Handout for Salt and Water lecture Flashcards
Hypernatremia - meaning
-means high concentration of sodium (excess sodium relative to water in the blood)
What is the most common cause of hypernatremia
Most common cause = lack of water rather than a gain of salt
Hyponatremia -meaning
Low concentration of sodium
Dehydration - meaning
Meaning high concentration of sodium
What determines the extracellular fluid content
How much sodium is in the body
Principal extracellular cation
- Na+ is the principal extracellular cation - makes up >95% of all cations in this space
How normal osmolarity is maintained
The [Na+] = ratio of Na+ to water needs to be maintained as close as possible to normal in order for osmolarity to be normal -disturbance in osmolarity will lead to H2O movement into or out of intracellular compartmentosmolarity to be normal -disturbance
What happens if remove 10% of Na+ from extracellular space
- intracellular space is free of Na+ so what ever forces removed 10% of the body sodium do not affect inside of the cell
- so get 10% drop in osmolarity of extracellular space
- leads to diffusion of water into cells to restore osmotic equilibrium
Sodium content
-The amount of sodium
How sodium content determines the extracellular fluid volume
-disturbances in sodium content trigger similar disturbances in water to keep the ratio of salt to water ([Na]) fixed
How sodium concentration determines intracellular fluid volume
-water shifts across cell membranes to equilibrate the osmolarity of the two compartments
-(and since Na+ is main cation in extracellular fluid its concentration is what determines extracellular osmolarity)
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Exception to rule of sodium concentration determining intracellular fluid volume and sodium content determining extracellular fluid volume
Severe hyperglycemia
-because glucose can between a factor in extracellular osmolarity and thereby lead to water shifts and changes in intracellular volume as well as dilution of serum sodium concentration)
What type of osmoles can be added to the extracelllular space without being fatal
Those that are permeable across cell membranes and thus do not lead to movement of water
Sodium concentration of edema + why
- is no barrier to salt or water movement between intravascular and interstitial compartments
- Na+ concentration of edema fluid = serum sodium concentration
Why edema is an example of disordered sodium content when serum sodium concentration is normal
- Na+ distributes between intravascular and interstitial compartments
- i.e. have to take into account the excess sodium in the interstitum –>person actually has sodium excess
Osmolarity of the ECFV
approx 292
How changes in sodium content lead to changes in water content and result in altered extracellular fluid volume -ex CHF
1) Poor effective circulating volume in CHF
2) Arterial sensors detect poor filing
3) renin, angiotensin ect are increased
4) Kidneys reabsorb more sodium (Ang II)
5) Increase in sodium content = increase in osmolarity
6) Increase in osmolarity detected by hypothalamus
7) Release of ADH
8) Increased water reabsorption and osmolarity returnss to normal
9) Gain in salt and water is proportional - is now volume overloaded
10) Since in case of CHF may still have low circulating (due to low cardiac output) may continue this process –> fluid pushed out into intersitium -= swollen ankles and orthopnea/PND
What can happen in severe CHF
- ADH levels can increase above those necessary to compensate for salt retention
- hyponatremia can result
Is it possible for same patient to have both edema and hyponatremia
- edema = sodium excess (leading to increased extracellular fluid volume)
- is possible that have increased sodium content –> but also too much water that is diluting the sodium leading to hyponatremia
Syndrome of inappropriate ADH secretion (SIADH)
- more ADH acting on kidneys that is necessary to maintain homeostasis the resul = more H2O reabsorbed then salt = hyponatremia
- increase in intravascular fluid will activate arterial and venous stretch receptors -causes a slight net excretion of sodium -not really clinically relevant
- since only osmolarity decreased but Na+ content remained pretty much unchanged –> wil not activate sensors/effectors of sodium content
