Water and Sodium

Question 1

How many litres of water in a 70kg male?

Answer 1

Total body water:
60% of body weight, 42L

Intracellular fluid:(in cells)
40% of body weight, 28L

Extracellular fluid:(in the plasma) 20% of body weight, 14L
Intravascular: plasma (circulates as the fluid component of blood) 3L
Intersitial: between cells (surrounds the cells, but does not circulate) 11L

Question 2

Water movement

Answer 2

Water is freely permeable through ICF and ECF
Determined by osmotic contents
Any change leads to a water shift from high osmolality to low osmolality so eventually:
Always equal = isotonic

Question 3

ECF

Answer 3

Sodium is the main contributor to ECF osmolality and volume

Anions chloride and bicarbonate.

Glucose and urea- contribute to osmolality

Protein = colloid osmotic pressure (oncotic)

Question 4

ICF

Answer 4

Predominant cation is potassium

Question 5

Plasma Osmolality

Answer 5

Largely determined by sodium and associated anions

Estimated plasma osmolality =
2[Na] + 2[K] + urea + glucose mmol/L

Intra- and extracellular osmolality are equal

Change in plasma osmolality pulls or pushes water across cell membranes

Question 6

Under normal circumstances fluid intake = fluid loss

Answer 6

slide 7 + 8

Water in: food and drink mainly, metabolic production

Water loss: skin, lungs, lots through urine, faeces

Question 7

Why don’t we give water intravenously?

Answer 7

It is hypo-osmolar/hypotonic vs cells (lower osmotic pressure and lower nutrients etc in blood compared to the cells)
So Water enters blood cells causing them to expand and burst: haemolysis
However, this only occurs in the vicinity of the intravenous cannula
If you could achieve instantaneous mixing it wouldn’t occur

Question 8

ECF Osmolality

Answer 8

Is very tightly regulated

Changes in ECF osmolality lead to a rapid response

Normal plasma osmolality 275-295 mmol/kg REMEMBER VALUE

Water deprivation or loss will lead to a chain of events

Question 9

ECF Volume

Answer 9

Changes in ECF volume cause a slower response compared to osmolality

Question 10

Water deprivation/Dehydration

Answer 10

Increase in the ECF osmolality
Firstly, The water will move from the cells (from ICF to ECF)

Other thing that happens is that the hypothalamus is stimulated and thirst centre is stimulated- increasing water intake

Another thing that happens is increase in ADH from the posterior pituitary. This affects the renal gland and so the kidneys.
ADH causes the walls of the distal convulated tubules and the collecting duct to become more permeable to water so more water leaks out back into the body.
So you end up with a very small volume of concentrated urine

Question 11

Renin-angiotensin- aldosterone system

Answer 11

Firstly, the kidney detects a fall in ECF volume because there’s a decrease in renal perfusion pressure

This acts on the juxtaglomerular apparatus causing the release of renin into the system

Renin now acts on a protein called angiotensinogen which is the precursor to angiotensin I. Renin cleaves angiotensinogen into angiotensin I

angiotensin I is now converted into angiotensin II by a protein called ACE (angiotensin converting enzyme). This conversion occurs in the lungs as that is where ACE is produced

Adrenal gland detects the angiotensin II and secretes aldosterone

Aldosterone acts on the distal tubes of the kidneys and so it changes the re absorption of potassium and sodium and water retention (increase in water retention). This changes the effect in circulating volume

Aldosterone can also act in another way- ADH secretion again by the pituitary gland so there is more absorption of through the collecting ducts of the kidneys and there will also be a system wide arterial vasoconstriction which will increase the blood pressure as well. This changes circulating volume

Process repeats again

Question 12

Causes of water depletion

Answer 12

reduced intake

sweating

vomiting
diarrhoea
diuresis/diuretics

Question 13

Dehydration

Answer 13

Thirst
Dry mouth
Inelastic skin
Sunken eyes
Raised haematocrit- proportion of blood that is made up
of the red blood cells
Weight loss
Confusion – brain cells
Hypotension

Question 14

Water excess

Answer 14

Decrease in ECF osmolality

Water moves from ECF to ICF, causing cells to swell.

No stimulation of thirst centre in hypothalamus

Inhibition of ADH from posterior pituitary gland so increase in dilute urine volume

Risk of cerebral over hydration if acute excessive intake (water intoxication)

Question 15

Consequences of water excess

Answer 15

Hyponatraemia

Cerebral overhydration
Headache
Confusion
Convulsions

Question 16

Hydrostatic pressure

Answer 16

Pressure difference between plasma and interstitial fluid

Water moves from plasma into interstitial fluid

Question 17

Oncotic pressure

Answer 17

Pressure caused by the difference in protein concentration between the plasma and interstitial fluid

Water moves from interstitial fluid into plasma

Question 18

Normal

Answer 18

slide 24-25

Loss from plasma= gain to plasma
No net change

Question 19

Oedema

Answer 19

Excess accumulation of fluid in interstitial space

Question 20

What causes oedema?

Answer 20

Disruption of the filtration and osmotic forces of circulating fluids

Obstruction of venous blood or lymphatic return
Inflammation;↑capillary permeability
Loss of plasma protein

Question 21

Serous effusion

Answer 21

Excess water in a body cavity

Question 22

Pathogenisis of oedema and serous effusion

Answer 22

Increased fluid leakage into interstitial spaces OR
Impaired reabsorption of fluid

Question 23

Different types of oedema

Answer 23

Inflammatory
Venous
Lymphatic
Hypoalbuminaemic

Question 24

Pleural effusions

Answer 24

The normal pleural space contains ~10 mL of fluid

Balance between
-hydrostatic and oncotic forces in the visceral and parietal pleural vessels.
-lymphatic drainage.

Pleural effusions result from disruption of this balance

Question 25

In a pleural effusion, different fluids can enter the pleural cavity

Answer 25

Transudate is fluid pushed through the capillary due to high pressure within the capillary.
Low protein content

Exudate is fluid that leaks around the cells of the capillaries caused by inflammation &↑permeability of pleural capillaries to proteins.
High protein content

Question 26

Why is pleural fluid protein measured?

Answer 26

Pleural fluid protein is measured to differentiate between exudative (eg malignancy, pneumonia) and transudative (eg LVF, cirrhosis, hypoalbuminaemia, peritoneal dialysis) effusions

Question 27

Disorders of plasma sodium: general principles

Answer 27

Normal (reference) range 135 -145 mmol/L

Concentration is a ratio, not a measure of total body content

High or low [Na] are more often due to gain or loss of water, rather than Na

Clinical effects are on the brain due to constrained volume (skull)

Rate of change is more important than absolute levels

Question 28

Hypernatraemia (↑ Na+) e.g. Caused by

Answer 28

Water deficit:
Poor intake
Osmotic diuresis (increased urination)
Diabetes insipidus (not related to diabetes)

Sodium excess:
Mineralocorticoid (aldosterone) excess
Salt poisoning

Effects: Cerebral intracellular dehydration (tremors, irritability, confusion)

Question 29

Hyponatraemia (↓ Na+) e.g. Causes by

Answer 29

Artefactual

Sodium loss
Diuretics
Addison’s disease

Excess water
IV fluids (iatrogenic)
SIADH

Excess water ++ and sodium +
Oedema

Effects: Cerebral intracellular over-hydration (headache, confusion, convulsions)