Fluid and electrolyte balance Flashcards
What are the features of Fluid Compartments ?
The fluid compartments are fairly tightly controlled in the body both in volume and composition
Major divisions of them are;
- Intracellular
- Extracellular
Extracellular can be further divided into;
- Plasma
- Interstitial
- (Synovial, intra-ocular, CSF, etc)
Composition is determined by the movement across the plasma membrane - ions must move through channels !
How do fluid compartments look in Men and Women, and what can they be broken down into?
Male;
- 60% fluids
- 40% solids
Female;
- 55% fluids
- 45% solids
Fluids can be broken down into;
- 2/3 Intracellular fluid (ICF)
- 1/3 Extracellular fluid (ECF)
Extracellular fluids can then be broken down into;
- 80% interstitial fluids
- 20% plasma
How does pressure change across a capillary and what does this cause?
Arterial end;
- Hydrostatic pressure > Osmotic pressure
- Get net filtration (moving out vessel into cells)
Venous end;
- Osmotic pressure > Hydrostatic pressure
- Get net absorption (moving into vessels from cells)
17L reabsorbed a day
3L leaks into lymphatic systems and is returned to venous circulation
How do the plasma, interstitial and intracellular compartments of the cell vary in concentration?
Plasma and interstitial are Extracellular and very similar, main difference is Plasma has slightly more protein because its harder for proteins to leak out of the plasma, than interstitial (interstitial has a little more ions)
There is a marked contrast between Extracellular and Intracellular;
- Intracellular has barely any sodium, calcium, chlorine and bicarb (all ions BUT potassium)
- Intracellular has a much higher potassium and protein levels
How much fluid do we Lose and Gain each day?
Lose and Gain 2200ml fluid a day, not net gain for loss (balanced)
Fluid loss;
- Urine 1300ml (59%)
- Skin 600ml (27%)
- Respiration 200ml (9%)
- Faeces 100ml (5%)
Fluid gain;
- Drink 1200ml (55%)
- Food 700ml (32%)
- Metabolic 300ml (14%)
What is the difference between Sweat and Insensible water loss?
Insensible water loss you are unaware that you are loosing fluid !
Sweat;
- From specialised skin appendages called sweat glands
- Lose a variable amount of solute (sweat tastes salty!)
- Regulates body temperature
Insensible water loss;
- From skin (transepithelial) and respiratory tracts
- Doesn’t secrete solutes
- Cannot be prevented
- Evaporation of insensible fluid is a major source of heat loss from the body each day but is not under regulatory control
What are the ways we can control Water Balance?
Control of Water Balance;
- Sensors/Central controller/Effectors
- Changes to gains/losses results in a change in osmolarity and/or volume (ADH)
How do sensors work in controlling Water Balance?
Sensors;
- Osmoreceptors in hypothalamus
- Low pressure baroreceptors in right atria and great veins
- High pressure sensors (carotid sinus/aorta)
NB - pressure is used as a surrogate marker of blood volume
How does the control of Antidiuretic hormone affect water balance ?
Increase blood osmolality or a decrease in blood volume;
- An increase in blood osmolality or a decrease in blood volume stimulates neurons in the hypothalamus, resulting in an increase in ADH release from the posterior pituitary
- ADH increases water reabsorption in the kidney, resulting in retention of a greater volume of water in the blood, a reduced urine volume, and a decreased blood osmolality. There is also an increase in blood volume
Decrease in blood osmolality or a increase in blood volume;
- A decrease in blood osmolality or a increase in blood volume inhibits neurons in the hypothalamus resulting in a decrease in ADH release from the posterior pituitary
- Reduced ADH decreases water absorption in the kidney, resulting in reduction of the volume of water in the blood, increased urine volume and increased blood osmolality. There is also a decrease in blood volume
What are the 4 things to consider with any Electrolyte Abnormality?
- What are the physiological mechanisms to control the concentration of ‘x’?
- What are the consequences for the patient if the levels are to high/low?
- Is it a change of intake/loss of ‘x’ or a change of location for dilutional (E.g same amount of Ribena, add more water = more dilute)
- Does it need to be corrected and if so how quickly and with what ?
E.g if sodium level falls doesn’t mean less sodium, can just be more diluted
What is the key driver of total volume and how can it affect volume?
Key driver of total volume is total sodium;
- If total sodium drops and osmolality (tightly regulated) stays the same, the total volume falls (including plasma volume)
- If total sodium rises and osmolality stays the same, the total volume will rise
- Compensatory mechanisms are really linked to low volume (low GFR, stimulation of Juxtaglomerular apparatus (JGA) r high volume (increased GFR and release of ANP)
(Think sodium drags water with it!)
How do we gain and loose sodium ?
- Gains: Food and drink (recommended max 2.4g Na+)
- Losses: Sweat (0.25g), faeces (0.25g), urine (2-10g - main loss is in urine)
- If you eat too much/too little salt, the only controllable route of loss is via urine: hormonal control
- May increase losses via the other routes in a non-hormonally ways (e.g exercise/heat etc causing increased sweating Na+ 50mml/l; diarrhoea - 130mmol/l, vomit - 50mml/l
- Intake is normally in excess of need, e.g processed food
What are the features of Hypovolaemia, Euvolaemia and Hypervolaemia ?
Pathophysiology - More water relative to sodium needs assessment of “-volaemia”
Hypovolaemia;
- Deficient in water an Na, Na greater than water due to excess sodium loss
Hypovolaemia Examples;
- Renal: Diuretics, Adrenocortical failure
- GI: Vomiting/diarrhoea (more water lost than sodium)
- Skin: Burns
Euvolaemia;
- Normal Na, excess water
Examples of Euvolaemia;
- Polydipsia
- Iatrogenic - IV fluids
- SIADH
Hypervolaemia;
- Water and Na retention
Examples of Hypervolaemia;
- Heart failure
- Cirrhosis
- CKD
What are the causes of Hypernatraemia ?
Hypernatraemia - High Sodium Level
Free water loss;
- Renal: Osmotic diuretic, e.g Manitol
- GI: Diarrhoea or vomiting
- Skin: Sweating
- Diabetes insipidus (cranial or nephrogenic)
Inadequate intake;
- Limited access to water
- Blunted thirst
Excess Na (rare);
- Excessive salt (IV/oral)
- Note - “Normal” saline = 154 mol/l of Na+
- Usually always caused by too much water - Too much Ribena not enough water
What are the Gains and Losses of Potassium?
Remember K+ is mainly intracellular;
- Gains: via food/drink
- Losses: Predominantly via urine. Little is lost in sweat or faeces in normal conditions
- Control is therefore at the kidney
- K+ is freely filtered through the glomerulus
- Predominantly absorbed again in the PCT with controlled section at the DCT (swaps sodium with potassium here)
- Secretion is linked to Na+ reabsorption (sodium pump)
How are different IV fluids distributed ?
Glucose 5%;
- 600ml Intracellular
- 328ml Interstitial and lymphatic (Part of extracellular)
- 72ml Plasma volume (Capillary membrane)
Sodium chloride 0.18% and glucose 4%;
- 480ml Intracellular
- 426ml Interstitial and lymphatic (Part of extracellular)
- 94ml Plasma volume (Capillary membrane)
Saline 0.9%;
- 820ml Interstitial and lymphatic (Part of extracellular)
- 180ml Plasma Volume (Capillary membrane)
Balanced Crystalloids;
- 820ml Interstitial and lymphatic (Part of extracellular)
- 180ml Plasma Volume (Capillary membrane)
Colloid;
- 1 litre Plasma Volume (Capillary membrane)
When would you consider a patient to be “Fluid Challenged”?
Fluid Challenge;
- Oliguria or hypotension and no signs of overload consider a fluid challenge
- Therapeutic & diagnostic
- 500mls balanced salt solutions
- QUICKLY ! (<15mins)
- Re-assess (What’s happening to BP, tissue perfusion)
- Can repeat up to 2000mls
