Residual volume in simple words is as its name implies remaining volume. The volume of air that remains in lungs as the end of forceful expiration.
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Breathing is a process of life. We breathe many times every minute for the entire life. From birth to death the process of breathing continues unstopped even when you sleep. Once you stop breathing life ends.
Breathing is the intake and output of air. Further the oxygen in air is exchanged with carbon dioxide in blood.
Blood further carries oxygen to various organs of the body. The blood flow is continuous to lungs and the air exchange too.
You breathe out that does not mean blood flow to lungs is stopped. The blood keeps coming.
So there should be oxygen rich air to exchange the gases when you breathe out.
This air volume in lungs at maximum expiration is residual volume.
The concept of residual volume is very important to understand.
Air like any fluid or gas moves from more pressure to less pressure. Your lungs have less pressure or negative pressure compared to air pressure outside the air flow is from outside to inside.
Lungs are like balloons. They are inflated when positive air pressure is applied from inside or negative air pressure is applied from outside.
At normal the air pressure outside is constant. It does not change when you breathe.
So we have a mechanism in the body to create negative pressure around lungs to take the air in the lungs.
When you breathe out air flows from inside lungs to outside. In this process more pressure is created around the lungs.
This entire process is carried out by the rib cage and muscles of respiration in the chest.
When there is no positive pressure in the airway or no negative pressure around, the lungs try to collapse.
The rib cage and chest muscles maintain negative pressure in lungs to prevent lungs collapse.
The alveoli in lungs are small sacs of air. Collectively lungs are collections of these small balloons. They have to be open all the time for the process of air exchange to happen continuously.
What is the function of residual volume?
Residual volume is an important phenomenon in day to day life.
It has following functions:
- Preventing lungs from collapse.
- Continued gas exchange.
- Keeping work of breathing at optimum levels.
Conditions with increased residual volume:
Conditions that are cause of increased lung volume are generally obstructive airway diseases. They are following.
- Chronic obstructive pulmonary disease.
- Hyper-reactive airway disease.
Conditions with reduced residual volume
These are comparatively rare diseases.
- Restrictive lung diseases.
- Diseases causing fibrosis in the lungs.
- Acute conditions like Acute respiratory distress syndrome (ARDS).
How to measure residual volume?
Residual volume can not be measured directly. It is the volume of air that does not move during normal respiration so can not be measured.
To measure this volume special tests are needed.
‘If the amount of gas is constant, the gas volume and pressure is in inverse proportion.’ This property of the gas is used to measure the residual volume.
Smaller the gas volume the more is the pressure. Lesser the gas volume less is the pressure.
For this purpose helium dilution test or nitrogen washout test is useful.
Helium dilution test
In this test a small amount of helium is added to respiratory gas.
You breathe in a small amount of helium with the air that you breathe.
Initial concentration of helium gas in breathing air is known.
Then the final concentration of gas in the air you breathe out is measured.
The residual volume dilutes this gas more and final concentration is less than initial.
Knowing the volume of air and concentration of helium gas while inhaling and exhaling, we can calculate residual capacity.
Nitrogen washout test
This test also is used to count functional residual capacity.
This test utilises nitrogen that forms 78 percent of our atmosphere.
Patient is given 100 oxygen for breathing. A two way valve is used. One valve opens at inspiration and closes at expiration, and the other valve vice versa.
The nitrogen concentration graph is plotted on a paper. The volume of expired air is counted and serial drop of nitrogen too.
This test also determines functional residual capacity.
This test is more expensive but more accurate in determining functional residual capacity.
For this test sits an enclosed chamber of plethysmography. Patient breathes air and exhales in the spirometer.
Spirometer measures pressure and volume at the expiration.
Plethysmography measures change in the entire chamber while breathing.
P1V1 = P2V2
After measuring functional residual capacity, residual volume is measured.
RV = FRC – ERV