Ventilation – Inhalation

Inhalation

Let’s diagram inhalation here.

We’ve got Patm out here, Ppul in here, and Pip over here.  When you inhale, the external intercostalsElevator Between ribs; lifts rib cage during breathing in. move your rib cage outward.  The parietal membrane of the pleural moves outward with the ribsCurved bones forming the rib cage; articulate with thoracic vertebrae and most with the sternum..  For a split second, the intrapleural space expands, increasing in volume.  According to Dr. Boyle, the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. in the intrapleural space or Pip decreases. 

Almost immediately, the visceral pleuraThe double-layered membrane surrounding the lungs and lining the thoracic cavity. is like, hey!  I go where you go!  And the visceral pleura starts to move superficially, closing the gap between it and the parietal pleuraThe outer layer of the pleura that lines the thoracic cavity.. 

Yeah, well, that visceral pleura is ultimately attached to all the connective tissue in the lungs.  So, as the visceral pleura moves outward, it expands all the alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. it can.

 This increases the volume of your alveoli which decreases the pressure in your alveoli.  In factA statement based on direct observation that is repeatedly confirmed., it decreases the pressure below that of the atmosphere. 

Oh!  And now I’ve created a pressure gradient along which air can passively flow.  And it does.

Factors Affecting Inhalation

Inhalation appears to quite simply follow the law of Boyle. By increasing the volume in the lungs we create a pressure gradient down which air can flow. There are a few things that can affect the process of inhalation. In our case study, the patient had bronchoconstriction and mucus production. These issues in the conducting respiratory systemThe organ system responsible for gas exchange (oxygen and carbon dioxide). can limit the volume of air reaching the lungs. As in our patient in the case study, they are young. They are healthy. They are an athlete. There is nothing wrong with their lungs. Their problem is truly a restrictive disease having to do with the delivery of the air to the lungs. There are other diseases that are known as obstructive diseases. These are diseases that struggle with the pressures of inhalation. For example, chronic obstructive pulmonary disease or COPD involves inhaling and exhaling unequal amounts of air. A patient with COPD takes a breath. They don’t exhale that same volume of air. The next breath they take does not have the same volume needed for external or alveolar respirationThe process of gas exchange, including ventilation, external and internal respiration.. This triggers responses by the respiratory system such as coughing and deeply inhaling or inhaling at a faster rate. These responses are triggered mostly by the lack of oxygen. So the deprivation of oxygen in COPD actually drives them to correct their breathing on subsequent breaths.  I only know this next tidbit from watching ER but I guess you don’t give patients with COPD a nebulizer. A nebulizer takes away their low oxygen and effectively takes away their ability to correct there is COPD.

Compliance in Inhalation

Another concept that affects your ability to change lung volume during inhalation is called complianceThe ease with which the lungs expand and contract during breathing.. This also applies to exhalation. Compliance is two-pronged subject. It involves the stretchability of your lungs during inhalation. It also concerns the recoil of your lungs during exhalation.

Stretchability and recoil together are known as distensibility. I put up this picture of someone blowing a bubble out of gum. Originally, I thought that they were blowing up a balloon.  Have you ever had to blow up the cheap balloons from the dollar store?  They are like rubber erasers, thick and unbendable.  You try to inflate one and your ears pop. It takes so much force to blow it up.  So now, it’s been inflated all day in the hot sun at some family picnic. You pop it to deflate it. You throw it away because you know how bad these balloons are for wildlife.  It barely changes shape.  Now, it’s all stretched and won’t go back to its original shape.  Both of these situations describe compliance.  In both analogies, the balloon has low compliance. 

Let’s go over some of the things that influence your compliance in inhalation and inhalation only.  In inhalation, you want some stretch and you want that stretch easy to achieve.

The thoracic cage has to be able to move outward during inhalation. It must also collapse inward during exhalation to achieve high compliance. The external intercostal muscles are between your ribs. When they contract they move the thoracic cage outward. Arthritis in the rib cage can curtail this movementA fundamental property of life involving motion of the body or its parts. of your thoracic cavityThe body cavity housing the heart and lungs.. This would effectively limit your ability to inhale the correct volume of air. Therefore somebody with arthritis of the rib cage would have low compliance. Alveolar surface tension is also a factor of compliance. This concept involves the stickiness or cohesion of waterThe universal solvent essential for life. moleculesGroups of atoms bonded together. on opposite side of an alveolus.  They want to stick or cohere to each other and produce surface tension, which is not good for inhalation.  You have to overcome alveolar surface tensionThe force exerted by the liquid lining the alveoli, which tends to collapse them; reduced by surfact in

Alveolar Surface Tension in Inhalation

Alveolar surface tension refers to the watery nature of your respiratory membrane that you can see. Little water molecules on one side of an alveolus like to cohere. They attach to other small water molecules on the opposite side. This surface tension of these water molecules prevents the alveoli from expanding. More effort by accessory respiratory muscles is required. They overcome the surface tension of the water molecules. This effort inflates the alveoli.

This is where those type 2 pneumocytes or great alveolar cellsThe basic structural and functional units of life. come into play. These cells secrete a substance called surfactantA substance secreted by Type II pneumocytes that reduces alveolar surface tension.. Surfactant is like a detergent, it is both polar and non polar at the same time. Surfactant is visible in this picture as a blue highlight outlining the simple squamous cells. It breaks up the alveolar surface tension. This makes it easier for the alveoli to expand. But what if you don’t make surfactant which is the case in premature babies. Surfactant isn’t made until about two weeks prior to birth. I mean, why would you want to make it? You’re not going to be breathing air for two weeks anyway. So … Premature babies cannot provide enough force to increase the volume of their chests. They cannot overcome the surface tension that is holding their alveoli closed. This is why premature babies will often experience a ventilator. Ventilators essentially blow air into your alveoli forcing them open. 

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