Inspiration and Expiration – Pulmonary Ventilation (Nursing)

00:01 So in the inhalation or inspiration this is going to actually be an active process and it involves multiple muscles.

00:12 In the diaphragm, which is this dome-shaped muscle just below the lungs when it contracts, instead of contracting in which we normally think of when we think of contractions the diaphragm actually flattens out and moves down.

00:28 This flattening and moving down causes it to increase the volume of the thoracic cavity.

00:37 Other actions include the actions of the intercostal muscles.

00:42 So when the external intercostal muscles contract these cause the rib cage to be lifted up and out.

00:50 This also is going to increase the thoracic volume.

00:56 So because the intrapleural pressure is always lower than the intrapulmonary pressure as the thoracic cavity volume increases the lungs are also stretched as they are pulled out with the thoracic cage.

01:11 This causes the pressure inside of the lungs or the intrapulmonary pressure to drop by 1 millimeter of mercury.

01:21 Remember the volume of the thoracic cavity increased so the pressure is going to decrease.

01:28 Because of the difference between the atmospheric pressure outside of the body and the intrapulmonary pressure, air is going to flow into the lungs down its pressure gradient.

01:40 It does this until it reaches equilibrium where the atmospheric pressure and the intrapulmonary pressure are the same.

01:49 During this same time, the intrapleural pressure is also lowered to about six millimeters of mercury less than the atmospheric pressure.

01:58 So again, I'm still pulling the lungs out with the thoracic cavity.

02:04 So to recap the sequences of events that occur during inspiration, the inspiratory muscles are going to contract including the diaphragm descending and the rib cage rising due to the external intercostal muscles.

02:19 This is followed by the thoracic cavity volume increasing.

02:24 The lungs are going to be stretched because the intrapulmonary volume has increased and this causes the intrapulmonary pressure to drop.

02:36 This then causes air or gases to flow into the lungs down their concentration gradient until the intrapulmonary pressure is equal to the atmospheric pressure, or they are at zero.

02:51 So while inhalation is an active process.

02:55 Exhalation or breathing out is a passive process and this process the inspiratory muscles are going to relax and with this the thoracic cavity volume is going to decrease and the lungs are going to recoil with the thoracic cavity.

03:14 The volume decrease causes the intrapulmonary pressure to now increase.

03:20 The intrapulmonary pressure is now greater than the atmospheric pressure.

03:25 So air is going to flow out of the lungs and down its concentration gradient until it reaches equilibrium.

03:34 Now while expiration is normally a passive process we can actively force air out.

03:42 This is referred to as forced expiration and this involves the use of the oblique and transverse abdominal muscles as well as our internal intercostal muscles.

03:56 So to recap the sequence of events that are occurring during expiration.

04:02 Our inspiratory muscles like our diaphragm and our intercostal muscles are going to relax.

04:08 This causes the diaphragm to rise and the rib cage descend due to the recoil of the costal cartilages.

04:17 Also, the thoracic cavity volume is going to decrease as well.

04:24 The elastic lungs are now going to recoil passively and this causes the intrapulmonary volume to decrease.

04:34 The decrease in intrapulmonary volume leads to an increase in intrapulmonary pressure and it rises to about 1 millimeter of mercury.

04:45 And then because of this air are gases are going to flow out of the lungs down their pressure gradient out into the atmosphere until the intrapulmonary pressure is equal to the atmospheric pressure.