Medical Physics Pressure - Part 2 PDF

9 Pressure – Part 2 By: Abdullah Munqith Pressure inside the skull Ø The brain contains approximately 150 cm3 of cerebrospinal fluid (CSF) in a series of interconnected openings called ventricles cerebrospinal fluid is generated inside the brain and flows through the ventricles into the spinal colum...

9 Pressure – Part 2 By: Abdullah Munqith Pressure inside the skull Ø The brain contains approximately 150 cm3 of cerebrospinal fluid (CSF) in a series of interconnected openings called ventricles cerebrospinal fluid is generated inside the brain and flows through the ventricles into the spinal column and into the circulatory system. One of the ventricles , the aqueduct , is especially narrow. If at birth this opening is blocked for any reason, the CSF is trapped inside the skull and increase the internal pressure. Ø The increased pressure causes the skull to enlarge.This serious condition , called hydrocephalus (water – head ) is common problem in infants. However, if the condition is detected soon enough , it can often be corrected by surgically. Measuring the CSF pressure It is not convenient to measure the CSF pressure directly. There are two methods: 1- Crude method: This method can measure the pressure inside the skull by measuring the circumference of the skull just above the ears. Normal values for newborn infants are from (32-37) cm, and a larger value may indicate hydrocephalus. 2- Transillumination: is a qualitative method of detection. Make use of the light –scattering properties of the rather clear CSF inside the skull. Eye pressure Ø The clear fluids in the eyeball that transmit the light to the retina , are under pressure and maintain the eyeball in a fixed size and shape. Ø The dimensions of the eye are critical to good vision – a change of only 0.1 mm in its diameter has a significant effect on the clarity of vision. Ø If you press on your eyelid with your finger you will notice the resiliency of the eye due to the internal pressure. Ø The pressure in normal eyes ranges from(12 to 23) mm Hg. Eye pressure Ø The fluid in the front part of the eye , the aqueous humor ,is mostly water. The eye continuously produces aqueous humor and a drain system allows the surplus to escape. Ø If a partial blockage of this drain system occurs, the pressure increases and the increased pressure can restrict the blood supply to the retina and thus affect the vision. This condition , called glaucoma, produces tunnel vision in moderate cases and blindness in severe cases. Ø Early physicians estimated the pressure inside the eye by " feel" Now pressure in the eye is measured with several different instruments, called tonometers. PRESSURE IN THE DIGESTIVE SYSTEM The body has an opening through it This opining , the digestive tract , The valves are designed to permit unidirectional flow of food. With some effort it is possible to reverse the flow, such as during vomiting The pressure is greater than atmospheric in most of the gastrointestinal , However , in the esophagus , the pressure is coupled to the pressure between the lungs and chest wall and is usually less than atmospheric. The pressure is sometimes determined by measuring the pressure in the esophagus, During eating the pressure in the stomach increases as the walls of the stomach are stretched , the increase in pressure is very slow, A more significant increase in pressure is due to air swallowed during eating. Air trapped in the stomach causes burping or belching. This trapped air is often visible on an x-ray of the chest. PRESSURE IN THE DIGESTIVE SYSTEM One valve, the pylorus, prevents the flow of food back into the stomach from the small intestine. Occasionally a blockage forms in the small or large intestine and pressure builds up between the blockage and the pylorus; if this pressure becomes great enough to restrict blood flow to the critical organs, it can cause death. Intubation, the passing of a hollow tube through the nose, stomach, and pylorus, is usually used to relieve the pressure. The pressure in the digestive system is coupled to that in the lungs through the flexible diaphragm that separates the two organs systems. When it is necessary or desirable to increase the pressure in the gut, such as during defecation, a person takes a deep breath, closes off the lungs at the glottis(vocal cord),and contracts the abdominal muscles. The increased pressure inside the GI system is due to 1- Accumulation of food increase the pressure inside stomach layers, the volume is directly proportional with the cube of the radius (R3) while the tension (stretching force) is proportional to (R) ,the increase in pressure is very low 2- Swallowing the air during eating food. Air trapped in the stomach causes burping and belching. 3- The gas generation due to the bacterial action increases the pressure. There are external factors increases the pressure inside the stomach and these factors are : 1- Belts. 2- Girdles. 3- Flying. 4- Swimming PRESSURE IN THE SKELETON Ø The highest pressures in the body are found in the weightbearing bone joints. The pressure in the knee joint may be more than 10 atm. The surface area of a bone at the joint is greater than its area either above or below the joint. Ø The larger area at the joint distributes the force, thus reducing the pressure, according to the equation: P= F/A PRESSURE IN THE SKELETON Ø The force is spread over a larger surface , this reduces the pressure in the tissues over the bone. Ø The skeletal system joints are the best bearing that any man can make. Ø The lubrication of it is due to synovial fluid.The large area of the joints , the shape of the bones , and the lubrication by synovial fluid is naturally designed to reduce the pressure on the joints. Pressure in the urinary bladder One of the most noticeable internal pressures is the pressure in the bladder due to accumulation of urine. The figure shows the typical pressure-volume curve for the bladder. The bladder stretches as the volume increases.\ For adult, the typical maximum volume in the bladder before voiding is 500ml.At some pressure (~30 cmH2O) the micturition (gotta go) reflex occurs. Pressure in the urinary bladder The resulting sizable muscular contraction in the bladder wall produces a momentary pressure of up to 150 cmH2O. Normal voiding pressure is fairly low (20 to 40 cmH2O), but for men who suffer from prostatic obstruction of the urinary passage it may be over 100 cmH2O. Pressure in the urinary bladder The pressure in the bladder can be measured by: 1. By passing a catheter with a pressure sensor into the bladder through the urinary passage (urethra). 2. By a needle inserted through the wall of the abdomen directly into the bladder. This technique gives information on the function of the exit valves (sphincters) that cannot be obtained with the catheter technique. The bladder pressure increases during coughing, straining, and sitting up. During pregnancy, the weight of the fetus over the bladder increases the bladder pressure and causes frequent urination. Pressure effects while diving We recall Boyles law : for a fixed quantity of gas at a fixed temperature the product of the absolute pressure and volume is constant (pv= constant). That is , if the absolute pressure is doubled , the volume is halved. So P1V1=P2V2 Pressure effects while diving ü Applications of Boyles law to scuba diving are given in Example : a- What volume of air at an atmospheric pressure of (1.01 × 105 N/m2 ) is needed to fill a (14.2 liter) scuba tank to a pressure of 1.45 × 107N/m2? P1v1= p2v2 (1.01 × 105)(v1) = (1.45 × 107)(14.2) V1= 2 × 103 liters Pressure effects while diving b- Since at sea level a diver uses about 14.2 liters (0.5 cu ft) of air per minute during moderate activity , the tank in ( a ) would last about 144 min. How long would the tank last at a depth of 10m ( 33 ft ) where the pressure is increased by 1 atmosphere , assuming the same volume us rate ? - Since the absolute pressure is twice as great ( 2 atm ) , the tank will last only 72 min. (However , no safety – conscious diver would completely empty his tank during a dive for then he would have to surface without air.) Pressure effects while diving Ø The middle ear is one air cavity that exists within the body. For comfort the pressure in the middle ear should equal the pressure on the outside of the eardrum. Ø This equalization is produced by air flowing through the Eustachian tube , which is usually closed except during swallowing , chewing , and yawning. Ø When diving , many people have difficulty obtaining pressure equalization and feel pressure on their ears. Ø A Pressure differential of 120 mm Hg across the eardrum , which can occur in about 1.7 m of water, can cause the eardrum to rupture. Ø Rupture can be serious since cold water in the middle ear can affect the vestibular or balance mechanism and cause nausea and dizziness. Pressure effects while diving Ø Breathing air at a depth of 30 m is also dangerous, this can produce serious problems : 1- Nitrogen narcosis which is an intoxication effect. While oxygen is transported by chemical attachment to red blood cells, nitrogen is dissolved in the blood and tissues. According to Henry’s law , the amount of gas that will dissolve in a liquid is proportional to the partial pressure of the gas in contact with the liquid. Thus more nitrogen is dissolved in the blood and from there into the tissues as a diver goes deeper. Pressure effects while diving 2- Bends "decompression sickness" which is an ascent problem. When the diver ascends , the extra nitrogen in the tissues must be removed via the blood and the lungs. The removal is a slow process , and if the diver ascends too fast , bubbles from in the tissues and joints. The bends(diver paralysis) are quit painful,. Bends occur because of sudden decrease in pressure when the diver ascends. Henrys law The amount of gas that will dissolve in a liquid is proportional to the partial pressure of the gas in contact with the liquid. Typical pressure in the normal body v Hyperbaric oxygen therapy (HOT) The body normally lives in an atmosphere that is about one fifth O2 and four-fifth N2. In some medical situations it is beneficial to increase the proportion of O2 in order to provide more O2 to the tissue. To greatly increase the amount of oxygen, medical engineers have constructed special high pressure (hyperbaric) oxygen chambers. Some are just large enough for a patient, while others are large enough to serve as operating rooms. Typical pressure in the normal body 1-Gas gangrene: The bacillus causes gas gangrene then it's treated with (HOT). That is due to bacillus cannot survive in the presence of oxygen. Typical pressure in the normal body 2-Carbon monoxide poisoning: Ø The red blood cells cannot carry O2 to the tissues because the carbon monoxide fastens to the hemoglobin at the places normally used by O2. Ø Normally the amount of O2 dissolved in the blood is about 2% of that carried on the red blood cells. Ø By using the (HOT) technique, the partial pressure of O2 can be increased by a factor of 15 , permitting enough O2 to be dissolved to fill the body’s needs. Typical pressure in the normal body 3-Treatment of cancer: (HOT) with radiation is given to the patient in transparent plastic tank. The theory was that more oxygen would make the poorly oxygenated radiation –resistant cell in the center of the tumor more susceptible to radiation damage. Hazards of HOT 1. The oxygen atmosphere makes fire a much greater hazard. 2. Risk of rupture of the tank due to the high pressures used.