Respiratory System Introduction

Next →
Author: Dr Peter de Souza
Last modified: 28 December 2020


Part 1 - Nose to Bronchi

Okay.    This tutorial is an introduction to respiratory anatomy.    The purpose of the respiratory system is essentially to deliver oxygen to the cells of the body and to remove carbon dioxide.    We’ll just from the start from the outside and work our way in. We'll start at the nose and mouth and work our way in towards the lungs and we'll talk about some of the structures which make up the respiratory system.

Air first enters into the body through the nose and the mouth.    The nostrils contain hair which filter the air. There's also a mucous membrane which lines the nose and the nasal cavities, which helps to warm and moisten the air as it passes through.


And then when the air passes through up into the nostrils and into the nasal cavities, so you could see these nasal cavities here, it passes into the pharynx.    Just behind the nasal cavity is the pharynx, which extends down to the larynx and the esophagus.


The other pathway for air is through the mouth. The air enters into the mouth and passes posteriorly into the pharynx.


The pharynx is this muscular tube posteriorly behind the nasal cavities and the mouth. It joins the nasal cavities and the mouth to the esophagus and the larynx below.


You’ve got three parts to the pharynx. You've got the nasopharynx at the top, the oropharynx, which is just behind the mouth and the laryngopharynx, which sits inferiorly.


If I just rotate around, you can see the esophagus sitting posteriorly. You can actually see the thyroid here. I'll just remove that and we'll take a look at the larynx.


The larynx is these set of structures here. It's also known as the voice box. It sits just behind the hyoid bone (below the hyoid bone). You could see the hyoid bone just here. You can see this set of structures, which is the larynx. This is important in voice production and also allowing air into and out of the lungs.


The larynx consists of several cartilages. You can see this big one here. This is the thyroid cartilage. And in males, this forms the Adam's apple. I'll do a separate tutorial on the larynx because there's quite a lot to learn.


The larynx connects the laryngopharynx above to the trachea below.    The trachea is this long pipe and it connects the larynx to the lungs. You can see on the trachea, there's these kind of rings. The trachea has these c-shaped cartilaginous rings, which help to form its structure.


If I just rotate around, you can see the esophagus behind.    The c-shaped rings, there's cartilage anteriorly and laterally on the trachea. And posteriorly, the trachea consists of smooth muscle.


Part 2 - Bronchial Tree and Lungs

The trachea then splits into two. It splits into a right and a left primary bronchus.    'Bronchi' is plural and 'bronchus' is singular.


I'll just remove one of the lungs so we can take a look at the bronchi. The place where the bronchus enters the lung is known as the hilum. At the hilum, there are a couple of other structures which enter the lung.


I've just brought the cardiovascular system and I'm going to rotate the model around posteriorly and you can see the other structures that enter the lung.    You’ve got the pulmonary veins and the pulmonary arteries which enter the lung at the hilum.


I've just switched to this diagram which shows the exact same view we're looking at. We're looking at the posterior view, so you can see the trachea coming down here branching off into the left and the right main bronchi. You can see the pulmonary arteries at the top and the pulmonary veins inferiorly entering into the lung at the hilum.


You’ve got the pulmonary arteries in blue carrying deoxygenated blood from the heart, to the lungs, to receive oxygen and you've got the pulmonary veins returning oxygenated blood from the lungs to the heart.


This diagram here just shows the hilum in a bit more detail. You can see the structures entering this area called the hilum of the lung.


Just coming back to the bronchi, once they've entered the lung, they divide further. They divide into lobar bronchi, which are also known as secondary bronchi. And the lobar bronchi divide further into segmental bronchi.


You’ve got the trachea dividing into the primary bronchus.    You’ve got a right and left primary bronchus or main bronchus. Then the primary bronchi divide into lobar bronchi, which are known as secondary bronchi. And the secondary bronchi divide into tertiary bronchi, which are segmental bronchi.


The bronchi just keep dividing and they end up in bronchioles, which are smaller and lack cartilage. And then these bronchioles eventually form alveolar ducts, which leads to alveolar sacs and form alveoli, which are responsible for gas exchange and they have a huge surface area for diffusion, have a rich blood supply and very thing, but we'll talk about that in another tutorial.


For now, just remember that the trachea divides into the right and left main bronchi, which divide into secondary bronchi, which divide into tertiary bronchi.


The secondary bronchi are called lobar bronchi because they supply the lobes of the lung with air. The right lung, which is this lung here, has three lobes. You've got a superior lobe, a middle lobe and then an inferior lobe. These lobes are separated by fissures.


On the right lung, you've got a fissure, which separates the superior and middle lobes and this is called the horizontal fissure. And you've also got an oblique fissure because it runs obliquely and this separates the middle from the inferior lobes and also the superior and inferior lobe at the back.


On the other side, the [left] lung, it only has one fissure.    It’s only got an oblique fissure, which separates a superior and inferior lobe.


Now that we know how many lobes there are in each lung, we know how many lobar bronchi there must be. There are only three right lobar bronchi and there are only two left lobar bronchi because there are three lobes on that right and two lobes on the left.


These lobes can actually be further divided into what is known as bronchopulmonary segments. Each lung has ten bronchopulmonary segments. These bronchopulmonary segments are supplied by the segmental bronchi, so the tertiary bronchi.


The lobar bronchi supply the lobes of the lungs and the segmental bronchi supply the bronchopulmonary segments.


You can see in this diagram the three lobar bronchi on the right and the two lobar bronchi on the left.    You can see them in different colors. You've got the superior in green; yellow, you've got the middle lobar bronchus; and in blue, you've got the inferior lobar bronchus. And you've got the superior and inferior lobar bronchus in green and blue on the left side.


The lungs themselves are surrounded by a pleural cavity. You can see the cavity here in pink.    The pleura or the pleural cavities are a serous membrane which line the lungs. There’s a visceral pleura and a parietal pleura.    The visceral pleura lies very closely to the lung and adheres closely to the lung tissue. The parietal pleural lines the thorax. Both these pleural layers are continuous at the hilum.


The visceral pleura and the parietal pleura is the pleural cavity. This contains pleural fluid.    This space between the visceral and the parietal pleura is only a potential space.    In normality, these layers lie in very close contact and you've got this thin layer of pleural fluid.


I've just switched over to this cross-section of the lungs. You can see anteriorly up here, posteriorly down here. You've got the left lung and the left lung. We'll just take a look at the – so you can see the pleura in this diagram.    You can see at the hilum, these two layers are continuous.    You’ve got the outer layer, which is the parietal pleura, which lines the thorax. And then the visceral pleural lies very closely to the lung and it goes into the fissures and everything like that. And then it is continuous with the parietal pleura at the hilum.


this area, remember, where all these structures enter – the bronchus, the bronchi, the pulmonary arteries and pulmonary veins entering at the hilum.    You can see how the viscera and parietal pleura are continuous with one another. And then you've got this space between the two layers, which is the pleural cavity, which contains pleural fluid.


That’s a very basic and broad overview of some of the structures in the respiratory system.