Dual role of diffusion in tissue gas exchange: blood-tissue equilibration and diffusion shunt

[Kidney medullary hypoxia: a key to understanding acute renal failure?]

The ability to produce a concentrated urine is imposed by a uniquely low ambient oxygen pressure in the renal medulla due to shunt diffusion within the vascular bundles. As the thick ascending limb of Henle's loop (TAL-segment) is able to glycolyse anaerobically, a phase of oxygen deficiency may be bridgespanned. It allows an exceptionally high oxygen extraction of 80% in this area. If oxygen capacity is reduced systematically, which can be effected in the isolated kidney model by using cell free perfusate, a typical pattern of lesions occur in TAL-segments. Segments near vascular bundles remain intact, as they take advantage from a radial oxygen diffusion originating from vascular bundles. The extent of lesions is increasing directed to the inner medulla due to the reduction of oxygen pressure, whereas lesions are not present in the inner medulla itself. Cells of TAL-segments are swelling during oxygen deficiency, when transport work surpasses the available energy necessary due to the luminal fluid inflow. Lesions could be prevented, when oxygen capacity was enhanced by adding erythrocytes or when transport was blocked by furosemide. Swollen cells in TAL-segments however are able to aggravate medullary hypoxia by an outflow block in vivo. Secondly, it can be demonstrated, that oxygen shunt diffusion is not only present in renal medulla but also within renal cortex especially as a preglomerular diffusion shunt for blood gases. Thus PCO2 has been measured to be 65 mmHg in the outermost cortical zone and thereby some 20 mmHg higher than renal venous blood.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of diffusion shunt in transfer of inert gases and O2 in muscle

Diffusion shunt is diffusive gas exchange between arterial and venous vessels. Evidence for diffusion shunt had been obtained in washout studies in the gastrocnemius muscle of the dog. According to models, diffusion shunt is expected to be enhanced at low blood flow, and for gases of high diffusivity. Shunting of O2 should be reduced in comparison to inert gases because of chemical binding in blood.