Investigations of Salt & Water and Acid/Base Balance Flashcards
How much water does our body contain?
Total body fluids = 60% of body weight
ECF: 20% ICF: 40% - Interstitial: 15% - Intravascular: 5% - Transcellular: 1% - Connective tissue: <1%
How is Water and Na+ balance determined?
By input and output of both
What are the main intake methods of Na+ and water?
Water Intake: Dietary intake (Thirst)
Na+ intake: Dietary (Western diet 100-200 mmol/day)
What are the obligatory water losses causing water output?
Obligatory losses
- Skin (sweat)
- Lungs (breath)
What are the controlled water losses
Controlled losses depend on:
- Renal function
- Vasopressin/ADH (antidiuretic hormone)
- Gut (main role of the colon)
What are the obligatory Na+ losses
Obligatory loss
- Skin (sweat)
Describe the controlled Na+ losses
Controlled losses / excretion
- Kidneys
- Mineralocorticosteroid Aldosterone
- GFR
- Gut - most sodium is reabsorbed; loss is pathological
How does redistribution occur in order to maintain Na+ and water balance?
determined by intravascular volume
How does aldosterone effect Na+ balance?
Aldosterone produced in the adrenal cortex: regulates sodium and potassium homeostasis
What is the role of natriuretic hormones in Na+ balance?
Natriuretic hormones (ANP cardiac atria, BNP cardiac ventricles) promote sodium excretion and decrease blood pressure
How does ADH maintain water balance?
ADH/vasopressin: synthesised in hypothalamus and stored in posterior pituitary. Release causes increase in water absorption in collecting ducts
What is the role of aquaporins in water balance?
Aquaporins (AQP1 proximal tubule and not under control of ADP) AQP2 and 3 present in collecting duct and under control of ADH
What hormones help maintain Na+/water balance?
Na+
- Aldosterone
- ANP/BNP
Water
- ADH/vasopressin
- AQP1/2/3
What is the effect of osmotic pressure on water movement?
Water moves from a high Ψ to low Ψ
How does osmotic potential control water movement in blood?
Osmotically active substances in blood may result in water redistribution to maintain osmotic balance but cause changes in other measured solutes
What processes occur in response to water loss
Water loss = inc. ECF osmolality
- ADH release stimulated
= renal water retention - Hypothalamic thirst centre
= inc. water intake - water redistribution from ICF
= inc. ECF water
Restoration of ECF osmolality
Which members of population are more susceptible to dehydration?
Dehydration common in elderly and v. young babies who can’t access water themselves
Describe what happens to Na+ in renal tubules
GFR ~95-98% of Na+ filtered by glomeruli undergoes obligatory reabsorption driven by renal perfusion
Where in the renal tubules is Na+ mainyl reabsorbed?
Most Na+ reabsorbed in proximal tubule
Distal tubule reabsorption due to aldosterone
Where does ADH produce its reabsorption effects?
ADH stimulates water reabsorption from the collecting duct
Outline the aldosterone mechanism in hypertension / Na+ depletion
- Juxtaglomerular cells produce renin
- Renin = conversion of angiotensinogen → angiotensin I
- Angiotensin I circulates body and (in lungs) ACE (angiotensin-converting enzyme) converts into angiotensin
- Angiotensin stimulates adrenal cortex to produce aldosterone
Helps maintain our circulating volume
How can we measure the osmometry of Na+?
Freezing point depression
Uses colligative properties of a solution
> More solute (Salt) – lower the freezing point
How can we measure electrolytes such as Na+?
- Indirect Ion selective electrodes (main lab analysers)
- Direct Ion selective electrodes (Blood gas analyser) - measure concentrations rather than ion activity
What is hypernatraemia?
Increased water loss and sodium gain
What is hyponatraemia?
Increased sodium loss and water gain
How do we assess a possible fluid / electrolyte disturbance?
- history
- Examination
- Fluid chart
What should a patient history determine?
Fluid intake / output
Vomiting/diarrhoea
Past history
Medication
What examination can be done on a patient with NA+/water imbalance?
Assess volume status
- Lying and standing BP
- Pulse
- Oedema
- Skin turgor/Tongue
- JVP / CVP
What risks should we consider before treating hyper/hyponatraemia?
Hyponatraemia
- Over-rapid correction may lead to central pontine myelinolysis (shrinkage)
Hypernatraemia
- Over rapid correction may lead to cerebral oedema (expansion)
How can we overcome the potential life damaging risks of treating Na+ imbalance?
Important to correct sodium at the same speed - no more than 10mmol/L per 24 hours sodium change
What are important lab investigations for Na+/water imbalance?
- Paired serum + urine osmolality and electrolytes
- Urea/creatinine ratio = inc. Urea = dehydration
- Serum osmolality
= Indicates presence of other osmotically active
substances
Urinary sodium – ignore reference interval
<20 mmol/L = conservation
>20 mmol/L = loss
Urinary osmolality - ignore reference interval
Relate to serum osmolality
Urine /serum osmolality
>1 = water conservation
< 1 = water loss
What is hypovolaemia?
Hypovolaemic - Low BP due to low circulating volume of blood, Na+ &/or water
What is menat byeuvolaemic?
Euvolemic - normal Na+ and bodily fluid stores and normal BP
What is hypervolaemia?
Hypervolemic - excess fluids (blood, water etc.)
What mechanisms are in place to determine Na+ balance?
- Baroreceptors = Blood pressure/volume sensed
- Renal perfusion pressure = Aldosterone produced
- Adrenal cortex = Action at DCT
- Sodium reabsorption = Loss of H+/K+
(reference range 133-145 mmol/L)
(Life threatening range <115 >160 mmol/L)
What causes build up of H+ in the body?
Large amounts of protons/hydrogen ions are an inevitable by-product of energy/ATP production
Why do we need to regulate H+ levels in the body?
Maintenance of extracellular [H+]/pH is essential to maintain protein/enzyme function
What is H+ balance dependent upon?
Balance depends on relative balance between acid production and excretion
- carbon dioxide production + excretion (respiration)
- hydrogen ion production + excretion (renal)
What are the threats to normal pH?
Carbonic acid = from burning carbohydrates (CO₂)
Non-Carbonic acids = from metabolising amino acids
How is H+ balance restored?
Majority of CO₂ removed via lungs
H+ excreted mainly through kidneys
Outline the pH calcualtion
pH = -log10[H+] or pH = log101/[H+]
pH from 7.4-7.1
[H+] 40-80
ie [H+] doubled for pH fall of 0.3
What is metabolic alkalosis?
Rate of H+ generation > excretion
[H+] increases, pH falls - acidosis