Medical Physics Pressure PDF

9 Pressure By: Abdullah Munqith pressure Ø Pressure is defined as the force per unit area in a gas or a liquid. For a solid the quantity of force per unit area is referred as stress. Ø Units of pressure: P = F /A (N /m2) Or (dynes/cm2) Ø Atmospheric pressure (1 atm)= 105 N /m2 = 105 N/m2 Ø The unit...

9 Pressure By: Abdullah Munqith pressure Ø Pressure is defined as the force per unit area in a gas or a liquid. For a solid the quantity of force per unit area is referred as stress. Ø Units of pressure: P = F /A (N /m2) Or (dynes/cm2) Ø Atmospheric pressure (1 atm)= 105 N /m2 = 105 N/m2 Ø The unit of pressure is Pascal (Pa) Ø In medicine, the unit of pressure is measured by the height of a column of mercury. Ø The pressure in a liquid depends on the depth at which we measure it. 1 Pa (Pascal) = 1 N/m2 pressure Pressure pressure Ø The pressure P under a column of liquid can be calculated from the following low: v If the unit is N/m2 v Or the unit is Dy/m2 P= ρ gh ρ = (g/cm3) ρ is the density of the liquid (kg/cm3) g = 980(cm/sec2) g is the acceleration due to the gravity 9.8 (m/sec2) h = (cm) h is height of the column liquid (m) pressure Note: 1 atmosphere (atm) = 1.01×105 N/m2 1 atmosphere (atm) = 1033 cmH2O 1 atmosphere (atm) =760 mmHg 1 cmH2O =0.735 mmHg Or Ø 1mmHg = 1.36 cmH2O Ø Ø Ø Ø pressure Ex: calculate the atmospheric pressure in N/m2 and in dyne/cm2, where pHg =13.6 g/cm3? 1 atm = 760 mm = 76 cm =0.76m pHg =13.6 g/cm3 or 13600 Kg/m3 The atmospheric pressure in N/m2 is equal The atmospheric pressure in dyne/cm2 is equal P = p g h =13600 Kg/m3 ×9.8m/sec2 ×0.76m P=101292.8 N/m2 P = p g h =13.6 g/cm3 ×980cm/sec2 ×76cm P=1012928 dyne/cm2 pressure Ex:-Find the pressure of 10 m of water in Dy/cm2 and N/m2 ? Sol:10 m= 1000 cm ρH20= 1g/cm 3 or 1000kg/m 3. P= ρ g h P=1×980×1000 P=980000 = 9.8×105 Dy/cm2 P=1000×9.8×10 P=9.8×104 N/m2 pressure The most common method of indicating pressure in medicine is the height of a column of mercury (Hg). For example , a peak (systolic) blood pressure reading of (120 ) mmHg indicates that a column of mercury of this height has a pressure at its base equal to the patients systolic blood pressure. Density of mercury is 13.6g/cm3, so that column of water has to be 13.6 times higher than a given column of mercury in order to produce the same pressure. pressure Ex:- Calculate the systolic pressure in N/m2 and Dy/cm2 In systolic pressure=120 mmHg =12 cmHg=0.12mHg P=(ρ g h)Hg = 13.6 ×980×12 1599936=1.6×105 Dy/cm2 P=(ρ g h)Hg = 13600 ×9.8×0.12 1.6×104 N/m2 pressure Ex: - what height of water will be produced the same pressure at 120 mmHg? P= ρ g h = 13.6x980x12 P Hg=PH2o For water: P= ρ g h Or 1.6x105 =1x980 x h So h=163cmH2o (ρ g h)Hg =( ρ g h)H2o ρ Hg hHg = ρH2o hH2o h H2o = (13.6x12)/1 =163 cmH2o Types of pressure v Gauge Pressure v Absolute Pressure The excess pressure over the atmospheric pressure. It is the type of pressure which takes vacuum or air-free space as the reference. v Negative Pressure (Vacuum) Any pressure, lower than atmospheric pressure. Types of pressure For example: The lung pressure during inspiration is a few centimeter of water negative a person drink through a straw the pressure in his mouth must be negative. There are numbers of places in the body where the pressure is lower than atmospheric pressure or negative. For example, when we breath inspires the pressure in the lungs must be lower than the atmospheric pressure. Types of pressure Different parts of the body Typical pressure (mmHg) Arterial blood pressure Maximum(systole) Minimum(diastole) 100-140 60-90 Venous blood pressure 3-7 Great veins