Heat and Cold.pdf

Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Heat and Cold in Medicine Physical basis of heat and temperature Matter is composed of molecules that are in motion. In a gas or liquid the molecules move about, hitting one an...

Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Heat and Cold in Medicine Physical basis of heat and temperature Matter is composed of molecules that are in motion. In a gas or liquid the molecules move about, hitting one another or the walls of container; even in a solid the molecules have some motion about the sites that they occupy within the crystal structure. The fact that the molecules move means that they have kinetic energy, and the kinetic energy is related to the temperature. The average kinetic energy of the molecules of an ideal gas can be shown to be directly proportional to the temperature; liquids and solids show similar temperature dependence. In order to increase the temperature of a gas it is necessary to increase the average kinetic energy of its molecules. This is can be done by putting the gas in contact with a flame. The energy transferred from the flame to the gas causing the temperature rise is called heat. Heat: is the total energy of molecular motion in a substance. Temperature: is a measure of the average energy of molecular motion in a substance. heat heat heat Solid Liquid Gas ions 1 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi If enough heat is added to a solid, it melts, forming a liquid. This liquid may be change to a gas by adding more heat. Adding still more heat converts the gas to ions. While adding heat to a substance increase its molecular kinetic energy, thereby increasing its temperature, the reverse is also true: heat can be removed from a substance to lower the temperature. Low temperatures are referred to as the cryogenic region. The ultimate in cold is "absolute zero” (-273.15°C), a temperature that is experimentally unattainable. Temperature Scales Temperature is difficult to measure directly, so we usually measure it indirectly by measuring one of many physical properties that change with temperature. We then relate the physical property to temperature by a suitable calibration. There are three types of Temperature Scales:  Fahrenheit (F) scale.  Celsius (°C) scale.  Kelvin (K) scale. The relationships between the different temperature scales are: Ko =273.15 + °C °C =(5/9)*(Fo-32) Fo = (9/5)* °C +32 2 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi H.W The temperature of the human body is normally about 98.6°F.calculate the temperature of the body in °C and °K? Thermometer Thermometer is a device used to measure the temperature. For the medical and biological purposes, there are several important types of thermometers are: 3 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Liquid Glass Thermometer Containing mercury, in general. The principle behind this thermometer is that the increase in temperature, mercury expands more than the glass thus the level of liquid increase in the glass. The expansion of liquid in thermometer is not large. It is about 1.8% of its volume in going from 0 to 100 °C. Therefore, the smaller the diameter of the capillary the greater sensitivity of the thermometer. Thermometer has diameter less than 0.1mm. Thermistors Thermistor: It is a special resistor that changes its resistance rapidly with temperature (about 5% /1 °C). Thermistors are used in medicine because of their sensitivity. It can measure temperature changes of 0.01 °C. 4 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Thermistors (bead type)  Thermistor has little effect on the temperature of the surrounding tissues and responds rapidly to temperature change because of its small mass.  Thermistors are placed in the nose to monitor the breathing rate of patient by showing the temperature change between inspired cool air and expired warm air. Thermocouple It consists of two junctions of two different metals. If the two junctions are at different temperature, a voltage is produced that depends on the temperature difference. One of the junctions is kept at reference temperature such as in an ice- water bath and the other for temperature measurement. It measure temperatures from -190 to 300 °C. It can be made small enough to measure the temperature of individual cells. 5 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Thermography  Measurement of body surface temperature indicates that the surface temperature varies from point to point depending upon external physical factors and internal metabolic and circulatory processes near the skin.  Thermography is a simple routine method of obtaining a surface temperature map. Thermography is most commonly used to detect breast cancer by comparing the heat pattern of the two breasts. 6 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Heat Therapy Two primary therapeutic effects take place in a heated area: 1. There is an increase in metabolism resulting in vasodilation. 2. There is an increase in blood flow as blood moves to cool the heated area. The physical methods of producing heat in the body are: 1. Conductive heating: Heat transfers from the warmer object to the cooler by conduction. The amount of transferred heat depends upon the area of contact, temperature difference, contact time and thermal conductivity of the materials. 2. Infrared (IR) radiant heating: It is also used for surface heating of the body. The used IR wavelength is 800-4000 nm. The waves penetrate the skin about 3mm and increase the surface temperature. IR is considered to be more effective than conductive heating because the heat penetrates deeper. 3. Radio wave heating (Diathermy): When alternating electric current passes through the body, tissue heating and electric shock take place. The amount of heat increases as the frequency of the current increases. Heat from diathermy penetrates deeper in the body than radiant and conductive heat. Short wave diathermy utilizes electromagnetic waves in the radio range (wavelength=10m). 4. Microwave Diathermy: It uses waves in the radar range (wavelength = 12cm). 5. Ultrasonic wave heating (ultrasonic diathermy): Ultrasonic waves are also used for deep heating of body tissue. They produce mechanical motion like audible sound waves. 7 Medical Physics Professor. Dr. Hamid Ibrahim Al-Tamimi Cold in Medicine  Cryogenics is the science and technology of producing and using very low temperature. The study of low temperature effects in biology and medicine is called cryobiology.  Low temperature has been used for long-term preservation of blood, sperm, bone marrow and tissues.  For medical uses the preservation at the temperature of liquid nitrogen (- 196°C) is much better than at the temperature of solid carbon dioxide (- 79°C). Cells survival after freezing is more dependent upon the cooling rate during the freezing cycle than on the warming rate during the thawing cycle.  The survival of some cells can be helped by adding a protective agent before cooling. Blood storage 1. Non-cryogenic method: The conventional non-cryogenic method of blood storage involves mixing whole blood with an anticoagulant and storing it at 4°C. 2. Cryogenic method: Blood can be stored for a much longer time if it is frozen after mixing with cryoprotective agents which are either intracellular or extracellular. Two techniques are used for this method slow cooling technique and the rapid cooling technique. 8