The skin is the only organ besides the lungs that is directly exposed to atmospheric oxygen. Apart from the stratum corneum, oxygen is consumed in all layers of the epidermis and dermis. The oxygen demand is partially satisfied by the blood: the dermis exhibits a vasculature that is arranged in two tiers that are parallel to the skin surface. The superficial plexus between the papillary and the upper reticular dermis deep plexus in the lower reticular dermis are connected by perpendicularly orientated communicating vessels. Arcades of capillaries loop upwards into the papillae from the subpapillary plexus (Braverman, 1989). In contrast, the epidermis has no vasculature, but is exposed directly to the atmosphere. As early as 1851, Gerlach was able to show that human skin takes up oxygen from the atmosphere.

Local relative measurements of the changes in cutaneous oxygen uptake from the atmosphere, the so‐called transcutaneous oxygen flux (tcJ O2 ), have become possible with the development of an oxygen fluxoptode (Holst, 1994; Holst et al. 1995). Measurements of tcJ O2 on the humidified skin of the volar forearm at normal skin temperature (33 °C) during artificially induced variations in blood perfusion have indicated the functional relevance of the external oxygen supply (Stücker et al. 2000a). The induction of hyperaemia in moist skin with a combination of nonivamide and nicoboxil resulted in a distinct decrease of tcJ O2 to 70 % of the resting values. These experiments clearly demonstrated that the oxygen supply of the corium is a balance between oxygen transport by the blood and uptake from the atmosphere. If the oxygen supply from the blood increases, a lower tcJ O2 suffices to cover the oxygen demand of the skin. Stopping capillary oxygen transport was compensated by an increase of tcJ O2 of only 9 %. This indicates that under normal conditions a substantial part of the upper skin is supplied by direct oxygen uptake from the atmosphere. Until now it has not been possible to determine the thickness of the layer (T) that characterises the contribution of tcJ O2 to the total skin oxygen supply.

In a theoretical analysis, Fitzgerald (1957) estimated a mean T of 48 μm, with a range of 34–84 μm, which would cover the main part or the whole of the epidermis. His calculations were based on data for the diffusion coefficient measured on the anterior abdominal wall of the frog following removal of the skin, because there were no comparable measurements on mammals. In fact, the true values for the oxygen permeability of skin tissue are an order of magnitude greater, whilst the oxygen consumption under normal conditions is about four times lower (actual data: 1470–2110 ml O 2 m−3 min−1; Fitzgerald used 7800 ml O 2 (ml tissue)−1 min−1). The approximate partial pressure of oxygen (P O2 ) of capillary blood, 95 Torr (1 Torr = 0.1333 kPa), was taken as the minimum P O2 of the skin. This is higher than the minimum value of 51 Torr that was measured in the skin in vivo using needle electrodes (Evans & Naylor, 1966a; Roszinski & Schmeller, 1995). A greater penetration depth T of the external oxygen is calculated using the latter values. Furthermore, Fitzgerald had to use data for the absorption of oxygen through the skin surface, which had a wide range of 0.4–2.9 ml O 2 m−2 min−1. This was due to different measuring locations and temperatures, large measuring areas and poor sensitivity of the measuring devices (for example, changes in the oxygen absorption caused by increased or decreased blood flow could not be detected).

et al. measured the intracutaneous profile of P O2 directly with needle electrodes. The skin surface of the lower limb was covered by a film of water, which resulted in a reduced skin surface P O2 of 78 Torr. Furthermore, the needle puncture probably produced a local hyperaemia and increased the oxygen supply by the blood. Under these conditions, with reduced skin surface P O2 and hyperaemia, the P O2 profile had a distinct minimum at a depth of about 100 μm, roughly at the level of the capillary loops ( J O2 is the oxygen flux, K the conductivity and ∂P O2 /∂x the pressure gradient. These measurements show, therefore, that in the upper 100 μm at least, there can only be a diffusion of oxygen from the skin surface to that depth instead of from the blood to the skin surface. In 1987, Baumgärtlmeasured the intracutaneous profile ofdirectly with needle electrodes. The skin surface of the lower limb was covered by a film of water, which resulted in a reduced skin surfaceof 78 Torr. Furthermore, the needle puncture probably produced a local hyperaemia and increased the oxygen supply by the blood. Under these conditions, with reduced skin surfaceand hyperaemia, theprofile had a distinct minimum at a depth of about 100 μm, roughly at the level of the capillary loops ( Fig. 1 ). These invasive measurements demonstrated a penetration depth of atmospheric oxygen into the skin, double that of Fitzpatrick's estimated values. According to Fick's law of diffusion:whereis the oxygen flux,the conductivity and ∂/∂the pressure gradient. These measurements show, therefore, that in the upper 100 μm at least, there can only be a diffusion of oxygen from the skin surface to that depth instead of from the blood to the skin surface.

Figure 1 Open in figure viewer PowerPoint Oxygen partial pressure (P O2 ) measured by a needle electrode inserted perpendicularly into the skin The depth z of the electrode is given in μm (skin surface at 0 μm). The skin surface was covered by a water film, resulting in a reduced skin surface P O2 (ssP O2 ) of 78 Torr. The P O2 profile has a distinct minimum at a depth of approximately 100 μm, roughly at the level of the dermo‐epidermal junction (according to Baumgärtl et al. 1987). The needle puncture probably resulted in a local hyperemia. Under more physiological conditions, it is expected that the minimum would occur at a greater depth.