Pressure 2024 (PDF)

Ch 6 : pressure Pressure is defined as the force per unit area in a gas or a liquid. Atmospheric pressure is 105 N/m2 (Pascal) ,some common units of pressure list in table 6.1 The pressure under column of liquid p=ρ gh and some times indicates pressure difference in the body in term of height of a c...

Ch 6 : pressure Pressure is defined as the force per unit area in a gas or a liquid. Atmospheric pressure is 105 N/m2 (Pascal) ,some common units of pressure list in table 6.1 The pressure under column of liquid p=ρ gh and some times indicates pressure difference in the body in term of height of a column of water (Example 6.1). Since we live in a sea of air ( p = 1 atm ) pressure measures relative to atmospheric pressure (gauge pressure ), for example if the pressure of a bicycle 60 lb/in2 (gauge pressure ) therefore the absolute pressure is 60 +14.7 = 74.7 lb/in2 When we breath in (inspiration ) ,the pressure is lower than atmospheric or negative (also in drinking) , some typical pressure in the body given in the table 6.2. Blood leaves the heart at about 120 mm Hg but its pressure continues to decrease (to almost 0) as it goes from the aorta to smaller arteries to small veins (see [link]). The pressure differences in the circulation system are caused by blood flow through the system as well as the position of the person. For a person standing up, the pressure in the feet will be larger than at the heart due to the weight of the blood. For a person standing up, the pressure in the feet will be larger than at the heart due to the weight of the blood. If we assume that the distance between the heart and the feet of a person in an upright position is 1.4 m, then the increase in pressure in the feet relative to that in the heart (for a static column of blood) is given by Measurement of pressure in the body Manometer of a u shaped is used to measure pressure (fig.6.1) ,clinicaly used sphygmomanometer ,either mercury or aneroid type. A crude pressure indicators are going up or down in an elevator or an airplane , swallowing the food, size of veins in the back of the hand. Pressure in the skull Brain contains approximately 150 cm3 of cerebrospinal fluid (CSF). CSF generated in the brain and flows through the ventricles into spinal column and eventually in to the circulatory system. Hydrocephalus ( water head ) in newborn → CSF trapped inside the skull → increases the internal pressure and measures indirectly by □ measuring the circumference of the skull (( Normal circumferences 32- 37 cm )) □ transillumination. In medicine transillumination generally refers to the transmission of light through tissues of the body. A common example is the transmission of light through fingers, producing a red glow due to red blood cells absorbing other wavelengths of light. Organs analysed include the sinuses, the breasts and the testes. It is widely used by pediatricians to shine light in bodies of infants and observe the amount of scattered light. Since their skeleton is not fully calcified, light can easily penetrate tissues. Common examples of diagnostic applications are: Finger Transillumination Maxillary Sinus Transillumination Ventricles of the brain: The ventricular system is a set of four interconnected cavities (ventricles) in the brain, where the cerebrospinal fluid (CSF) is produced. Within each ventricle is a region of choroid plexus, a network of ependymal cells involved in the production of CSF, The ventricular system is composed of 2 lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle There are four of them. Two lateral, a third and a fourth, in descending order. In the lateral ventricles are choroid plexii. These pump out cerebrospinal fluid 24/7. About 500cc a day. We have about 150cc at any one time, so you can see it is made and reabsorbed in pretty large quantities. CSF is similar to salt water and it bathes the brain, inside and out. CSF is maintained at a constant temperature, pressure and salinity. The balance of which is quite important. Blockage of the flow of CSF anywhere (especially at the 3d or 4th, will cause an increase in intracranial pressure. We call this Hydrocephalus, or "water on the brain." People, kids usually, get real sick when this happens. You have to put in a shunt to remove the excess. CSF is critically imprtant to the proper function of the human brain, and the ventricles play a key role in its production, storage and regulation. pressure in the EYE pressure in the EYE Change in eye diameter o.1mm → significant effect on the clarity of vision. Normal eye pressure 12- 23 mmHg. eye pressure increased → Glaucoma (tunnel vision) in moderate ) or blindness if (sever cases). Tonometer is used to measure eye pressure. pressure in the Digestive system valves and sphincters of the digestive system is shown in (fig. 6.3). Most of gastrointestinal (GI) system has a pressure higher than atmospheric however - In the esophagus is coupled to the pressure between the lungs and chest wall ( intrathoracic pressure ) and is usually less than atmospheric. A more significant increase in pressure during eating is due to air swallowed, this trapped air often → burping or belching. In gut (gas) generated by bacterial action increases the pressure. pressure in the skeleton Highest pressure in the weight bearing bones joints. If the weight of the body is on one leg (when walking) then the pressure in the knee joints even more than 10 times. hence to reduce the effects □ synovial fluids □ The surface area of a bone at the joint is greater than its area above or below the joint. □ Bearing weight bones are flat and cylindrical so the applied force spread over a large surface. pressure in the bladder Pressure in urinary bladder is due to accumulation of urine (fig.6.6) For adults the typical maximum volume of bladder before voiding 500 ml. At some pressure 30 cm H2O the micturition reflex occur. □ The resulting sizable muscular contraction in the bladder wall produces a momentary pressure up to 150 cm H2O. Normal voiding pressure is fairly low ( 20 – 40 cm H2O) except the men how suffer from prostatic problems. pressure in the bladder (cont.) pressure in the bladder can be measured by passing catheter with a pressure sensor into the bladder through urethra. In direct systometry the pressure is measured by means of needle inserted through the wall of the abdomen directly into the bladder (fig.6.7), this technique gives information on the function of exit valves. Bladder pressure increase during coughing , straining, sitting up , during pregnancy , stressful situation and in some cases study for exams. Pressure effects while diving sudden pressure changes on gas cavities in the body can cause profound effects. Why ? Bayle's law : for a fixed quantity of gas at a fixed temperature the product of the absolute pressure and volume is constant (PV= constant ) (Example 6.3 ) The middle ear is one air cavity that exists within the body P in the middle ear = P out side of the eardrum { this equalization is produced by Eustachian tube which is usually closed except while chewing , yawning and swallowing } Pressure effects while diving (cont.) when diving many people have difficulty obtaining pressure equalization and feel pressure on their ears. A pressure differential of 120 mm Hg a cross the eardrum can cause eardrum to rupture. and hence cold water cause nausea and dizziness. ☼ one method of equalization used by diver is to raise the pressure in the mouth by holding the nose and trying to blow out.as the pressure equalizes the diver can hear ears ((pop)). Deep-Sea Diving (High pressure) descending beneath the sea, increased the pressure around the diver's body Relationship of Pressure to Sea Depth: a person 33 feet beneath the ocean surface is exposed to 2 atmospheres pressure At 66 feet the pressure is 3 atmospheres, and so forth.. Figure 44-1 Effect of sea depth on pressure (top table) and on gas volume (bottom). Downloaded from: StudentConsult (on 2 February 2013 07:43 AM) Deep-Sea Diving (High pressure) cont.. Effect of Sea Depth on the Volume of Gases-Boyle's Law: descending beneath the sea is compression of gases to smaller and smaller volumes a principle of physics called Boyle's law At 33 feet beneath the sea, where the pressure is 2 atmospheres, the volume has been compressed to only one-half litter increased pressure can collapse the air chambers of the diver's body, especially the lungs, and often causes serious damage. Figure 44-1 Effect of sea depth on pressure (top table) and on gas volume (bottom). Nitrogen Narcosis at High Nitrogen Pressures About four fifths of the air is nitrogen. At sea-level pressure, the nitrogen has no significant effect on bodily function but at high pressures it can cause varying degrees of narcosis Nitrogen narcosis has characteristics similar to those of alcohol intoxication Nitrogen Narcosis at High Nitrogen Pressures cont… divers remaining beneath the sea for an hour will develop following symptom At about 120 feet --Mild narcosis At 150 to 200 feet the diver becomes drowsy At 200 to 250 feet, his or her strength wanes considerably, and too clumsy to perform the work required Beyond 250 feet (8.5 atmospheres pressure) become useless decompression sickness and how it can be avoided. Breathes air under high pressure for a long time--nitrogen dissolved in the body fluids increases And because nitrogen is not metabolized by the body, it remains dissolved in all the body tissues until the nitrogen pressure in the lungs is decreased back to some lower level, however, this removal often takes hours to occur and is the source of multiple problems collectively called decompression sickness decompression sickness and how it can be avoided. Cont… if diver suddenly comes back to the surface of the sea, nitrogen bubbles can develop in the body fluids either intracellularly or extracellularly decompression sickness. Due to decompression gases can escape from the dissolved state and form actual bubbles, in both tissues and blood where they plug many small blood vessels. The bubbles may not appear for many minutes to hours because sometimes the gases can remain dissolved in the "supersaturated" state for hours before bubbling. Symptoms of Decompression Sickness Tissue ischemia or tissue death due to gas bubbles blocking many blood vessels Pain in joints, muscles of the legs and arms, affecting 85 to 90 % In 5 to 10 % nervous system symptoms occur, ranging from dizziness in about 5 % to paralysis or collapse and unconsciousness in as many as 3 %. The paralysis may be temporary, but in some instances, damage is permanent. Treatment of Decompression Sickness Acute decompression sickness is treated by placing the affected person in a high pressure chambers or pressurized tank also called hyperbaric oxygen therapy. to force the gas back into solution then subsequent slow decompression permits gradual gas resorption and exhalation ,so the obstructed bubbles do not form. Treatment of Decompression Sickness cont… In very deep dives, helium is usually used in the gas mixture instead of nitrogen for three reasons: (1) it has only about one-fifth the narcotic effect of nitrogen; (2) only about one half as much volume of helium dissolves in the body tissues as nitrogen, and the volume that does dissolve diffuses out of the tissues during decompression several times as rapidly as does nitrogen, thus reducing the problem of decompression sickness; and (3) the low density of helium (one seventh the density of nitrogen) keeps the airway resistance for breathing at a minimum, which is very important in very deep dives it is important to reduce the oxygen concentration in the gaseous mixture because otherwise oxygen toxicity would result elevated partial pressures of O2