pressure

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Pressure
Introduction:
Pressure is defined as the force per unit area in a gas or liquid. For a
solid the quantity force per unit area is referred to as stress. In the
metric system pressure is measured in dynes per square centimeter
(Dy/cm2) or Newton per square meter (N/m2) or Pascal (Pa).
If the unit is Dy/cm2
P = ? g h ? = density of liquid (g/cm3)
g=980(cm/sec2) acceleration of gravity
h= in (cm) the height of liquid
Or the unit is N/m2
P = ? g h ? = (kg/m3)
g= 9.8 m/ sec2
h= In (m)
Example -1-
Find the pressure of 10 m of water in Dy/cm2 and N/m2?
10 × 100 = 1000 cm 1m=100cm
... P = ? g h =1×980×1000=980000=9.8×105 Dy/cm2
P = ? g h =1000 × 9.8 ×10 =9.8×104 N/m2
The most comm0n method of indicating pressure in medicine is by
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 .
Example -2-
Calculate the systolic pressure in Dy/cm2 and N/m2?
In systolic pressure =120 mmHg=12 cmHg
= 0.12 m Hg
... P = ? Hg g h Hg =13.6×980×12=159936=1.6×105 Dy/cm2
P = ? Hg g h Hg =13600×9.8×0.12=1.6×104 N/m2
The Instrument that measures pressure is called a manometer. A
common type of manometer is U- shaped tube containing a fluid that
is connected to the pressure to be measured Fig (1).
P0= atmospheric pressure
h= height of liquid
P=the pressure of container.
Fig (1)
The most common clinical instrument used in measuring pressure is
the sphygmomanometer , which measures blood pressure. Two types
of pressure gauges are used in sphygmomanometers.
1- Mercury manometers.
2- Aneroid.
Recently another type used (digital).
Gauge pressure :is defined as the excess pressure over atmospheric
pressure.
Gauge pressure =? g h
=1000 x 9.8x10
=105 N/m2 = 1 atm
Absolute pressure=atmospheric pressure+gauge pressure
=1+ 1= 2 atm
P
P0
h
Or In N/m2
=105 + 105= 2x 105 N/m2
Atmospheric pressure = ?Hg g hHg =13600x 9.8 x0.76
=105 N/m2
0
Negative pressure:
Any pressure lower than atmospheric pressure. For example ,
when we breathe in (inspire)the pressure in the lungs must be
somewhat lower than atmospheric pressure or the air would not
flow in.
Pressure inside the skull :
The brain contains approximately 150 cm3 of cerebrospinal fluid
(CSF) in a series of interconnected openings called ventricles.
If at birth this opening is blocked for any reason, the CSF is trapped
inside the skull and increased the internal pressure. This serious
condition, called hydrocephalus.
Measurement of hydrocephalus
1- Crude method: - In this method the circumference of the skull
just above the ears .Normal values of newborn infants are from
32-37 cm, and larger than this may indicate hydrocephalus.
2- qualitative method (transillumination):-
In this method light – scattering properties is used.
h=10 m
P0
Spinal Cord

Eye pressure: The clear fluids in the eye ball (aqueous and vitreous
humors) that transmit the light to retina (the light sensitive part of the
eye). Are under pressure and maintain the eye ball in fixed size and shape.
Normal eye
change in diameter of the eye
Only 0.1 mm
If a partial blockage of the drain system occurs, the pressure increase then
restrict the blood the blood supply to the retina then affect the vision. This
condition, called glaucoma.
Glaucoma : a. Moderate ------------tunnel vision
b . Sever---------------blindness
The pressure in normal eyes ranges from (12 –23) mm.Hg
Measuring the eye pressure :
1- By (feel)the physician estimate the pressure inside the eye by (feel)
as they pressed on the eye with their fingertips
2- Tonometers . ch.15
Pressure in skeleton:
The highest pressures in the body are found in the weight bearing bone
(joints).
The pressure in the knee joint may be more
Than 10 atm. P=F/A---------------- (1)
the surface area of a bone at the joint is greater than its area either above
or below the joint. The larger area at the joint distributes the force thus
reducing the pressure according to the equation 1.
Bone has adapted in another way to reduce pressure the finger bones are
flat rather than cylindrical on the gripping side and the force is spread
Fluids eye
ball good
vision
Effect on
the clarity
of vision
over a large surface this reducing tissues over the bones according to
P=F/A .
Pressure in the urinary bladder :
The internal pressure in the bladder is due to the accumulation of the
urine. The figure below shows the typical pressure - volume curve for the
bladder, which stretches as the volume increase.
Pressure H2O cm
150
30
V ml
Fig 2 :Pressure in the Urinary bladder
or adults, the typical max. Volume in the bladder before voiding is (500)
ml. at some pressure (~30 cm H2o) the micturition reflex occurs .
The resulting sizable muscular contraction in the bladder wall produces a
momentary pressure of up to 150 cmH2O.
The pressure in the bladder can be measured:
*By passing a catheter with a pressure sensor into the bladder through the
urinary passage (urethra).
100 300 500
*By a needle inserted through the wall of the abdomen directly into the
bladder.
This technique gives information about the function of the exit valves that
can not be obtained with the catheter technique.
The bladder pressure increases during coughing, straining, sitting up, also
during pregnancy the weight of the fetus over the bladder increase the
bladder pressure and causes frequent urination.
Normal voiding pressure is fairly low (20-40)cmH2O but for
Men who suffer from prostate obstruction of the urinary passage it may be
over 100cmH2o.
The bladder pressure increase……
Boyle s law:
For a fixed quantity of gas at a fixed temperature the product of the
absolute and volume is constant.(PV=constant)
P1 V1 =P2 V2
That is ,if the absolute pressure is double, the volume is halved.
The middle ear is one of the air cavities that exist within the body. For
comfort the pressure in the middle ear should be equal
To the pressure on the outside of the eardrum.
ear drum
Middle ear
Eustachian tube
Pressure effect while diving
Pmiddle ear = Poutside the eardrum , this equalization is produced by air flowing
through the Eustachian tube , which is usually closed except during
swallowing , chewing ,and yawning.
*When diving many people have difficulty obtaining pressure
equalization and feel pressure on their ears.
*(120mmHg) across the eardrum, which can occur in about 1.7 m of
water, can cause the damage (rupture) to the eardrum.
One method of equalization used by a diver is to raise the pressure in the
mouth by holding the nose and trying to blow out.
The pressure in the lung:-
Pin the lung at any depth > Pin the lung at sea level
This means that the air in the lung is denser under water and that the
partial pressures of all the air components are proportionately higher .
1- The higher partial pressure of O2 causes more O2 molecules to be
transformed into the blood , and oxygen poisoning results if the
partial pressure of O2 gets high .Partial pressure of O2 is (0.8 atm)
and absolute air pressure is (4 atm) at depth of (30 m).
2- Breathing air at a depth of (30m) is also dangerous because it
may result in excess N2 in the blood and tissues ,there is a possibility
of having :
• Nitrogen narcosis (intoxication effect).
• The bends or decompression sickness (a scant problem).
*O2 is attached to red blood cells , while N2 is dissolved in the blood and
tissues .
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.
1- Gas gangrene :-
The bacillus causes gas gangrene then its treated with (HOT) . That is
due to bacillus cannot survive in the presence of oxygen (O2).
2- Carbon Monoxide poisoning :-
• The red blood cells cannot carry O2 to the tissues because the
carbon monoxide fasters to the hemoglobin at the places normally
used by O2.
• Normally the amount of O2 dissolved in the blood is about 2% of
that carried on the red blood cells.
• By using the (HOT) technique , the partial pressure of O2can be
increased by a factor of 15, permitting enough O2 to be dissolved
to fill the body’s need .
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 .
Tumor
Hot radiation
Transparent plastic tank center
(Poorly oxygenated
Q: If the pressure of a man is 8 cm Hg, Does the man have prostate or
not?
A/: PHg= PWater
PghHg=?ghwater
The (g) is canceled from both sides
13.6x8=1xhwater
HWater=108.8 cm.so the man has prostate
Note:
? If hwater =100…..the man has prostate
? If hwater >100…..the man has prostate
? If hwater <100…..the man is normal