Handout for Salt and Water lecture

Question 1

Hypernatremia - meaning

Answer 1

-means high concentration of sodium (excess sodium relative to water in the blood)

Question 2

What is the most common cause of hypernatremia

Answer 2

Most common cause = lack of water rather than a gain of salt

Question 3

Hyponatremia -meaning

Answer 3

Low concentration of sodium

Question 4

Dehydration - meaning

Answer 4

Meaning high concentration of sodium

Question 5

What determines the extracellular fluid content

Answer 5

How much sodium is in the body

Question 6

Principal extracellular cation

Answer 6

1. Na+ is the principal extracellular cation - makes up >95% of all cations in this space

Question 7

How normal osmolarity is maintained

Answer 7

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

Question 8

What happens if remove 10% of Na+ from extracellular space

Answer 8

• 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

Question 9

Sodium content

Answer 9

-The amount of sodium

Question 10

How sodium content determines the extracellular fluid volume

Answer 10

-disturbances in sodium content trigger similar disturbances in water to keep the ratio of salt to water ([Na]) fixed

Question 11

How sodium concentration determines intracellular fluid volume

Answer 11

-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)
-

Question 12

Exception to rule of sodium concentration determining intracellular fluid volume and sodium content determining extracellular fluid volume

Answer 12

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)

Question 13

What type of osmoles can be added to the extracelllular space without being fatal

Answer 13

Those that are permeable across cell membranes and thus do not lead to movement of water

Question 14

Sodium concentration of edema + why

Answer 14

• is no barrier to salt or water movement between intravascular and interstitial compartments
• Na+ concentration of edema fluid = serum sodium concentration

Question 15

Why edema is an example of disordered sodium content when serum sodium concentration is normal

Answer 15

• 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

Question 16

Osmolarity of the ECFV

Answer 16

approx 292

Question 17

How changes in sodium content lead to changes in water content and result in altered extracellular fluid volume -ex CHF

Answer 17

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

Question 18

What can happen in severe CHF

Answer 18

• ADH levels can increase above those necessary to compensate for salt retention
• hyponatremia can result

Question 19

Is it possible for same patient to have both edema and hyponatremia

Answer 19

• 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

Question 20

Syndrome of inappropriate ADH secretion (SIADH)

Answer 20

• 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