MCAT Basics Podcast

Respiratory System

Sam Smith reviews the basics of the Human Respiratory System. He outlines the anatomy, the functions, and how the entire system is controlled.

  • [1:24] The Anatomy of The Respiratory System
  • [2:08] The Parts of the Upper Respiratory System
  • [4:55] The Parts of the Lower Respiratory Tract
  • [10:32] The Functions of the Respiratory System
  • [11:56] Gas Exchange
  • [31:46] Thermoregulation
  • [37:10] Particle Filtration
  • [40:26] pH Control
  • [42:15] How the Respiratory System is Controlled

The Anatomy of The Respiratory System

The respiratory system can be divided into two regions: (1) The Upper Respiratory Tract and (2) The Lower Respiratory Tract.

The upper respiratory tract is composed of several structures including the nose, the nasal cavity, the sinus, the larynx, and the trachea.

The Upper Respiratory Tract

The nose is the structure that is located mainly on your face. It is found directly in front of the nasal cavity.

Within your nasal cavity, are your sinuses. These are hollowed, air pockets located on each side of the nose, and on your forehead. One of its functions is mucus secretion.

The   Larynx, or the voice box, on the other hand, is an organ located at the top of the neck. It is involved in breathing, producing sound and speech, and protecting the trachea and lower respiratory tract from food particles.

Lastly, the trachea (Windpipe) is a tube connecting the larynx and the lungs. One of its functions is to moisten air before it enters the lungs.

The Parts of the Lower Respiratory Tract

The lower respiratory tract is, essentially, the lungs. The lungs are composed of three main components: the alveoli, the bronchi, and the bronchioles.

The lungs are the main organ of the respiratory system. Its function is to transport oxygen into the blood and remove carbon dioxide from it.

The alveoli are small functional units of the lungs that allow the exchange of gases. It is located at the end of the bronchioles.

The Bronchi are small branches of tubes from the trachea into the lungs.

The Bronchioles are smaller branches of tubes connected from the bronchi that lead directly into alveoli.

The Functions of the Respiratory System

The respiratory system has five main functions:

  1. Gas exchange
  2. Thermoregulation
  3. pH control
  4. Particle filtration
  5. Speech production

Gas exchange function is the most important function. It removes carbon dioxide from and oxygenates the blood. The second function is thermoregulation. It helps control our body temperature, (e.g., cool down body temperature when it is too high). The third function is it helps balance body pH – acid-base regulation. It also functions for sound or speech production. Lastly, it helps to filter the air that we breathe in.

Gas Exchange

For gas exchange to occur, the first thing you need to do is pull in air to your lungs and breathe out. The breathing process starts in the diaphragm, a dome-shaped, thin-sheet muscle located right beneath the lungs. Upon its contraction, it moves from being dome-shaped to flat. This enlarges the chest cavity and increases its volume. When the diaphragm relaxes, it goes back to its original shape and the chest cavity shrinks and its volume decreases.

The intercostal muscles that are located within the ribs also help in breathing. They expand or contract to further up the volume of the chest cavity and the lungs.

When your lungs are at rest after an exhale, the pressure within your lungs is equal to atmospheric pressure (760 mm Hg or 1atm). As you inhale, the volume in your lungs increases. As a result, the pressure of the gas inside your lungs decreases.

Boyle’s Law explains this. It says that there is an inverse relationship between the pressure of a gas and the volume that the gas is contained within. If the volume thereby increases, the pressure of the gas decreases.

Surface Tension and The Alveoli

The alveolar lining is coated by a watery substance. Water is of course a very polar molecule. It has two charged regions: a partial negative charge and a partial positive charge. As a result, water molecules can form strong intermolecular attractions – hydrogen bonds. Surface tension is a phenomenon that occurs at water-air interfaces because of hydrogen bonding. Basically, the water molecules pull hard on each other, but not on the air.

Since water lines the inner surface of the alveoli, surface tension from this water pulls the alveoli inwards. Without a mechanism to counter this force, the alveoli would collapse.

The Laplace law explains this further. It says that the smaller a spherical structure is, the more pressure it’s going to feel from surface tension. Since alveoli are very small, they feel a ton of surface tension from the fluid that coats their inner surface.

Pulmonary surfactants (surfactant = surface acting agents) are what counteracts the surface tension to prevent the alveoli from collapsing. These surfactants are made up of 90% lipids and 10% protein. Mostly, it’s made of phospholipids. These molecules in pulmonary surfactant separate individual water molecules so they cannot hydrogen bond as well and thus do not pull inwards with the same force.

The Elasticity and Compliance of The Lungs

Compliance is defined as the lungs’ ability to stretch and expand. Elasticity is very much like compliance. One thing that differs it from compliance is that elasticity means that the lungs can spring back to their original shape without being distorted. Elasticity and compliance is aided by a protein called elastin. Elastin is a 70kDa protein that crosslinks with other elastin proteins to form an insoluble polymeric protein known as an elastomer. The elastomer is part of the extracellular matrix that surrounds the individual alveoli.

There are a few diseases that affect the compliance and elasticity of the lungs. One is Chronic Obstructive Pulmonary Disease (COPD) which is characterized by more compliance within the lungs. In other words, the lungs get more stretchy than usual. Another is Fibrosis, the scarring of the lung tissue or the hardening of the lungs. This leads to less compliance within the lungs, therefore making them less stretchy.

Gas Exchange: Oxygen and Carbon Dioxide

Oxygen is required for cells to produce energy based on the oxidative-phosphorylation pathway.

On the other hand, carbon dioxide is produced during metabolism. This is something that needs to be excreted because too much of it can acidify the blood.

Altogether, we need to get oxygen in our system and carbon dioxide out. This is the overall goal of gas exchange and it happens within the alveoli.

The Microanatomy of the Alveoli

The alveoli are lined with simple squamous epithelium, a line of thin, flattened cells. It’s composed of a single layer of cells surrounded by an extracellular matrix that has a little bit of elastin protein in it. A big network of capillaries is also found surrounding the alveoli.

This anatomy (single layer of cells) allows for gas molecules to freely diffuse from the lungs into the capillaries surrounding each individual alveolus. This also happens vice versa; gas can travel from the capillaries into the lungs where it gets breathed out.

Partial Pressure

Partial pressure is the driving force that pushes oxygen into the capillaries and carbon dioxide out of the capillaries. These gases strictly follow their own partial pressure gradient. They move from areas of high partial pressure to areas of low partial pressure.

Partial pressure is defined as the pressure of individual gasses in a mixture.

Oxygen has high partial pressure in the gas that’s inside the alveoli. On the other hand, it has a relatively low partial pressure in the capillaries surrounding the alveoli. The difference in their pressure is the driving force that moves oxygen from the alveoli into the capillaries. The opposite happens to carbon dioxide.

 What happens when gas enters your bloodstream?

The gas that enters your bloodstream dissolves in your blood.

Henry’s Law tells us that the amount of gas that dissolves in a liquid is proportional to the partial pressure of the gas that is above that liquid.

Therefore, the amount of oxygen that dissolves in the blood is proportional to the partial pressure of oxygen that is in the blood.

Since the partial pressure of blood in the pulmonary veins is about 100 mm Hg (high), there is going to be a lot of dissolved oxygen in the blood. That dissolved oxygen is going to become attached to hemoglobin. Hemoglobin can then carry and deliver the oxygen to tissues around the body.


The respiratory system helps play a role in maintaining our body temperature.

Thermoregulation is the process in which your body maintains an acceptable body temperature. Your body temperature usually ranges from somewhere 97°F (36.1°C) to 99°F (37.2°C).

In humans, thermoregulation occurs through respiration and evaporation.

Evaporation is basically a phase change of liquid to gas.  For evaporation to happen, you need to have heat. This means for evaporation to occur; your sweat must have enough heat to change its state from liquid to gas. What does that do? It draws out a lot of heat from your body.

The respiratory system plays only a little role in thermoregulation as compared to sweating in humans. It only helps you remove warm air from your body to help cool you down.

Particle Filtration

Particles that come inside the respiratory system may not be good for you. That is why we have a particle filtration mechanism that allows us to counteract or prevent particles from entering our lungs or external passages.

External particles may possibly get deposited in the alveoli and block gas exchange. Also, you may breathe in bacteria, viruses, and other pathogens.

There are two ways that the respiratory system filters particles:

  1. Nasal hairs are hairs inside the nose that filters large particles (dust, dirt)
  2. The mucus-cilia system is the combination of mucus and hairlike projections that lines the walls of the respiratory tract. This system is responsible for filtering smaller particles.

pH Control

Blood pH refers to how basic or acidic your blood is.

When your blood pH is too high it means your blood is too basic. This can lead to a condition called alkalosis.

When your blood pH is too low it means your blood is too acidic. This can lead to a condition called acidosis.

These are the reasons why your body needs to maintain a constant pH level. The normal values are usually at 7.35 and 7.45.

The respiratory system helps maintain the blood pH by removing excess carbon dioxide in the blood that lowers the blood pH. When carbon dioxide dissolves in the blood it is readily converted to carbonic acid – a compound that can increase the concentration of H+ ions in the blood. 

How the Respiratory System is Controlled

You involuntarily breathe all the time – whether you’re sleeping, awake, walking, or whatever you’re doing. You’re not trying to breathe. It’s just happening. However, there is a voluntary override in the system. This happens when you’re swimming or when you have sleep apnea where you hold your breath.

So, the respiratory system involves involuntary contractions and voluntary contractions. There are not a lot of organs that can do this.

The breathing centers of the brain are in the upper part of the brain. That includes the medulla oblongata and the pons.

The medulla oblongata controls muscles that cause breathing like the diaphragm and the intercostal muscles. The pons on the other hand controls our breathing rate. The brainstem is involved in many involuntary functions. This makes sense how the respiratory system is controlled in the brainstem.

Voluntary breathing, on the other hand, is controlled by the cerebral cortex.

Sam Smith

Sam completed his Bachelors of Science in Chemical and Biological Engineering at the University of Colorado Boulder. Following his graduation, he worked at the National Institutes of Health Vaccine Research Center studying HIV. Meanwhile, with a microphone in his garage, Sam founded the MCAT Basics podcast. The podcast has grown to become the top rated MCAT podcast on iTunes. In addition to podcasting, Sam enjoys the outdoors, sports, and his friends and family.

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