Evidence is reviewed for the concept that the body employs essentially the same lubrication system in many sites in the body where tissues slide over each other with such ease. This system consists of fluid adjacent to surfaces coated with an oligolamellar lining of surface-active phospholipid (SAPL) acting as a back-up boundary lubricant wherever the fluid film fails to support the load--a likely event at physiological velocities. Particular attention is paid to the load-bearing joints, where the issue of identifying the vital active ingredient in synovial fluid is reviewed, coming down--perhaps predictably--in favour of SAPL. It is also explained how Lubricin and hyaluronic acid (HA) could have 'carrier' functions for the highly insoluble SAPL, while HA has good wetting properties needed to promote hydrodynamic lubrication of a very hydrophobic articular surface by an aqueous fluid wherever the load permits. In addition to friction and wear, release is included as another major role of boundary lubricants, especially relevant in environments where proteins are found, many having adhesive properties. The discussion is extended to a mention of the lubrication of prosthetic implants and to disease states where a deficiency of boundary lubricant is implicated, particular attention being paid to osteoarthritis.

A new ultrasonic Doppler device has been used noninvasively over the femoral artery of anesthetized dogs to prove that it can detect carefully calibrated microbubbles of 14--189 micrometers diam when these are infused directly into the aorta. The same evaluated technique has then been employed to detect any bubbles escaping into the arterial system when gas was infused into the venous system either as microbubbles or as a bolus. Results from 18 dogs showed that, under normal conditions, the lungs are a superb filter for bubbles and that any cutoff diameter is less than 22 micrometers. However, bubbles escaped entrapment when the lungs were severely overloaded with gas (20 ml) or were pretreated with a pulmonary vasodilator (aminophylline). The dog preparation and arterial Doppler device appear to be ideal for future studies to determine what other factors might compromise the capability of the lungs to filter microbubbles. Physiological parameters showed dramatic changes when bubbles were detected as escaping into the arterial system by comparison with their effect when retained within the lungs. Changes in respiration profile indicated that they may offer a useful index of the degree of venous embolization and, hence, a warning of impending overload leading to arterial embolization.

The contact angle subtended between a droplet of aqueous fluid and nonwettable surfaces provides a direct estimation of their degree of hydrophobicity. The mean contact angle recorded in dogs at the oxyntic mucosal surface was 85.2 degrees, a value characteristic of acid-resistant substances such as polyethylene. This indicates that the mucosal surface of the stomach has a hydrophobic lining that may be attributed to the surface-active phospholipids known to be present in both the gastric mucosa and juice. Barrier breakers such as bile and aspirin were found virtually to eliminate the hydrophobicity. Hydrophobicity was found to be different in the esophagus, antrum, proximal and distal duodenum, and the colon but consistent with their resistance to acid attack. Endogenous surfactants are discussed for their capability to provide a cohesive and strongly adsorbed protective monolayer--a physical model for the gastric mucosal barrier compatible with the major properties of the gastric lining and many features of ulcerogenesis, including the protection afforded by prostaglandins.

To resolve the apparent conflict between a lubricating glycoprotein, 'lubricin', as the active ingredient in synovial fluid (SF) and surface-active phospholipid (SAPL) present in SF (and adsorbed to articular cartilage) as the boundary lubricant reducing friction to such low physiological levels, lubricin was isolated from bovine SF following the original procedure of Swann et al. (Arthritis Rheum 1981;24:22-30). Analysis of the lipid extract by thin-layer chromatography and phosphorus determination demonstrated a phospholipid component of 11.1 +/- 1.7% (N = 5) which corresponds very closely to the 9.2-13.0% of lubricin which had hitherto remained unidentified and which has previously been shown to be transferable to the articular surface to impart lubrication. These results would appear to resolve any theoretical conflict in that lubricin is, indeed, an active ingredient within SF. Yet, as a large water-soluble molecule, it really functions as a carrier for the highly insoluble SAPL which it deposits on the articular surface as the oligolamellar layer visualized in previous studies. However, it is this deposited SAPL, rather than lubricin, which actually lubricates.