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CVR Lectures > Water and sodium homeostasis > Flashcards

Water and sodium homeostasis Flashcards

1
Q

How many litres of water are there in a 70kg male?

A

60 % of body weight so 42L

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2
Q

How many litres of intracellular fluid?

A

40 % of body weight so 28L

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3
Q

How many litres of extracellular fluid?

A

20 % of body weight so 14L

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4
Q

Of the extracellular fluid how much is intravascular and interstitial?

A

Intravascular (in plasma/blood stream) = 3L
Interstitial (between cells) = 11L

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5
Q

Is water permeable in ICF and ECF?

A

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)

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6
Q

What is in the ECF?

A
  • Sodium (Cation) is the main contributor to ECF osmolality and volume
  • Chloride (anion)
  • Glucose and urea
  • Protein = colloid osmotic pressure
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7
Q

What is in the ICF?

A
  • Predominant cation is potassium
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8
Q

Interstitial fluid vs plasma

A
  • Interstitial fluid surrounds the cells, but does not circulate
  • Plasma circulates as the fluid component of blood
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9
Q

How is plasma osmolality determined?

A
  • 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
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10
Q

What is the equation for the estimated plasma osmolality?

A

2(Na) + 2(K) + urea + glucose mmol/L

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11
Q

What are some ways we can gain water?

A
  • Food and drink
  • Metabolism
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12
Q

What are some ways we can loose water?

A
  • Skin
  • Lungs
  • Urine
  • Feces
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13
Q

How does fluid intake compare to fluid loss?

A

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

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14
Q

Why don’t we give water intravenously?

A
  • 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
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15
Q

How does water homeostasis react to changes in the ECF?

A
  • 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
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16
Q

What happens if we have water deprivation or dehydration?

A
  • 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
17
Q

How does ADH work?

A
  • 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.
18
Q

What is the first stage of the RAA (Renin- Angiotensin- aldosterone) system?

A
  • 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
19
Q

What happens after Renin has been released in the RAA?

A
  • 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
20
Q

What does Angiotensin II and Aldosterone do?

A
  • 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
21
Q

What are some causes for dehydration?

A
  • Reduced intake of water
  • Vomiting/ diarrhoea/ Diuretics (drug that increases urination)
  • Sweating
22
Q

What are the consequences of dehydration?

A
  • thirst
    -dry mouth
  • inelastic skin
  • sunken eyes
    -raised haematocrit (viscosity of blood)
  • weight loss
  • confusion
  • hypotension
23
Q

What are some causes for water excess?

A
  • High intake/ decreased loss of water
  • excess ADH
24
Q

What are the consequences of water excess?

A
  • 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),
25
Q

What is hydrostatic pressure?

A

Pressure difference between plasma and interstitial fluid
Water moves from plasma into interstitial fluid

26
Q

What is oncotic pressure?

A

Pressure caused by the difference in protein concentration between the plasma and interstitial fluid
- Water moves from interstitial fluid into plasma

27
Q

How does water move at the arteriole and venule end of capillaries?

A

Arteriole: more pressure so H20 moves out
Venule: less pressure so H20 moves in

28
Q

What is oedema?

A
  • 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
29
Q

What is serous effusion?

A

Excess water in body cavity e.g. lungs

30
Q

Inflammatory oedema and the capillaries?

A

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

31
Q

Venous oedema and the capillaries

A

Venous (increased end pressure) - due to increased venous pressure or venous obstruction from a thrombus.
- Causes pooling of blood in calves

32
Q

Lymphatic oedema and capillaries?

A

Lymphatic (blocked) - Obstructions from a tumour/parasite

33
Q

Hypoalbuminaemic oedema and capillaries?

A

Hypoalbuminaemic - lower oncotic pressure. Body doesn’t produce enough albumin

34
Q

What is a pleural effusion?

A
  • 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
35
Q

What can enter the pleural cavity during a pleural effusion?

A

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

36
Q

What is Hypernatraemia?
Causes and consequences

A
  • 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
37
Q

What is Hyponatraemia?
Causes and consequences

A
  • 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

CVR Lectures (28 decks)

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