Patient 1 and 2 have the same plasma sodium concentration of 140mmol/L.
Patient 1 has an urine sodium concentration of 60 mmol/L.
Patient 2 has an urine sodium concentration of 200 mmol/L.
How can the plasma sodium concentration can be identical in these two patients, yet the urine sodium concentrations can be different.
This is due to the ability of the kidney to produce very dilute or very concentrated urine. As the filtrate goes up the ascending limb toward the CCD, it becomes extremely hypovolemic, due to the active sodium potassium pump pumping sodium out of the filtrate. Other filtrates follow, but the thick ascending limb is impermeable to water. This causes the filtrate to lose solute but not water, thus making it hypovolemic. As the filtrate flows down the collecting duct, the presence of ADH determines if the urine is concentrated or not. This is because from the cortex to the medulla, the concentration ranges from 60 mmol from the outermost part of the cortex to 200mmolL in the innermost part of the medulla. The extreme sodium concentration is due to the countercurrent multiplier effect, which is generated by the ascending and descending limbs of the loop of henle. This concentrates the sodium in the medulla, as it is only able to leave through the ascending limb of the vasa recta.
Hence as a result of that, as water flows down the collecting duct, it has the potential to be concentrated to 200mmol/L. The system that controls the concentration is the ADH system. The ADH system is very sensitive to the osmolality of blood. If the osmolality of blood begins to fall, then in order to prevent it, a lot of water will not be absorbed. However, if the osmolality of the blood begins to rise, in order to prevent it, water will be absorbed by the action ADH, which up-regulates the amount of aquaporins (water channels) in the collecting tubules. This causes water to be absorbed, ensuring that the urine becomes very concentrated.
Sodium concentration is primarily regulated by aldesterone and ADH.
Aldosterone is secreted when someone is hypovolemic, inhibited when someone is hypervolemic
ADH secreted when someone is hyperosmotic, inhibited when osmolality is very low.
In heart failure, if the heart failure is due to essential high blood pressure. High blood pressure à high volume, body attempts to reduce volume by inhibiting aldosterone, and later ADH. Thus body removes fluid + electrolytes, normal sodium concentration
If the heart failure is due to other problems, cardiac output drops. As cardiac output drops, Aldosterone, ADH and everything else is upregulated, as the body senses through the baroreceptors. The body accepts this as a decrease in effective circulating volume. Total body sodium increases, and TBW increases to a greater extent. The ECF is increased markedly, with the presence of edema.
