Acid base Flashcards
What’s the total body water in men? And women?
Why does it decrease with age?
60% men and 50% women. Decreases due to an increase in body fat with age.
How much water is intra vs extracelular?
ECW 1/3 of TBW
ICW 2/3 of TBW
How much from the ECV is located in the interstitium vs plasma?
Interstitium = 3/4 of ECV
Plasma = 1/4 of ECV
How is the osmolality in the interstitium vs plasma?
All the same because water passes freely between them
What determines the EXTRACELLULAR FLUID VOLUME?
Sodium status
What determines the Serum Sodium concentration?
Water status
Total body sodium = the amount of Sodium present in the ECFV (although there is Na is the intracellular space and other places but are judged to be osmotically inactive).
An increase in TBNa will increase ECFV (edema, effusion, ascites…). A decrease in TBNa will decrease ECFV (ortho hypotension, lightheaded…)
Which mechanisms control Na excretion and concentration?
1- receptors in Juxtaglomerular cells if kidney sense changes in renal perfusion and responds through release of renin.
2- volume receptors in great veins and atria that sense High Volume and Respond by releasing atrial natriuretic peptide to excrete Na in case of volume overload.
3- pressure receptors in aorta and carotid sinus that sense hypotension and increase renal production by sympathetic reflex.
What are the 3 mechanisms that increase production of Renin?
Decreased pressure in renal Afferent artery.
Decreased sodium delivery to distal nephron.
Sympathetic nervous stimulation.
What’s the formula for serum osmolality?
(2xNa) + (Glu/18) + (BUN/2.8)
What is tonicity and how is that different to osmolality?
Osmolality is the total solute concentration in a fluid compartment.
Tonicity is the ability of the combined solutes to generate an osmotic driving force to cause water movement between compartments. So in order to increase tonicity of the ECV, the solute cannot be able to cross to the ICV (must be confined to where the water should flow).
What are effective osmoles?
Substances that generate tonicity (unable to cross compartments so able to generate water draw to where they are).
Glucose
Sorbitol
Mannitol
Sodium
What is the OSMOLAL GAP?
It is the difference between the Measured serum Osmolality - Calculated serum Osmolality.
The lab measures the Measured osmolality utilizing other substances as osmotically active and these do not enter our formula for the Calculated one.
So if there is a difference greater than 10 between the measured and the calculated, we know such a substance has to be present in that blood sample.
These are substances that could cause this:
- ethanol
- isopropanol
- methanol
- ethylene glycol
- propylene glycol
- sorbitol
- mannitol
At what GFR does the kidney start to experience problems with dilution and concentration?
20% of normal
Why do we see HypoNa in patients with fluid overload (edema states: cirrhosis, nephrosis, HFREF) AND in patient with volume down (vomiting)?
Because the regulating step for concentration of urine is located at the DISTAL tubules and in these cases, the PROXIMAL tubules absorb too much water (edema states) or as much as they can (volume down), leading to too little water going through the distal tubules.
Hence, distal tubules perceive urine as very concentrated and retains free water (worsening hypoNa).
