Physiology of the Respiratory System

There are 3 main processes in respiration:

1. Pulmonary Ventilation
2. External Respiration
3. Internal Respiration

Pulmonary Ventilation is the process of taking in air from the atmosphere into the lungs (inspiration) and then releasing it back to the atmosphere (expiration). The purpose of this is to take in oxygen which is needed by every cell in the body and to release carbon dioxide which is toxic if it builds up in the body. Normally we inspire and expire air quietly, i.e. passively. In this case, we use only two sets of respiratory muscles the diaphragm and external intercostals. When these muscles contract air is taken into the lungs and when they relax air is expelled. To see the anatomy of the external intercostals watch this (from 1:08-1:50). To see the anatomy of the diaphragm watch this.

These two sets of muscles increase the size (volume) of the thoracic cavity when they contract and they decrease its volume when they relax. To understand how this change in volume causes air to be sucked into the lungs or pushed out, we need to understand Boyle's Law:

P * V = k or in more simply, pressure is inversely proportional to volume. This means that if volume increases, pressure decreases, and vice versa.

Another thing to consider is that the lungs are surrounded by a pleural membrane which is a serous membrane (serous b/c it doesn't open to outside) and this membrane is filled with fluid. This is called the pleural cavity.

The pressure-volume relationship in these 3 spaces determines whether air is inhaled or exhaled.

• Patm = 760mmHg -- the pressure in the atmosphere (always)
  
• Ppul = 760mmHg -- this is the pressure in the lungs at rest (between breaths) when no air is moving
• Pip = 756mmHg -- the pressure in the pleural cavity when lungs at rest (no breathing/air movement)

First lets look at inspiration. When the diaphragm and external intercostals contract and increase the volume of the thoracic cavity they pull on the parietal (attached to thoracic wall) portion of the pleural membrane causing the pleaural cavity volume to increase, and since pressure is inversely proportional to volume this decreases the pressure here from 756mmHg to 754mmHg. Now the volume in the pleural cavity increases not only b/c of the pull from the expansion, but also because the visceral portion (attached to lungs) doesn't move when the thoracic cavity expands, i.e. the lungs resist this expansion. However, soon the lungs feel an increased pressure gradient (Ppul>>Pip) and to reduce this difference, the lungs expand. Since an increase in volume means a reduction in pressure, the pressure in the lungs decreases from 760mmHg to 758mmHg. This causes a pressure gradient or difference between the lungs and the atmosphere, so air moves from the area of higher pressure (Patm=760) to the area of lower pressure (Ppul=758) until the there is no longer a pressure gradient, Ppul=Patm.

Expiration is the same process but in the reverse direction, but keep in mind starting point is the end of inspiration so Pip=754, Ppul=Patm=760. Here the diaphragm and external intercostals relax decreasing thoracic cavity volume --> pleaural cavity shrinks to resting size, Pip becomes 756 --> lungs feel pressure gradient and shrink to resting, Ppul becomes 762 --> pressure gradient between atm and lungs causes air to move out till Patm=Ppul=760.

Forced inspiration and expiration are active processes and recruit additional muscles. In forced inspiration the sternocleidomastoids, scalenes, and pectoralis minors are recruited and these all contract to further increase the size of the thoracic cavity causing even more of a pressure gradient and therefore more air to be taken into the lungs. In forced expiration besides relaxing the diaphragm and external intercostals, the internal intercostals, and abdominal muscles are contracted. The internal intercostals pull the ribs down (opposite action of externals) and the abdominal muscles compress the abdomen pushing viscera up into the diaphragm. This causes and even greater decreases in thoracic cavity size causing even more of a pressure gradient for air to move out of the lungs.

Clinically Significant Terms and Concepts

• Pulmonary or Lung Compliance = change in volume / change in pressure
  
• Elastic Recoil
• Airway resistance
  

Compliance is the ability of your lungs to stretch and expand (increase in volume) when a change in pressure occurs (think inspiration). The more a pair of lungs can expand for a given pressure change the more compliance they have. Whereas recoil is the ability to return to resting size after expansion or stretch (think expiration).

Healthy lungs are very stretchy and can recoil easily because they have elastic connective tissue and surfactant (slippery fluid) on the alveoli. Diseases such as Mesothelioma (lung disease due to asbestos) the lungs become stiff as asbestos fibers damage the connective tissue and their compliance is reduced making it difficult to breathe. In another illness called respiratory distress syndrome, premature/preterm babies (usually less the 7months gestation) have little or no surfactant causing their alveoli to collapse and thus reducing their lung compliance. This means it is very difficult for them to inhale and expand their lungs, this can lead to exhaustion and even death.