fluid, any substance that is able to flow. Of the four states of matter, only a solid is not a fluid, since it has a definite shape that is not readily changed. Any liquid, gas, or plasma is classed as a fluid.
or seminal fluid

Whitish viscous fluid emitted from the male reproductive tract that contains sperm and liquids (seminal plasma) that help keep them viable. Sperm cells, produced by the testes in humans, represent 2–5percnt of semen volume; fluids from tubules, glands, and storage areas of the reproductive system bathe them as they travel down, nourishing them, keeping them motile, or participating in certain chemical reactions. During ejaculation, liquids from the prostate gland and seminal vesicles dilute the sperm and provide a suitable, slightly alkaline environment. An average ejaculation of a human male expels 0.1–0.3 cu inches (2–5 ml), containing 200–300 million sperm.

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Study of the effects of forces and energy on liquids and gases. One branch of the field, hydrostatics, deals with fluids at rest; the other, fluid dynamics, deals with fluids in motion and with the motion of bodies through fluids. Liquids and gases are both treated as fluids because they often have the same equations of motion and exhibit the same flow phenomena. The subject has numerous applications in fields varying from aeronautics and marine engineering to the study of blood flow and the dynamics of swimming.

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Any liquid or gas that cannot sustain a shearing force when at rest and that undergoes a continuous change in shape (see flow) when subjected to such a stress. Compressed fluids exert an outward pressure that is perpendicular to the walls of their containers. A perfect fluid lacks viscosity, but real fluids do not.

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Clear, colourless liquid that surrounds the brain and spinal cord and fills the spaces in them. It helps support the brain, acts as a lubricant, maintains pressure in the skull, and cushions shocks. Analysis of CSF obtained by a spinal tap (lumbar puncture) helps diagnose a number of disorders, including meningitis and hemorrhage in the central nervous system.

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A fluid is defined as a substance that continually deforms (flows) under an applied shear stress. All liquids and all gases are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. The term "fluid" is often used as being synonymous with "liquid". This can be erroneous and sometimes clearly inappropriate—such as when referring to a liquid which does not or should not involve the gaseous state. For example, "brake fluid" is hydraulic oil which will not perform its required function if gas is present. The medical profession relies on the term "fluids" in dietary references ("take plenty of fluids") where the presence of gases is irrelevant or even possibly dangerous.

Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and fluid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered to behave like a solid or a fluid, depending on the time period over which it is observed. However Silly Putty is correctly termed a viscoelastic fluid.

Fluids display such properties as:

  • not resisting deformation, or resisting it only lightly (viscosity), and
  • the ability to flow (also described as the ability to take on the shape of the container).

These properties are typically a function of their inability to support a shear stress in static equilibrium.

Solids can be subjected to shear stresses, and to normal stresses - both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of rate of strain. A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state.

Depending on the relationship between shear stress, and the rate of strain and its derivatives, fluids can be characterized as:

  • Newtonian fluids : where stress is directly proportional to rate of strain, and
  • Non-Newtonian fluids : where stress is proportional to rate of strain, its higher powers and derivatives.

The behavior of fluids can be described by the Navier-Stokes equations - a set of partial differential equations which are based on:

The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.

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