Exhalation

Exhalation

Before we begin, let’s remind ourselves of some abbreviations. We use them to indicate partial pressures during this process of ventilation. Patm Is the abbreviation that we will use for the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. in the atmosphere.  actually the pressure in the atmosphere is about 760 mmHg.  when we talk about atmospheric pressure in relationship to ventilation we usually set atmospheric pressure at 0 .  This helps us easily determine whether the pressures in the alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. are higher or lower than atmospheric pressure. It helps us find out if the pressures in the intrapulmonary space are higher or lower than atmospheric pressure. We also determine the state of the pleural space compared to atmospheric pressure. It allows us to easily use positives and negatives. We do not always have to subtract 760 from whatever number you get.

We also defined Ppul as the pressure inside of your alveoli. Then to make things even more complicated we added Pip which is the pressure in your intrapleural space. However, we remember that the intrapleural space will not be gaining any gases during ventilation. It is used to create a pressure difference.  

Let’s quickly diagram exhalation.  In exhalation, the diaphragm moves up and it relaxes, making the thoracic cavityThe body cavity housing the heart and lungs. smaller. The external intercostalsElevator Between ribs; lifts rib cage during breathing in. also relax, making the thoracic cavity smaller.  Your alveoli are squishy and porous. They are inflated with a large volume. They recoil because of those elastic fibers. These movements involve the primary musclesThe diaphragm and external intercostals, responsible for quiet breathing. of respirationThe process of gas exchange, including ventilation, external and internal respiration..  Remember, the diaphragm and intercostals are not contracting here.  They are passively relaxing. Because the alveoli get smaller, we know, via Boyle’s Law that the pressure in the alveoli will rise.  It will rise enough over the pressure in the atmosphere and create a gradient.  Air will flow down that gradient in exhalation.  For both inhalation and exhalation, you are experiencing volume changes in your thoracic cavity.  These changes are creating gradients, down which air will flow.  It’s all about the gradients!

Alveolar Surface Tension in Exhalation

During exhalation alveolar surface tensionThe force exerted by the liquid lining the alveoli, which tends to collapse them; reduced by surfact is a good thing it contributes to your lungs complianceThe ease with which the lungs expand and contract during breathing.. Its contribution increases the ability of your alveoli to collapse. This collapse lowers their volume and increases the pressure in the alveolus or Ppul. It creates a gradient down which air can flow out of the lungs.

To reference the example I used in inhalation about a baby being on a ventilator. The ventilator forcefully expelled air to expand the lungs. There is not really a need for the ventilator to control the collapse of the baby’s lungs. The alveolar surface tension will take care of that collapse. But since this is a baby, they don’t have much ability to breathe actively or forcibly. They cannot overcome the alveolar surface tension.

Alveolar surface tension seems to be negative when we’re talking about inhalation. It is a force that must be overcome to inhale. However, alveolar surface tension is quite important in exhalation. It is one of the forces that encourage your alveoli to collapse passively during exhalation. So I think it’s a love-hate relationship here with alveolar surface tension. You want just the right amount, not too much, not too little.

Compliance in Exhalation

Scleroderma is an autoimmune disease in which auto antibodies attack connective tissues of the body . One of the main areas of attack are the elastic tissues in the lungs. As the disease progresses the patient is unable to expand their lungs to the correct volume for inhalation. Their lungs are also incapable of recoiling during exhalation to expel all of the carbon dioxide rich air.

A patient with this disease experiences low compliance for both inhalation and exhalation.  Overall, the distensibility of their lungs is low. They can’t inflate. They refuse to go back to their original shape when they do inflate.  But, recognize that one aspect of ventilation can be different in inhalation and exhalation.  Alveolar surface tension is a great example.  You want it in exhalation, you don’t want it in inhalation.