We all heard someone say that he or she had water "on" the lungs or even, water "in" the lung.

What is it exactly? Should we say "on" the lungs or "in" the lung?
Both situations exist.

Water "in" the lungs

The true term for water "in" the lung is pulmonary edema, that is, water in the alveoli of the lung.

Water "on" the lungs

In the case where there is water "on" the lungs, we mean there is fluid in the lung envelope: a left or right pleural effusion. When there is fluid on both sides, it is simply called pleural effusion.

The causes are multiple. Water "on" lungs may be associated with water "in" the lung or may be associated with a lung disease.


Better understand

Where does this water come from? Normally, there isn’t any...

Let's go back to the basics.


Lungs and breathing

The lungs are where the exchanges of oxygen (O2) and carbon dioxide (CO2) occur.

The venous blood filled with carbon dioxide is brought to the lungs to fill up with oxygen and then brings the blood back to the heart so that it can be pumped to the organs and oxygenate them.

The arrival of "used" blood, heavy with carbon dioxide (CO2)

"Used" blood comes from the right ventricle to the lungs through the pulmonary artery.

The pulmonary artery branches into a multitude of small arterioles and capillaries to direct the blood into all the lungs.


Pulmonary capillaries, microscopic vessels

The pulmonary capillaries are the junction points between the arteries and the pulmonary veins.

They are microscopic vessels in which the blood cells pass in a single file.



The pulmonary alveoli, these little air sacs

The lungs are composed of a multitude of small air sacs called alveoli.

There are more than 600 million of them in the lungs. One alveolus has a diameter of 0.2 mm and its wall is made of a very thin layer of cells. The pulmonary capillaries cover all the cells.

Junction of alveoli and pulmonary capillaries

It is at this microscopic location that the "breathing" really takes place.

Fluid exchange at the capillaries

Capillaries are like thin, membrane-covered drains that prevent proteins and blood cells from coming out, but allow blood fluids to escape or resume circulation.





Normally, the balance of what goes out and enters is preserved. There is no accumulation of fluid in the lung.





But where does the balance between the water coming out the capillaries and the water returning to it come from?


The cells of the lungs are in a liquid

Alveolus cells are in a liquid to ensure their proper functioning and to allow the exchange of oxygen (O2) to the blood and carbon dioxide (CO2) to the alveoli and out of the lung through expiration.



Two factors control the water.

Two factors influence the entry and exit of the capillary water. One tries to keep it inside and the other allows it to escape.

Oncotic factor

Let's call the first factor, the oncotic pressure.

"On" cotic to help us remember "on" for "in" or inside.

The retention of water in the capillary is exerted by the blood cells and blood proteins that can not cross the membrane of the capillaries. They remain in this microscopic vessel.



Osmotic factor

The second factor that influences the water entering and exiting the capillaries is called the osmotic pressure.

The term osmotic comes from the word osmosis. This term describes the passage of water in and out of the capillairies freely to ensure the balance of concentrations.

This allows for the cells to bathe without any overflow.



Consequence of a loss of balance

Any alterations to these factors can result in a surplus of water in the environment where the cells bathe. This excess water will slow down the exchange of oxygen and carbon dioxide and result in a more pronounced shortness of breath during an effort.

This is the case of anemia, the drop in hemoglobin that alters the oncotic factor. There is less water retention in the capillary. The resulting shortness of breath is further amplified by a decrease in the number of red blood cells, these small boats that carry oxygen.

The same effect will be present with a decrease of proteins in the blood due to diseases that decrease their production in the liver or other intestinal or renal diseases that exaggerate their loss in the intestine or the kidneys.
These diseases are usually chronic, that is, they develop gradually.

But what does the heart have to do with this balance?

You will probably say that these two factors have nothing to do with the heart and its role with the formation of the pulmonary edema.
You are right. There is a third factor. This factor is related to the blood pressure in the capillary.

3rd factor: Blood pressure

Blood pressure is generated by the contraction of the heart to move blood throughout the human body, including the lungs.

Normal blood pressure at the level of the pulmonary capillaries is less than 20mmHg. Increasing this pressure will create blood congestion in the pulmonary capillaries.


An ice jam in the river

Take the example of a river in which an ice jam has formed. The water rises in front of this jam. The river comes out of its bed and floods the surrounding lands.

Water invades the surrounding land

At the pulmonary level, water rises in the "bathtub" of the lung cells. Its overflow finally causes the water to invade the alveoli.
The presence of a surplus of water between the cells of the alveoli already deteriorates the exchanges of oxygen and carbon dioxide. The shortness of breath appears.

The impact is even more important when the water invades the small air sacs of the lungs that are the alveoli.


Capillaries are the nerve center of the impacts of congestion.

The pulmonary capillaries are at the mercy of all the diseases of the heart which bring congestion to their level. We could name here all the problems related to the aortic and mitral valves, any damage to the left ventricle without forgetting certain arrhythmias that cause the heart beat to be too fast and so on.


The symptom of breathlessness evolules with time.

It is related to the amount of water in the environment where bathe the cells and even worse when there is water in the alveoli.



It is the same for two other symptoms related to this loss of equilibrium of the exchanges of liquid in the pulmonary capillaries.

Shortness of breath at bedtime

Some patients are no longer able to sleep without raising their heads with several pillows. If they don’t, they feel like they are choking. This symptom is called orthopnea.

What is it?

When lying down, the gravitating blood in the legs is now released and recirculated.
Normally, the individual feels no impact of this redistribution.

In people with certain heart problems, the weakened pump has difficulty managing this excess volume. There is therefore overload at the pump.

This increase in volume in the cavities of the heart creates congestion in the left ventricle and atrium and affects the capillaries, the nerve center of oxygen (O2) and carbon dioxide (CO2) exchanges.

The inclination of the chest by placing several pillows decreases the effect of the redistribution of blood that is released from the legs. The patient breathes better.

An increase in the number of pillows to maintain this comfort is a sign of a deterioration of the heart condition. It must be mentioned to your doctor.


Shortness of breath at night

It is possible that the redistribution of blood that was attracted by gravity in the legs is done more gradually.
In this case, the patient lies down and does not perceive shortness of breath. Shortness of breath will wake up the patient later in the night.

Sitting in bed with your feet on the floor, or getting up to get some fresh air relieves breathlessness within minutes.

This symptom is called "paroxysmal nocturnal dyspnea" and should also be presented to the doctor.

Chronic heart disease

Some heart diseases are chronic and the symptoms develop gradually or will fluctuate over time. Some events will be more important than others. We are talking about recurrent pulmonary edema.

Acute pulmonary edema

Other heart conditions rapidly cause progressive shortness of breath and can put the life of the individual at risk.

The blood oxygenates less well. Carbon dioxide accumulates. The patient lacks air. His breathing becomes difficult or even laborious.

He or she is drowning and chokes.

Anxiety is at its peak.




Cold sweats are present and fatigue because of breathing becomes a problem. This is called acute pulmonary edema.

It's a medical emergency - Call 911.

In front of any symptomatology which worries you, consult.


Water "on" the lungs

Now, how can water "in" the lungs also be found in its envelope?

Let's go to another example to image everything.

Just put a sponge in a glass. By spraying this sponge gradually, it fills up with water. The surplus accumulates at the bottom of the glass.

It is the same with the lungs. The congestion of the pulmonary capillaries floods the lung, which floods the lung envelope.


This flood causes pleural effusions opposite the two lungs or pleural, or on one side.


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