Water and sodium homeostasis Flashcards
How many litres of water are there in a 70kg male?
60 % of body weight so 42L
How many litres of intracellular fluid?
40 % of body weight so 28L
How many litres of extracellular fluid?
20 % of body weight so 14L
Of the extracellular fluid how much is intravascular and interstitial?
Intravascular (in plasma/blood stream) = 3L
Interstitial (between cells) = 11L
Is water permeable in ICF and ECF?
Yes but it is determined by osmotic contents (e.g. ions K+ etc). Any change in ion concentration means there will be a water shift as the end result always needs to be isotonic (equal)
What is in the ECF?
- Sodium (Cation) is the main contributor to ECF osmolality and volume
- Chloride (anion)
- Glucose and urea
- Protein = colloid osmotic pressure
What is in the ICF?
- Predominant cation is potassium
Interstitial fluid vs plasma
- Interstitial fluid surrounds the cells, but does not circulate
- Plasma circulates as the fluid component of blood
How is plasma osmolality determined?
- Largely by sodium associated anions
- Intra and extracellular osmolality are equal
- Change in plasma osmolality pulls or pushes water across cell membranes to allow cells to function properly
What is the equation for the estimated plasma osmolality?
2(Na) + 2(K) + urea + glucose mmol/L
What are some ways we can gain water?
- Food and drink
- Metabolism
What are some ways we can loose water?
- Skin
- Lungs
- Urine
- Feces
How does fluid intake compare to fluid loss?
Under normal circumstances they should balance each other out
For example: Intake (2.2L/day) + Metabolic production (0.3L/day) - Output (2.5 L/day) = 0
Why don’t we give water intravenously?
- It is hypo-osmolar / hypotonic vs cells
- Water enters blood cells causing them to expand and burst: haemolysis
-However, only occurs in the vicinity of the intravenous cannula
How does water homeostasis react to changes in the ECF?
- ECF osmolality is very tightly regulated
- Changes in ECF lead to a rapid response
- Normal plasma osmolality is 275-295 mmol/kg
- Water deprivation/ loss will lead to chain of events
What happens if we have water deprivation or dehydration?
- This will increase the ECF osmolality
- Stimulate the thirst centre in the hypothalamus to hopefully make us drink if we can
- Water will move from the ICF to the ECF
- ADH is released from posterior pituitary, which will make sure we absorb more water rather than pee/poo it out
How does ADH work?
- Called Antidiuretic Hormone
- Causes the collecting tubes in the kidney to be more permeable to water so water can leave and we produce a small volume of concentrated urine as we have tried to retain as much water as possible.
What is the first stage of the RAA (Renin- Angiotensin- aldosterone) system?
- There is a decrease in water in ECF = decrease in effective circulating volume = decrease in renal blood flow
- Results in the release of renin from the juxtaglomerular kidney cells in the kidneys
What happens after Renin has been released in the RAA?
- Renin converts angiotensinogen to angiotensin I,
- Then an angiotensin converting enzyme (ACE) then converts angiotensin I into angiotensin II, which in turn
triggers the release of aldosterone from the adrenal cortex above the kidneys
What does Angiotensin II and Aldosterone do?
- increase Na+ reabsorption in the kidneys in exchange for potassium or hydrogen excretion
- Also stimulates the release of ADH
- Na reabsorption brings water with it to return water levels in the ECF to normal
What are some causes for dehydration?
- Reduced intake of water
- Vomiting/ diarrhoea/ Diuretics (drug that increases urination)
- Sweating
What are the consequences of dehydration?
- thirst
-dry mouth - inelastic skin
- sunken eyes
-raised haematocrit (viscosity of blood) - weight loss
- confusion
- hypotension
What are some causes for water excess?
- High intake/ decreased loss of water
- excess ADH
What are the consequences of water excess?
- Water moves from ECF into cells causing them to swell
- No thirst stimulation
- Inhibition of ADH so increased urine volume
- Risk of cerebral over hydration if acute excessive intake
- Hyponatraemia (low sodium levels),
What is hydrostatic pressure?
Pressure difference between plasma and interstitial fluid
Water moves from plasma into interstitial fluid
What is oncotic pressure?
Pressure caused by the difference in protein concentration between the plasma and interstitial fluid
- Water moves from interstitial fluid into plasma
How does water move at the arteriole and venule end of capillaries?
Arteriole: more pressure so H20 moves out
Venule: less pressure so H20 moves in
What is oedema?
- Excess accumulation of fluid in interstitial space
- Disruption of the filtration and osmotic forces of circulating fluids
- obstruction of venous blood or lymphatic return
- so inflammation increases and there is loss of plasma proteins
What is serous effusion?
Excess water in body cavity e.g. lungs
Inflammatory oedema and the capillaries?
Inflammatory (leakage) - proteins (e.g. albumin) leak out due to increased vascular permeability - they bring in water, thereby diluting the toxins - fibrinogen polymerises to form a fibrin mesh and immunoglobulins collect
Venous oedema and the capillaries
Venous (increased end pressure) - due to increased venous pressure or venous obstruction from a thrombus.
- Causes pooling of blood in calves
Lymphatic oedema and capillaries?
Lymphatic (blocked) - Obstructions from a tumour/parasite
Hypoalbuminaemic oedema and capillaries?
Hypoalbuminaemic - lower oncotic pressure. Body doesn’t produce enough albumin
What is a pleural effusion?
- Normal plural space contains 10mL of fluid
- an effusion is a result from a disruption of the balance between hydrostatic and oncotic forces in the visceral and parietal pleural vessels
What can enter the pleural cavity during a pleural effusion?
Different fluids:
- Transudate (fluid pushed through the capillary due to high pressure)
- Exudate (fluid that leaks around cells caused by inflammation and increased permeability of pleural capillaries to proteins)
So exudates have a high protein level compared to transudates
What is Hypernatraemia?
Causes and consequences
- High sodium
- Causes: renal failure, mineralocorticoid excess (sodium excess), osmotic diuresis (increased urine rate due to high amount of water) and diabetes insipidus.
- Consequences: cerebral intracellular dehydration - high
sodium = low H2O which then dehydrates the brain as there is a lower water concentration since H2O leaves intracellular to go extracellular as solute concentration increases - osmosis
What is Hyponatraemia?
Causes and consequences
- Low sodium
- Causes: diuresis (increase urine rate), Addison’s
disease, excess IV fluids & oedema. - Consequences: Intracellular over hydration -
hypotension since H2O goes intracellular as solute concentration increases - osmosis
