A Parent's Guide to Respiratory Physiology
TL;DR
The lungs basically do two things - get oxygen in, and get carbon dioxide out
Oxygen is important because at the end of the day it allows our. bodies’ cells to perform cellular respiration efficiently, which means it allows cells to generate energy
The brain controls the depth and rate of breathing based on carbon dioxide levels
Shallow water blackout is a dangerous phenomenon that can occur during breath-holding underwater
Children have much more pliable (compliant) chest walls than adults; this means that their lungs are a little more collapsed and prone to atelectasis (mild collapse of parts of the lung) when they get sick
Introduction
The most common reason children end up in the ICU is because of respiratory failure, which is a fancy way of saying that their lungs can’t get enough oxygen in or enough carbon dioxide out to stay safe and healthy. I know firsthand how frightening it is for parents to watch their child struggle to breathe. While we can't prevent all children from getting sick, I have found that parents who understand a little more about how their child breathes are better equipped to recognize when something is amiss and make healthy decisions for them.
Air Basics
The air we breathe is a mixture of gases, mainly:
21% oxygen (O2) - essential for life, used by our cells to produce energy
78% nitrogen (N2) - important for plant growth but not used by our bodies
0.04% carbon dioxide (CO2) - a waste product that we exhale
Used under Creative Commons license; original image here.
Why We Breathe: Oxygenation and Ventilation
At the end of the day, our cells stay alive because they are able to make energy from food and depend on getting oxygen in and carbon dioxide out to do this
The lungs inhale oxygen (O2) and get it into the blood; getting oxygen in is oxygenation
The heart then pumps the blood from the lungs out to the rest of the body (muscle, brain, other organs) where the cells use oxygen to carry out cellular respiratory (generate energy)
The cells make carbon dioxide (CO2) waste, that goes back into the blood, and returns to the lungs so it can be removed (exhaled); the term for blowing carbon dioxide out is ventilation
Without a constant supply of oxygen and removal of carbon dioxide, cells can’t make energy efficiently which leads to death
Used under Creative Commons License. Original work by domdomegg.
The Urge to Breathe: CO2 is Key
Even though we think about oxygen as the critical thing our body needs, the brain actually continuously monitors carbon dioxide (CO2) levels in the blood to decide when to breathe faster or slower
When CO2 rises above a certain threshold, the brain triggers the urge to breathe - when you’re under water or holding your breath and you feel that incredible feeling in your chest telling you that you want to breathe, this is because CO2 is building up in your blood (not as much because O2 levels are low)
Hyperventilation is not an accidental term!
When you breathe very fast, you might think you’re increasing oxygen levels in your body, but you’re really not - instead you’re decreasing CO2 levels - hence the term hyperventilate (hyper- a lot, ventilate - remove CO2)
So when someone is freaking out, breathing into a paper bag makes a lot of sense - you want to rebreathe the same CO2 that you’re breathing out so that you don’t breathe out too much CO2 and pass out
The Dangers of Shallow Water Blackout
Have you heard of shallow water blackout? It's a little-known phenomenon that can lead to drowning without warning; I think it’s super interesting and explains a lot about respiratory physiology.
Say you’re planning to do a competition for who can swim the furthest under water (hold your breath the longest, or drive through a tunnel without breathing) - one common strategy is to take huge and rapid breaths just before holding because people feel like they can hold their breath longer - this actually does allow you to hold your breath longer, but it can be very dangerous!
A child hyperventilates (breathes rapidly and deeply) before going underwater
Hyperventilation blows off a lot of CO2, reducing the urge to breathe, but like we said above, it doesn’t really increase O2 levels much at all
The child swims underwater, using up oxygen faster than CO2 accumulates
Because their CO2 level is so low to start with, they don’t get that severe urge to surface and breathe before the oxygen levels drop down
Once a critical shortage of oxygen to the brain occurs, the swimmer suddenly loses consciousness underwater and drowns
To prevent this silent killer:
Teach kids to never swim alone
Discourage prolonged underwater breath-holding contests and explain why you shouldn’t hyperventilate prior to doing them!
The Unique Respiratory Physiology of Children
Specific aspects of unique pediatric anatomy are reviewed in a separate post, but I did want to talk about a critical aspect of the infant’s body here - the compliant chest wall!
Infants and young children have much more pliable (compliant) chest walls compared to adults. This means that their chest wall is less rigid and provides less support to keep the lungs expanded.
We all keep our lungs partially inflated even after breathing out; this makes sure that when we breathe in again it will be easy to open the lungs back up. In kids, their compliant chest wall means that their lungs are more prone to collapsing (atelectasis) with each breath, especially when they get sick. To compensate for this, infants have a unique breathing mechanism called laryngeal braking.
Laryngeal braking is when infants briefly close their vocal cords (in the voice box / larynx) at the end of each exhalation. This creates a little extra pressure in the lungs to prevent them from collapsing too much. You might sometimes hear this as a small grunting noise with each breath out. It's a normal finding in healthy babies, but it can also be a sign of respiratory distress.
When children get sick with lung infections like pneumonia or bronchiolitis, they have to work harder to breathe. Their compliant chest wall and tendency for atelectasis puts them at a disadvantage. You may see signs of increased work of breathing like:
Nasal flaring: widening the nostrils to try to get more air in
Retractions: skin sucking in around the ribs, neck, or collarbone
Head bobbing: infants' heads may bob with each labored breath
Grunting: an exaggerated noise at the end of each breath as they try to keep air in the lungs. Grunting can sound like a baby is just fussy or upset, but in the context of illness, it's often a sign they are struggling to breathe and need extra medical support. In the ICU, we often see children "grunt" as they fatigue, signaling they may need more breathing support from a machine.
So if your little one gets sick and you see them persistently grunting with each breath out, it's important to have them checked out right away. Their body is trying to tell us they need some help to keep those lungs open and working well!
When to Worry
As a parent, you are the expert on your child. Trust your instincts if something seems off with their breathing. Some red flags to watch for:
Consistently fast or labored breathing
Persistent coughing or wheezing
Skin color changes (blue/gray lips or fingers)
Retractions or grunting with each breath
Don't hesitate to seek prompt medical care if you notice any of these signs or have other concerns. We would much rather reassure you that everything is okay than miss a serious problem.
Further Reading
American Lung Association - Lung Anatomy and Function Link: https://www.lung.org/lung-health-diseases/how-lungs-work/lung-anatomy-and-function
KidsHealth - Your Lungs & Respiratory System Link: https://kidshealth.org/en/kids/lungs.html
Children's Hospital of Philadelphia - Signs of Respiratory Distress in Children: Signs of Respiratory Distress in Children | Children's Hospital of Philadelphia (chop.edu)
American Academy of Pediatrics - Bronchiolitis and Your Young Child Link: Bronchiolitis and Your Young Child | Pediatric Patient Education | American Academy of Pediatrics (aap.org).
World Health Organization - Children's Environmental Health: Air Pollution Link: https://www.who.int/teams/environment-climate-change-and-health/settings-populations/children/air-pollution