Pressure gradients during compression

The more recent thoracic pump theory is based on the belief that blood flow during CPR results from intrathoracic pressure alterations induced by chest compressions. During compression (systole), a pressure gradient develops between the intrathoracic arteries and extrathoracic veins, causing forward blood flow from the lungs into the systemic circulation. After compression ends (diastole), intrathoracic pressure declines and blood flow returns to the lungs. 

For higher temperature operation, a polybenzimid-azole-based polymer electrolyte may be preferred. The PEMFC structures have good mechanical integrity under compression and expansion from differential temperature and pressure gradients that occur during operation. This system has minimal materials problems, except for the cost and operation characteristics of the membrane. The PEMFC operates at 1 A/cm at 0.7 V. The electrode reactions in acidic media have been discussed above. 

The data is interpreted to indicate that the tissue is very permeable to the flow of fluid, normal to its surface, at the pressure gradients and flow rates generated during compression. 

Cracks can also appear during the pressure release in the autoclave. In the supercritical drying process, the gel is subjected to high temperature and high pressure. When the critical point is reached, the pressure of the autoclave is decreased while the temperature is kept constant. At this instant, the pressure applied to the supercritical fluid is equal to that within the pores. The supercritical fluid has a very low density and viscosity compared with that of the liquid at room temperature however, the low permeability of the gel resists the flow of the supercritical fluid out of the gel. In other words, if the supercritical fluid release is performed too fast a pressure gradient appears. In this case the supercritical fluid within the gel, which is in compression, suddenly expands and the solid part suffers tensile stress. Experiments show that cracking depends on the pressure release rate, on the nature of the gel (basic or neutral), and on its geometrical dimensions. 

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