Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property heat or some chemical, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or solid powders, or any combination of those. For example, in a distillation column, the vapors bubble up through the downward flowing liquid while exchanging both heat and mass; the maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than co-current exchange because countercurrent maintains a declining difference or gradient. In cocurrent exchange the initial gradient is higher but falls off leading to wasted potential. For example, in the adjacent diagram, the fluid being heated has a higher exiting temperature than the cooled fluid, used for heating. With cocurrent or parallel exchange the heated and cooled fluids can only approach one another; the result is that countercurrent exchange can achieve a greater amount of heat or mass transfer than parallel under otherwise similar conditions.
See: flow arrangement. Countercurrent exchange when set up in a circuit or loop can be used for building up concentrations, heat, or other properties of flowing liquids; when set up in a loop with a buffering liquid between the incoming and outgoing fluid running in a circuit, with active transport pumps on the outgoing fluid's tubes, the system is called a countercurrent multiplier, enabling a multiplied effect of many small pumps to build up a large concentration in the buffer liquid. Other countercurrent exchange circuits where the incoming and outgoing fluids touch each other are used for retaining a high concentration of a dissolved substance or for retaining heat, or for allowing the external buildup of the heat or concentration at one point in the system. Countercurrent exchange circuits or loops are found extensively in nature in biologic systems. In vertebrates, they are called a rete mirabile the name of an organ in fish gills for absorbing oxygen from the water, it is mimicked in industrial systems.
Countercurrent exchange is a key concept in chemical engineering thermodynamics and manufacturing processes, for example in extracting sucrose from sugar beet roots. Countercurrent multiplication is a similar but different concept where liquid moves in a loop followed by a long length of movement in opposite directions with an intermediate zone; the tube leading to the loop passively building up a gradient of heat or solvent concentration while the returning tube has a constant small pumping action all along it, so that a gradual intensification of the heat or concentration is created towards the loop. Countercurrent multiplication has been found in the kidneys as well as in many other biological organs. Countercurrent exchange along with cocurrent exchange and contra-current exchange comprise the mechanisms used to transfer some property of a fluid from one flowing current of fluid to another across a barrier allowing one way flow of the property between them; the property transferred could be heat, concentration of a chemical substance, or other properties of the flow.
When heat is transferred, a thermally-conductive membrane is used between the two tubes, when the concentration of a chemical substance is transferred a semipermeable membrane is used. In the cocurrent flow exchange mechanism, the two fluids flow in the same direction; as the cocurrent and countercurrent exchange mechanisms diagram showed, a cocurrent exchange system has a variable gradient over the length of the exchanger. With equal flows in the two tubes, this method of exchange is only capable of moving half of the property from one flow to the other, no matter how long the exchanger is. If each stream changes its property to be 50% closer to that of the opposite stream's inlet condition, exchange will stop when the point of equilibrium is reached, the gradient has declined to zero. In the case of unequal flows, the equilibrium condition will occur somewhat closer to the conditions of the stream with the higher flow. A cocurrent heat exchanger is an example of a cocurrent flow exchange mechanism.
Two tubes have a liquid flowing in the same direction. One starts off hot at 60 °C, the second cold at 20 °C. A thermoconductive membrane or an open section allows heat transfer between the two flows; the hot fluid heats the cold one, the cold fluid cools down the warm one. The result is thermal equilibrium: Both fluids end up at around the same temperature: 40 °C exactly between the two original temperatures. At the input end, there is a large temperature difference of much heat transfer. If the equilibrium—where both tubes are at the same temperature—is reached before the exit of the liquid from the tubes, no further heat transfer will be achieved along the remaining length of the tubes. A similar example is the cocurrent concentration exchange; the system consists of two tubes, one with brine, the other with freshwater, a semi permeable membrane which allows only water to pass between the two, in an osmotic process. Many of the water molecules pass from the freshwater flow in order to dilute the brine, while the concentration of salt in the freshwater grows.
This will continue, until both flows reach a similar dilution, with a concentration somewhere close to midwa
Flying and gliding animals
A number of animals have evolved aerial locomotion, either by powered flight or by gliding. Flying and gliding animals have evolved separately many times, without any single ancestor. Flight has evolved at least four times, in the insects, pterosaurs and bats. Gliding has evolved on many more occasions; the development is to aid canopy animals in getting from tree to tree, although there are other possibilities. Gliding, in particular, has evolved among rainforest animals in the rainforests in Asia where the trees are tall and spaced. Several species of aquatic animals, a few amphibians and reptiles have evolved to acquire this gliding flight ability as a means of evading predators. Animal aerial locomotion can be divided into two categories -- unpowered. In unpowered modes of locomotion, the animal uses aerodynamics forces exerted on the body due to wind or falling through the air. In powered flight, the animal uses muscular power to generate aerodynamic forces. Animals using unpowered aerial locomotion cannot maintain altitude and speed due to unopposed drag, while animals using powered flight can maintain steady, level flight as long as their muscles are capable of doing so.
These modes of locomotion require an animal start from a raised location, converting that potential energy into kinetic energy and using aerodynamic forces to control trajectory and angle of descent. Energy is continually lost to drag without being replaced, thus these methods of locomotion have limited range and duration. Falling: decreasing altitude under the force of gravity, using no adaptations to increase drag or provide lift. Parachuting: falling at an angle greater than 45° from the horizontal with adaptations to increase drag forces. Small animals may be carried up by the wind; some gliding animals may use their gliding membranes for drag rather than lift. Gliding flight: falling at an angle less than 45° from the horizontal with lift from adapted aerofoil membranes; this allows falling directed horizontal movement, with streamlining to decrease drag forces for aerofoil efficiency and with some maneuverability in air. Gliding animals have a lower aspect ratio than true flyers. Powered flight has evolved only four times.
It uses muscular power to replace energy lost to drag. Flapping: moving wings to produce lift and thrust. May ascend without the aid of the wind, as opposed to parachuters. Ballooning and soaring are not powered by muscle, but rather by external aerodynamic sources of energy: the wind and rising thermals, respectively. Both can continue as long. Soaring is only seen in species capable of powered flight, as it requires large wings. Ballooning: being carried up into the air from the aerodynamic effect on long strands of silk in the wind. Certain silk-producing arthropods small or young spiders, secrete a special light-weight gossamer silk for ballooning, sometimes traveling great distances at high altitude. Soaring: gliding in rising or otherwise moving air that requires specific physiological and morphological adaptations that can sustain the animal aloft without flapping its wings; the rising air is due to ridge lift or other meteorological features. Under the right conditions, soaring creates a gain of altitude without expending energy.
Large wingspans are needed for efficient soaring. Many species will use multiple of these modes at various times. While gliding occurs independently from powered flight, it has some ecological advantages of its own. Gliding is a energy-efficient way of travelling from tree to tree. An argument made is that many gliding animals eat low energy foods such as leaves and are restricted to gliding because of this, whereas flying animals eat more high energy foods such as fruits and insects. In contrast to flight, gliding has evolved independently many times. Worldwide, the distribution of gliding animals is uneven as most inhabit rain forests in Southeast Asia. Additionally, a variety of gliding vertebrates are found in Africa, a family of hylids lives in South America and several species of gliding squirrels are found in the forests of northern Asia and North America. Various factors produce these disparities. In the forests of Southeast Asia, the dominant canopy trees are taller than the canopy trees of the other forests.
A higher start farther travel. Gliding predators may more efficiently search for prey; the lower abundance of insect and small vertebrate prey for carnivorous animals in Asian forests may be a factor. In Australia, many mammals possess, to prehensile tails. Powered flight has evolved unambiguously only four times—birds, bats and insects. In contrast to gliding, which has evolved more but gives rise to only a handful of species, all three extant groups of powered flyers have a huge number of species, suggesting that flight is a successful strategy once evolved. Bats, after rodents, have the most species of any mammalian order, about 20% of all mammalian species. Birds have the most species of any class of terrestrial vertebrates
Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled below 0° C, it will tend to freeze into ice. Volumes of water cooled only a few degrees below 0° C stay free of ice for long periods. At these conditions, nucleation of ice does not occur at all. However, at lower temperatures ice crystals appear after no delay. At these conditions ice nucleation is fast. Nucleation is how first-order phase transitions start, it is the start of the process of forming a new thermodynamic phase. By contrast, new phases at continuous phase transitions start to form immediately. Nucleation is found to be sensitive to impurities in the system; these impurities may be too small to be seen by the naked eye, but still can control the rate of nucleation.
Because of this, it is important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at nucleation sites on surfaces in the system. Homogeneous nucleation occurs away from a surface. Nucleation is a stochastic process, so in two identical systems nucleation will occur at different times; this behaviour is similar to radioactive decay. A common mechanism is illustrated in the animation to the right; this shows nucleation of a new phase in an existing phase. In the existing phase microscopic fluctuations of the red phase appear and decay continuously, until an unusually large fluctuation of the new red phase is so large it is more favourable for it to grow than to shrink back to nothing; this nucleus of the red phase grows and converts the system to this phase. The standard theory that describes this behaviour for the nucleation of a new thermodynamic phase is called classical nucleation theory. However, the CNT fails in describing experimental results of vapour to liquid nucleation for model substances like Argon by several orders of magnitude.
For nucleation of a new thermodynamic phase, such as the formation of ice in water below 0° C, if the system is not evolving with time and nucleation occurs in one step the probability that nucleation has not occurred should undergo exponential decay as seen in radioactive decay. This is seen for example in the nucleation of ice in supercooled small water droplets; the decay rate of the exponential gives the nucleation rate. Classical nucleation theory is a used approximate theory for estimating these rates, how they vary with variables such as temperature, it predicts that the time you have to wait for nucleation decreases rapidly when supersaturated. It is not just new phases such as crystals that form via nucleation followed by growth; the self-assembly process that forms objects like the amyloid aggregates associated with Alzheimer's disease starts with nucleation. Energy consuming self-organising systems such as the microtubules in cells show nucleation and growth. Heterogeneous nucleation, nucleation with the nucleus at a surface, is much more common than homogeneous nucleation.
For example, in the nucleation of ice from supercooled water droplets, purifying the water to remove all or all impurities results in water droplets that freeze below around - 35 C, whereas water that contains impurities may freeze at - 5 C or warmer. Thus here, we have direct evidence that nucleation of ice on impurities can occur at much higher temperatures than without impurities; this observation that heterogeneous nucleation can occur when the rate of homogeneous nucleation is zero, is understood using classical nucleation theory. This predicts that the nucleation slows exponentially with the height of a free energy barrier ΔG*; this barrier comes from the free energy penalty of forming the surface of the growing nucleus. For homogeneous nucleation the nucleus is approximated by a sphere, but as we can see in the schematic of macroscopic droplets to the right, droplets on surfaces are not complete spheres and so the area of the interface between the droplet and the surrounding fluid is less than a sphere's 4 π r 2.
This reduction in surface area of the nucleus reduces the height of the barrier to nucleation and so speeds nucleation up exponentially. Nucleation can start at the surface of a liquid. For example, computer simulations of gold nanoparticles show that the crystal phase nucleates at the liquid-gold surface. Classical nucleation theory makes a number of assumptions, for example it treats a microscopic nucleus as if it is a macroscopic droplet with a well-defined surface whose free energy is estimated using an equilibrium property: the interfacial tension σ. For a nucleus that may be only of order ten molecules across it is not always clear that we can treat something so small as a volume plus a surface. Nucleation is an inherently out of thermodynamic equilibrium phenomenon so it is not always obvious that its rate can be estimated using equilibrium properties. However, modern computers are powerful enough to calculate exact nucleation rates for simple models; these have been compared with the classical theory, for example for the case of nucleation of the crystal phase in the model of hard spheres.
This is a model of hard spheres in thermal motion, is a simple model of some colloids. For the crystallization of hard spheres the classical theory is a reasonable approximate theory. So for the simple models w
A hydrogen bond is a electrostatic force of attraction between a hydrogen atom, covalently bound to a more electronegative atom or group the second-row elements nitrogen, oxygen, or fluorine —the hydrogen bond donor —and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor. Such an interacting system is denoted Dn–H···Ac, where the solid line denotes a covalent bond, the dotted line indicates the hydrogen bond. There is general agreement that there is a minor covalent component to hydrogen bonding for moderate to strong hydrogen bonds, although the importance of covalency in hydrogen bonding is debated. At the opposite end of the scale, there is no clear boundary between a weak hydrogen bond and a van der Waals interaction. Weaker hydrogen bonds are known for hydrogen atoms bound to elements such as chlorine; the hydrogen bond is responsible for many of the anomalous physical and chemical properties of compounds of N, O, F. Hydrogen bonds can be intramolecular.
Depending on the nature of the donor and acceptor atoms which constitute the bond, their geometry, environment, the energy of a hydrogen bond can vary between 1 and 40 kcal/mol. This makes them somewhat stronger than a van der Waals interaction, weaker than covalent or ionic bonds; this type of bond can occur in inorganic molecules such as water and in organic molecules like DNA and proteins. Intermolecular hydrogen bonding is responsible for the high boiling point of water compared to the other group 16 hydrides that have much weaker hydrogen bonds. Intramolecular hydrogen bonding is responsible for the secondary and tertiary structures of proteins and nucleic acids, it plays an important role in the structure of polymers, both synthetic and natural. It was recognized that there are many examples of weaker hydrogen bonding involving donor Dn other than N, O, or F and/or acceptor Ac with close to or the same electronegativity as hydrogen. Though they are quite weak, they are ubiquitous and are recognized as important control elements in receptor-ligand interactions in medicinal chemistry or intra-/intermolecular interactions in materials sciences.
Thus, there is a trend of gradual broadening for the definition of hydrogen bonding. In 2011, an IUPAC Task Group recommended a modern evidence-based definition of hydrogen bonding, published in the IUPAC journal Pure and Applied Chemistry; this definition specifies: The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation. Most introductory textbooks still restrict the definition of hydrogen bond to the "classical" type of hydrogen bond characterized in the opening paragraph. A hydrogen atom attached to a electronegative atom is the hydrogen bond donor. C-H bonds only participate in hydrogen bonding when the carbon atom is bound to electronegative substituents, as is the case in chloroform, CHCl3. In a hydrogen bond, the electronegative atom not covalently attached to the hydrogen is named proton acceptor, whereas the one covalently bound to the hydrogen is named the proton donor.
In the donor molecule, the H center is protic. The donor is a Lewis base. Hydrogen bonds are represented as H · · · Y system. Liquids that display hydrogen bonding are called associated liquids; the hydrogen bond is described as an electrostatic dipole-dipole interaction. However, it has some features of covalent bonding: it is directional and strong, produces interatomic distances shorter than the sum of the van der Waals radii, involves a limited number of interaction partners, which can be interpreted as a type of valence; these covalent features are more substantial when acceptors bind hydrogens from more electronegative donors. Hydrogen bonds can vary in strength from weak to strong. Typical enthalpies in vapor include: F−H···:F, illustrated uniquely by HF2−, bifluoride O−H···:N, illustrated water-ammonia O−H···:O, illustrated water-water, alcohol-alcohol N−H···:N, illustrated by ammonia-ammonia N−H···:O, illustrated water-amide HO−H···:OH+3 The strength of intermolecular hydrogen bonds is most evaluated by measurements of equilibria between molecules containing donor and/or acceptor units, most in solution.
The strength of intramolecular hydrogen bonds can be studied with equilibria between conformers with and without hydrogen bonds. The most important method for the identification of hydrogen bonds in complicated molecules is crystallography, sometimes NMR-spectroscopy. Structural details, in particular distances between donor and acceptor which are smaller than the sum of the van der Waals radii can be taken as indication of the hydrogen bond strength. One scheme gives the following somewhat arbitrary classification: those that are 15 to 40 kcal/mol, 5 to 15 kcal/mol, >0 to 5 kcal/mol are considered strong, moder
Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities, in places where the pressure is low. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate an intense shock wave. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion; this results in surface fatigue of the metal causing a type of wear called "cavitation". The most common examples of this kind of wear are to pump impellers, bends where a sudden change in the direction of liquid occurs. Cavitation is divided into two classes of behavior: inertial cavitation and non-inertial cavitation; the process in which a void or bubble in a liquid collapses, producing a shock wave, is called inertial cavitation. Inertial cavitation occurs in nature in the strikes of mantis shrimps and pistol shrimps, as well as in the vascular tissues of plants.
In man-made objects, it can occur in control valves, pumps and impellers. Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field; such cavitation is employed in ultrasonic cleaning baths and can be observed in pumps, etc. Since the shock waves formed by collapse of the voids are strong enough to cause significant damage to moving parts, cavitation is an undesirable phenomenon, it is often avoided in the design of machines such as turbines or propellers, eliminating cavitation is a major field in the study of fluid dynamics. However, it is sometimes useful and does not cause damage when the bubbles collapse away from machinery, such as in supercavitation. Inertial cavitation was first observed in the late 19th century, considering the collapse of a spherical void within a liquid; when a volume of liquid is subjected to a sufficiently low pressure, it may rupture and form a cavity. This phenomenon is coined cavitation inception and may occur behind the blade of a rotating propeller or on any surface vibrating in the liquid with sufficient amplitude and acceleration.
A fast-flowing river can cause cavitation on rock surfaces when there is a drop-off, such as on a waterfall. Other ways of generating cavitation voids involve the local deposition of energy, such as an intense focused laser pulse or with an electrical discharge through a spark. Vapor gases evaporate into the cavity from the surrounding medium; such a low-pressure bubble in a liquid begins to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapor within increases; the bubble collapses to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism which releases a significant amount of energy in the form of an acoustic shock wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand kelvin, the pressure several hundred atmospheres. Inertial cavitation can occur in the presence of an acoustic field.
Microscopic gas bubbles that are present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size and rapidly collapse. Hence, inertial cavitation can occur if the rarefaction in the liquid is insufficient for a Rayleigh-like void to occur. High-power ultrasonics utilize the inertial cavitation of microscopic vacuum bubbles for treatment of surfaces and slurries; the physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local temperature of the liquid reaches the saturation temperature, further heat is supplied to allow the liquid to sufficiently phase change into a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature. In order for cavitation inception to occur, the cavitation "bubbles" need a surface on which they can nucleate.
This surface can be provided by the sides of a container, by impurities in the liquid, or by small undissolved microbubbles within the liquid. It is accepted that hydrophobic surfaces stabilize small bubbles; these pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake's threshold. The vapor pressure here differs from the meteorological definition of vapor pressure, which describes the partial pressure of water in the atmosphere at some value less than 100% saturation. Vapor pressure as relating to cavitation refers to the vapor pressure in equilibrium conditions and can therefore be more defined as the equilibrium vapor pressure. Non-inertial cavitation is the process in which small bubbles in a liquid are forced to oscillate in the presence of an acoustic field, when the intensity of the acoustic field is insufficient to cause total bubble collapse; this form of cavitation causes less erosion than inertial cavitation, is used for the cleaning of delicate materials, such as silicon wafers.
Hydrodynamic cavitation describes the process of vaporisation, bubble generation and bubble implosion which occurs in a flowing liquid as a result of a decrease and su
Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it transports nutrients; the word "xylem" is derived from the Greek word ξύλον, meaning "wood". The term was introduced by Carl Nägeli in 1858; the most distinctive xylem cells are the long tracheary elements. Tracheids and vessel elements are distinguished by their shape. Xylem contains two other cell types: parenchyma and fibers. Xylem can be found: in vascular bundles, present in non-woody plants and non-woody parts of woody plants in secondary xylem, laid down by a meristem called the vascular cambium in woody plants as part of a stelar arrangement not divided into bundles, as in many ferns. In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although a vascular bundle will contain primary xylem only; the branching pattern exhibited by xylem follows Murray's law.
Primary xylem is formed during primary growth from procambium. It includes metaxylem. Metaxylem develops before secondary xylem. Metaxylem has wider tracheids than protoxylem. Secondary xylem is formed during secondary growth from vascular cambium. Although secondary xylem is found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the two main groups in which secondary xylem can be found are: conifers: there are some six hundred species of conifers. All species have secondary xylem, uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood. Angiosperms: there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem is rare in the monocots. Many non-monocot angiosperms become trees, the secondary xylem of these is used and marketed as hardwood; the xylem and tracheids of the roots and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plants.
The system transports water and soluble mineral nutrients from the roots throughout the plant. It is used to replace water lost during transpiration and photosynthesis. Xylem sap consists of water and inorganic ions, although it can contain a number of organic chemicals as well; the transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees. Three phenomena cause xylem sap to flow: Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals; the phloem pressure can rise to several MPa, far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere creates a negative pressure at the top of a plant. This causes millions of minute menisci to form in the mesophyll cell wall; the resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil. Root pressure: If the water potential of the root cells is more negative than that of the soil due to high concentrations of solute, water can move by osmosis into the root from the soil; this causes a positive pressure. In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is highest in the morning before the stomata allow transpiration to begin. Different plant species can have different root pressures in a similar environment; the primary force that creates the capillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits.
Capillary action provides the force that establishes an equilibrium configuration, balancing gravity. When transpiration removes water at the top, the flow is needed to return to the equilibrium. Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves; this evaporation causes the surface of the water to recess into the pores of the cell wall. By capillary action, the water forms concave menisci inside the pores; the high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. Transpirational pull requires that the vessels transporting the water be small in diameter, and as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots the gases come out of solution and form a bubble – an embolism forms, which will spread to other adjacent cells, unless bordered pits are present (these have
Hemorheology spelled haemorheology, or blood rheology, is the study of flow properties of blood and its elements of plasma and cells. Proper tissue perfusion can occur only when blood's rheological properties are within certain levels. Alterations of these properties play significant roles in disease processes. Blood viscosity is determined by plasma viscosity and mechanical properties of red blood cells. Red blood cells have unique mechanical behavior, which can be discussed under the terms erythrocyte deformability and erythrocyte aggregation; because of that, blood behaves as a non-Newtonian fluid. As such, the viscosity of blood varies with shear rate. Blood becomes less viscous at high shear rates like those experienced with increased flow such as during exercise or in peak-systole. Therefore, blood is a shear-thinning fluid. Contrarily, blood viscosity increases when shear rate goes down with increased vessel diameters or with low flow, such as downstream from an obstruction or in diastole.
Blood viscosity increases with increases in red cell aggregability. Blood viscosity is a measure of the resistance of blood to flow, it can be described as the thickness and stickiness of blood. This biophysical property makes it a critical determinant of friction against the vessel walls, the rate of venous return, the work required for the heart to pump blood, how much oxygen is transported to tissues and organs; these functions of the cardiovascular system are directly related to vascular resistance, preload and perfusion, respectively. The primary determinants of blood viscosity are hematocrit, red blood cell deformability, red blood cell aggregation, plasma viscosity. Plasma's viscosity is determined by water-content and macromolecular components, so these factors that affect blood viscosity are the plasma protein concentration and types of proteins in the plasma. Hematocrit has the strongest impact on whole blood viscosity. One unit increase in hematocrit can cause up to a 4% increase in blood viscosity.
This relationship becomes sensitive as hematocrit increases. When the hematocrit rises to 60 or 70%, which it does in polycythemia, the blood viscosity can become as great as 10 times that of water, its flow through blood vessels is retarded because of increased resistance to flow; this will lead to decreased oxygen delivery. Other factors influencing blood viscosity include temperature, where an increase in temperature results in a decrease in viscosity; this is important in hypothermia, where an increase in blood viscosity will cause problems with blood circulation. Many conventional cardiovascular risk factors have been independently linked to whole blood viscosity. Anemia can reduce blood viscosity. Furthermore, elevation of plasma viscosity correlates to the progression of coronary and peripheral artery diseases. In pascal-seconds, the viscosity of blood at 37 °C is 3 × 10−3 to 4 × 10−3 3 - 4 centipoise in the centimetre gram second system of units. Μ = ⋅ 10 − 3 P a ⋅ s ν = μ ρ = ⋅ 10 − 3 1.06 ⋅ 10 3 = ⋅ 10 − 6 m 2 s Blood viscosity can be measured by viscometers capable of measurements at various shear rates, such as a rotational viscometer.
Viscoelasticity is a property of human blood, due to the elastic energy, stored in the deformation of red blood cells as the heart pumps the blood through the body. The energy transferred to the blood by the heart is stored in the elastic structure, another part is dissipated by viscosity, the remaining energy is stored in the kinetic motion of the blood; when the pulsation of the heart is taken into account, an elastic regime becomes evident. It has been shown that the previous concept of blood as a purely viscous fluid was inadequate since blood is not an ordinary fluid. Blood can more be described as a fluidized suspension of elastic cells; the red blood cells possess elastic properties. This elastic property is the largest contributing factor to the viscoelastic behavior of blood; the large volume percentage of red blood cells at a normal hematocrit level leaves little room for cell motion and deformation without interacting with a neighboring cell. Calculations have shown that the maximum volume percentage of red blood cells without deformation is 58%, in the range of occurring levels.
Due to the limited space between red blood cells, it is obvious that in order for blood to flow, significant cell to cell interaction will play a key role. This interaction and tendency for cells to aggregate is a major contributor to the viscoelastic behavior of blood. Red blood cell deformation and aggregation is coupled with flow induced changes in the arrangement and orientation as a third major factor in its vi