I1i: Describe how body detects and responds to H2O deficit

Define/Quantify

  • TBW = 60% body weight = 42L
  • TBW = ECF + ICF
  • ECF is regulated
  • ICF is dependent on ECF regulation, as H2O moves freely across PM
  • Regulation of H2O balance is via NEGATIVE FEEDBACK

Regulation

Sensors

Osmoreceptors

BaroR

  • High P
  • Low P
  • Intrathecal

Control

Hypothalamus

Effectors

ADH

Renin

Aldosterone

SNS

Sensors

Osmoreceptors

    • ↑osmolality of ECF > 2% (Neutral osmolality = 280 – 295 mOsm/L)
    • ↑linear firing from osmoreceptors cause ↑ADH release from SO & PV nuclei of hypothalamus

Low P Baroreceptors

  • Walls of atria & great veins
  • ≥ 10% volume depletion (4L water, 300mL of plasma)
  • Less sensitive (>10%) cf. OsmoR but more potent because can override OsmoR & take volume over tonicity

High P Baroreceptors

    • Carotid Sinus & Aortic Arch
      • ↓MAP
      • ↓BaroR firing → vasomotor & cardiac centres of Medulla
      • ↑symp & ↓ output

→ VC, ↑HR, ↑FoC, ↑CO → restore MAP

Intrarenal P Receptors

  • BaroR in JG cells
  • ↓MAP = ↓afferent arteriole stretch
  • ↑RENIN from JG cells

Effectors

Effector

ADH

(3 pronged approach!)

Trigger

↑Osmolarity

Stress/surgery

Nausea

AII

Standing

Mechanism

V1 → VC

V2 → of cortical & medullar C ∆

V2 receptor on BL side

GS   

↑cAMP

Promote Aquaporin 2 channel insertion into luminal membrane

V3 → thirst

Result

→ ↓GFR = ↓water filtration

→ ↑water reabsorption

Max urine osmolarity 1400mOsm/L

→ ↑water intake

Effector

Renin

Trigger

β1 stimulation

JG receptor

Mechanism

Renin

Stimulates aldosterone

Stimulates thirst

VC

↑Na+ reabsorption from PCT

Result

↑Na+ reabsorption

↑H2O intake

Effector

Aldosterone

Trigger

AII (Renin)

ACTH

↑K

Sympathetic stimulation

Mechanism

Acts on Mineralocorticoid receptor of PRINCIPLE CELLS

  1. Upregulates Na/K/ATPase
  2. Upregulates apical Na+ channel
  3. ↑Renin peripheral VC

Result

↑Na+ reabsorption

Effector

SNS

Trigger

Stimulation

Mechanism

↑Renin

Peripheral VC

Result

↑MAP

3 Cell Membranes Impermeable to H2O

  1. Ascending LoH
  2. Collecting Ducts in absence of ADH
  3. Bladder Epithelium
  • Thirst = the conscious drive to obtain H2O, stimulated by:
    1. Hypovolaemia (low P BaroR)
    2. Hypotension (High P BaroR)
    3. Hyperosmolarity (OsmoR)
    4. AII
    5. Physical stimulation i.e. dry mouth