Laterally accreting sinuous channels and their deposits: The Goldilocks of deep-water slope systems

ABSTRACT Channels with a sinuous planform are common in both continental and deep-marine environments on Earth, and similarly in high-resolution images of the surface of Mars. Whereas common in rivers, continuous lateral channel migration and point-bar deposition appear to be much less common in the deep sea. In the bends of rivers, near-bed flow driving point bar growth results from a cross-flow superelevation of the water surface that sets up a lateral hydrostatic pressure gradient driving an inward-directed flow near the bed. However, in deep-marine systems the surface between the turbidity current and overlying ambient fluid sits well above the channel margins, and therefore precludes a similar lateral superelevation of the current top. Here it is argued that the cross-flow component is related to a density gradient that mimics the effect of the hydrostatic pressure gradient in rivers, and develops as coarse suspended particles that experience little uplift, and therefore negligible overspill, become concentrated along the outer bank. This condition develops best in a two-part suspension made up of a highly concentrated, unstratified basal plug of coarse sediment overlain sharply by a dilute cloud of much finer sediment—a density structure that differs from the more typical upward exponential decrease in density. The abundance of coarse and fine sand, but depletion in the intermediate grain size fraction, is related to transgressive shelf processes and its influence on sediment supplied to the system, and in turn, the flow structure of the current. It is under these seemingly uncommon granulometric conditions that continuous laterally migrating channels, and accordingly, riverine-like point-bar deposition, is most common in the deep sea.

Discriminating transudates and exudates in dogs with pleural effusion: diagnostic utility of simplified Light’s criteria compared with traditional veterinary classification

ObjectivesTo determine whether the simplified Light’s criteria (ie, pleural effusion lactate dehydrogenase concentration and serum total protein) can identify the pathophysiology of pleural effusion formation in dogs, and to assess whether these criteria were more accurate than the traditional veterinary classification based on pleural effusion total protein (TPp) and nucleated cell count (TNCCp).MethodsThis is a cross-sectional study including 100 dogs with pleural effusion. The aetiology of effusion was used to classify the pathophysiology of its formation. Parameters measured included the simplified Light’s criteria, TPp and TNCCp. The diagnostic utility of the two methods in classifying pleural effusion formation was evaluated.ResultsSeven transudates due to decreased colloid osmotic pressure, 18 transudates due to increased hydrostatic pressure gradient and 75 exudates were included in the study. The simplified Light’s criteria misclassified 2 of 75 exudates (98 per cent overall accuracy). The traditional veterinary classification scheme misclassified 31 of 75 exudates and 12 of 18 increased hydrostatic pressure gradient transudates (57 per cent overall accuracy). The frequency of agreement between the simplified Light’s criteria and the traditional veterinary classification with the true nature of the pleural effusion was significantly different (P<0.001).Clinical significanceThe simplified Light’s criteria were highly accurate in discriminating exudates from transudates, while TPp and TNCCp had no diagnostic value in doing so.

On the dynamics of a thin viscous film spreading between a permeable horizontal plate and an elastic sheet

The two-dimensional dynamics of a thin film of viscous fluid spreading between a permeable horizontal plate and an overlying thin elastic sheet is explored. We use a lubrication model to describe the balance between the elastic stress, the hydrostatic pressure gradient and the viscous resistance of the flow, as fluid spreads laterally from a source and simultaneously drains through the plate. A family of asymptotic solutions are described in which the flow is dominated by either the hydrostatic pressure gradient or the elastic stress associated with the deformation of the sheet. In these solutions, although the deformation of the sheet above the porous plate arises from the fluid flow below the sheet, the fluid typically separates from the sheet a short distance upstream of the full extent of the draining zone, with the region of flow being driven purely by the hydrostatic pressure gradient. As a result, an air gap develops below the sheet up to the point where it touches back down onto the plate. With a very light or stiff elastic sheet, this touchdown point may extend far beyond the fluid draining zone, but otherwise it is similar to the extent of the draining zone.

Putting It All Together—Manned Space Flight

There are 3 main sources of pressure in physiology: atmospheric, hydrostatic, and mechanical. Unless a feasible method to generate ‘artificial gravity’ is developed, the astronauts on board will experience about 888 days of weightlessness during a Mars mission. What does this mean physiologically? The mechanical pressures generated by the heart, blood vessels, and the muscles of respiration will remain unchanged, except for whatever atrophy occurs during the mission. One interesting and apparently intractable problem of reduced gravity is muscle wasting. The skeletal muscles of respiration will not atrophy because they still will be constantly used and exercised. What about hydrostatic pressures? Without gravity, there is no possibility of a hydrostatic pressure gradient. What is the practical effect of losing the hydrostatic pressure gradient? Apparently, it has very little effect because astronauts survive and their brains seem to remain perfused during exposure to weightless environments. There is another physiologic challenge in space, a decrease in total blood volume, which results in orthostatic intolerance upon returning to Earth.

Direct measurements of sieve element hydrostatic pressure reveal strong regulation after pathway blockage

According to the Münch hypothesis, solution flow through the phloem is driven by a hydrostatic pressure gradient. At the source, a high hydrostatic pressure is generated in the collection phloem by active loading of solutes, which causes a concomitant passive flow of water, generating a high turgor pressure. At the sink, solute unloading from the phloem keeps the turgor pressure low, generating a source-to-sink hydrostatic pressure gradient. Localised changes in loading and unloading of solutes along the length of the transport phloem can compensate for small, short-term changes in phloem loading at the source, and thus, maintain phloem flow to the sink tissue. We tested directly the hydrostatic pressure regulation of the sieve tube by relating changes in sieve tube hydrostatic pressure to changes in solute flow through the sieve tube. A sudden phloem blockage was induced (by localised chilling of a 1-cm length of stem tissue) while sieve-tube-sap osmotic pressure, sucrose concentration, hydrostatic pressure and flow of recent photosynthate were observed in vivo both upstream and downstream of the block. The results are discussed in relation to the Münch hypothesis of solution flow, sieve tube hydrostatic pressure regulation and the mechanism behind the cold-block phenomenon.