Disorders of Water Balance: Physiology Flashcards
What determines water balance?
Intake (oral fluids, food, metabolism, IV fluids) and output.
What is the range of plasma osmolality maintained in the body?
280-295 mOsm/kg H₂O.
Which axis is involved in maintaining plasma osmolality?
Hypothalamic-pituitary-ADH axis.
What are the key regulators of water balance?
- Thirst * ADH * Kidney response
What stimulates osmotic thirst?
↑ plasma osmolality (osmoreceptor shrinkage in anterolateral hypothalamus).
What triggers hypovolemic thirst?
Angiotensin II.
Do ineffective osmolytes stimulate thirst?
No.
What is the structure of ADH?
Nonapeptide (arginine vasopressin/AVP).
Where is ADH synthesized?
In supraoptic/paraventricular hypothalamic nuclei.
What is the half-life of ADH?
10-25 minutes.
What are the main stimuli for ADH release?
- Osmotic: ↑ plasma osmolality (>295) * Nonosmotic: Nausea, pain, hypoglycemia, hypoxia
What substances decrease ADH release?
- Ethanol * Phenytoin * PGE₂
What is copeptin?
Stable surrogate biomarker for ADH.
In which conditions is copeptin elevated?
- SIADH * Hypovolemia * Stress
In which conditions is copeptin low?
- Primary polydipsia * Cortisol deficiency
What is the clinical use of copeptin?
Diagnosing polyuric syndromes, predicting ADPKD progression.
What is the function of AQP-1?
Constitutive water reabsorption.
Where is AQP-2 located and what regulates it?
Collecting duct (apical), regulated by ADH.
What is the primary action of ADH in the kidney?
↑ Water permeability in collecting ducts.
What happens to urine concentration when ADH is low?
Dilute urine (50 mOsm/kg).
What are the components of free water clearance (Cₕ₂₀) calculation?
C_{H_2O} = V - C_{osm}.
What does a negative free water clearance indicate?
Water retention.
What characterizes hyponatremia?
Water excess (↓ osmolality <280).
What characterizes hypernatremia?
Water loss (↑ osmolality >295).
What was a key finding in the case of SIADH?
Serum Na⁺ 114, urine osmolality 540.
What can worsen hyponatremia in SIADH?
Hypotonic saline.
What is the role of AQP-3/4?
Water exit from the collecting duct (basolateral).
What is the effect of ADH on urea transport?
↑ Urea transport (medullary recycling).
What activates the Na/K/Cl cotransporter?
ADH.
Fill in the blank: The primary action of ADH is to increase _______ in the collecting ducts.
water permeability.
