Normal level of mercury pressure. How does low atmospheric pressure affect people? Relationship between atmospheric and blood pressure

As if modern man Although he tried to isolate himself from nature, to show himself as an independent unit, the environment exerts its influence on him. This was established in ancient times, although the connection between well-being and atmospheric pressure was not immediately proven.

Why does this happen, what atmospheric pressure is considered normal for humans?

Why is it necessary to know atmospheric pressure indicators?

For a long time, people thought of air as something absolutely weightless, even though its pressure was used for very clear purposes: to inflate the sail of a ship, to start the work of the blades of a mill. Only in the middle of the 17th century, a barometer was invented by a student of Galileo - a device that allows you to track air vibrations. It was then that it became clear that on every square centimeter of the Earth’s surface the air presses with a force of 1.033 kg, and if we take into account the size of the body, then about 16,000 kg of air exerts pressure on a person every day. Discomfort does not occur only because this volume is distributed evenly and, in addition, meets resistance from the inside, from internal organs, which also contain oxygen in dissolved form.

• The barometer displays the measurement result in millimeters mercury- abbreviated as “mm.Hg.” Normal atmospheric pressure for humans is in the range of 750-760 units. This is the most optimal corridor, taking into account the Earth's topography.

Established norm atmospheric pressure varies for each region: for Moscow the average is 747-748 mmHg, but in St. Petersburg the norm is much higher - it is 753-755 mmHg. However, this does not mean that every city resident will perceive such indicators correctly: some need those same 750-760 mmHg, regardless of place of residence - temporary or permanent. Moreover, in summer the numbers are always higher than in winter.

• During the day, changes in atmospheric pressure of 1-2 units in any direction are considered normal and do not affect the human condition. Deterioration in well-being is observed with distortions of 2-3 units in 3 hours.
• Normal atmospheric pressure over the entire surface of the globe is impossible: it is associated with the topography and the distance (in height) from sea level, so it drops seriously in mountainous areas. In addition, the closer to the northern or south pole, the stronger these changes are felt. In the Equator zone, on the contrary, due to the flat terrain, there are almost no such jumps.
• It is noteworthy that even an increase of 100 m, which often happens to those who are forced to stay in high-rise buildings, already leads to entering a zone of changes in atmospheric pressure. But a person who is often exposed to this quickly adapts.

The human body is very flexible; with proper training, it can adapt to fluctuations in atmospheric pressure (within certain limits), and a long-term decrease or increase in pressure will be painless. Athletes, due to altered indicators of physical endurance, can for a long time be in conditions of low atmospheric pressure and feel good. But a common person Often he feels all the fluctuations on himself, especially if they occur within 2-3 units, and happen in a short period of time.

Acclimatization after a long flight, i.e. changing time zones and climate zones is one of the most simple examples effects on the body of changes in atmospheric pressure.

How does atmospheric pressure affect humans??

When the force of air gravity exerted on the body increases or decreases sharply, the activity of internal resistance must also change. Thus, a reaction occurs in the vessels in which oxygen is mixed with blood. In response to fluctuations in atmospheric pressure, fluctuations in blood pressure begin inside a person. If the body is healthy, the blood vessels adapt quickly and smoothly, no special problems will arise, the changes will “pass by.” But if they contract and unclench very sluggishly, the normal flow of blood is disrupted: it thickens, comes in jerks, or, on the contrary, barely goes its way. This is typical for people with pathologies of the cardiovascular system.

• In order not to overreact to such situations, doctors advise paying attention to strengthening blood vessels and increasing their adaptation: contrast showers, walking, gymnastics, physical activity - all this naturally trains the cardiovascular system.

However, such a step does not always save you from weather dependence. Moreover, the influence of atmospheric pressure on a person’s blood pressure is not the only negative aspect. There is also a relationship between respiratory system and the heaviness of the air, especially for a person living in a metropolis, where the situation is aggravated by gas pollution, oxygen deficiency due to the abundance of “concrete boxes” and the almost complete absence of green spaces. Also suffers the immune system, since the proportion of leukocytes decreases, which leads to weakening protective functions body. An accidentally introduced virus can cause a long and severe illness.

• The main risk group for weather dependence is hypertensive patients, people with heart pathologies, intracranial pressure disorders, asthmatics, and allergy sufferers. There is also a high probability of reacting sharply to fluctuations in atmospheric pressure office workers who have to stay in stuffy rooms and at high altitudes.

The impact of natural instability affects both the physical and psychological state of a person:

• The inability to take a full breath, a feeling of lack of oxygen, is the most common complaint recorded by doctors. In addition, shortness of breath may be added with minimal physical activity(up to a regular walk on level ground), arrhythmia, tachycardia.
• Headaches appear (most often migraines, although a feeling of a “hoop” or pain in the back of the head is possible), weakness, loss of concentration, drowsiness, and a feeling of heaviness in the limbs.
• Some people react to changes in atmospheric pressure with intestinal upset and/or pain in the epigastric region. Poor circulation can lead to loss of sensation or coldness of the extremities.

Man is part of nature. It constantly influences each of us. The more time passes, the more weather-sensitive people appear. Today there are approximately 4 billion such people living on the planet. The factor that most strongly influences a person’s health and well-being is atmospheric pressure, or rather its norm. What atmospheric pressure is considered normal depends on geographical location the area where a person spends most of the time.

What is atmospheric pressure?

The Earth is habitable due to a number of factors. The first of these is the availability of breathing air. Our planet, like a dome, is covered with an atmosphere consisting of many layers, each of which performs a specific important function. Air mass exerts constant pressure on everything that is on Earth, including humans, which is why it is so important to know what its norm is. Every day, each of us withstands a load of approximately 15,000 kg. We do not feel this burden due to the unique structure of our body. But he can't always cope natural phenomena. Sometimes there is an imbalance in the organ system human body, and then a person becomes dependent on changes in atmospheric pressure.

Atmospheric pressure, which is the norm for a person living in middle lane Russia, is 750-760 mmHg. This is the indicator at which most people do not feel any discomfort in their health.

Any deviation of a person’s atmospheric pressure from the norm by 5-10 units or higher will be painfully accepted by our body.

Atmospheric pressure measurement

To measure the normal atmospheric pressure for a person, a specially designed device is used - a barometer. Science has established that the force of atmospheric pressure per 1 sq.cm. surface of the Earth, corresponds to the height of the mercury column of 760 mm. This indicator was taken as the norm of atmospheric pressure for humans. If the barometer reading is above this mark, it is customary to talk about high blood pressure from the norm. If the atmospheric pressure is below normal for a person, then it is considered low. Barometer readings in different points planets are different due to differences in topography, temperature, etc.

The normal atmospheric pressure for a person is measured in mm of mercury (mmHg). Other units may be used, such as pascals (Pa). An indicator of 760 mmHg will be equal in this case to 101325 Pa. However, in everyday life, measuring normal atmospheric pressure for humans in pascals has not taken root. Any weather forecast informs us about the state of atmospheric pressure using mmHg.

What is weather sensitivity?

Many people have so-called weather sensitivity. This is a peculiar reaction of the body to a change in the normal atmospheric pressure for a person. It can be expressed, depending on the presence of various health problems, in the appearance of irritability, pain in various parts of the body, a general decrease in performance, and insomnia. A change in normal atmospheric pressure for a person can manifest itself in mental disorders, for example, a state of anxiety, depression, unreasonable fear.

According to statistics, during sudden changes in weather, even the number of crimes and accidents in transport, as well as man-made disasters, increases several times.

The human body is a kind of chemical laboratory that works normally at the appropriate standard of atmospheric pressure for a person. As soon as these conditions change in any direction, the body reacts with painful manifestations. He lacks something, for example, oxygen. Or vice versa, there is something in excess.

The causes of weather sensitivity are not only health problems, but also poor lifestyle choices. Sedentary activity plays an important role poor nutrition with subsequent acquisition of excess weight, stress.

The influence of human atmospheric pressure

In the vessels and cavities of the human body there are specific receptors that are very sensitive to any changes in atmospheric pressure standards for humans. For example, people with musculoskeletal disorders always “predict” weather changes by aching joints. Hypertensive patients due to headaches in the temples, etc.

The well-being of heart patients also worsens with changes in normal atmospheric pressure for humans. They feel pain in the heart and head, rapid heartbeat and other unpleasant symptoms.

High atmospheric pressure forces the human body to equalize the imbalance that has arisen. How does this happen? By reducing blood pressure. At the same time, the vessels relax, the speed of blood flow changes. Malaise, headaches, and stuffy ears occur. With increased atmospheric pressure, changes occur in chemical composition blood, in particular, the level of leukocytes, the main fighters against infections and viruses, decreases.

Low atmospheric pressure creates conditions for the body similar to climbing a mountain. Under such conditions, there is a deficiency of oxygen, and, consequently, the brain and other organs suffer from hypoxia. The person feels difficulty breathing, pain in the temporal region, and pressure in the head.

Scientists have discovered the dependence of atmospheric pressure on air temperature. With warming, atmospheric pressure decreases. This is unfavorable for hypotensive patients and asthmatics.

With the onset of cold weather and the establishment of clear weather, the norm becomes high. Hypertension sufferers, allergy sufferers, and kidney sufferers.

The most dangerous thing is a sharp increase in atmospheric pressure (by 1 mmHg in 2-3 hours). The patient sharply feels negative symptoms and feels very bad. It is imperative to monitor your blood pressure and immediately take the medications prescribed by your doctor.

A person cannot influence the weather, but he can help himself survive difficult periods.

The first thing you should do as much as possible is to reduce physical activity.

Secondly, consult a doctor about prescribing normal medications for those who are especially hard-pressed by weather changes.

The optimal combination of weather indicators for a person is as follows:

• Atmospheric pressure is normal - 760 mm Hg.
• Air temperature is the norm 18-20 ͦ C.
• Humidity is normal 50-55%.

At different points on the Earth, the pressure rate may be different. Recorded fluctuations at sea level are 641-816 mmHg. The average value is exactly 760 mmHg at 20 degrees Celsius. This does not mean that for every person this reading should be equal to the norm. If a person was born and raised in the mountains, the normal numbers for him will naturally be different. Climbing into the mountains will reduce the altitude by about 13% for every kilometer.

The average atmospheric pressure for St. Petersburg, located in northwestern Russia, is only 748 mmHg.

It is time that can greatly influence a patient’s barometric pressure. At night it is higher than during the day. That is why most attacks in heart patients occur at night.

Since 1982, a standard pressure of 100 kPa has been adopted.

Naturally, what to choose for yourself ideal conditions impossible. How many people, so many problems. Everyone is concerned about certain disturbances, so it is important to prepare yourself and your body for weather changes and take care of your health.

We are taught about what atmospheric pressure is at school during natural history and geography lessons. We get acquainted with this information and safely throw it out of our heads, rightly believing that we will never be able to use it.

But after years of stress and ecological situation environment will have a sufficient impact on us. And the concept of “geodependence” will no longer seem nonsense, because pressure surges and headaches will begin to poison life. At this moment you will have to remember what it is like in Moscow, for example, in order to adapt to new conditions. And move on with your life.

School basics

The atmosphere that surrounds our planet, unfortunately, literally puts pressure on all living and nonliving things. There is a term to define this phenomenon - atmospheric pressure. This is the force of the air column acting on the area. In the SI system we talk about kilograms per square centimeter. Normal atmospheric pressure (optimal indicators for Moscow have long been known) affects the human body with the same force as a weight weighing 1.033 kg. But most of us don't notice this. There are enough gases dissolved in body fluids to neutralize all unpleasant sensations.

Atmospheric pressure standards in different regions are different. But 760 mmHg is considered ideal. Art. Experiments with mercury turned out to be the most revealing at a time when scientists were proving that air has weight. Mercury barometers are the most common devices for determining pressure. It should also be remembered that the ideal conditions for which the mentioned 760 mm Hg are relevant. Art., is a temperature of 0 ° C and the 45th parallel.

In the international system of units, it is customary to define pressure in Pascals. But for us, the use of mercury column fluctuations is more familiar and understandable.

Relief features

Of course, many factors influence the value of atmospheric pressure. The most significant are the relief and proximity to the magnetic poles of the planet. The norm of atmospheric pressure in Moscow is fundamentally different from the indicators in St. Petersburg; and for residents of some remote village in the mountains, this figure may seem completely abnormal. Already at 1 km above sea level it corresponds to 734 mm Hg. Art.

As already noted, in the region of the Earth’s poles the amplitude of pressure changes is much higher than in equatorial zone. Even during the day, atmospheric pressure varies slightly. Insignificantly, however, only by 1-2 mm. This is due to the difference between day and night temperatures. At night it is usually cooler, which means the pressure is higher.

Pressure and man

For a person, in essence, it does not matter what atmospheric pressure is: normal, low or high. These are very conditional definitions. People tend to get used to everything and adapt. The dynamics and magnitude of changes in atmospheric pressure are much more important. On the territory of the CIS countries, in particular in Russia, there are quite a lot of zones. Often, local residents do not know about it.

The norm of atmospheric pressure in Moscow, for example, may well be considered as a variable value. After all, every skyscraper is a kind of mountain, and the higher and faster you go up (or go down), the more noticeable the difference will be. Some people may well pass out while riding a high-speed elevator.

Adaptation

Doctors almost unanimously agree that the question “what atmospheric pressure is considered normal” (is it Moscow or any locality planets - not the point) is incorrect in itself. Our body adapts perfectly to life above or below sea level. And if the pressure does not have a detrimental effect on a person, it can be considered normal for the area. Doctors say that the standard atmospheric pressure in Moscow and other major cities is in the range from 750 to 765 mmHg. pillar

The pressure drop is a completely different matter. If within a few hours it rises (falls) by 5-6 mm, people begin to experience discomfort and pain. This is especially dangerous for the heart. Its beating becomes more frequent, and a change in the frequency of breaths leads to a change in the rhythm of oxygen supply to the body. The most common ailments in such a situation are weakness, etc.

Meteor dependence

Normal atmospheric pressure for Moscow may seem like a nightmare to a visitor from the North or the Urals. After all, each region has its own norm and, accordingly, its own understanding of the stable state of the body. And since in life we ​​do not concentrate on exact pressure indicators, weather forecasters always focus on whether the pressure is high or low for a given region.

After all, not every person can boast that they do not notice the corresponding changes. Anyone who cannot call himself lucky in this matter must systematize his feelings during pressure changes and find acceptable countermeasures. Often a cup of strong coffee or tea is enough, but sometimes more serious help in the form of medications is needed.

Pressure in the metropolis

Residents of megacities are the most weather-dependent. It is here that a person experiences more stress, lives life at a high pace and experiences environmental degradation. Therefore, knowing what normal atmospheric pressure is for Moscow is vital.

The capital of the Russian Federation is located on the Central Russian Upland, which means that here is a priori a zone low blood pressure. Why? It's very simple: the higher you are above sea level, the lower the atmospheric pressure. For example, on the banks of the Moscow River this figure will be 168 m. And the maximum value in the city was recorded in Teply Stan - 255 m above sea level.

It is quite possible to assume that Muscovites will experience abnormally low atmospheric pressure much less frequently than residents of other regions, which, of course, cannot but make them happy. And yet, what atmospheric pressure is considered normal in Moscow? Meteorologists say that it usually does not exceed 748 mm Hg. pillar This means little, since we already know that even a quick ride in an elevator can have a significant impact on a person's heart.

On the other hand, Muscovites do not feel any discomfort if the pressure fluctuates between 745-755 mm Hg. Art.

Danger

But from the point of view of doctors, not everything is so optimistic for the residents of the metropolis. Many experts quite reasonably believe that by working on the upper floors of business centers, people expose themselves to danger. Indeed, in addition to the fact that they live in a zone of low pressure, they also spend almost a third of the day in places with

If we add to this fact violations of the building’s ventilation system and the constant operation of air conditioners, it becomes obvious that employees of such offices turn out to be the most incapacitated, sleepy and sick.

Results

Actually, there are a few things to remember. Firstly, there is no single ideal value for normal atmospheric pressure. There are regional standards that can vary significantly in absolute terms. Secondly, the characteristics of the human body make it easy to experience pressure changes if they happen rather slowly. Thirdly, the more healthy image We lead our lives and the more often we manage to maintain a daily routine (getting up at the same time, getting a long night's sleep, following a basic diet, etc.), the less susceptible we are to weather dependence. This means they are more energetic and cheerful.

Atmospheric pressure is the force with which a column of air presses on a certain unit area of ​​the Earth. It is often measured in kilograms per square meter, and from there it is converted to other units. By to the globe atmospheric pressure varies - it depends on geographical location. Normal, habitual blood pressure is extremely important for the human body to function properly. You need to understand what atmospheric pressure is normal for a person, and how its changes can affect well-being.

When you rise to a height, the atmospheric pressure decreases, and when you go down, it increases. Also, this indicator may depend on the time of year and humidity in a particular area. In everyday life it is measured using a barometer. It is customary to indicate atmospheric pressure in millimeters of mercury.

The ideal atmospheric pressure is considered to be 760 mmHg, but in Russia and most of the planet in general, this figure is far from this ideal.

The normal force of air pressure is considered to be that at which a person feels comfortable. Moreover, for people from different places habitats in which normal well-being is maintained will be different. A person usually gets used to the indicators of the area in which he lives. If a resident of the highlands moves to the lowlands, he will experience discomfort for some time and gradually get used to the new conditions.

However, even at permanent place residence, the atmospheric pressure may change. This usually happens with changing seasons and sudden changes in weather. In this case, people with a number of pathologies and congenital weather dependence may experience discomfort, and old diseases may begin to worsen.

It is worth knowing how you can improve your condition if there is a sharp drop or increase in atmospheric pressure. You don’t have to immediately run to the doctor - there are home techniques that have been tested by many people that can help you start feeling better.

Important! It is worth noting that people sensitive to changing weather conditions should be more careful when choosing places to spend their holidays or move.

What indicator is considered normal for a person?

Many experts say: normal pressure for a person the figure will be 750–765 mmHg. It is easiest to adapt to indicators within these limits. For most people living on plains, small hills, and lowlands, they will be suitable.

It is worth noting that the most dangerous thing is not increased or decreased indicators, but their sudden change. If changes happen gradually, most people won't notice them. Abrupt change can lead to negative consequences: Some people may faint when climbing a mountain sharply.

Pressure norm table

In different cities of the country, the indicators will be different - this is the norm. Usually detailed weather reports tell you whether the atmospheric pressure is above or below normal. this moment time. You can always calculate the norm for your place of residence yourself, but it is easier to refer to ready-made tables. For example, here are the indicators for several cities in Russia:

It is worth noting that for some cities and regions large pressure drops are normal. Local residents are usually well adapted to them; only a visitor will feel unwell.

Important! If weather dependence occurs suddenly and has never been observed before, you should consult a doctor - this may indicate heart disease.

Effect on the body

For people with certain medical conditions and increased sensitivity to weather changes, pressure changes can negatively affect them, in some cases limiting their ability to work. Experts note: women are slightly more likely than men to react to weather changes.

People have different reactions to changes. Some people feel slight discomfort that easily goes away on their own after some time. Others require special medications to avoid exacerbation of any disease that may occur due to changing weather conditions.

The following groups of people are most prone to negative experiences during pressure changes:

1. With various lung diseases, which include bronchial asthma, obstructive and chronic bronchitis.
2. With heart and vascular diseases, especially hypertension, hypotension, atherosclerosis, and other disorders.
3. With brain diseases, rheumatic pathologies, diseases of the musculoskeletal system, especially osteochondrosis.

It is also believed that changes in weather conditions provoke allergy attacks. In completely healthy people, the changes usually do not have a pronounced effect.

People with weather dependence experience headaches, drowsiness, a feeling of fatigue, and pulse irregularities that are not observed under normal conditions. In this case, it is advised to consult a doctor to exclude the development of heart disease and nervous system.

In addition to headaches and fatigue, people with various diseases may experience discomfort in the joints, changes in blood pressure, numbness in the lower extremities, and muscle pain. In case of exacerbation of chronic diseases, you should take medications prescribed by your doctor.

What to do if you are weather dependent

If increased sensitivity to changing weather conditions is present, but there are no diseases leading to it, then the following recommendations will help you cope with unpleasant sensations.

In the morning, it is advised to take a contrast shower, then drink a cup of good coffee to keep yourself in good shape. It is recommended to drink more tea during the day. Better - green with lemon. It will be useful to do exercises, several times a day.

Towards evening it is advised to relax. Herbal teas and decoctions with honey, valerian infusion and other mild sedatives will help with this. It is recommended to go to bed early and eat less salty foods during the day.

The earth's atmosphere can be compared to a blanket of air that envelops the planet and, with a certain force, presses on everything that is on it. It has been estimated that a person is constantly exposed to air pressure weighing more than ten tons. Yes, yes, just such a colossal load does not have any effect on us due to the fact that the gases dissolved in the liquids washing the tissues of the human body balance the load. But here everything is not so simple either, because this pattern is only valid when a person is at flat surface, rather than diving deep underwater or climbing mountains.

So, what is atmospheric pressure, and on what conditions does this indicator depend? As already mentioned, this is the force with which a column of air exerts pressure on a certain unit of area. The normal atmospheric pressure is 760 mmHg. in an area located at sea level. However, this value is largely influenced by relief and changes in altitude - the higher above sea level the territory is, the lower the atmospheric pressure there.

The time of day and air temperature also matter: since it is usually colder at night than in daytime, atmospheric pressure indicators are slightly higher. True, the increase in pressure is so small that it does not cause negative changes in well-being. In addition, the values ​​of this indicator fluctuate noticeably at the Earth's poles, while on the equatorial plains they are barely noticeable.

How do changes in atmospheric pressure affect health?

As you know, each person reacts individually to pressure changes and weather changes. Some do not notice any changes at all and are able to travel to different destinations without any problems. climatic zones. And others, and these are the majority, can predict weather changes based on their well-being, without even leaving home. Painful symptoms that appear in response to changes in atmospheric pressure, humidity, and other weather conditions are called weather dependence. This condition is found in approximately four billion people around the globe.

The question is often asked, what atmospheric pressure is considered normal for humans? Taking into account the fact that people are adapted to life in different climatic conditions, in areas located at different altitudes, it is impossible to clearly answer it. For a person, normal atmospheric pressure is one that does not worsen his well-being. This figure ranges from 750 to 765 mmHg.

When there are changes in atmospheric pressure, weather-dependent people experience:

• Headache.
• Vascular spasms.
• Circulatory disorders.
• Increased fatigue and weakness.
• Drowsiness.
• Dizziness.
• Joint pain.
• Numbness of arms and legs.
• Tachycardia.
• Psycho-emotional disorders and irritability.
• Dyspnea.
• Intestinal disorders.
• Nausea.
• Decreased visual acuity and clarity.

Deterioration in well-being is associated with the work of baroreceptors that respond to changes in pressure. In the human body they are located in the pleural and abdominal cavities, joints, blood vessels. Therefore, on the eve of weather changes, such people feel pain in the joints, disturbances in the functioning of the heart, and heaviness in the chest. If a person has digestive problems, he suffers from flatulence and intestinal disorders. Those who have previously suffered a traumatic brain injury or people with aneurysms experience unbearable migraines. With a significant decrease in pressure, the headache is caused by a lack of oxygen in the brain cells.

Emotionally unstable people react very sharply to changes in atmospheric pressure, feeling causeless anxiety, irritability, and sleep disturbances. It is noted that during pressure fluctuations there is large quantity offenses, accidents on highways and high-risk industries.

The influence of atmospheric pressure on human blood pressure deserves special attention. In hypertensive patients, an increase in atmospheric pressure, at which sunny, clear weather sets in, causes narrowing and spasms of blood vessels, painful headache, nausea, hypertensive crises. Also these weather cause a significant decrease in the activity of the immune system.

Hypotensive people react especially painfully to a decrease in atmospheric pressure, at which there is high humidity, rain, and cloudiness. At such pressure in the air, the concentration of oxygen decreases, and there is an insufficient supply of it to the brain, which causes circulatory disorders. People with low blood pressure suffer from migraines, shortness of breath, tachycardia, lack of air, weakness.

How to become less weather sensitive?

Conducted studies have shown that the development of various health disorders in response to weather changes is based on the following factors:

• Insufficient physical activity.
• Poor nutrition leading to extra pounds gain.
• Instability of the nervous system due to constant stress and nervous tension.
• Lack of oxygen and poor ecology.

• Consume foods rich in vitamin B6, magnesium and potassium, vegetables, fruits, lactic acid products, honey.
• Eat less meat, fried and salty foods, sweets, and spices.

It is necessary to limit the consumption of alcoholic beverages and stop smoking. You should make it a rule to do exercises every day, take walks fresh air, sleep at least seven to eight hours. It is also useful to use natural adaptogens, such as tinctures of Eleutherococcus, ginseng, and preparations based on antlers. reindeer. But, since these drugs have some contraindications and may have side effects, you should consult your doctor about the advisability of taking them.