Body Fluids and Electrolytes Flashcards
Distribution of total body water
Total body water
- ~42L in 70kg male. ~60% of weight. (600mLs/kg)
- This can be measured by tritium oxide - a radioactive isotope of hydrogen.
○ Equilibrates to all compartments within 3-4hrs
Distribution of TBW
- Intracellular Fluid (66%)
- Extracellular Fluid (33%)
○ Physiologically active/Functional ECF
–> Interstitial fluid (20%)
–> Intravascular fluid (7.5%)
–> Transcellular fluid (2.5%) - (fluid which is formed from the transport activities of cells). This fluid is in contact with intracellular fluid through epithelial membranes (CSF, Bile, Bladder urine, Aqueous humor, Joint fluid, Fluid in body cavities, Fluid in bowel)
Note: ratio of ICF:functional ECF = 55:30 (almost 2:1)
Control of body water
Sensors
* Osmoreceptors - located in the hypothalamus
— Circumventricular organs - ie nil BBB
— Mechanoreceptors –> changes firing rate secondary to osmotic swelling or shrinking
— Afferent limb: Via organum vasculosum lamina terminalis (OVLT) –> hypothalamus
* High pressure baroreceptors - located in carotid sinus & aortic arch - detect changes in systemic blood pressure
— Afferent limb via glossopharyngeal nerve
Central control
* Hypothalamus is responsible for overall control of water balance
— Supraoptic nucleus + periventricular nucleus - this is where magnocellular neuron cell bodies lie (vasopressin producing cells)
Effectors
* Water output: Efferent signal is vasopressin (released from nerve terminals in posterior pituitary
— Secretion minimal if Osmolality <285mOsm/kg
— Increases up to 20x normal value when ECF osm increases, and increases massively (1000x) in presence of hypotension
— ADH binding to V2 receptors in renal collecting duct –> expression of apical aquaporins (water retention, urine is concentrated)
* Water input: Thirst - hypothalamic neurons –> anterior cingulate gyrus –> thirst sensation + behavioural changes
TBW is also affected by:
- RAAS - AII increases Na+/H+ exchange in PCT –> Na+ retention, thus H2O retention
- Aldosterone - release stimulated by AII –> mineralocorticoid receptor in DCT & increases expression of eNAC –> Na+ reabsorption –> H2O retention
- Natriuetic peptides - Volume receptors (low- pressure baroreceptors) - located in the walls of RA & large veins - sensitive to changes in volume by detecting changes in CVP (stretch receptors) –> decreased volume –> decreased ANP, BNP –> increases Na+ & H2O retention
