Ventilation

Definitions & Key takeaways

Ventilation refers to the process of moving air in and out of the lungs. Alveolar ventilation refers to the amount of air that reaches the alveoli in the lungs for gas exchange. This determines the amount of oxygen that is available for the body to use, and the amount of carbon dioxide that is eliminated from the body.

Alveolar ventilation is determined by the tidal volume and the amount of dead space in the respiratory system. The formula for alveolar ventilation is (tidal volume - physiological dead space) x respiratory rate. Tidal volume is the amount of air that enters or leaves the lungs during a single normal breath; and physiological dead space refers to the portion of the respiratory system where air exchange does not occur, resulting in wasted ventilation.

Chapters:

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Introduction0:00–0:27

The main job of the lungs is gas exchange, pulling oxygen into the body and getting rid of carbon dioxide. Normally, during an inhale - the diaphragm and chest muscles contract to pull open the chest and suck in air like a vacuum cleaner, and then during an exhale - the muscles relax, allowing the lungs to spring back to their normal size pushing that air out.

Minute ventilation0:27–1:17

Ventilation rates measure the volume of air moving in and out of the lungs, over a period of time. During normal quiet breathing, each breath of air that enters and leaves the lungs is about half a liter, which is called the tidal volume.
The respiratory rate is the number breath a person takes per minute. In an adult this is normally around 15 breath per minute at rest.
So the minute ventilation is the amount of air moved in and out of the lungs in a minute. So minute ventilation is given by Minute Ventilation = (Tidal Volume) X (Respiratory Rate) In a normal healthy adult, this means 500 ml per breath times 15 breaths per minute, or about 7.5 litres per minute.

Alveolar ventilation1:17–6:07

However, not all the air that we breathe in reaches the alveoli, where gas exchange actually takes place. Some air is trapped in the airways - an area called the anatomical dead space.
Also, some of the alveoli may be defective and can’t even participate in gas exchange. When you add the volume of air lost in these malfunctioning alveoli to the anatomical dead space, you get the physiological dead space.
So to calculate alveolar ventilation, it’s the tidal volume minus the physiologic dead space and that volume gets multiplied by the respiratory rate: Alveolar ventilation = [(Tidal volume) - (Physiological dead space)] X (Respiratory Rate) In a normal healthy person, almost all the alveoli are functioning properly, and the physiological dead space is about equal to the anatomic dead space which is about 150 ml.
So the alveolar ventilation comes to about (500 - 150) ml or 350 ml per breath, times 15 breaths per minute or about 5.2 litres per minute.
A way of measuring the alveolar ventilation without actually measuring the dead spaces is by knowing inspired air contains almost zero carbon dioxide and all the carbon dioxide in the expired air comes from the functioning alveoli.
If we call the alveolar ventilation, VA. That’s the amount of air going in and out of the alveoli in a minute.
A fraction of this volume is carbon dioxide, so let’s call that fraction FCO2. So, the volume of carbon dioxide, VCO2, is: VCO2 = VA X FCO2 Or, VA = (VCO2) / (FCO2) Now in any mixture of gases, the partial pressure of one of the gases is proportional to the fractional concentration of the gas in that mixture.
Think of the gas molecules as little balls of different colors, constantly moving around in a container. If there are more balls of a particular color in that mixture, then those balls are more likely to hit the walls of the container.
As gas molecules hit the walls of a container they exert pressure on it, so that’s why partial pressure is proportional to the fractional concentration of that gas in the mixture.
In fact, the relationship between the two is based on the constant K, which assumes that the gases are all saturated with water vapor at normal body temperature and at normal sea-level atmospheric pressure.
So for partial pressure of carbon dioxide in the alveolar air, let it be PCO2, and the equation becomes: PCO2 = FCO2 X K Now, plugging this back into the equation for alveolar ventilation, we get, VA = [(VCO2) / (PCO2)] X K Now, carbon dioxide is highly soluble in water and therefore also in blood.
As soon as carbon dioxide in the alveoli contacts the blood in the capillaries, it diffuses across the capillary wall, and the pressure of CO2 in the blood equals the pressure of CO2 in the alveolar air.
So we can replace the pressure of CO2 in the alveolar air, that is PCO2, with the pressure of CO2 in the arterial blood, let it be PaCO2.
The equation then becomes: VA = [(VCO2) / (PaCO2)] X K This is known as the alveolar ventilation equation and it describes the inverse relationship between alveolar ventilation and the partial pressure of CO2 in the alveolar air as well as the pulmonary arteries.
This inverse relation is kind of intuitive as well, air in the alveoli helps remove CO2 from the blood. More air ventilating the alveoli means less CO2 in the blood and vice versa.
One physiological implication of this is that when a person is breathing in and out more frequently than normal, that is hyper-ventilating, the concentration of carbon dioxide in blood starts to fall.
This leads to alkalosis of the blood and can cause dizziness and anxiety. On the flip side, when we start producing too much carbon dioxide, like while exercising, we need to get it out.
In that situation, the respiratory rate and tidal volume increase and that makes the alveolar ventilation go up. All right, as a quick recap… Minute ventilation is the total volume that enters and exits the lungs in a minute, and alveolar ventilation is the same thing after correcting for the physiologic dead space.

Review6:07–6:30

The alveolar ventilation equation describes the inverse relationship between the alveolar ventilation and the carbon dioxide content of the alveoli and the arterial blood.