What is the normal range for plasma osmolality?
285-295 mosmol/kg
At what levels does hyperkalaemia occur?
Plasma [K+] concentration > 5.5mM
Where are ROMK channels expressed?
In the principal cells of the collecting duct
What does ECF refer to?
Plasma in vascular system and interstitial fluid that surrounds the cells
What can increase the ECF osmolarity?
- water deprivation
- solute ingestion
- diarrhoea
What receptors detect an increase in osmolarity?
osmoreceptors located close to the supraoptic and paraventricular nuclei in the hypothalamus of the brain
Effect of stimulating osmoreceptors
- send signals to posterior pituitary gland to release ADH into the blood
What does ADH do?
Makes the cells of the collecting duct more permeable to water causes water retention by the kidneys
What level of osmolarity do the lateral preoptic receptors detect?
changes in osmolarity > 295 mosm/l
Cause a thirst
What decreases ECF osmolarity
excessive fluid ingestion
- detected by the osmoreceptors in the supraoptic and paraventricular nucleus of the hypothalamus to suppress ADH release from posterior pituitary
- to make cells of the CD impermeable to water
- more water excretion via the kidneys–> diuresis
Osmoreceptors in the lateral pre-optic area suppress thirst
How is the volume of ECF regulated?
- osmolarity of ECF is tightly controlled
- volume of ECF is determined by the volume of solute (mainly NaCl)
- regulation of volume is all about sodium balance
Quantity of dietary salt intake?
Intake varies from 0.05-25g/day
Average salt intake is 2.3g/day
What is an important determinant of blood pressure in the veins, cardiac chambers and arteries?
plasma volume!!
low total body sodium –> low plasma volume –> low cardiovascular pressures
Size of intracellular fluid compartment?
twice the size of ECF
10L of interstitial fluid + 4L of plasma
Main solute in ECF and ICF
sodium in ECF
potassium in ICF
Through what organ does the body directly control osmolarity and volume of ECF in vascular system?
kidneys
affects the osmolarity and volume of other compartments
What do osmoreceptors control?
- water renal excretion by altering ADH release
- water intake by altering thirst
Equation for amount of sodium excreted?
sodium excreted= sodium filtered- sodium reabsorbed
How does the kidney control Na+ content?
By controlling GFR and sodium reabsorption
regulate them separately
What process is frequently ignored?
- small passive secretion process of Na in the descending limb of the loop of Henle
What does the GFR depend on?
- combination of opposing starling forces (hydrostatic and colloid osmotic)
- hydraulic permeability
- characteristics of capillaries
- surface area
regulated by both neural and hormonal input
can be controlled extrinsically and intrinsically
What reduces the GFR
- constricting afferent arteriole
- decreases blood flow
- decreases hydrostatic pressure gradient
- reduces GFR
Describe the extrinsic control of GFR to maintain systemic blood pressure.
- Activation of sympathetic nervous system (baroreceptor response- stretch receptors in carotid and aortic sinus)
- If there is a decrease in stretch, they will send signals to the cardiovascular centre in the brain which activates the sympathetic nervous system
- SN vasoconstricts afferent arteriole which reduces GFR
- Reduces surface area of filtration barrier via mesangial cells which reduces GFR
- ^ activates the mesangial cells that lie over the glomerular capillaries- when activated the actin contract reducing the SA of filtration barrier
- Also increases renin release
- Reduction in GFR will conserve sodium and water and increase blood volume/ pressure!!
- As tubular flow has decreased, it allows time for Na and water reabsorption processes to occur
Describe the intrinsic control of GFR to protect capillaries from hypertensive damage and increase blood volume/ pressure
Autoregulation’ within the kidney can control afferent arteriole constriction
• Mechanisms include
o Myogenic response by the renal smooth muscle cells that surround arterioles (vasoconstriction in response to stretch)
o Tubuloglomerular feedback by the juxtaglomerular apparatus (controls vasoconstriction of AA and renin release)
- Renal blood flow (RBF) and GFR remain constant for arterial pressures between 90- 200 mmHg
- Afferent arterioles constrict when BP is raised and dilate when BP is lowered, thus maintaining constant capillary pressure and glomerular blood flow.
- Purpose: to protect glomerular capillaries from hypertensive damage and maintain a healthy GFR.
