Mass flow, also known as mass transfer and bulk flow, is the movement of fluids down a pressure or temperature gradient,
[Moyes & Schulte (2008). Principles of Animal Physiology. Pearson Benjamin Cummings. San Francisco, California] particularly in the life sciences. As such, mass flow is a subject of study in both fluid dynamics and biology. Examples of mass flow include
blood circulation
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
and transport of
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
in vascular plant tissues. Mass flow is not to be confused with diffusion which depends on concentration gradients within a medium rather than pressure gradients of the medium itself.
Plant biology
In general, bulk flow in plant biology typically refers to the movement of water from the soil up through the plant to the leaf tissue through
xylem
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
, but can also be applied to the transport of larger solutes (e.g. sucrose) through the
phloem
Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...
.
Xylem
According to
cohesion-tension theory
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
, water transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via
hydrogen bonding
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
combined with the high water pressure of the plant's substrate and low pressure of the extreme tissues (usually leaves).
[Taiz, Lincoln; Zeiger, Eduardo; Moller, Ian Max; Murphy, Angus (2015). Plant Physiology and Development. Sunderland, MA: Sinauer Associates, Inc. p. 66. .]
As in blood circulation in animals,
(gas) embolisms may form within one or more xylem vessels of a plant. If an air bubble forms, the upward flow of xylem water will stop because the pressure difference in the vessel cannot be transmitted. Once these embolisms are
nucleated
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, ...
, the remaining water in the capillaries begins to turn to water vapor. When these bubbles form rapidly by
cavitation
Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
, the "snapping" sound can be used to measure the rate of cavitation within the plant .
[Pockman, W.T., Sperry, J.S., & O'Leary, J.W. 1995. Sustained and significant negative water pressure in xylem. 'Nature' 378: 715-716] Plants do, however, have physiological mechanisms to reestablish the capillary action within their cells .
Phloem
Solute flow is driven by a difference in hydraulic pressure created from the unloading of solutes in the sink tissues.
[Lambers, Hans (2008). Plant Physiological Ecology. 233 Spring Street, New York, NY: Springer Science+Business Media, LLC. p. 153. .] That is, as solutes are off-loaded into sink cells (by active or passive transport), the density of the phloem liquid decreases locally, creating a pressure gradient.
See also
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Countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each ...
*
Pounds Per Hour
Pound per hour is a mass flow unit. It is abbreviated as PPH or more conventionally as lb/h. Fuel flow for engines is usually expressed using this unit. It is particularly useful when dealing with gases or liquids, as volume flow varies more wit ...
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Fluid Dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...
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Mass flow rate
In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot (''ṁ ...
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Hemorheology
Hemorheology, also spelled haemorheology (from Greek ‘αἷμα, ''haima'' 'blood' and rheology, from Greek ῥέω ''rhéō'', ' flow' and -λoγία, ''-logia'' 'study of'), or blood rheology, is the study of flow properties of blood and its ...
*
Flying and gliding animals
A number of animals are capable of aerial locomotion, either by powered flight or by gliding. This trait has appeared by evolution many times, without any single common ancestor. Flight has evolved at least four times in separate animals: inse ...
References
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Fluid dynamics