Hello and welcome! My name is Dr. Alexandra Kapelovic and today we will be discussing the process of ventilation. Ventilation is a mechanical process of air inhalation and exhalation. Typically it occurs at 12 to 20 times per minute at rest.
We can divide ventilation into quiet and forced. Quiet ventilation occurs during sedentary activities. and requires low metabolic demand. Force ventilation occurs during strenuous activities and typically requires rapid and voluminous exchange of air. Ventilation is driven by active and passive forces.
The change in intrathoracic volume drives the change in air pressure. Let's review Boyle's law. Given a fixed temperature and mass, we find that there is an inverse relationship between the volume and the pressure of gas.
Therefore, we know if there is an increased intrathoracic volume, we will see a decrease in the pressure of gas. So let's apply this to inspiration and expiration mechanics. During inspiration, we see an increase in intrathoracic volume. That happens by contraction of the muscles that attach to the ribs and sternum. As the thorax expands, the pressure within the intrapleural space, which is already negative, is further reduced, creating suction that expands the lungs.
Resulting expansion of the lungs reduces alveolar pressure. Below atmospheric. This draws the air from the atmosphere to the lungs.
Quiet expiration is typically a passive process, meaning it requires no muscular activity. The process is exactly the opposite of inspiration. A decrease in intrathoracic volume will cause an increase in intrapleural pressure.
Lungs will recoil. We will see an increase in alveolar pressure. And once again, the pressure will go from high concentration to low concentration.
Therefore, it will flow from alveoli to the atmosphere. Let's examine the axis of motion of the rib cage. Specific path of movement of ribs depends on the shape and spatial orientation of axis of rotation.
The length, shape, and downward angle of each rib is unique and therefore the axis of rotation for each rib is slightly different. Axis of rotation of the upper ribs lie closer to the frontal plane as illustrated in the first image on the left. You can see that this is closer to the frontal plane orientation. This will allow motion predominantly in the sagittal plane.
This movement is typically referred to as a pump handle movement, demonstrated in the first image on the right. Pump handle, you can see how the pump handle moves from inferior position to a superior position. The same thing happens to the upper ribs.
When the sternum moves from inferior position to a superior position. Now let's examine the axis of rotation of the lower ribs. The axis of rotation for the lower ribs move towards the sagittal plane.
You can see how the orientation changes closer to the sagittal plane. This allows for motion of those ribs closer to the frontal plane. So if the axes are anterior to posterior, we know that the that the movement will occur in the frontal plane. In the lower ribs, this movement is referred to as a bucket handle movement associated with the movement of the bucket handle.
It moves more from inferior to superior in the lateral direction. So here in the superior rib cage, we see predominant movement in the sagittal plane While in the inferior portion of the rib cage, the predominant movement is in the frontal plane. Let's take a look at the biomechanics of inspiration. I divided it into a few different components. Let's start with ribs and take a look at how the body of the rib moves during inspiration.
The body of the rib will typically move through elevation whether it's upper or lower ribs upper ribs as i mentioned in the previous slide go through a mechanism of pump handle moving into interior and superior direction lower ribs move through the mechanism of bucket handle illustrated right here this means that the ribs will move in the lateral and superior direction the bucket handle movement will increase intra-thoracic volume this will increase anterior to posterior diameter demonstrated in this image right here as well as medial to lateral diameter once again going back to our mechanics of inspiration increasing intra-thoracic volume will allow all these processes to occur so it's very important that the ribs follow proper axis of movement and actually the biomechanics of movement. Now let's take a look at different part of the rib, the costoveterial joint. This is the joint demonstrated right here. As you can see from this zoomed in image, with inspiration the costoveterial joints move through a posterior rotation. Now not demonstrated in these images is the movement of sternum.
You can palpate your own sternum and see how it moves with inspiration and expiration. It's pretty simple to feel. With inspiration, you will see that the sternum moves in interior and superior direction.
And finally, with inspiration, forced inspiration, the last thing you will find is that thoracic spine will move into extension. The Z-joints of thoracic spine will move into closing. Now, biomechanics of expiration.
The muscles will relax. Ribs and sternum will return to their pre-inspiration position. We will see the lowering of the body of the rib and the sternum will return to its original position.
It will move through inferior posterior movement. Now, let's take a look at muscles responsible for inspiration. We have three muscles responsible for inspiration. It is the diaphragm, the scalenes, and their intercostals.
Let's start with the diaphragm. diaphragm is the primary muscle responsible for inspiration it performs 60 to 80 percent of ventilatory process work during quiet inspiration and take a look at this image right here follow the arrows down and we will notice that the dome will drop about 1.5 centimeters down towards the lumbar spine During forced inspiration, the dome will typically drop about 6 to 10 centimeters down. What it does, it increases the intrathoracic volume in all three diameters.
Vertical, medial and lateral, and anterior and posterior. So let's take a look at how the muscle actually works. We remember that the muscle can work, can contract from proximal to distal.
or from distal to proximal. It really depends which part is being stabilized. First, diaphragm flattens down towards the ribs.
So when the lower ribs are stabilized by the abdominal muscles here, the dome of the diaphragm, illustrated in this picture, drops down, flattens. This will increase the vertical diameter of the ribcage. abdominal resistance will stabilize the dome inferior after the first step this resistance and stabilization will allow diaphragm to move its distal attachment and elevate the lower six ribs so the arrows will go into the opposite direction the lower six ribs will be elevated while the proximal portion of the diaphragm will be stabilized The elevation of the lower six ribs will increase anterior to posterior and medial to lateral diameters and that we examined with the biomechanics process earlier right here.
So increase in the medial to lateral and anterior to posterior diameter diameters. Now let's take a look at scalenes. We have three scalene muscles anterior, middle and posterior.
They all start from the transverse processes from C3 to C6. Anterior and middle scalenes attach to the first rib, while the posterior scalene attaches to the second rib. Remember that brachial plexus courses between the anterior and middle scalenes.
Hypertrophy, spasm, or excessive stiffness of these muscles can compress brachial plexus. and cause motor or sensory disturbances in the upper extremity. The function of the scalenes, just like we examined the function of the diaphragm, depends on which skeletal attachment will be stabilized. So, here, if we have cervical spine stabilized, it will drive elevation of the upper ribs and the sternum, creating inspiratory movement.
This will increase intrathoracic volume and allow the lungs to expand and once again going back to our original process of inspiration increasing the intrathoracic volume and allowing the air to flow from atmosphere to the lungs the opposite is true when we stabilize the first two ribs inferior attachment of the scalenes the scalenes are able to laterally flex cervical spine Okay, and the last two muscles of inspiration are our intercostal muscles. Conventionally, it's thought that the external intercostal muscles drive inspiration, while internal drive expiration, forced expiration. Recent research has actually divided internal intercostal muscles into two separate fiber divisions. parasternal fibers and interosseous fibers.
Parasternal fibers occupy the region of the sternocostal joints, so closer to the sternum and the cartilage. Interosseous occupy more of the lateral and posterior lateral spaces. So now we know that the external intercostal muscles is the primary muscle of inspiration together with the diaphragm and the scalenes.
Internal intercostal muscles, especially parasternal fibers, join the external intercostal muscles in the process of inspiration, while the interosseous fibers, once again located posterior and lateral, are responsible for forced expiration.