2001A07 Describe how the body detects and responds to water deficit.

· Introduction

· Detection: receptors

· Responses: ADH and thirst

Water distribution	· Total body water 60% of body weight (~42L) · 2/3 ICF (28L) · 1/3 ECF (14L) – ¼ plasma, ¾ ISF
Effects of water deficit	· Loss of free water occurs from the ECF · But water crosses membranes freely · Hence both ICF and ECF water volumes contract in proportion · Hence ↑osmolality, ↓blood volume, ↓mAP
Measurement problem	· Can’t measure total body water directly · Use tonicity, CVP, mAP as surrogates
Priorities	· ADH responds to ∆2% osmolality · ADH responds to ∆10% or 500mL blood volume (~4L total body fluid deficit), but overrides osmolality

Osmoreceptors

· Anterior hypothalamus, circumventricular organs (subfornical organ, organum vasculosum laminae terminalis)

· (note: responsive to tonicity, not osmolality)

· Normal 285-295mOsmol/L

Low pressure baroreceptors

· Stretch-sensitive mechanoreceptors

· Walls of atria and great veins

· ↓CVP -> ↓inhibitory afferents

High pressure baroreceptors

· Same

· In the carotid and aortic sinuses

· ↓mAP -> ↓inhibitory afferents

ADH

Source:

· Nonapeptide produced by supra-optic and paraventricular hypothalamic nuclei

· Transported down pituitary stalk

· Secreted by posterior pituitary (neurohypophysis)

Stimuli:

· ↑Osmolality: sensitive ∆2%

· ↓blood volume: sensitive ↓10% = ↓500mL = ↓TBW 4L; overrides osmolality

· ↓mAP

· Angiotensin 2

· Stress: e.g. surgery

· Drugs: e.g. SSRI

Effects:

· Insertion of aquaporin 2 into apical membrane of collecting ducts -> ↑H₂O reabsorption

· Insertion of urea transporters -> ↑ medullary interstitial osmolarity

· ↑Na⁺ reabsorption in thick ascending loop of Henle

Thirst

Controller: hypothalamus

Stimuli:

· ↑osmolality

· ↓blood volume

· ↓mAP

· Angiotensin 2 at SFO and OVLT

Effect:

· ↑Free H₂O intake