| | When fluids (liquids and gases) are filled into tanks, the pressure is higher if there is more of the fluid in the tank. We know this from air balloons, from the open tanks seen above, from medicine, and many other examples. An important medical example is the eye. High pressure is detrimental, so it is important to understand the effects of changes of the volume of liquid in the eye. The pressure rises with volume in a manner similar to what we can expect for pressure vessels such as balloons or the kinds used in engineering (Figure 1). The pressure rises more steeply for larger volumes. The relation between fluid pressure and volume stored is called the capacitive characteristic of the storage element.
Interpretation In the case of an open tank (or a lake or the ocean), the added weight of the added fluid leads to a rising pressure. In pressure vessels (membrane accumulators used in hydraulic systems, the eye, the skull with a brain, the aorta of the blood circulatory system), the elastic container wall is stressed more strongly if there is more fluid in the storage element. Moreover, typical walls get stiffer when stressed more strongly which explains the steeper rise of the pressure with increasing volume. Rubber balloons are an exception to the rule that the pressure rises with increasing volume. Rubber has an unexpected property: At small volumes, the pressure indeed rises with increasing volume. However, there is a range of volumes when the pressure drops with increasing volume before rising again (Figure 2). | 
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