Acoustics and Fluids PDF

# Acoustics Acoustics is a branch of physics that studies mechanical waves, including vibrations, sound, infrasound and ultrasound. Acoustics observes generation, propagation of mechanical waves, their interaction with the environment and the process of hearing. ## What is a mechanical wave? A me...

# Acoustics Acoustics is a branch of physics that studies mechanical waves, including vibrations, sound, infrasound and ultrasound. Acoustics observes generation, propagation of mechanical waves, their interaction with the environment and the process of hearing. ## What is a mechanical wave? A mechanical wave is a spreading disturbance in an elastic medium. The disturbance is caused by a force that displaces particles in the medium. Mechanical waves transfer energy but not mass. They can take the form of an elastic deformation in solids or a variation of pressure in gases. ## What is sound? Sound is defined as any mechanical (or MW) vibrations that produce hearing perception. ## Totality of mechanical waves can be illustrated in a diagram spectrum of mechanical waves consisting of: - Low frequency mechanical waves - INFRASOUND. - Acoustic range (sound) - mechanical waves with frequencies from 20 Hz to 20 kHz. - High frequency mechanical waves - ULTRASOUND. ## Sound propagation can occur in elastic media only! The vibrations in elastic matter are transmitted consecutively from particle to particle, causing recurrent alteration of density. If the source of the sound is located in an air environment, its vibrations produce additional movement of the particles, which results in variations of pressure: $P(t) = ΔPsin(φ)$. Where: - $ΔP$ is the maximal pressure increase/decrease. - $φ$ is the phase of the given mechanical wave, showing the rate of alterations. ## Typical sources of sound are: - Sound is a mechanical wave. - Sound can only be transmitted through elastic media. - Sound transfers energy but not mass. ## Mechanical waves propagate as two forms of matter vibration: - Longitudinal and Transversal. ### Longitudinal sound waves - Are typical of gases and liquids. - Particle movement is illustrated below: ### Transverse waves - Are typical of solids, where particles displacement occurs perpendicularly to wave propagation. ## Physical characteristics of sound. ### 1. Sound intensity $I$ $I = E/t.S$ DEF: The sound energy transmitted per second through unit area, placed perpendicularly to the sound propagation. Intensity is measured in watt per square meter. [$W/m^2$]. ### 2. Sound pressure $P$ $P = ∆Psino$ added to atmospheric $P_{atm}$ The relation between intensity and pressure: $I = p^2/2Za$ ### 3. Sound frequency $f$ DEF: Frequency is the number of vibrations (full alterations of the sound pressure or repetitions of motion of the particles) per second. It is measured in Hz - 1 Hz equals to 1 vibration per second. A quantity which is directly related with $f$ is the period: the time duration of 1 vibration. ### 4. Sound velocity (SV) (or speed of sound) is the quantity which depends on the properties of the medium through which sound propagates. For example, at 20° C, SV in the air is about 340 m/s, but in water, SV is about 1500 m/s. The exact relationship between sound speed and matter properties treats elasticity as a major factor. #### Temperature dependence of the SV: ### 5. Sound wavelength: The distance between two consecutive sound fronts. $λ = v.T = v/f$ ### 6. Acoustic impedance Z $Z = ρ.v$, ρ-density of the given medium, v – sound velocity. Sound propagation through a certain medium is determined from acoustic impedance $Z$ of the latter. When the sound wave encounters a border between two different media, a part of the wave reflects, and the other part passes due to differences in acoustic impedances $Z_1$ and $Z_2$. The degree of reflection/transmission depends on the difference between $Z_1$ and $Z_2$: $a_{r}^2 = (Z_2-Z_1)^2/(Z_2+Z_1)^2$ ## The sounds are of two types: - Simple tones and complex tones. ### DEF: - Sound is called a simple tone if the vibration has a sinus shape and possesses a certain frequency (on figure below, two simple sounds - at $100$ and $500$ Hz). - Sound is called complex when it consists of several simple tones, as a superposition of mechanical waves (on figure below - third axis wave). Each complex sound can be illustrated by a graph representing its acoustic spectrum: a combination of certain amplitudes and frequencies, proportional to participant simple waves frequencies. The minimal frequency is called the basic frequency $f_0$ (on the pictures above - $100$ and $200$ Hz) for the two given sounds. The basic soundhas always maximal amplitude. The other components of the vibration are $2f_0$ (two-fold higher), $3f_0$ (three-fold higher)... and their amplitudes are smaller. ## Psychophysical characteristics of sound. Each physical (objective) characteristic of sound corresponds to the respective psychophysical (subjective) analogue. Human perception of sound is based on fundamental physiological Weber-Fechner law: $Perception \sim log(stimulus)$ (Perceptions increase logarithmically with the stimulus) Intensity of the sound has a subjective analogue: Sound Intensity level $Ε$ ($Ε$ is defined at 1000 Hz sound frequency only). $Ε=k.Ig(I/I_0)$, where: - $I$ is the intensity of the sound. - $I_0$ is the threshold at 1000 Hz. The threshold is approximately $10^{-12} W/m^2$ - the lowest power vibration audible for human ears. Intensity level is measured in bel $B$/ decibel $dB$ (at $k=1$ and $k=10$ respectively). ### Loudness of the sound $L$ is the quantity representing subjective human perception of the sound magnitude. Loudness is defined by the equation: $L= k. Ig(I/I_0)$ where $k$ is a coefficient depending on frequency, $I$ is the intensity of the given sound, $I_0$ is the threshold of audibility of this sound. $L$ is measured in Phon. On figure bellow red lines represent equal-loudness contours. Each point of these curves corresponds to certain frequency and sound level in $dB$, making the relation loudness-intensity level accessible. ## Weber-Fechner low explanation: Why is it that doubling the sound intensity to the ear does not produce a dramatic increase in loudness? We cannot give answers with complete confidence, but it appears that there are saturation effects. Nerve cells have maximum rates at which they can fire, and it appears that doubling the sound energy to the sensitive inner ear does not double the strength of the nerve signal to the brain. This is just a model, but it seems to correlate with the general observations which suggest that something like ten times the intensity is required to double the signal from the inner ear. One difficulty with this “rule of thumb” for loudness is that it is applicable only to adding loudness for identical sounds. If a second sound is widely enough separated in frequency to be outside the critical band of the first, then this rule does not apply at all. While not a precise rule even for the increase of the same sound, the rule has considerable utility along with the just noticeable difference in sound intensity when judging the significance of changes in sound level. ## Audibility area. On the diagram below, the closed area between the threshold of audibility and threshold of pain/feeling represents the audibility area. All the sounds we can perceive are inside this area. Our ears are most sensitive to sounds from 1000 – 4000 Hz range. Human speech is spread over 400-1000 Hz frequency range. ## Pitch of the sound. Pitch of the sound is a subjective characteristic, corresponding to sound frequency. The higher the frequency, the higher the pitch. The pitch of the human voice is a physiological feature. It is determined by the vibrating characteristics of vocal cords (their basic resonant frequency). For example, male basic frequency is in the range of 80-100 Hz and men’s voice sounds with lower pitch, whereas females basic frequency of vocal cords is about 400-500 Hz and this is the reason for the higher pitch of the female voice. ## Timbre. Timbre characterizes complex sounds. Timbre allows different sound sources to be distinguished by their acoustic spectra. ## Sound as a diagnostic instrument. ### Auscultation. The basic tool for auscultation is a stethoscope. Mechanical stethoscopes amplify sound due to standing wave phenomenon. There are optimal recording sites (sites to place a chest piece of a stethoscope) for the various heart sounds (figure below). Sound conductivity of a stethoscope is characterized by some features: - firm application of the chest piece makes the diaphragm taut with pressure, thereby causing an attenuation of low frequencies; - loose fitting earpiece causes leakage, which reduces the coupling between the chest wall and the ear. #### Heart Valves and Heart Sounds: - Closure of the AV valves creates the 1st heart sound ('lub'). - Closure of the semilunar valves creates the 2nd heart sound ('dupp'). Placement of a stethoscope varies depending on which heart sounds and valves are of interest. ### Phonocardiography. Heart sounds and murmurs have extremely small amplitudes with frequencies from 0.1 to 2000 Hz. Thus the recording device must be carefully selected for a wide band frequency response characteristics. Specially designed acoustically quiet environments are needed for noise-free recording of heart sounds. The device used for recording these tones and their transformation into audible sounds is called the phonocardiograph. #### Phonocardiogram. Phonocardiogram (picture above) is a recording of the heart sounds and murmurs. The method allows to eliminate subjective interpretation of the heart sounds. It enables evaluation of the heart sounds and murmurs regarding electric and mechanical events in the cardiac cycle. Evaluation of the result is based on changes in the wave shape and various timing parameters. ### Audiometry Audiometry is a method for experimental examination/determination of the personal threshold of audibility. It is carried out by means of a generator of simple tones with a certain frequencies (125, 250, 500, 1000, 2000, 4000, 8000 Hz). Audiometry is used to determine the relation between objective sound intensity and subjective loudness. ## Sound therapy ### Extracorporeal Lithotripsy: EL is based on the concept of remote destruction of kidney and bladder stones by sound waves. When a mechanical wave (of type shock wave) passes trough the stone, the latter starts to vibrate. A resonance occurs and the amplitude of vibration reaches critical value at which the structure stability is impaired. The stone has been destroyed to small pieces. The latter is due to rapid increase of pressure inside the stone and impairing of its structure stability. #### fig. A - lithotripsy set #### fig. B - operation principle ## Ultrasound Ultrasound (US) waves are spread over a wide range of frequencies: 20 kHz - 20 MHz. US waves carry higher energy compared to the same amplitude sound (due to their high frequency). - US waves can interact with biological tissues (due to the similarity in size of different bio-structures and US wavelength), which determines its significance in medicine. - US vibrations are well absorbed by tissues. - Being reflected or scattered by small objects, US serves as an information carrier in diagnostic imaging. ### Production of US US is produced by means of the Piezoelectric effect. PEE occurs in some crystals due to their specific crystal lattice: normally electric charges are placed symmetrically, but if the crystal is being deformed, the symmetry of the lattice is impaired and polarization occurs. This phenomenon is called direct piezoelectric effect. Conversely, if the electric charge is loaded on the opposite walls of the crystal, it experiences electric forces deforming the lattice (**reverse piezoelectric effect**). If this deformation appears at more than 20000 times per second, the crystal will vibrate with US frequency, and it becomes a US source. The crystals of this type can simultaneously generate and receive US waves. The device built on a piezo-crystal is called a transducer and serves as both a generator and a receiver of US. ### Physical properties of US determining its application as a diagnostic instrument: - The wavelength of US is shorter than that of the audible sound (example: if two sound waves with frequencies of 2 KHZ and 2 MHz are spreading in air, their wavelengths will be 75 cm and 0.75 mm respectively). - Short wavelength enables US to interact with small structures (dimensions of mm and less) in human tissues. - As a consequence, US can produce resonant vibration within tissues and being reemitted or reflected by them, it carries useful information about their location, intactness, constitution. ## US imaging US sonography is based on the analysis of US reflected by tissues. Reflections occur when US encounters a boundary between different types of tissues (due to a difference in acoustic impedances). The distribution of reflected/penetrated US beam is determined by: - **Reflection coefficient**: $a_r^2 = (Z_2-Z_1)^2/(Z_2+Z_1)^2$. When US falls from the air to the patient’s skin, $a_r$ equals approximately 1 ($Z_2$ skin >> $Z_1$ air), therefore almost 100% of the wave reflects back to the air. Thus, the US wave does not penetrate into the body and no diagnostic information is received. This problem is abolished by the use of contact gel. The gel has the same acoustic impedance as the skin (tissues). The transducer is immersed in gel and US penetrates into the body without losses when it crosses the border gel-skin: ### US sonography types: - **A**- Amplitude scan. The image represents a straight line with spikes. Each spike corresponds to a border between tissues. The main diagnostic parameter is the amplitude of spikes - the larger the difference between acoustic impedances of tissues, the greater the amplitude. This method is applicable to simple structures. - **B**- Brightness scan images represent each small detail of the tissue observed as a bright/dark spot, depending on the value of acoustic impedance. This method is suitable for compound structures examination. This is the default mode that is produced by any ultrasound/echo machine. It is a 2 dimensional cross-sectional view of the underlying structures and is made up of… This is the most intuitive of all modes to understand. The field of view is the portion of the organs or tissues that are intersected by the scanning plane. Depending on the probe used, the shape of this field could be a sector - commonly seen with Echo and abdominal ultrasound probes - or rectangular or trapezoid - seen with superficial or vascular probes. - Multiple images of the field or frames are generated every second on the screen, giving an illusion of movement. A frame rate of at least 20 frames per second is needed to give a realistic illusion of motion. - On a grey scale, high reflectivity (bone) is white; low reflectivity (muscle) is grey and no reflection (water) is black. Deeper structures are displayed on the lower part of the screen and superficial structures on the upper part. - The main uses for 2-D mode are to measure cardiac chamber dimensions, assess valvular structure and function, estimate global and segmental ventricular systolic function, and improve accuracy of interpretation of Doppler modalities. - While this mode is useful to accurately represent the 2-dimensional structure of the underlying tissues, it does not resolve rapid movements well and may misrepresent 3-dimensional nature of structures. ### M-mode This mode allows investigation of movable structures. Initially, a 2-D image of the object is produced and a single scan line is placed along the area of interest. The M-mode will then show how the structures intersected by that line move toward or away from the probe over time. The M-mode has good temporal resolution, so it is useful in detecting and recording rapid movements. We can also correlate and time events with ECG or respiratory pressure waveforms traced alongside the M-mode tracings. The M-mode is commonly used for measuring chamber dimensions and calculating fractional shortening and ejection fraction. ## Doppler's effect The Doppler effect is named after its inventor Christian Doppler. Doppler effect is called the change in frequency of a wave as perceived by an observer moving relative to the source of the wave. Doppler’s effect can occur in any process involving waves, including ultrasound. Let’s consider a person (receiver) who is moving toward a sound source with velocity $v$. The receiver will perceive sound with higher frequency than the source frequency (respectively - the wavelength will be shorter than at the source) because moving against the sound propagation, he/she will encounter more than one wave front per second. If the person (receiver) is going away from the source - the effect is reverse: perceived sound has a lower frequency and a longer wavelength, compared with the source. These phenomena are called blue shift and red shift respectively and are present in other wave phenomena such as light. Measurement of Doppler shift allows calculation of blood stream velocity. The frequency of reflected US changes with: $Δf = 2.vcosθ.f_0/c$ where: - $Δf = f_0+ f_r$ - $f$ - reflected wave frequency - $v$ - velocity of the blood stream - $c$ - US velocity in soft tissues (~1540 m/s) - $f_0$ - US frequency - $θ$ - the angle between blood flow and US axis of propagation (Doppler's angle). ## Ultrasound applications in therapy US can influence tissues producing favorable therapeutic effects upon them. The character of the effects derived is related to US physical properties and the type of tissues. Different influences can be achieved selecting the US mode of generation - continuous or pulsed. ## Pulsed US (duty cycle of 20%) cannot produce thermal effects because of dissipation of energy absorbed during the pause of the cycle. Ultrasound energy creates also mechanical forces independent of thermal effects, thereby causing biologic effects that are not related to temperature rise alone, such as cavitation, torque forces, oscillatory shear, radiation, pressure and microstreaming. In general, pulsed US is used to produce: - increase of skin and cell membranes permeability resulting in calcium influx enhancement, - also increase of: mast cell degranulation, macrophage activity, rate of protein synthesis, oppose inflammatory processes. ### Thermal effects. The thermal action of US is caused due to oscillations of tissues (cells, molecules) when US is passing through them. The amount of heat produced depends on the intensity of the ultrasound, the time of exposure, and the specific absorption characteristics of the tissue. As much as 70% of the total temperature increase associated with ultrasound occurs within the first minute of exposure, but temperature continues to rise as exposure time is prolonged. Better thermal effect is achieved in highly absorbing structures (rich in collagen as the main absorber) - joints, bones, connecting tissue. The relative protein content of each tissue, since absorption coefficients of tissues are directly related to protein content; absorption coefficients vary between 1 (skin, tendon, spinal cord) and 10 (bone) dB/cm MHz. Such effect is derived by high intensity, high frequency, continuous US. Since muscles are well vascularized, they do not undergo sizable heating up because blood flow continuously carries away a large part of heat produced. Main effects of heating are: - acceleration of metabolic processes, - alteration of nerve conductivity, - enhanced blood circulation, - improved extensibility of soft tissues incl. muscle elasticity. The mentioned phenomena are beneficial in US diathermy (heating therapy) to: - heat up bones and joints - treat of arthritis - strengthen bones Having confirmed benefits on the ossification process, ultrasound therapy offers promising treatment options for a variety of dental procedures. Tooth decay (caries) and diseases of the dental pulp result in the loss of tooth vitality and function, requiring invasive treatment to restore the tooth to health. Low intensity pulsed ultrasound has been shown to accelerate bone fracture healing, indicating that ultrasound may be used as a tool to facilitate hard tissue regeneration. Low intensity low frequency ultrasound may stimulate endogenous coronal tooth repair by stimulating dentine formation from existing odontoblasts or by activating dental pulp stem cells to differentiate into new reparative dentine-producing cells. Ultrasound therapy also has the potential of alleviating dentine hypersensitivity by inducing occlusion of dentinal tubules. ## US sonophoresis Ultra-sonophoresis is the method for drug delivery into localized areas assisted by collimated ultrasound beam directed toward these areas. Pharmaceuticals are inserted through the skin (per cutis) and can penetrate into the tissues in depths of 4-6 cm. Medications are impasted on the skin in the form of gel or cream. ## Infrasound IS - vibrations in elastic media with frequency from 0 to 20 Hz. Humans do not hear these vibrations. Natural sources of IS are: earthquakes, volcanoes, sea waves (tsunami), typhoons, waterfalls. Some animals use IS to communicate - elephants, hippopotamus. Artificial sources: some machines, transport systems, factories, large air conditioners. IS is absorbed weakly due to its long wavelength. For example, if two waves with frequencies of 1000 Hz and 10 Hz respectively propagate through the same medium - the latter will be absorbed ten times less! Therefore IS penetrates deeper than audible sound. Because of its low frequency, IS causes vibrations of large objects (machines, buildings, ships) and also can produce resonance vibrations in the human body - internal organs and body cavities have the characteristic resonant frequencies: - Heart - 1 to 1.5 Hz. - Circulation of the blood flow across the body - 1.2 Hz. - Vestibular apparatus - 0.5 to 13 Hz. - Most of Internal organs - 2 to 8 Hz. - Head - up to 20 Hz. Exposure to infrasound has been demonstrated to effect recipients with symptoms including fear, sorrow, depression, anxiety, nausea, chest pressure and hallucination. It can cause objects to move through vibration, and some believe the body’s internal organs can be effected. It is suggested that levels above 80 decibels at frequencies between 0.5 to 10Hz may start to effect the vestibular of the inner ear thus causing disorientation. Any high-volume sound can trigger the body to react by increasing respiration, heart rate and blood pressure, but when they cannot actually hear the sound, recipients are left with no explanation for the sudden onset of these symptoms. This may then lead to further effects caused by the mind’s possible reaction to the unknown, as outlined below. Ocean waves are known to sometimes generate infrasound, and it has been suggested to have been a possible “trigger” causing ships crews to abandon their craft in fear, only to have the ship later found mysteriously drifting about unmanned. The range of infrasound is generally accepted to be between 0-20 hertz with a specific area of interest between 17 and 19 hertz. Tests by NASA have revealed that the human eyeball resonates at around 18Hz, to which infrasound exposure may cause a reaction and lead to hallucinations. Infrasound occurs quite naturally at some locations and possible causes include storms, earthquakes, waterfalls, volcanoes, ocean waves and wind reacting with structures such as chimneys. Some animals are sensitive to these low frequency vibrations and may appear to “foresee” approaching storms and earthquakes. Elephants are known to use infrasound as a form of communication over long distances. Subsonic sound can travel long distances, pass through walls and may be amplified in tunnel-like structures. Standard hearing protection is of little use for subsonic sound as it often can pass straight through and may even be amplified. ## Processes of transfer ### Diffusion DEF: Diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential). Quantitatively, the mass of the transferred substance can be evaluated using Fick’s laws of diffusion: $J = D.dc/dx$, where: - $J$ is a mass flux, the mass flowing per second through the unit area ($M/S.t$). - $D$ is the diffusion coefficient. - $dc/dx$ - concentration gradient. or: $M=-D.dc/dx.S.t$ ### Gradient Gradient is a vector quantity representing the rate of change of a given scalar quantity over the distance. Its value equals to the difference in the quantity’s values in two points/regions and direction - from low toward high (denoted in the formula by “-"). Particularly in diffusion, the concentration gradient $dc/dx$ represents difference in concentration over the distance. The substance flows against the gradient and down the gradient/osmotic forces. Diffusion is a spontaneous process (no input of energy is required). It can also be called a passive process. ### Diffusion applications in medicine Diffusion across membrane (transfusion) is the physical basis of hemodialysis. Hemodialysis is called the process of (out of body) purification of blood from toxins by controlled transfusion. For this purpose, a high concentration gradient is used. This therapy is applied in cases of kidney deficiency. Toxin molecules diffuse through the membrane separating two counter-fluxes - blood from a special solution named dialysis fluid. The permanent flow of dialysis fluid provides a high concentration gradient/difference (figure below). ### Internal friction $Impulse$ - physical quantity equal to the product of velocity and mass of a moving object: $p = m.v$ When neighbouring parts of a fluid (for example two layers of a liquid shown below) move with different velocities $v1$ and $v2$, the transfer of impulse is occurred between two layers. The faster layer “drags” the slower one transferring impulse to the latter. In the same time, the faster one experiences decelerating action of the slower layer. This phenomenon is called internal friction. The forces of internal friction are evaluated by the equation: $F_i = -n.dv/dx.S.t$ ,where: - $dv/dx$ - difference in velocities over the distance between layers - $η$ - viscosity is a quantity depending on the molecular interaction within the given fluid. Take Attention: - In contrast to other transfer processes, the Force of internal friction acts perpendicularly to the velocity “gradient” ### Heat conduction Another type of physical transfer appears when thermal non-homogeneities exist in a given matter medium. Heat is transferred when there is a difference in the temperature of two contacting objects or two parts of one medium (see image: temperature gradient is presented as the difference $T_a > T_b$). The amount of heat energy transferred is calculated by: $Q = -k.dT/dx.S.t$ where: - $dT/dx$ - temperature gradient, - $k$ - thermal conductivity coefficient, - $S$ - contacting area. ### Medical applications of heat transfer and thermal conduction. The body temperature can be controlled by heat transfer decrease or increase of the temperature depending on the physiological status. Thermal therapy - warming pads and compresses, baths with warm liquids, paraffin. A heating pad is a pad used for warming of parts of the body in order to manage pain. Localized application of heat causes the blood vessels in that area to dilate, enhancing perfusion to the targeted tissue. Many episodes of pain come from muscle exertion or strain, which creates tension int he muscles and soft tissues. This tension can constrict circulation, sending pain signals to the brain. Heat application eases pain by: - dilating the blood vessels surrounding the painful area. Increased blood flow provides additional oxygen and nutrients to help heal the damaged muscle tissue. - stimulating sensation in the skin and thereby decreasing the pain signals being transmitted to the brain. - increasing the flexibility (and decreasing painful stiffness) of the soft tissues surrounding the injured area, including muscles and connective tissue. Some more specific heat-exchange procedures are referred to as Cardiac arrest management: case of whole-body ischemia and subsequent reperfusion injury. This injury mechanism along with pre-arrest comorbidities cause enormous biochemical, structural, and functional insults, which in a complex interrelated process leads to progressive cell destruction, multiorgan dysfunction, neuronal apoptosis, and death. Many of these processes are temperature sensitive. Hypothermia has been shown to attenuate or ameliorate many deleterious temperature-sensitive mechanisms, thereby contributing to protection of the brain and heart. The pathophysiologic mechanisms involved in hypothermia are incompletely understood but have been studied in cellular, animal, and human models. The following actions are associated with hypothermia: - Reducing cerebral metabolism (approximately 6-8% per 1°C). - Reducing excitatory amino acids (glutamate release). - Attenuation and/or reversibility of ischemic depolarization of the CNS, leading to membrane stabilization, electrolyte redistribution, and normalization of intracellular water concentration and intracellular $pH$ (stabilization of the blood-brain barrier). - Attenuation of oxygen free radical production and lipid peroxidation. - Restoration of normal intracellular signaling mechanisms (including calcium modulation) and inhibition of deleterious signaling mechanisms, such as apoptotic signaling. - Restoration of protein synthesis and gene expression. - Inhibition of deleterious inflammatory products (ie, cytokines, interleukins, arachidonic acid cascade end products). - Attenuation of CSF platelet-activating factor (PAF). - Inhibition of cytoskeletal breakdown. In the heart, hypothermia may decrease the area of injury, promote epicardial reflow, decrease myocardial metabolic demand, and preserve intracellular high-energy phosphate stores. ## Cryosurgery Cryosurgery - application of extreme cold to destroy or impair diseased tissues. Cryosurgery is a minimally invasive procedure. Tissue cooling is done by liquid nitrogen and more rarely - carbon dioxide. Modern cryosurgery uses Argon gas sprayed from ultra-thin needles. Cryosurgery works by taking advantage of the destructive force of freezing temperatures on cells. Once the temperature falls below -40°C, ice crystals may form within the cells. Once it occurs, cell death is almost certain. During cryosurgery, progressive failure of microcirculation occurs due to a cascade of events: endothelial layer destruction causing vessel walls to become porous, interstitial edema, platelet aggregation, microthrombi, an ultimately vascular congestion and obliteration. It was theorized that during cryosurgery, the immune system of the host became sensitized to the tumor being destroyed by the cryosurgery. Any primary tumor tissue undamaged by the cryosurgery and the metastases were destroyed by the immune system after cryosurgery. This response was termed the “Cryo-Immunological response”. Transplantation of organs requires low temperature for conservation. Cold suppresses enzyme activity of tissues and provides their intactness. ## Fluids ### Free surface of a liquid Free surface of a liquid is called the outside surface contacting with the surrounding medium. The free surface is the area on which many specificities of liquids are expressed. ### Surface tension Surface tension is such force that aspires to screw/decrease the free surface. At the free surface of liquids, surface tension results from the greater attraction of liquid molecules to each other (cohesion interaction), than to the molecules in the air (adhesion interaction). The net effect is an inward force at its surface that causes the liquid to behave as if its surface were covered with a stretched elastic membrane. Thus, the surface becomes under tension from the imbalanced forces. Surface tension has the dimension of force per unit length, or of energy per unit area. Because of the relatively high attraction of water molecules to each other through hydrogen bonds, water has a higher surface tension (72.8 millinewtons per meter at 20 °C) compared to that of most other liquids. Surface tension is an important factor in the phenomenon of capillarity. We will consider further. ### Forces of surface tension Forces of surface tension are determined of quantity called surface tension coefficient $σ$. The value of $σ$ is specific for a given fluid. $F$ equals a product of the $σ$ and the length of the boundary surface. $F_{sur} = σ.Ι$ DEF: $σ$ is equal to the amount of energy required for increase of the boundary surface of the fluid by one unit. ### Surfactants Surfactants are substances that lower surface tension. Pulmonary surfactant is a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions. By adsorbing to the air-water interface of alveoli, with hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine (DPPC), reduces surface tension. Main functions of surfactant are: - To increase pulmonary compliance. - To prevent atelectasis (collapse of the lung) at the end of expiration. ### Additional pressure (Ap) The surface tension is the cause for appearance of specific phenomena that arise when liquid is placed in a narrow tube or moves within. Interaction between the liquid and tube causes curvature of the liquid surface (see fig below). The curvature determines additional surface pressure. This pressure is directed perpendicularly to the liquid boundary surface - inward or outward depending on the type of the surface - convex or concave. ### Laplace law: Additional pressure Δp arises on the boundary surface between two fluids if it (boundary) is curved. Quantitatively, additional pressure is measured as: $Δp = 2 σ/r$ where: - $r$ - radius of curvature (tube, blood vessel) - $σ$ - coefficient of surface tension - specific property of the given fluid. ### Capillary effect due to additional pressure: ### Additional pressure and embolism This phenomenon occurs when air bubbles or fat drops enter the blood vessels: pressure difference that drives blood causes different curvatures on the two opposite sides of the bubble: with smaller radius at low pressure side and bigger at the other side. Therefore, the resulting additional pressure acts against blood stream and at certain value it can equilibrate it causing blockage of vessel. ## Rheology. Cardiovascular system (CVS) dynamics Rheology observes the motion of blood in the cardiovascular system CVS. Blood belongs to the group of real fluids. These fluids are characterized with changeable density and viscosity. ### Types of fluids Ideal fluid has: Real fluid has: - $p$ = const. - $η$ = 0 - $p$ = const. - $η$ = const. (newtonian) - $p \neq$ const. - $η \neq$ const. (non-newtonian) ### Motion of fluids Fluids move inter two manners: as a laminar flow or as a turbulent flow. Each type of flow possesses particular properties, such as: energy dissipation, generation of mechanical vibrations and spatial velocity distribution. Which type of flow will occur depends on the quantity called Reynolds number $R$, evaluated from the fluid properties: density $p$, velocity $v$, diameter of the stream (cross-sectional diameter) $d$, and the viscosity of the fluid $η$. $R= v. d.p / η$ When viscosity interactions predominate, the motion is smooth - laminar, but if inertial forces determine the stream it will be turbulent. The fluid will move as: - laminar - when $Re < 2000$ - transitional - when $2000 < Re < 4000$ - turbulent - when $Re > 4000$ Laminar blood flow occurs in most of the blood vessels. It is characterized by: - Erythrocytes move in parallel layers. - Low energy loss. - No sound is generated. ### Turbulent blood flow: - Vortical motion. - High energy loss. - Sound generation. Turbulent motion is observed in conditions such as: - Stenosis (blood flow). - Cardiac shunts (blood flow). - Upper respiratory tracts inflammation (air flow). ### Elasticity and blood flow Blood vessels are capable to accumulate mechanical energy during blood propagation. When the heart pumps out a portion of blood, the arterial vessels expand, thus holding potential energy in their walls. Further, these expanded walls act upon the blood with elastic forces pushing it toward the capillaries. This phenomenon provides smooth time-distribution of energy use up and respectively - uniform blood flow. The mentioned elastic deformation propagates along the arterial vessel wall as a transverse wave called Pulse wave (velocity of 6-8 m/s.), fig bellow. ### Steady flow DEF: steady flow is a fluid flow for which the following relation is satisfied: the product of the fluid cross section and velocity is constant along the vessel/pipe. $S.v = Q = constant$ ### Poiseuille's law Poiseuille’s Law: at fixed pressure conditions, the flow is dependent on pipe/vessel size only - small vessels contribute significantly on the flow due to their higher resistance. Vessel disorders such as stenoses can influence blood stream sizably. Regulation of hydrodynamic