Control and Abnormalities of Body Water

Question 1

As a recap, list the percentages of the fluid compartments.

Answer 1

• BODY WATER: 60% of the body weight
• INTRACELLULAR WATER (ICFV): 40% of the body weight
• EXTRACELLULAR WATER (ECFV): 20% of the body weight

Question 2

What does osmosis determine?

Answer 2

Movement of fluid between the ICFV and the ECFV.

Question 3

What do changes in the plasma [Na] suggest?

Answer 3

Excess or deficit of body water

Question 4

What is osmolality?

Answer 4

Number of particles per unit volume of fluid.

Question 5

How is osmolality different from osmolarity?

Answer 5

Osmolality: measured per unit weight of fluid
Osmolarity: measured per unit volume of fluid

Question 6

What does hyponatraemia signal?

Answer 6

Hypo-osmolality (too little water)

Question 7

What does hypernatraemia signal?

Answer 7

Hyperosmolality (too much water)

Question 8

How is the osmolality of the ECFV adjusted?

Answer 8

Using osmoreceptors

Question 9

What are osmoreceptors?

Answer 9

• Sensory receptors located in the hypothalamus
• Sense changes in osmolality of the ECFV.

Question 10

What does an increase in osmolality cause?

Answer 10

Stimulates thirst and ADH secretion

Question 11

Describe how ADH (vasopressin) acts as the osmoregulation hormone.

Answer 11

Regulates plasma osmolality primarily by controlling water excretion and reabsorption

Question 12

What must the kidney be able to do in response to ADH?

Answer 12

• Excrete urine that is either hyperosmotic or hypo-osmotic with respect to the ECF

Question 13

Describe the mechanism of action of ADH in the distal tubule and collecting duct.

Answer 13

• Vasopressin binds to the membrane receptor.
• Receptor activates the cAMP secondary messenger system.
• Cell inserts AQP2 water pores into the apical membrane.
• Water is absorbed by osmosis into the blood.

Question 14

ADH secretion is regulated by two major physiological mechanisms.
What are they?

Answer 14

• baroreceptor input
• RAAS

Question 15

What are the abnormalities following water excess?

Answer 15

• excessive water intake
• impairment of renal water excretion

Question 16

What are the abnormalities following water depletion?

Answer 16

• insufficient water intake
• impairment of renal water absorption

Question 17

Continued water intake with failure to suppress ADH can lead to water overload and hyponatremia.
List some examples of such a scenario.

Answer 17

• vomiting, diarrhoea
• certain drugs (MDMA, ‘ecstasy’ promotes ADH secretion)
• ectopic secretion of ADH
• hypocortisolism
• primary adrenal insufficiency

Question 18

Describe the Syndrome of Inappropriate ADH secretion (SIADH).

Answer 18

• Reduces the urinary excretion of water.
• Water excess
• Low plasma sodium and osmolality
• High urine osmolality

Question 19

What are the major causes of SIADH?

Answer 19

• TUMOUR: ectopic production of ADH, such as a small cell carcinoma of the lung
• CNS DISTURBANCE: enhanced ADH release due to trauma
• DRUGS: enhanced release of ADH or response to ADH, such as carbamazepine, Prozac

Question 20

Describe who water depletion (dehydration) can occur in.

Answer 20

• infants
• elderly (demented, stroke, etc)
• individuals in coma
• individuals with no access to water

Question 21

What can water depletion lead to?

Answer 21

• diabetes mellitus
• impairment in ADH release and/or action

Question 22

What are the two types of diabetes insipidus?

Answer 22

• Central diabetes insipidus
• Neurogenic diabetes insipidus

Question 23

What is the difference between central and neurogenic diabetes insipidus?

Answer 23

• CENTRAL: Lack of ADH secretion
• NEUROGENIC: Impaired response to ADH

Question 24

What are the causes of central diabetes insipidus?

Answer 24

• genetic mutations
• head trauma
• disease of the hypothalamus

Question 25

What are the causes of neurogenic diabetes insipidus?

Answer 25

- mutation of the ADH receptor - mutation of the ADH-dependant H2O channels - renal disease

Question 26

What is the equation for estimating plasma osmolarity?

Answer 26

2[Na+] + 2[K+] + [glucose] + [urea]

Question 27

What happens during diabetes mellitus?

Answer 27

- glucose concentration rises - contributes to the osmolality - high glucose concentration is filtered into the kidney tubule

Question 28

What does concentrated urine tell you about ADH levels?

Answer 28

→ ADH relatively high

Question 29

Describe the loop of Henle. PART 1

Answer 29

→ Tubular fluid is iso-osmotic in the PCT → along the descending limb there is active reabsorption of Na+ → Water moves out passively

Question 30

Describe the loop of Henle. PART 2

Answer 30

→The loop has very concentrated interstitial fluid → in the ascending loop there is dilution of the tubular fluid as there is reabsorption of NaCl → at the DCT the tubular fluid is the most dilute

Question 31

What does a large drop in arterial pressure cause?

Answer 31

ADH release

Question 32

Why is ADH secreted during hypovolaemia?

Answer 32

→ retention of water to increase blood volume

Question 33

What happens to plasma osmolality during severe haemorrhage?

Answer 33

→ Loss of BP is sufficient to stimulate ADH → decrease in plasma osmolality

Question 34

What is hyperosmotic thirst?

Answer 34

→ occurs after eating a lot of

Question 35

What is hypovolemic thirst?

Answer 35

→ Occurs after losing a lot of blood

Question 36

What is missing in people with Addisons?

Answer 36

→ Loss of cortisol → Loss of aldosterone

Question 37

What is the effect of Addisons?

Answer 37

→ Lack of sodium retention → Water is lost with it → hyponatremia due to water intake

Question 38

How do you estimate the solute load?

Answer 38

→ body weight x 10

Question 39

How do you work out how much water is needed to excrete the solute load?

Answer 39

→ divide the urine concentration by the solute load