Na+ and H2O Balance

Question 1

Which hormone controls…

• Water balance
• Sodium balance?

Answer 1

• Water balance: anti-diuretic hormone (ADH)

- Sodium balance: steroids (released from the adrenals)

Question 2

When is ADH released and from where?

Answer 2

When blood H2O is low (i.e., blood is too concentrated), ADH is released from the posterior pituitary gland

Question 3

What is the effect of ADH release?

Answer 3

It acts on the kidneys to cause water to be reabsorbed from the renal tubules

This increases blood volume

Question 4

High ADH = small/large volume of dilute/concentrated urine

?

Answer 4

High ADH = small volume of concentrated urine

Question 5

Low ADH = small/large volume of dilute/concentrated urine

Answer 5

Low ADH = large volume of dilute urine

Question 6

Urine concentration is measured as urine osmolality.

Concentrated urine = high osmolality. T/F?

Answer 6

True

Concentrated urine = high osmolality

Dilute urine = low osmolality

Question 7

Which is the main steroid hormone involved in Na+ balance?

Answer 7

Aldosterone (a mineralocorticoid released from the zona glomerulosa)

Question 8

The effect of steroids on Na+ balance is known as mineralocorticoid activity. Describe mineralocorticoid activity

Answer 8

MA = Na+ is reabsorbed in the renal tubules in exchange for K+/H+

Question 9

In the body, too much mineralocorticoid activity results in a sodium loss. T/F?

Answer 9

False

Excess MA = sodium gain

Too little MA = sodium loss

Question 10

What molecule follows Na+ that is reabsorbed in the renal tubules?

Answer 10

Water

Question 11

What equation links H2O and Na+ balance?

Answer 11

Na+ conc. = the ratio of Na+ to water:

[Na+] = (mmol Na+) / (1L H2O)

Question 12

Using the equation:
[Na+] = (mmol Na+) / (1L H2O)

Name 2 causes of hyponatraemia

Answer 12

Too little Na+

Too much H2O

Question 13

Using the equation:
[Na+] = (mmol Na+) / (1L H2O)

Name 2 causes of hypernatraemia

Answer 13

Too much Na+

Too little H2O

Question 14

Loss/gain of H2O is from/to…

a) Intracellular fluid
b) Extracellular fluid
c) All body compartments

Answer 14

c) All body compartments

Question 15

Loss/gain of sodium is from/to…

a) Intracellular fluid
b) Extracellular fluid
c) All body compartments

Answer 15

b) Extracellular fluid

due to control by the Na+/K+ ATPase

Question 16

Loss of Na+ from the ECF results in water loss from…

a) The ECF
b) All body compartments

Answer 16

a) The ECF

Question 17

Changes in fluid compartment volumes are greater when caused by changes in...
a) Na+
b) H2O
?
Why?

Answer 17

a) Na+

This is because changes caused by Na+ are confined to the ECF and so the effect on that compartment is greater

Question 18

Why does change in H2O volume have less effect than Na+?

Answer 18

Changes caused by H2O have less effect as they are shared across all body compartments

Question 19

Hypernatraemia due to excess Na+ is common. T/F?

Answer 19

False

It is actually very rare

Hypernatraemia is usually caused by too little H2O e.g., in patients who are unable to eat or drink

Question 20

Clinical signs of hyper or hyponatraemia are seen when Na+ imbalance is the cause. Why doesn’t H2O imbalance cause the same signs?

Answer 20

H2O imbalance is spread across all body compartments and so does not have enough of an effect to cause clinical signs

Question 21

List some clinical signs of hyponatraemia (decreased ECF volume)

Answer 21

• Tachycardia
• Dry mucous membranes
• Decreased skin turgor
• Decreased consciousness
• Decreased urine output
• Postural hypotension
• Soft/sunken eyeballs

Question 22

List some clinical signs of hypernatraemia (increased ECF volume)

Answer 22

• Abdominal swelling
• Peripheral oedema
• Pulmonary oedema
• Pleural effusion
• Coughing
• SOB
• Tiredness
• Increased pumping of heart

Question 23

List some causes of hyponatraemia (caused by too little Na+ AND too much H2O)

Answer 23

Too little Na+:

• Loss from gut e.g., vomiting, diarrhoea
• Loss from skin e.g., sweating, burns
• Reduced food or drink intake

Too much H2O:

• SIADH (most common, causes reduced H2O excretion)
• Compulsive water drinking

Question 24

List some causes of hypernatraemia (caused by too much Na+ AND too little H2O)

Answer 24

Too much Na+: (rare)

• Some IV meds
• Near drowning

Too little H2O:

• Diabetes insipidus
• Reduced food and drink intake e.g., young or elderly

Question 25

List the workup to treating hyponatraemia

Answer 25

• Work out cause
• Treat too little Na+ by giving Na+ e.g., IV saline or oral Na+ salt
• Treat excess H2O by fluid restricting

Question 26

List the workup to treating hypernatraemia

Answer 26

• Work out cause
• Treat excess Na+ by removing Na+ (using loop diuretics)
• Treat too little H2O by giving H2O as IV dextrose

Question 27

IV saline increases the volume of all body compartments. T/F?

Answer 27

False

Dextrose increases fluid volume across all body compartments

Saline increases fluid volume in the ECF compartment only