School Encyclopedia. Pressure for dummies: definition, explanation in simple words

To understand what pressure is in physics, consider a simple and familiar example. Which?

In a situation where it is necessary to cut a sausage, we will use the most sharp object- with a knife, not with a spoon, comb or finger. The answer is obvious - the knife is sharper, and all the force we apply is distributed along the very thin edge of the knife, bringing maximum effect in the form of a separation of a part of an object, i.e. sausages. Another example - we are standing on loose snow. Legs fail, walking is extremely uncomfortable. Why, then, do skiers rush past us with ease and at high speed, without drowning and not getting entangled in the same loose snow? It is obvious that snow is the same for everyone, both for skiers and for walkers, but the effect on it is different.

With approximately the same pressure, that is, weight, the surface area pressing on the snow varies greatly. The ski area is much more area shoe soles, and, accordingly, the weight is distributed over a larger surface. What helps or, on the contrary, prevents us from effectively influencing the surface? Why does a sharp knife cut bread better, and flat wide skis hold better on the surface, reducing penetration into the snow? In the seventh grade physics course, the concept of pressure is studied for this.

pressure in physics

The force applied to a surface is called pressure force. And pressure is a physical quantity that is equal to the ratio of the pressure force applied to a specific surface to the area of ​​this surface. The formula for calculating pressure in physics is as follows:

where p is pressure,
F - pressure force,
s is the surface area.

We see how pressure is denoted in physics, and we also see that with the same force, the pressure is greater when the support area, or, in other words, the contact area of ​​interacting bodies, is smaller. Conversely, as the area of ​​support increases, the pressure decreases. That is why a sharper knife cuts any body better, and nails driven into a wall are made with sharp tips. And that is why skis hold on the snow much better than their absence.

Pressure units

The unit of pressure is 1 newton per square meter - these are quantities already known to us from the seventh grade course. We can also convert pressure units N / m2 to pascals, units of measurement named after the French scientist Blaise Pascal, who derived the so-called Pascal's Law. 1 N/m = 1 Pa. In practice, other units of pressure are also used - millimeters of mercury, bars, and so on.

Imagine an air-filled sealed cylinder with a piston mounted on top. If you start to put pressure on the piston, then the volume of air in the cylinder will begin to decrease, the air molecules will collide with each other and with the piston more and more intensively, and the pressure of compressed air on the piston will increase.

If the piston is now abruptly released, then the compressed air will abruptly push it up. This will happen because with a constant piston area, the force acting on the piston from the compressed air will increase. The area of ​​the piston remained unchanged, and the force from the side of the gas molecules increased, and the pressure increased accordingly.

Or another example. A man stands on the ground, stands with both feet. In this position, a person is comfortable, he does not experience inconvenience. But what happens if this person decides to stand on one leg? He will bend one of his legs at the knee, and now he will lean on the ground with only one foot. In this position, a person will feel some discomfort, because the pressure on the foot has increased, and about 2 times. Why? Because the area through which gravity now presses a person to the ground has decreased by 2 times. Here is an example of what pressure is and how easy it is to detect in everyday life.

From the point of view of physics, pressure is called physical quantity, numerically equal to the force acting perpendicular to the surface per unit area of ​​this surface. Therefore, in order to determine the pressure at a certain point on the surface, the normal component of the force applied to the surface is divided by the area of ​​the small surface element on which this force acts. And in order to determine the average pressure over the entire area, the normal component of the force acting on the surface must be divided by total area this surface.

Pressure is measured in pascals (Pa). This pressure unit got its name in honor of the French mathematician, physicist and writer Blaise Pascal, the author of the basic law of hydrostatics - Pascal's Law, which states that the pressure exerted on a liquid or gas is transmitted to any point unchanged in all directions. For the first time, the unit of pressure "pascal" was put into circulation in France in 1961, according to the decree on units, three centuries after the death of the scientist.

One pascal is equal to the pressure exerted by a force of one newton, evenly distributed, and directed perpendicular to a surface of one square meter.

In pascals, not only mechanical pressure (mechanical stress) is measured, but also the modulus of elasticity, Young's modulus, bulk modulus of elasticity, yield strength, proportionality limit, tensile strength, shear strength, sound pressure and osmotic pressure. Traditionally, it is in pascals that the most important mechanical characteristics of materials in the strength of materials are expressed.

Atmosphere technical (at), physical (atm), kilogram-force per square centimeter(kgf/cm2)

In addition to the pascal, other (off-system) units are also used to measure pressure. One such unit is the “atmosphere” (at). A pressure of one atmosphere is approximately equal to atmospheric pressure on the Earth's surface at sea level. Today, “atmosphere” is understood as the technical atmosphere (at).

The technical atmosphere (at) is the pressure produced by one kilogram-force (kgf) distributed evenly over an area of ​​one square centimeter. And one kilogram-force, in turn, is equal to the force of gravity acting on a body with a mass of one kilogram under conditions of free fall acceleration equal to 9.80665 m/s2. One kilogram-force is thus equal to 9.80665 Newton, and 1 atmosphere turns out to be equal to exactly 98066.5 Pa. 1 at = 98066.5 Pa.

In atmospheres, for example, the pressure in automobile tires is measured, for example, the recommended pressure in the tires of a GAZ-2217 passenger bus is 3 atmospheres.

There is also the "physical atmosphere" (atm), defined as the pressure of a column of mercury, 760 mm high at its base, given that the density of mercury is 13595.04 kg / m3, at a temperature of 0 ° C and under conditions of a gravitational acceleration of 9, 80665 m/s2. So it turns out that 1 atm \u003d 1.033233 atm \u003d 101 325 Pa.

As for the kilogram-force per square centimeter (kgf/cm2), this non-systemic unit of pressure is equal to normal atmospheric pressure with good accuracy, which is sometimes convenient for assessing various effects.

The non-systemic unit "bar" is approximately equal to one atmosphere, but is more accurate - exactly 100,000 Pa. In the CGS system, 1 bar is equal to 1,000,000 dynes/cm2. Previously, the name "bar" was carried by the unit, now called "barium", and equal to 0.1 Pa or in the CGS system 1 barium \u003d 1 dyn / cm2. The word "bar", "barium" and "barometer" come from the same Greek word"gravity".

Often, to measure atmospheric pressure in meteorology, the unit mbar (millibar), equal to 0.001 bar, is used. And to measure pressure on planets where the atmosphere is very rarefied - microbar (microbar), equal to 0.000001 bar. On technical pressure gauges, most often the scale has a graduation in bars.

Millimeter of mercury column (mm Hg), millimeter of water column (mm of water column)

The non-systemic unit of measure "millimeter of mercury" is 101325/760 = 133.3223684 Pa. It is designated "mm Hg", but sometimes it is designated "torr" - in honor of the Italian physicist, a student of Galileo, Evangelista Torricelli, the author of the concept of atmospheric pressure.

The unit was formed in connection with convenient way measurement of atmospheric pressure with a barometer, in which the mercury column is in equilibrium under the influence of atmospheric pressure. Mercury has high density about 13600 kg/m3 and is characterized by low pressure saturated steam at room temperature, which is why mercury was chosen for barometers at one time.

At sea level, atmospheric pressure is approximately 760 mm Hg, it is this value that is now considered to be normal atmospheric pressure, equal to 101325 Pa or one physical atmosphere, 1 atm. That is, 1 millimeter of mercury is equal to 101325/760 pascals.

In millimeters of mercury, pressure is measured in medicine, meteorology, and aviation navigation. In medicine, blood pressure is measured in mmHg; in vacuum technology, it is graduated in mmHg, along with bars. Sometimes they even just write 25 microns, meaning microns of mercury, if we are talking about evacuation, and pressure measurements are carried out with vacuum gauges.

In some cases, millimeters of water column are used, and then 13.59 mm of water column \u003d 1 mm Hg. Sometimes it is more expedient and convenient. A millimeter of a water column, like a millimeter of a mercury column, is an off-system unit, equal in turn to the hydrostatic pressure of 1 mm of a water column, which this column exerts on a flat base at a column water temperature of 4 ° C.

Everything is quite simple. It is one of the main indicators of activity of cardio-vascular system. Let's look at this issue in more detail.

What is BP?

Blood pressure is the process of squeezing the walls of capillaries, arteries and veins under the influence of blood circulation.

Types of blood pressure:

• upper, or systolic;
• lower, or diastolic.

When determining the level blood pressure Both of these values ​​must be taken into account. The units of its measurement remained the very first - millimeters of a mercury column. This is due to the fact that mercury was used in the old devices to determine the level of blood pressure. Therefore, the BP indicator looks like this: upper blood pressure (for example, 130) / lower blood pressure (for example, 70) mm Hg. Art.

Circumstances that directly affect the range of blood pressure include:

• the level of strength of contractions performed by the heart;
• the proportion of blood pushed out by the heart during each contraction;
• wall resistance blood vessels which turns out to be a blood stream;
• the amount of blood circulating in the body;
• pressure fluctuations in chest that are caused by the respiratory process.

Blood pressure levels can change throughout the day and with age. But for most healthy people characterized by stable blood pressure.

Definition of types of blood pressure

Systolic (upper) blood pressure is a characteristic general condition veins, capillaries, arteries, as well as their tone, which is caused by contraction of the heart muscle. It is responsible for the work of the heart, namely, with what force the latter is able to expel blood.

Thus, the level of upper pressure depends on the strength and speed with which heart contractions occur.

It is unreasonable to assert that arterial and cardiac pressure is one and the same concept, since the aorta also participates in its formation.

The lower characterizes the activity of blood vessels. In other words, this is the level of blood pressure at the moment when the heart is maximally relaxed.

Lower pressure is formed as a result of the contraction of peripheral arteries, through which blood enters the organs and tissues of the body. Therefore, the state of blood vessels is responsible for the level of blood pressure - their tone and elasticity.

How to know the level of blood pressure?

You can check your blood pressure level with special device called "blood pressure monitor". This can be done both at the doctor's (or nurse's) and at home, having previously bought the device at the pharmacy.

Distinguish the following types tonometers:

• automatic;
• semi-automatic;
• mechanical.

A mechanical tonometer consists of a cuff, a pressure gauge or display, a pear for pumping air and a stethoscope. Principle of operation: put the cuff on your arm, put a stethoscope under it (while you should hear the pulse), inflate the cuff with air until it stops, and then start to lower it gradually, unscrewing the wheel on the pear. At some point, you will clearly hear pulsating sounds in the stethoscope headphones, then they will stop. These two marks are the top and bottom blood pressure.

Consists of a cuff, an electronic display and a pear. Principle of operation: put on the cuff, pump up the air to the maximum with a pear, then let it out. The electronic display shows the upper and lower values ​​of blood pressure and the number of beats per minute - the pulse.

The automatic blood pressure monitor consists of a cuff, an electronic display and a compressor that performs inflation and deflation manipulations. Principle of operation: put on the cuff, start the device and wait for the result.

It is generally accepted that a mechanical tonometer gives the most exact result. It's also more affordable. At the same time, automatic and semi-automatic blood pressure monitors remain the most convenient to use. Such models are especially suitable for older people. Moreover, some types have the function of voice notification of pressure indicators.

It is worth measuring blood pressure indicators no earlier than thirty minutes after any physical exertion (even minor ones) and an hour after drinking coffee and alcohol. Before the measurement process itself, you need to sit quietly for a couple of minutes, catch your breath.

Blood pressure - the norm by age

Each person has an individual which may not be associated with any diseases.

The level of blood pressure is determined by a number of factors that are of particular importance:

• age and gender of the person;
• personal characteristics;
• life style;
• lifestyle features, preferred type of vacation, and so on).

Blood pressure also tends to rise with unusual physical activity and emotional stress. And if a person constantly performs physical exercise(for example, an athlete), then the level of blood pressure may also change both for time and for a long period. For example, when a person in stressful condition, then his blood pressure can rise to thirty mm Hg. Art. from the norm.

However, there are still certain limits of normal blood pressure. And even every ten points of deviation from the norm indicate a violation of the body.

You can also calculate the individual value of blood pressure using the following formulas:

1. For men:

• upper blood pressure = 109 + (0.5 * number full years) + (0.1 * weight in kg);
• lower BP \u003d 74 + (0.1 * number of full years) + (0.15 * weight in kg).

2. For women:

• upper BP \u003d 102 + (0.7 * number of full years) + 0.15 * weight in kg);
• lower blood pressure \u003d 74 + (0.2 * number of full years) + (0.1 * weight in kg).

The resulting value is rounded to an integer according to the rules of arithmetic. That is, if it turned out to be 120.5, then when rounded it will be 121.

Elevated blood pressure

High blood pressure is high level at least one of the indicators (lower or upper). It is necessary to judge the degree of its overestimation, taking into account both indicators.

Regardless of whether the lower blood pressure is high or upper, it is a disease. And it's called hypertension.

There are three degrees of the disease:

• the first - GARDEN 140-160 / DBP 90-100;
• the second - SAD 161-180 / DBP 101-110;
• the third - GARDEN 181 and more / DBP 111 and more.

It is worth talking about hypertension when there is a high level of blood pressure values ​​for a long period.

Statistically, an overestimate systolic pressure most often observed in women, and diastolic - in men and the elderly.

Symptoms of high blood pressure can be:

• decrease in working capacity;
• appearance of fatigue;
• frequent feelings of weakness;
• morning pain in the back of the head;
• frequent dizziness;
• occurrence of bleeding from the nose;
• noise in ears;
• decreased visual acuity;
• appearance at the end of the day.

Causes of high blood pressure

If lower arterial, then most likely this is one of the symptoms of the disease thyroid gland, kidneys, adrenal glands, which began to produce renin in large quantities. It, in turn, increases the tone of the muscles of the blood vessels.

Elevated lower blood pressure is fraught with the development of more more serious illnesses.

High top pressure indicates too frequent contractions of the heart.

A jump in blood pressure can be caused by a number of reasons. This is for example:

• vasoconstriction due to atherosclerosis;
• overweight;
• diabetes;
• stressful situations;
• malnutrition;
• excessive consumption of alcohol, strong coffee and tea;
• smoking;
• lack of physical activity;
• frequent weather changes;
• some diseases.

What is low BP?

Low blood pressure is vegetovascular dystonia or hypotension.

What happens with hypotension? When the heart contracts, blood enters the vessels. They expand and then gradually narrow. Thus, the vessels help the blood to move further along circulatory system. The pressure is normal. For a number of reasons, vascular tone may decrease. They will remain expanded. Then there is not enough resistance for the movement of blood, because of which the pressure drops.

The level of blood pressure in hypotension: upper - 100 or less, lower - 60 or less.

If the pressure drops sharply, then the blood supply to the brain is limited. And this is fraught with such consequences as dizziness and fainting.

Symptoms may be:

• increased fatigue and lethargy;
• darkening in the eyes;
• frequent shortness of breath;
• cold feeling in hands and feet;
• hypersensitivity to loud sounds and bright light
• muscle weakness;
• motion sickness in transport;
• frequent headaches.

What is the reason for low blood pressure?

Poor joint tone and low blood pressure (hypotension) can be present from birth. But more often the culprits reduced pressure become:

• Severe fatigue and stress. Congestion at work and at home, stress and lack of sleep cause a decrease in vascular tone.
• Heat and stuffiness. When you sweat, it leaves the body a large number of liquids. For the sake of maintaining water balance it pumps water out of the blood that flows through the veins and arteries. Its volume decreases, vascular tone decreases. The pressure drops.
• Taking medication. Heart drugs, antibiotics, antispasmodics and painkillers can “drop” the pressure.
• emergence allergic reactions anything with possible anaphylactic shock.

If you have not had hypotension before, do not leave unpleasant symptoms without attention. They can be dangerous "bells" of tuberculosis, stomach ulcers, complications after a concussion and other diseases. Contact a therapist.

What to do to normalize the pressure?

These tips will help you feel whole hearty day if you are hypotonic.

1. Don't rush to get out of bed. Wake up - do a little warm-up lying down. Move your arms and legs. Then sit down and stand up slowly. Perform actions without sudden movements. they can cause fainting.
2. Accept cold and hot shower in the morning for 5 minutes. Alternate water - a minute warm, a minute cool. This will help to cheer up and is good for blood vessels.
3. A good cup of coffee! But only a natural tart drink will raise the pressure. Drink no more than 1-2 cups per day. If you have heart problems, drink coffee instead green tea. It invigorates no worse than coffee, but does not harm the heart.
4. Sign up for a pool. Go at least once a week. Swimming improves vascular tone.
5. Buy a tincture of ginseng. This natural "energy" gives tone to the body. Dissolve 20 drops of tincture in ¼ cup of water. Drink half an hour before meals.
6. Eat sweets. As soon as you feel weak - eat ½ teaspoon of honey or a little dark chocolate. Sweets will drive away fatigue and drowsiness.
7. Drink clean water. Daily 2 liters of pure and non-carbonated. This will help maintain pressure on normal level. If you have a sick heart and kidneys, drinking regimen must be prescribed by a doctor.
8. get enough sleep. A rested body will work as it should. Sleep at least 7-8 hours a day.
9. Get a massage. According to experts oriental medicine, there are special points on the body. By acting on them, you can improve your well-being. Pressure is controlled by the point between the nose and upper lip. Gently massage it with your finger for 2 minutes in a clockwise direction. Do this when you feel weak.

First aid for hypotension and hypertension

If you feel dizzy severe weakness, tinnitus, call an ambulance. In the meantime, the doctors go, act:

1. Open the collar of your clothes. The neck and chest should be free.
2. Lie down. Lower your head down. Place a small pillow under your feet.
3. Smell ammonia. If it is not available, use table vinegar.
4. Have some tea. Definitely strong and sweet.

If you feel the approach of a hypertensive crisis, then you also need to call the doctors. In general, this disease should always be supported preventive treatment. As first aid measures, you can resort to the following actions:

1. Organize foot bath with hot water, in which mustard was previously added. An alternative would be to apply mustard compresses to the area of ​​the heart, back of the head and calves.
2. Lightly bind the right, and then the left arm and leg for half an hour each side. When the tourniquet is applied, a pulse should be felt.
3. Have a drink from chokeberry. It can be wine, compote, juice. Or eat jam from this berry.

To reduce the risk of occurrence and development of hypotension and hypertension, you should adhere to a regimen healthy eating, prevent the appearance excess weight, exclude harmful products from the list, move more.

Pressure should be measured from time to time. When observing a trend of high or low blood pressure, it is recommended to consult a doctor to determine the causes and prescribe treatment. Prescribed therapies may include methods to normalize blood pressure, such as taking special medications and herbal infusions diet, exercise, and so on.

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