fluid

(noun)

Any substance which can flow with relative ease, tends to assume the shape of its container, and obeys Bernoulli's principle; a liquid, gas or plasma.

Examples of fluid in the following topics:

  • Fluid Compartments

    • The major body fluid compartments include: intracellular fluid and extracellular fluid (plasma, interstitial fluid, and trancellular fluid).
    • The intracellular fluid of the cytosol or intracellular fluid (or cytoplasm) is the fluid found inside cells.
    • Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all body fluid outside of cells.
    • It is the intravascular fluid part of extracellular fluid (all body fluid outside of cells).
    • Examples of this fluid are cerebrospinal fluid, and ocular fluid, joint fluid, and the pleaural cavity which contain fluid that is only found in their respective epithelium-lined spaces.

  • Flow Rate and Velocity

    • Flow velocity and volumetric flow rates are important quantities in fluid dynamics used to quantify motion of a fluid and are interrelated.
    • Fluid dynamics is the study of fluids in motion and corresponding phenomena.
    • Fluid velocity can be affected by the pressure of the fluid, the viscosity of the fluid, and the cross-sectional area of the container in which the fluid is travelling.
    • The magnitude of the fluid flow velocity is the fluid flow speed.
    • Fluid flow velocity effectively describes everything about the motion of a fluid.

  • Poiseuille's Equation and Viscosity

    • Virtually all moving fluids exhibit viscosity, which is a measure of the resistance of a fluid to flow.
    • It describes a fluid's internal resistance to movement and can be thought of as a measure of fluid friction.
    • The greater the viscosity, the ‘thicker' the fluid and the more the fluid will resist movement.
    • Different fluids exhibit different viscous behavior yet, in this analysis, only Newtonian fluids (fluids with constant velocity independent of applied shear stress) will be considered.
    • In analyzing the properties of moving fluids, it is necessary to determine the nature of flow of the fluid.

  • What is a Fluid?

    • A fluid is a substance that continually deforms (flows) under an applied shear stress.
    • A fluid is a substance that continually deforms (flows) under an applied shear stress.
    • The distinction between solids and fluid is not entirely obvious.
    • It is best described as a viscoelastic fluid.
    • This also means that all fluids have the property of fluidity.

  • Body Fluid Composition

    • This means that fluid composition varies between body compartments.
    • The pH of the intracellular fluid is 7.4.
    • The concentrations of the other ions in cytosol or intracellular fluid are quite different from those in extracellular fluid.
    • The extracellular fluid is mainly cations and anions.
    • Ocular fluid in the eyes contrasts cerebrospinal fluid by containing high concentrations of proteins, including antibodies.

  • Flow Rate and the Equation of Continuity

    • The flow rate of a fluid is the volume of fluid which passes through a surface in a given unit of time .
    • where Q is the flow rate, v is the velocity of the fluid, and a is the area of the cross section of the space the fluid is moving through.
    • The equation of continuity applies to any incompressible fluid.
    • Since the fluid cannot be compressed, the amount of fluid which flows into a surface must equal the amount flowing out of the surface.
    • Using the known properties of a fluid in one condition, we can use the continuity equation to solve for the properties of the same fluid under other conditions.

  • Movement of Fluid Among Compartments

    • Movement of fluid among compartments depends on several variables described by Starling's equation.
    • Extracellular fluid is separated among the various compartments of the body by membranes.
    • To prevent a build up of tissue fluid surrounding the cells in the tissue, the lymphatic system plays a part in the transport of tissue fluid.
    • If positive, fluid will tend to leave the capillary (filtration).
    • If negative, fluid will tend to enter the capillary (absorption).

  • Static Equilibrium

    • For a fluid at rest, the conditions for static equilibrium must be met at any point within the fluid medium.
    • Thus for any region within a fluid, in order to achieve static equilibrium, the pressure from the fluid below the region must be greater than the pressure from the fluid above by the weight of the region.
    • At the same time, there is an upwards force exerted by the pressure from the fluid below the object, which includes the buoyant force. shows how the calculation of the forces acting on a stationary object within a static fluid would change from those presented in if an object having a density ρS different from that of the fluid medium is surrounded by the fluid.
    • The appearance of a buoyant force in static fluids is due to the fact that pressure within the fluid changes as depth changes.
    • This figure is a free body diagram of a region within a static fluid.

  • Application of Bernoulli's Equation: Pressure and Speed

    • Bernoulli's equation states that for an incompressible and inviscid fluid, the total mechanical energy of the fluid is constant .
    • (An inviscid fluid is assumed to be an ideal fluid with no viscosity. )
    • The kinetic energy of the fluid is stored in static pressure, $p_s$, and dynamic pressure, $\frac{1}{2}\rho V^2$, where \rho is the fluid density in (SI unit: kg/m3) and V is the fluid velocity (SI unit: m/s).
    • Static pressure is simply the pressure at a given point in the fluid, dynamic pressure is the kinetic energy per unit volume of a fluid particle.
    • Syphoning fluid between two reservoirs.

  • Torricelli's Law

    • Torricelli's law is theorem about the relation between the exit velocity of a fluid from a hole in a reservoir to the height of fluid above the hole.
    • Torricelli's law is theorem in fluid dynamics about the relation between the exit velocity of a fluid from a sharp-edged hole in a reservoir to the height of the fluid above that exit hole .
    • This relationship applies for an "ideal" fluid (inviscid and incompressible) and results from an exchange of potential energy,
    • Due to the assumption of an ideal fluid, all forces acting on the fluid are conservative and thus there is an exchange between potential and kinetic energy.
    • The exit velocity depends on the height of the fluid above the exit hole.