Do you know about our respiratory tract?

Hello everyone.

Just a few weeks ago we have gone through a special day of our lungs.

It is World Lung Cancer Awareness day that was on 1st August, to raise awareness about the threats posed by lung cancer.

But was such a day ever needed?

We all know that lungs are our breathing components. Without it, we would be like…. Well humans without lungs.

So, today we will discuss about lungs. Not its function, yet. I mean, what is a function without its origin and development. Therefore, today, we will learn about its

  • Embryological development
  • Respiratory tract

Along the journey, we will answer some interesting questions.

So, be ready for another interesting article.

Without further ado, lets get on…

Lungs development:

It occurs in 5 stages inside embryo. They are- Embryonic, Pseudoglandular, Canalicular, Saccular, Alveolar. Menmonic- Elf People Can See Air.

  • Embryonic(3 to 6 weeks)- Lungs are developed from lung bud i.e. respiratory diverticulum which is an outgrowth of foregut i.e. future oesophagus. This occurs during 4th week of development. Development pattern goes like this.
    • Lung bud -> trachea -> bronchial buds -> bronchi -> secondary -> tertiary bronchi.
  • Pseudoglandular(6 to 16 weeks)- It is name as such because during this stage, lungs resembles an endocrine gland. In this, terminal bronchioles are formed which are part of conducting airways. Means, if premature baby is born at this stage, baby will be incompatible with life as conducting airways do not exchange air in between blood and outside atmosphere.
  • Canalicular(16 – 26 weeks)-
    • Terminal bronchioles -> respiratory bronchioles -> alveolar ducts.
    • Also, air passage widens.
    • Pronounced growth of vascularisation continues after capillary growth starts in pseudoglandular stage. This is the earliest developmental stage a baby can survive.
  • Saccular(26 – 36 week)-
    • Alveolar ducts -> terminal sacs which is last part of air conduction.
    • Each terminal sac is divided by 1o septae.
  • Alveolar(36 week – 8 years)-
    • Terminal sacs -> alveoli(2o septae present).
    • During this period, immense growth of alveoli occurs called alveolarization. Alveoli number increases from average 50 million to 400 million.

Question for you: Q1. Does a fetus breath inside womb?

A1: Looking above, you might say. Sure! As in end of canalicular stage i.e. 26 weeks of fetal development, alveolar ducts are present which can provide gaseous exchange. But let me tell you. Inside mother’s womb, it is the role of placenta to carry such process. Moreover, placenta carries nutrients and wastage to and from fetus respectively.

So, what happens after 25-26 weeks?

Actually, fetus has breathing capable lungs at the end of 26th week. But during that period, it aspirates amniotic fluid(A1) and not gases. This practice is essential for the baby as it pushes growth of alveolar sacs and lung development.  Just consider an elastic party balloon. Think that you have a balloon that is completely sucked out and is very tightly rolled on itself. You have taken all world’s air into your lungs and rocket forced it into the balloon, yet nothing is happening. So, what you will do is fill it with water at first so that it can stretch as much as it can and then it will be able to get filled with air.

About: A1. Amniotic fluid- It is a yellowish fluid that surrounds the developing fetus. It is present within amniotic sac(fetus-surrounding sac). It serves to work as a cushion, provide antibodies and nutrients to fetus. Right now, we are concerned with its development. Initially, it is produced from components of mother’s body which during 2nd trimester of gestation i.e. 20 weeks, is completely replaced by fetal urine and lung secretions. So it means, proper functional kidneys are also essential for such process. Most of the time we don’t have to worry about kidney’s duty as it completes its development in 12 weeks and starts producing urine.

A2. Oligohydramnios- It is a condition when there is less than normal(5cm to 25cm is normal) amniotic fluid. This can cause lots and lots of harms to baby in different forms. But, why not we stick to our present topic?

After going through its embryological development, we will speak about its 2 important epithelial cells(A3).

These are Type I and Type II pneumocytes/alveolar cells.

Type I alveolar cells:

  • Squamous cells(A3).
  • Covers more than 95% of alveolar surface
  • Lines alveoli for gaseous exchange

Type II alveolar cells:

  • Cuboid cells(A3)
  • Progenitor of both alveolar cells (type I and II). Also perform as stem cell(A4) precursors for proliferation during lung damage.
  • Main function is to secrete surfactants.
  • Surfactant- Consists phospholipids and proteins. Phospholipids > proteins.
    • Reduces alveolar surface tension. This is necessary to reduce the effort to open the airways. Means, higher the surface tension, higher the force needed to open the lungs.
    • Increases compliance. It is degree to which lungs can expand. Therefore, less the tension, more the compliance will be and the more alveoli can expand.
    • This compliance is inverse to lung recoil commonly called elastic recoil. Lung recoil- Tendency of lung to deflate after lung inflation.
    • Synthesis occurs after 24 weeks and achieves adequacy around week 34-35.

About: A3. Epithelial cells- Cells that line our body surfaces and hollow organs. There are 3 types.

  • Squamous cell- Scale-like flat cells. Also found in blood vessels. Ideal for passing substance across, through diffusion and secretion.
  • Cuboidal cells- Symmetrical width and height. Function is to secrete and absorb. Found in salivary gland, nephron etc.
  • Columnar cells- More height than width. Same function as cuboidal cells. Found in digestive tract, bladder, bronchi etc.

A4. Stem cells- These are the cells that can develop and differentiate into other cell types when there is damage or anything. Stem cells in embryo can grow into any cell i.e. pluripotent.

Now let’s understand respiratory tract structure in simple terms.

Respiratory tract (RT) is divided into 2 parts.

  • Upper RT- Nose, nasal passage, paranasal sinuses(A5), pharynx and larynx.
  • Lower RT- Everything left beyond i.e. trachea, bronchial tree and lungs.

A5. Paranasal sinuses-

  • These are air-filled spaces that surround our nasal cavity.
  • There are 4 pairs- Maxillary, frontal, sphenoid and ethmoid according to the bone in which they are found.
  • Provides various functions such as- humidifying and heating air, reducing head weight, providing resonance for loudness and richness.
  • It is covered by pseudostratified ciliated epithelium that secretes mucus(by goblet cells that are present inbetween) to moisturise the inside and trap pollutants, microbes and dirt.

According to physiological role, it is divided as

Conducting zone:

  • Function is to humidify and filter air, warming the air suitable for body’s temperature.
  • From nose to terminal bronchioles.
  • Further divided into large and small airways.
  • Large airways- Nose, pharynx, larynx, trachea and bronchi. Provides high resistance to air passage.
  • Small airways- Bronchioles and terminal bronchioles. Least airway resistance(Q2).

Q2. Why small airways have less resistance?

A2. You may know that cylindrical structures provide resistance inversely to their diameter (Poiseuille’s law). Means, bronchioles should provide more resistance than the bronchi/trachea. And, honestly they do. But as the branching increases down the path, we have huge number of structures in parallel which compensates the total resistance to airflow of large airways. In other words, individual small airway resistance is still more than large airway resistance but the total resistance to airflow is less in small airways.

  • Carries anatomic dead space. It is the volume of air that doesn’t take part in gas exchange. In other words, amount of air that fills when inspired but doesn’t get expired. Amounts average 150 ml.
  • We see transition of cell types. It is like this- From bronchus to start of terminal bronchioles where pseudostratified ciliated columnar cells(A6) are found which beyond the terminal bronchioles get replaced by cuboidal cells.
  • Important for trapping and clearing of debris and mucus from lungs. This is a protective process where foreign irritants or pathogens are removed. It is known as mucociliary clearance/escalator.
  • Airway smooth muscle(ASM) tissue that regulates bronchus tone, is extended till the last of terminal bronchioles.

A6. Pseudostratified ciliated columnar cells- Stratified epithelium contains more than 1 cell layers(contrast to simple epithelium where only one cell layer is present). Pseudostratified cells mean those cells that appear to be multilayered but in actual are not. This pseudo appearance is due to their nucleus that is present at different height, appearing as many layers. These cells contain cilia on their surface that beat in regular rhythm to move fluid or particles in a particular direction. Imagine many boat paddlers who direct boat in a rhythmic manner.

Respiratory zone:

  • Respiratory bronchioles to alveolar ducts to alveoli.
  • As the name says, contributes to respiratory exchange. Means, it is the lung parenchyma(functional component of lungs).
  • Cells are- in respiratory bronchioles(cuboidal cells) to alveoli(simple squamous cells for gaseous exchange).
  • Smooth muscle cells are still found.

Throughout the respiratory system, macrophages are found. These are the first line of defence against pathogenic microbes i.e. provide immune response. These also clear debris.

Now, as we have gone through the respiratory tract. We must go again. We will sum up air passage as we go down.

  • We will enter through nostrils
  • Nasal passage and cavity where some of the air will be filtered and humidified
  • Pharynx (common passage for food and air)
  • Trachea or windpipe (that will divide into main bronchi at 5th thoracic vertebral level. The division ridge is called carina.)
  • Primary bronchi that will enter into lungs through hilum (it also contains many structures entering and exiting lungs)
  • Secondary lobar bronchi
    • In right lung, primary bronchi divides into superior (branches out before entering into hilum), middle and inferior bronchi.
    • In left lung, bronchi divides into superior and inferior bronchi
  • Tertiary or segmental bronchi (you must know each lung has 10 segments)
  • Bronchioles i.e. terminal bronchioles (last conducting part)
  • Respiratory bronchioles  (1st part of respiratory division)
  • Alveolar ducts
  • Alveoli (main site of gaseous exchange)

Q3. Why right side is more susceptible to infection or foreign object introduction?

A3. Right lung is shorter, wider and straighter than left lung. Hence, provides a better path for foreign objects to enter. The end point of object depends on our position and gravity. Eg,

  • Supine position (lying down with face and torso up)- Object will settle in Superior segment of Right lower lobe
  • Upright – Right lower lobe
  • Lateral on right side- Right upper lobe

Wow! Quite a journey. We have gone from outside through our nose to inside our lungs. So, what about lungs. Well, that is for next time.

Comment below if you have any suggestions and query. Later.

Published by signaturedoctor

I am a doctor-to-be pursuing my medical studies. I want to share my knowledge to fellow medical students and to other interested people.

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