We have all different types of saline: Isotonic saline, hypotonic saline (1/2 normal saline, ¼ normal saline, 5% dextrose in water), and hypertonic saline (3%, 5%); normal saline is 0.9%. We are referring to normal tonicity of the plasma, which is controlled by the serum Na. These are the three types of tonicity (iso, hypo, and hyper). Serum Na is a re5lection of total body Na divided by total body H20. For
example: hypernatremia is not just caused by increased total body Na; it can also be caused by decreasing total body water with a normal total body Na, therefore there is an increase in serum Na concentration. It is really a ratio of total body Na to total body H20. To determine serum Na, just look at serum levels. With different 5luid
abnormalities, can lose or gain a certain tonicity of 5luid.
1. Isotonic loss of Uluid – look at ratio of total body Na and water; in this case, you are losing equal amounts of water and Na, hence ISOtonic. This 5luid is mainly lost from the ECF. The serum Na concentration is normal when losing isotonic 5luid. ECF would look contracted. There would be no osmotic gradient moving into or out of the ECF. Clinical conditions where there is an isotonic loss of 5luid: hemorrhage, diarrhea.
If we have an isotonic gain, we have in equal increase in salt and water; ie someone getting too much isotonic saline; normal serum Na, excess isotonic Na would be in the ECF, and there would be no osmotic gradient for water movement.
2. Hypotonic solutions – by de5inition, it means hyponatremia. Hypoglycemia will not produce a hypotonic condition. MCC of low osmolality in plasma is hyponatremia. How? Lose more salt than water, therefore, serum Na would be decreased. If losing more salt than water, kidney is probably the location of where/ why it is happening. Main place to deal with sodium (either to get rid of it or to get it back) is in kidney, esp when dealing with diuretics (furosemides and HCTZ). The tonicity of solution you lose in your urine is HYPERtonic, so that’s how you end up with hyponatremia with a hypotonic condition. ECF concentration is low with hyponatremia, therefore the water will move into the ICF compartment. (Osmosis-‐ remember low to high)
Example: If you gained pure water, and no salt, you have really lowered your serum Na: MCC = SIADH – in small cell carcinoma of the lung; you gain pure water b/ c ADH renders the distal and the collecting tubule permeable to free water. With ADH present, will be reabsorbing water back into the ECF compartment, diluting the serum Na, and the ECF and ICF will be expanded. The ECF is expanded due to water reabsorption, and the ICF is expanded b/c it has a high concentration levels (its levels are not diluted). This can lead to mental status abnormalities. Therefore, the more water you drink, the lower your serum Na levels would be. The treatment is by restricting water. Don’t want to restrict Na b/c the Na levels are normal. When ADH is present, you will CONCENTRATE your urine b/c taking free water out of urine; with absent ADH, lose free water and the urine is diluted. Therefore, for with SIADH, water stays in the body, goes into the ECF compartment, and then move into the ICF compartment via osmosis. The lowest serum sodium will be in SIADH. On the boards, when serum Na is less than 120, the answer is always SIADH. Example: pt with SIADH, not a smoker (therefore not a small cell carcinoma), therefore, look at drugs – she was on chlorpropramide, oral sulfylureas produce SIADH.
Example: Gain both water and salt, but more water than salt, leading to hyponatremia – these are the pitting edema states – ie RHF, cirrhosis of the liver. When total body Na is increased, it always produces pitting edema. What compartment is the total body Na in? ECF What is the biggest ECF compartment? Interstial compartment. Therefore, increase in total body Na will lead to expansion of interstial compartment of the ECF, water will follow the Na, therefore you get expansion via transudate and pitting edema; seen in right HF and cirrhosis.
Example: hypertonic loss of salt (from diuretic) leads to hyponatremia Example: SIADH (gaining a lot of water) leads to hyponatremia
Example: gaining more water than salt will lead to hyponatremia: pitting edema 3. Hypertonic state – by de5inition, have too much Na (hypernatremia) or have hyperglycemia (ie pt with DKA has a hypertonic condition, which is more common than hypernatremia). With hypernatremia, what does ICF look like? It will always be contracted or shrunken.
Primary aldosteronsim – gain more salt and water.
Diabetes insipidus – Lose pure water (vs. gaining pure salt in SIADH). If you lose more water than salt in the urine, you have osmotic diuresis – mixture. When there is glucose and mannitol in the urine, you’re losing hypotonic salt solution in urine.
Example: Baby diarrhea = hypotonic salt solution (adult diarrhea is isotonic), therefore, if baby has no access to water and has a rotavirus infection, serum sodium should be high because losing more water than salt, leading to hypernatremia. However, most moms give the baby water to correct the diarrhea; therefore the baby will come in with normal serum Na or even hyponatremia b/c the denominator
(H2O) is increased. Treatment is pedialyte and Gatorade – these are hypotonic salt
solution (just give them back what they lost). What has to be in pedialyte and what has to be in Gatorade to order to reabsorb the Na in the GI tract? Glucose b/c of the co-‐transport. With the co-‐transport, the Na HAS to be reabsorbed with glucose or galactose. Example: cholera, in oral replacement, need glucose to reabsorb Na b/c co-‐transport pump located in the small intestine. Gatorade has glucose and sucrose (which is converted to fructose and glucose).
Sweat = hypotonic salt solution; if you are sweating in a marathon, you will have hypernatremia