Body water, sodium and potassium regulation Flashcards
Describe water regulation
Water regulation
* Required to prevent excess or depletion of intra and extra-cellular (intravascular and extravascular) volume
* Central to maintaining stable BP and tissue perfusion
* Extracellular intravascular volume is most dynamic component - in close contact with kidney and heart so signals TBW content
* Key effector organ: kidney
* Key affector organ: heart - senses intravascular volume depletion and expansion
* Volume expansion: stretched atrium releases AMP; major vessel stretch baroreceptors → reflex excretion of salt and water by kidney
* Volume depletion: major blood vessels constrict, increasing sympathetic tone and causing peripheral vasoconstriction = RAAS
2 players determine homeostatic control:
* Osmocontrol (ADH): hormonal control of NaCl and H2O reabsorption and secretion
- If body preferentially reabsorbs water, extra water dilutes total salt in intravascular and extravascular spaces
- when [Na+] ↓,
osmostat in brain senses ↓ Na+ ∴ ↑ salt reabsorption and ↓ water reabsorption
- Volume control:
- Tubuloglomerular feedback acts on single nephron level
- Nerves and RAAS works on kidneys systemically
- Volume control takes precedence in severe hypovolaemic states (e.g. ↓ BP affecting cerebral and renal perfusion)
- Osmocontrol drives homeostasis in mild hypovolaemia
- osmocontrol prevails over volume control (much stronger and faster response)
Describe the role of baroreceptors
- Arterial baroreceptors: sense pressures in aorta and arteries → send information to brainstem vasomotor centre
- Total peripheral resistance
- Cardiac performance (HR)
- Sympathetic drive to kidney
- Venous compliance (either dilates or contracts)
- Cardiopulmonary receptors: sense pressure in cardiac atria and pulmonary arteries → brainstem vasomotor centre
- Same effects as arterial baroreceptors
- Regulate ADH secretion from hypothalamus
- Intrarenal receptors: located in macula densa
- Drive local paracrine RAAS system → GFR, salt and water reabsorption
Describe volume control measures
BODY WATER, NA+ AND K+
Water regulation
Volume control
* Hypovolaemia
* ↓ ECF volume
* ↓ Pressure detected by atrial and pulmonary baroreceptors
* ↑ secretion of ADH
* ↑ aquaporins in principal cells of collecting ducts
* ↑ water reabsorption
* ↓ water excretion
- Hypervolaemia
- Pure water load
- ↓ osmolality detected by hypothalamic osmoreceptors
- ↓ secretion of ADH
- ↓ aquaporins in collecting ducts (rapidly induced)
- ↓ water reabsorption
- ↑ water excretion
- Severe sweating: removal of hypo osmotic fluid
- ↓ ECF volume → activation of RAAS → ↑ angiotensin II and aldosterone → Na+ reabsorption
- ↑ plasma osmolality (from ↑ salt loss compared to NaCl loss) → ↑ ADH → ↑ sodium and water reabsorption
List factors that modulate urine concentration
- Osmotic gradient of renal medulla interstitium
- Lengths of loops of Henle (longer loops = more
concentrated urine) - Rate of active NaCl reabsorption (Na+ delivery and
Na+/K+ pump activity) - Protein content of diet
- Medullary blood flow (high blood flow washes out
solutes) - Degree of water permeability (of collecting tubules and
ducts) - Luminal flow in LOH and CD - high flow rates reduce
efficacy of countercurrent multiplier - Pathophysiology - central or nephrogenic diabetes
insipidus
Decreases in ECF volume (e.g. hemorrhage or decreased Na+) cause a decrease in renal perfusion pressure, which in turn increases secretion of the enzyme…
renin
The major action of ADH (vasopressin) is to increase […] of the principal cells in the late distal convoluted tubule and collecting duct
…
water permeability (and thus reabsorption)
What receptors sense an increase in plasma osmolarity, providing stimuli to increase ADH secretion?
Osmoreceptors in the anterior hypothalamus
What receptors sense a decrease in plasma volume, providing stimuli to increase ADH secretion?
Baroreceptors
Hypovolemia or volume […] is another potent stimulus for ADH secretion
depletion
sensed by baroreceptors; information transmitted through the vagus nerve to the hypothalamus
The most important physiologic stimulus for increasing antidiuretic hormone (ADH) secretion is […]
increased plasma osmolarity
sensed by osmoreceptors in the anterior hypothalamus
… is a hormone that increases renal Na+ reabsorption, thereby restoring ECF volume and blood volume to normal
Aldosterone
Mineralocorticoids (aldosterone) cause increased renal […]
secretion via action on the principal cells of the late distal tubule and collecting duct
potassium
What is the action of aldosterone?
Aldosterone increases Na+ and H2O reabsorption at the distal tubules and collecting duct
What effect do normal levels of angiotensin II have on GFR?
Increased GFR
due to preferential constriction of efferent arterioles
**
What is the effect on ADH secretion if a patient has low volume AND low osmolarity?
Volume trumps osmolarity and ADH is secreted
