Control and Abnormalities of Body Water

Question 1

What’s the difference between sodium and water balance?

What is the role of Na in ECF osmolality, and what is the equation used?

What is Hypernatraemia and Hyponatraemia?

Answer 1

• Sodium balance occurs by removing water from the body. Water balance simply occurs by intake and output.
• [Na+] is the main determinant of ECF osmolality:
  Osmolality = 2[Na+] + 2[K+] + [glucose] + [urea]
• Hypernatraemia - hyperosmolality due to too little water = high blood [Na+]
  Hyponatraemia - hypoosmolality due to too much water = low blood [Na+]

Question 2

Why is osmoregulation important?

What are used to detect these changes?

Answer 2

• Important as changes in ECF osmolality has major effects on brain cells:
  Hypotonicity (low osmolality) = swelling of brain cells
  Hypertonicity (high osmolality) = shrinking of brain cells
• Can lead to confusion, headaches, coma
• Sensory Osmoreceptors in the hypothalamus

Question 3

What occurs when there’s an increase in ECF Osmolality?

How is ADH secreted?

What does ADH act on?

Answer 3

• Osmoreceptors detect ↑ and stimulate Thirst and ADH secretion.
• Produced by Magnocellular neurons in hypothalamus and sent to the Posterior pituitary - it puts ADH in vesicles, ready for secretion
• Cells of late DCT/CD

Question 4

How does ADH work to produce concentrated urine during water deficit?

What are the 2 main factors that control ADH secretion?

Answer 4

• 1. ADH binds to V2 receptors on basolateral membrane of distal tubule = activates cAMP system
     
     2. ↑cAMP = ↑Aquaporin channel insertion on the luminal membranes
     3. ↑Water reabsorption as result = Concentrated urine
• Plasma Osmolality - ↑Osmolality = ↑ADH secretion - uses osmoreceptors in hypothalamus
  Arterial BP - ↓↓↓Arterial BP = ↑ADH secretion - uses baroreceptors in carotid bodies

Question 5

What is the role of thirst?

What are the 2 types of thirst?

How is the hypothalamic thirst centre stimulated?

Answer 5

• First line of defence against dehydration
• Hyperosmotic thirst - due to high osmolality
  Hypovolemic thirst - due to low BV
• Dehydration causes ↓Salivary flow, Stimulation of hypothalamic osmoreceptors, and ↓BV which stimulates the RAAS.

Question 6

Abnormalities of water balance:
1. Water excess:
What is it due to?

What are some of the consequences?

Give examples where this will occur?

Answer 6

• Excessive water intake or impairment in renal water excretion
• Continued water uptake with failure to prevent excess ADH secretion = Water overload and Hyponatraemia. Excessive ADH despite a decreasing plasma osmolality will lead to a decrease in urine production.
• Addison’s disease, Vomiting, Diarrhoea, Drugs, Ectopic ADH secretion

Question 7

2. Water Depletion:
   What is it due to?

What is Diabetes Insipidus (DI), what does it cause, and what are its 2 types?

Answer 7

• Insufficient water intake (infants, elderly) or impairment in renal water absorption (diabetes, poor ADH release/action)
• DI is the dysfunction of ADH, leading to ↓urine osmolality and ↑plasma osmolality:
  1. Central DI - lack of ADH secretion
  2. Nephrogenic DI - impaired response to ADH