Respiratory Cycle – Breathing and Lung Mechanics

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.