Control and Abnormalities of Body Water Flashcards
As a recap, list the percentages of the fluid compartments.
- BODY WATER: 60% of the body weight
- INTRACELLULAR WATER (ICFV): 40% of the body weight
- EXTRACELLULAR WATER (ECFV): 20% of the body weight
What does osmosis determine?
Movement of fluid between the ICFV and the ECFV.
What do changes in the plasma [Na] suggest?
Excess or deficit of body water
What is osmolality?
Number of particles per unit volume of fluid.
How is osmolality different from osmolarity?
Osmolality: measured per unit weight of fluid
Osmolarity: measured per unit volume of fluid
What does hyponatraemia signal?
Hypo-osmolality (too little water)
What does hypernatraemia signal?
Hyperosmolality (too much water)
How is the osmolality of the ECFV adjusted?
Using osmoreceptors
What are osmoreceptors?
- Sensory receptors located in the hypothalamus
- Sense changes in osmolality of the ECFV.
What does an increase in osmolality cause?
Stimulates thirst and ADH secretion
Describe how ADH (vasopressin) acts as the osmoregulation hormone.
Regulates plasma osmolality primarily by controlling water excretion and reabsorption
What must the kidney be able to do in response to ADH?
- Excrete urine that is either hyperosmotic or hypo-osmotic with respect to the ECF
Describe the mechanism of action of ADH in the distal tubule and collecting duct.
- 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.
ADH secretion is regulated by two major physiological mechanisms.
What are they?
- baroreceptor input
- RAAS
What are the abnormalities following water excess?
- excessive water intake
- impairment of renal water excretion
What are the abnormalities following water depletion?
- insufficient water intake
- impairment of renal water absorption
Continued water intake with failure to suppress ADH can lead to water overload and hyponatremia.
List some examples of such a scenario.
- vomiting, diarrhoea
- certain drugs (MDMA, ‘ecstasy’ promotes ADH secretion)
- ectopic secretion of ADH
- hypocortisolism
- primary adrenal insufficiency
Describe the Syndrome of Inappropriate ADH secretion (SIADH).
- Reduces the urinary excretion of water.
- Water excess
- Low plasma sodium and osmolality
- High urine osmolality
What are the major causes of SIADH?
- 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
Describe who water depletion (dehydration) can occur in.
- infants
- elderly (demented, stroke, etc)
- individuals in coma
- individuals with no access to water
What can water depletion lead to?
- diabetes mellitus
- impairment in ADH release and/or action
What are the two types of diabetes insipidus?
- Central diabetes insipidus
- Neurogenic diabetes insipidus
What is the difference between central and neurogenic diabetes insipidus?
- CENTRAL: Lack of ADH secretion
- NEUROGENIC: Impaired response to ADH
What are the causes of central diabetes insipidus?
- genetic mutations
- head trauma
- disease of the hypothalamus
What are the causes of neurogenic diabetes insipidus?
- mutation of the ADH receptor
- mutation of the ADH-dependant H2O channels
- renal disease
What is the equation for estimating plasma osmolarity?
2[Na+] + 2[K+] + [glucose] + [urea]
What happens during diabetes mellitus?
- glucose concentration rises
- contributes to the osmolality
- high glucose concentration is filtered into the kidney tubule
What does concentrated urine tell you about ADH levels?
→ ADH relatively high
Describe the loop of Henle. PART 1
→ Tubular fluid is iso-osmotic in the PCT
→ along the descending limb there is active reabsorption of Na+
→ Water moves out passively
Describe the loop of Henle. PART 2
→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
What does a large drop in arterial pressure cause?
ADH release
Why is ADH secreted during hypovolaemia?
→ retention of water to increase blood volume
What happens to plasma osmolality during severe haemorrhage?
→ Loss of BP is sufficient to stimulate ADH
→ decrease in plasma osmolality
What is hyperosmotic thirst?
→ occurs after eating a lot of
What is hypovolemic thirst?
→ Occurs after losing a lot of blood
What is missing in people with Addisons?
→ Loss of cortisol
→ Loss of aldosterone
What is the effect of Addisons?
→ Lack of sodium retention
→ Water is lost with it
→ hyponatremia due to water intake
How do you estimate the solute load?
→ body weight x 10
How do you work out how much water is needed to excrete the solute load?
→ divide the urine concentration by the solute load
