabolic alkalosis
What happens in bilious vomiting?
- loss of K, HCO3, Na
- cause hypokalemia, acidosis (colour of vomit is important)
In panting, what are the losses?
- loss of free water
- but no electrolyte loss
What can promote ion loss in the free water gain situation?
dilution and diuresis
What is by far the most important indication for fluid and electrolyte replacement?
dirrhoea
What enhances intestinal absorption of sodium and water?
by glucose (other carbs)
What is the solution to replacement of fluid/electrolytes lost through diarrhea?
Oral rehydration therapy (ORT) - give solutions containing Na, K, and glucose (other carbs, starch)
What is a result of the oral rehydration solution?
- enhance absorption of water/electrolytes
- replace electrolyte deficit
- contain alkalinising agent to counter acidosis
- be slightly hypo-osmolar (250mmol/L) to prevent possible induction of osmotic diarrhoea
- simple to use in hospital/at home
- palatable and acceptable to children esp.
- readily available
What is WHO’s rehydration salt formulation containing?
NaCl = 2.6g
KCl = 1.5g
Na citrate = 2.9g
anhydrous glucose = 13.5g
(But solutions are lower in sodium for UK since patients suffer less sodium loss)
How long is the rehydration process?
3-4hrs
What is the maximum rate of sweating?
50mls/min
2000mls/hr
8000mls/day??
What is the % losses of TBW through sweating during stress?
25%
What is lost through sweating?
- fluid (hot environment, physically active)
- solute (acclimatisation)
- heat (high latent evaporisation of water)
How much of sweat is solute in %?
0.2-1%
Where is sweat loss from?
sweat glands, specialised skin appendages
Where is insensible fluid loss from?
- skin
- respiratory tract
What is the role of sweat?
regulate body temperature
Under extreme conditions, how much sodium can be lost in a day?
350mmols/day
What is the central controller of water balance?
hypothalamus
What are the key parts of the hypothalamus involved in water balance?
- osmoreceptors
- thirst center
- OVLT and SFO (respond to angiotensin II)
- supraoptic and paraventricular nuclei (ADH synthesis)
What are the 2 types of thirst?
hypovolemic and osmotic
What are the 4 major stimuli to thirst?
- Hypertonicity
- Hypovolaemia
- Hypotension
- Angiotensin II
What is hypertonicity?
cellular dehydration, via osmoreceptor mechanism
What is hypovolaemia?
low volume sensed via the low pressure baroreceptors in great veins and right atrium
What is hypotension?
high pressure baroreceptors in carotid sinus and aorta provide the sensors for this input
what is angiotensin II?
produced consequent to release of renin by kidney (response to renal hypotension)
What is ADH?
antidiuretic hormone
- nonapeptide synthesised in hypothalamus
- vasopressin because vasopressor response to pharmacological doses
What is ADH’s major role?
regulation of water balance by its effect on kidneys
What is the half life of intravascular ADH?
15mins, being rapidly metabolised in liver/kidney to inactive products
What properties do both caffeine and alcohol have?
diuretic properties
- work by restricting ADH activity
- inhibit pituitary secretion of anti-diuretic hormone
At what levels of caffeine would there be diuretic effect?
excess of 300mg
