Conventional concepts indicate that Na+ within the body is maintained within narrow limits for fluid and blood pressure homeostasis. However, some studies have shown that substantial amounts of Na+ can be retained or removed from the body without a corresponding loss of water, and the skin can act as a major salt reservoir.
Recently, a study published in the journal Hypertension showed that the skin is hypertonic compared to plasma salt storage, which also applies to the skin interstitial fluid. Even a small electrolyte gradient between plasma and interstitial fluid can represent an enhanced force for edema generation.
Because water accumulation was modest, the researchers decided to re-examine in rats using an alternative method whether the tissue fluid is hypertonic during salt accumulation induced by a high-salt diet (8% NaCl and 1% saline drink) or deoxycorticosterone acetate pellet implantation.
These treatments led to increased systemic blood pressure, as well as increased skin salt and water accumulation and skin hypertonicity. However, the tissue fluid separated from the implanted core and the lymphatic exudate were isotonic, and the Na+ concentration in the fluid separated by centrifugation and in the lymph did not differ from that in plasma.
Interestingly, by eluting the skin layers, an osmotic and urea gradient exists from the epidermis to the dermis.
This study indicates that the fluid leaving the skin via the lymph is isotonic to plasma, but the skin can also control its own electrolyte microenvironment by generating local gradients that may be functionally important.