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Respiratory Cycle – Breathing and Lung Mechanics

by Thad Wilson, PhD
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    00:01 In this particular lecture, we’re going to cover ventilation.

    00:05 And we have a number of learning goals to achieve.

    00:08 And the first will be to describe what the respiratory cycle is.

    00:13 The next will be to apply the various methods for us to understand ventilation and these two methods are minute ventilation and alveolar ventilation.

    00:23 Then, we will also talk about the resistance to ventilation.

    00:28 You will also understand how the work of breathing works and how compliance and surface tension affect work of breathing.

    00:36 Finally, we will predict how lung diseases such as restrictive lung disease or an obstructive lung disease affect these various components of work of breathing.

    00:47 Okay, let’s get right into ventilation.

    00:51 To breathe, we have a specific respiratory cycle.

    00:55 And we’re going to use pressure differentials to allow air to flow from outside the mouth into the air sacs or alveoli.

    01:04 What is kind of interesting to think about is that you always start off with a negative or subatmospheric pleural pressure.

    01:14 So pleural pressure in our example in this diagram is -5 cm of water.

    01:20 So what this means is there’s always a negative pressure in a certain portion of the pleural space of your lungs.

    01:27 And the reason for that is that the chest wall wants to expand and the lungs want to collapse and they are pulling on each other and that creates a subatmospheric or negative pressure.

    01:40 In this case, it’s at 5.

    01:44 When a person breathes in, negative pressure in the pleural space increases.

    01:50 And what this will do or how this works is places like the diaphragm pull down.

    01:56 And as you pull the diaphragm down, you stretch on the pleural membrane and create a greater negative pressure.

    02:05 This then allows the lungs to expand and air to rush in.

    02:12 So let’s take this now into more of a graphic format from kind of our cartoon.

    02:19 When someone breathes in, you’re going to have an increase volume in the lung and that can be seen here as a volume increasing and then during exhalation, volume decreases.

    02:35 Intrapleural pressure usually looks almost opposite of that.

    02:40 As you breathe in, pleural pressure decreases.

    02:44 And upon exhalation, it returns back to normal, but notice that it’s still negative.

    02:52 It just becomes more negative during inspiration and less negative during exhalation.

    03:00 How does this work for the pressure within the air sacs themselves? If you got from A to B, you see that there is, during inspiration, lower pressure within the alveoli.

    03:14 This is very important because airflow only occurs via pressure differential.

    03:21 So to get air into the air sacs, you need to have a lower pressure than what mouth pressure is.

    03:28 And if you get a lower pressure, air will then want to travel in from outside the body, into that air sac.

    03:36 During the midpoint of your inspiratory cycle, that is the lowest amount of pressure that you have within the alveolar space.

    03:46 As you return to a resting position whereas denoted here as C, alveolar pressure will be back to 0.

    03:54 When you try to get air out of the air sacs, pressure has to be positive compared to mouth pressure.

    04:01 And positive pressure will create a differential from the alveolar sac to the mouth and air will flow out.

    04:08 So it’s all about creating a pressure differential between the air sacs and atmospheric pressure or what’s outside the mouth.

    04:17 If there’s a negative pressure in the air sac, volume will flow in.

    04:23 If it’s a positive pressure, volume will flow out.


    About the Lecture

    The lecture Respiratory Cycle – Breathing and Lung Mechanics by Thad Wilson, PhD is from the course Respiratory Physiology.


    Included Quiz Questions

    1. More negative (i.e., < -5).
    2. Higher than -5 but still less than 0.
    3. Positive (i.e., above zero).
    4. Intrapleural pressure does not change.
    1. Pleural pressure decreases.
    2. Pleural pressure increases.
    3. Diaphragm is pulled upwards.
    4. Decrease volume in the lung.
    5. The alveolar pressure becomes zero.

    Author of lecture Respiratory Cycle – Breathing and Lung Mechanics

     Thad Wilson, PhD

    Thad Wilson, PhD


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