<< Chapter < Page Chapter >> Page >

Systolic pressure

Systolic pressure is the maximum blood pressure.

Diastolic pressure

Diastolic pressure is the minimum blood pressure.

U.S. Army Spc. Monica Brown takes a soldier's blood pressure reading at the hospital on Forward Operating Base Salerno, Afghanistan, March 10, 2008.
In routine blood pressure measurements, an inflatable cuff is placed on the upper arm at the same level as the heart. Blood flow is detected just below the cuff, and corresponding pressures are transmitted to a mercury-filled manometer. (credit: U.S. Army photo by Spc. Micah E. Clare\4TH BCT)

Calculating height of iv bag: blood pressure and intravenous infusions

Intravenous infusions are usually made with the help of the gravitational force. Assuming that the density of the fluid being administered is 1.00 g/ml, at what height should the IV bag be placed above the entry point so that the fluid just enters the vein if the blood pressure in the vein is 18 mm Hg above atmospheric pressure? Assume that the IV bag is collapsible.

Strategy for (a)

For the fluid to just enter the vein, its pressure at entry must exceed the blood pressure in the vein (18 mm Hg above atmospheric pressure). We therefore need to find the height of fluid that corresponds to this gauge pressure.


We first need to convert the pressure into SI units. Since 1.0 mm Hg = 133 Pa ,

P = 18 mm Hg × 133 Pa 1.0 mm Hg = 2400 Pa . size 12{P="18"`"mm"`"Hg" times { {"133"`"Pa"} over {1 "." 0`"mm"`"Hg"} } ="2400"`"Pa" "." } {}

Rearranging P g = hρg size 12{P rSub { size 8{g} } =hρg} {} for h size 12{h} {} gives h = P g ρg size 12{h= { {P rSub { size 8{g} } } over {ρg} } } {} . Substituting known values into this equation gives

h = 2400 N /m 2 1 . 0 × 10 3 kg/m 3 9 . 80 m/s 2 = 0.24 m. alignl { stack { size 12{h= { {"2400"`"N/m" rSup { size 8{2} } } over { left (1 "." 0 times "10" rSup { size 8{3} } `"kg/m" rSup { size 8{3} } right ) left (9 "." "80"`"m/s" rSup { size 8{2} } right )} } } {} #" "=" 0" "." "24"`m "." {} } } {}


The IV bag must be placed at 0.24 m above the entry point into the arm for the fluid to just enter the arm. Generally, IV bags are placed higher than this. You may have noticed that the bags used for blood collection are placed below the donor to allow blood to flow easily from the arm to the bag, which is the opposite direction of flow than required in the example presented here.

Got questions? Get instant answers now!

A barometer is a device that measures atmospheric pressure. A mercury barometer is shown in [link] . This device measures atmospheric pressure, rather than gauge pressure, because there is a nearly pure vacuum above the mercury in the tube. The height of the mercury is such that hρg = P atm size 12{hρg=P rSub { size 8{"atm"} } } {} . When atmospheric pressure varies, the mercury rises or falls, giving important clues to weather forecasters. The barometer can also be used as an altimeter, since average atmospheric pressure varies with altitude. Mercury barometers and manometers are so common that units of mm Hg are often quoted for atmospheric pressure and blood pressures. [link] gives conversion factors for some of the more commonly used units of pressure.

Mercury barometer has an evacuated glass tube inverted and placed in the mercury container. The height of the mercury column in the inverted tube is determined by the atmospheric pressure.
A mercury barometer measures atmospheric pressure. The pressure due to the mercury’s weight, hρg size 12{hρg} {} , equals atmospheric pressure. The atmosphere is able to force mercury in the tube to a height h size 12{h} {} because the pressure above the mercury is zero.
Conversion factors for various pressure units
Conversion to N/m 2 (Pa) Conversion from atm
1.0 atm = 1 . 013 × 10 5 N/m 2 size 12{1 "." 0`"atm"=1 "." "013" times "10" rSup { size 8{5} } `"N/m" rSup { size 8{2} } } {} 1.0 atm = 1 . 013 × 10 5 N/m 2 size 12{1 "." 0`"atm"=1 "." "013" times "10" rSup { size 8{5} } `"N/m" rSup { size 8{2} } } {}
1.0 dyne/cm 2 = 0 . 10 N/m 2 size 12{1 "." 0`"dyne/cm" rSup { size 8{2} } =0 "." "10"`"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 1 . 013 × 10 6 dyne/cm 2 size 12{1 "." 0`"atm"=1 "." "013" times "10" rSup { size 8{6} } `"dyne/cm" rSup { size 8{2} } } {}
1 . 0 kg/cm 2 = 9 . 8 × 10 4 N/m 2 size 12{1 "." 0`"kg/cm" rSup { size 8{2} } =9 "." 8 times "10" rSup { size 8{4} } `"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 1 . 013 kg/cm 2 size 12{1 "." 0`"atm"=1 "." "013"`"kg/cm" rSup { size 8{2} } } {}
1 . 0 lb/in . 2 = 6 . 90 × 10 3 N/m 2 size 12{1 "." 0`"lb/in" "." rSup { size 8{2} } =6 "." "90" times "10" rSup { size 8{3} } `"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 14 . 7 lb/in . 2 size 12{1 "." 0`"atm"="14" "." 7`"lb/in" "." rSup { size 8{2} } } {}
1.0 mm Hg = 133 N/m 2 size 12{1 "." 0`"mm"`"Hg"="133"`"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 760 mm Hg size 12{1 "." 0`"atm"="760"`"mm"`"Hg"} {}
1 . 0 cm Hg = 1 . 33 × 10 3 N/m 2 size 12{1 "." 0`"cm"`"Hg"=1 "." "33" times "10" rSup { size 8{3} } `"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 76 . 0 cm Hg size 12{1 "." 0`"atm"="76" "." 0`"cm"`"Hg"} {}
1 . 0 cm water = 98 . 1 N/m 2 size 12{1 "." 0`"cm"`"water"="98" "." 1`"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 1 . 03 × 10 3 cm water size 12{1 "." 0`"atm"=1 "." "03" times "10" rSup { size 8{3} } `"cm"`"water"} {}
1.0 bar = 1 . 000 × 10 5 N/m 2 size 12{1 "." 0`"bar"=1 "." "000" times "10" rSup { size 8{5} } `"N/m" rSup { size 8{2} } } {} 1 . 0 atm = 1.013 bar size 12{1 "." 0`"atm"=1 "." "013"`"bar"} {}
1.0 millibar = 1 . 000 × 10 2 N/m 2 size 12{1 "." 0`"millibar"=1 "." "000" times "10" rSup { size 8{2} } `"N/m" rSup { size 8{2} } } {} 1.0 atm = 1013 millibar

Section summary

  • Gauge pressure is the pressure relative to atmospheric pressure.
  • Absolute pressure is the sum of gauge pressure and atmospheric pressure.
  • Aneroid gauge measures pressure using a bellows-and-spring arrangement connected to the pointer of a calibrated scale.
  • Open-tube manometers have U-shaped tubes and one end is always open. It is used to measure pressure.
  • A mercury barometer is a device that measures atmospheric pressure.

Conceptual questions

Explain why the fluid reaches equal levels on either side of a manometer if both sides are open to the atmosphere, even if the tubes are of different diameters.

Got questions? Get instant answers now!

[link] shows how a common measurement of arterial blood pressure is made. Is there any effect on the measured pressure if the manometer is lowered? What is the effect of raising the arm above the shoulder? What is the effect of placing the cuff on the upper leg with the person standing? Explain your answers in terms of pressure created by the weight of a fluid.

Got questions? Get instant answers now!

Considering the magnitude of typical arterial blood pressures, why are mercury rather than water manometers used for these measurements?

Got questions? Get instant answers now!


Find the gauge and absolute pressures in the balloon and peanut jar shown in [link] , assuming the manometer connected to the balloon uses water whereas the manometer connected to the jar contains mercury. Express in units of centimeters of water for the balloon and millimeters of mercury for the jar, taking h = 0 . 0500 m size 12{h=0 "." "0500"`m} {} for each.


P g = 5.00 cm H 2 O, P abs = 1.035 × 10 3 cm H 2 O. alignl { stack { size 12{P rSub { size 8{g} } =5 "." "00"`"cm"`H rSub { size 8{2} } "O,"} {} #P rSub { size 8{"abs"} } =1 "." "035" times "10" rSup { size 8{3} } `"cm"`H rSub { size 8{2} } O "." {} } } {}


P g = 50.0 mm Hg , P abs = 710 mm Hg. alignl { stack { size 12{P rSub { size 8{g} } = - "50" "." 0`"mm"`"Hg,"} {} #P rSub { size 8{"abs"} } ="710"`"mm"`"Hg" "." {} } } {}

Got questions? Get instant answers now!

(a) Convert normal blood pressure readings of 120 over 80 mm Hg to newtons per meter squared using the relationship for pressure due to the weight of a fluid ( P = hρg ) size 12{ \( P=hρg \) } {} rather than a conversion factor. (b) Discuss why blood pressures for an infant could be smaller than those for an adult. Specifically, consider the smaller height to which blood must be pumped.

Got questions? Get instant answers now!

How tall must a water-filled manometer be to measure blood pressures as high as 300 mm Hg?

4.08 m

Got questions? Get instant answers now!

Pressure cookers have been around for more than 300 years, although their use has strongly declined in recent years (early models had a nasty habit of exploding). How much force must the latches holding the lid onto a pressure cooker be able to withstand if the circular lid is 25.0 cm size 12{"25" "." 0`"cm"} {} in diameter and the gauge pressure inside is 300 atm? Neglect the weight of the lid.

Got questions? Get instant answers now!

Suppose you measure a standing person’s blood pressure by placing the cuff on his leg 0.500 m below the heart. Calculate the pressure you would observe (in units of mm Hg) if the pressure at the heart were 120 over 80 mm Hg. Assume that there is no loss of pressure due to resistance in the circulatory system (a reasonable assumption, since major arteries are large).

Δ P = 38.7 mm Hg, Leg blood pressure = 159 119 . alignl { stack { size 12{ΔP="38" "." 7`"mm"`"Hg,"} {} #size 12{"Leg"`"blood"`"pressure"= { {"159"} over {"119"} } "." } {} } } {}

Got questions? Get instant answers now!

A submarine is stranded on the bottom of the ocean with its hatch 25.0 m below the surface. Calculate the force needed to open the hatch from the inside, given it is circular and 0.450 m in diameter. Air pressure inside the submarine is 1.00 atm.

Got questions? Get instant answers now!

Assuming bicycle tires are perfectly flexible and support the weight of bicycle and rider by pressure alone, calculate the total area of the tires in contact with the ground. The bicycle plus rider has a mass of 80.0 kg, and the gauge pressure in the tires is 3 . 50 × 10 5 Pa size 12{3 "." "50" times "10" rSup { size 8{5} } `"Pa"} {} .

22 . 4 cm 2 size 12{"22" "." 4`"cm" rSup { size 8{2} } } {}

Got questions? Get instant answers now!

Questions & Answers

what is wave
Charity Reply
a wave is a distirbance that transmits energy from one place ro another within or without a medium
wave is the transfer of energy from one medium to another without the transfer of particles
wave is a disturbance which transfer energy from one medium to another without causing any permanent displacement by itself
wave is a disturbance or oscillation that travel through space and matter,accompanied by a transfer of energy
A wave is any disturbances in an elastic medium which carries energy from one point to another through a medium
what is harmonic motion
is a restoring force
what is thermodynamics
Williams Reply
what is thermodynamics
Are the antimatters of Hadrons also Hadrons?!Does the same rule apply to Leptons?
Daniel Reply
yes. Hadrons are the elementary particles that take part in stong, electromagnetic and weak interactions. Infact only Hadrons are involved in Strong interactions and when an anti-particle of any hadron is produced, it would be a hadron-conservations laws. Leptons are involved in weak int and follow
what is physics
physic is a pure science that deal with behavior of matter,energy & how it related to other physical properties
Owk. But am are Art student.
What happens when an aeroplanes window is opened at cruise altitude?
Theophilus Reply
what is the minimum speed for any object to travel in time?
Pankaj Reply
as per theory of relativity, minimum speed will be the speed of light
what is physics
Lote Reply
it is just a branch of science which deals with the reasons behind the daily activities taking place everyday in our lives. it clearly states the reason in the form of laws.
like Newton's laws , Kepler's laws etc....
physics is the study of motion or moving things. Usually the moving things are normal items like vars or planets but sometimes it's electricity or heat that moves.
what happens when an aeroplane takes off?
Kofi Reply
it flies
the lift generated by the wing overcome the weight of the plane(in Newton)and a net force of upward is created
it is a direct application of Magnus effect (which helps in throwing curve balls) the wings of plane are made in such a way that the net flow of air is more below them rather than on their upper side. So when the plane accelerates, the flaps produce the upward lift when enough velocity is obtained
then due to lower pressure on upper part of wings helps producing an additional lift because air flows from areaof lower to the area of higher pressure
The engines located under the wings generate thrust .. in relation thrust is a force ... which ovwrcomes or becomes greater than the weight of the plane.. remember weight is a force Weight = m x g-2 So therefore F(thrust) becomes greater than F(weight) Even if by 1Newton the plane starts lifting o
what happens when a ship moves
What is the sign of an acceleration that reduces the magnitude of a negative velocity? Of a positive velocity?
Conwil Reply
If it reduces the magnitude of the velocity, the acceleration sign is the opposite compared to the velocity.
what is accerelation
John Reply
an objects tendency to speed up over time
acceleration is the change in velocity over the change in time it would be written delta-v over delta-t.
the change in velocity V over a period of time T.
Delta means "change in"...not period of
just kidding. it all works mathematically
except doesn't time really only change if the instantaneous speeds vary...?
and I assume we are all talking average acceleration
Hey shiii 😀
the rate of change of velocity is callaed acceleration
a=delta v/delta t
the rate of change in velocity with respect to time is acceleration
nana you r right
what is meant by lost volt
Hardeyyemih Reply
Lost volt. Lol. It is the electrical energy lost due to the nature or the envirommental conditions (temperature and pressure) that affect the cable across which the potential difference is measured.
What is physics?
Bedabyas Reply
physics is brance science concerned with nature and properties of matter and energy
physics is study of the natural phenomenon on the basis of certain laws and principles. it's like watching a game of chess and trying to understand its rules how it's played.
physics is study of nature and it's law
physics is a branch of science that deals with the study of matter ,properties of matter and energy
Branch of science (study) of matter, motion and energy
what is a double-slit experiment?Explain.
Daniel Reply
when you pass a wave of any kind ie sound water light ect you get an interface pattern forming on a screen behind it, where the peaks and troughs add and cancel out due to the diffraction caused by a wave traveling through the slits
double slit experiment was done by YOUNG. And it's to give out monochromatic coherent, if an incoherent wave is passing through it. And then the waves form interference fringes. The screen placed in front of the double slit is preferably a film and then in the middle where "p=0" a brighter color
is formed and then the constructive interferences occur at 0 (which is the brightest band)... then a sequence of bright band (constructive interference) and dark band (destructive interference) happens and the further from the central band the lower the intensity of bright band(constructive interfe
what is photoelectric effect
Godwin Reply
the emission of electrons in some materials when light of suitable frequency falls on them
The phenomenon that involves the emission of electrons (photoelectrons) when light of appropriate wavelength and frequency is incident on the surface of a metal.
what is regelation
oladipupo Reply
is the process of melting under pressure and freezing when pressure is reduce
poisons ratio is which chapter
Regelation is the phenomenon of melting under pressure and freezing again when the pressure is reduced
how do i convert energy in MeV/c2 to GeV/c2 and vice versa?
Daniel Reply
And also from J/s to MeV?I don't quite understand what is in the book,particle physics just in case.
Practice Key Terms 4

Get the best College physics course in your pocket!

Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?