Azrial's Anatomy: Today, We Learn About the Respiratory System

Azrial's Anatomy is a series of apprehensible lessons in human anatomy. These lessons will cover the anatomy of organs, briefly describe functions and some diseases. Italic text in parentheses are medical term.

This lesson covers the respiratory system.

The function of the respiratory system is to bring oxygen to the blood and to remove unwanted gasses (e.g. carbondioxide CO2) from the blood. The main organs in the respiratory system are the lungs (pulmones). We will get to these, but first I will describe the airways. The airways are split into the upper and the lower airways.

Upper Airways

The upper airways consist of the mouth, nose, sinuses, and larynx (above the vocal cords).

There is not much to say about the mouth and nose. The triangular part of the nose that is visible on the face has hairs on the inside. These hairs catch the largest particles one might inhale and protect the airways further down. Inside the nose (a cavity that can't be seen from the outside) are three folds on each side (conchae nasales; "the nasal mussels"). When the air flows past these folds (made of bone and mucosa) it is heated and humidified. The nasal cavity is connected to the sinuses.

The sinuses are hollow areas in the bones of the skull that are connected to the nasal cavity. They are lined with mucosa just like the nose). There are three sinuses on each side of the head and a group of small sinuses in the middle. The sinuses are named after the bones they are located inside. People are still discussing the purpose of the sinuses, but some things they agree upon: The sinuses are related to our voice and speech (due to resonances) and they reduce the weight of the skull.

Sinus maxillares are the largest of the sinuses. They can be found next to the nose. Due to anatomical characteristics (that I won't bore you with), these are the ones most likely to be infected (sinusitis).

Sinus frontales are located above the nose.

Toward the back of the nose are the sinus sphenoidales.

The group of sinuses is called sinus ethmoidales.

(Sinus locations; picture from chronicsinusdisease.com)

The larynx is a tube of muscle, cartilage and mucosa. Air flow through here. The larynx is the home of the vocal cords, which form sounds when air passes by.

Lower Airways

The lower airways consist of the larynx (below the vocal cords), the trachea (or "wind pipe"), the bronchi, bronchioli and alveoli.

As stated above, the larynx is a tube for conducting the air onwards to the trachea.

Trachea, the bronchi and bronchioli are all tubes made of varying amounts of muscle, cartilage and mucosa.

The trachea branches into two main bronchi. These branch into more, smaller bronchi. With each branching, more and smaller tubes form (at some point, they get so small that they are called bronchioles instead of bronchi). Once the branches reach bronchioles, there is no cartilage to be found in the tubes.

After travelling from the nose, through the larynx, trachea, bronchi and bronchioli, the air reaches the alveoli. These are tiny, hollow "beads". This is the final part of the airways, and it is here the exchange of gasses between blood and air takes place. There are MANY alveoli in the lungs. In fact, the combined surface area of the alveoli approximates that of a tennis court (wow, huh?)

(Alveoli; picture from Flickr).

Asthma (asthma bronchiale) can be triggered by allergies or physical exercise.The disease is chronic, but comes in attacks where the patient has trouble breathing and often gets a characteristic wheezing breathing. The chronic part lies in the airways being inflamed (thickening the walls). When an attack is triggered, the muscles in the airways contract, reducing the space inside the airways. This in turn makes it difficult to breathe.

Interesting fact: During an asthmatic attack, the patient actually has difficulty exhaling; not inhaling.

Protecting the Airways

On the cells that lines the inside of the airways are tiny, hair-like folds called cilia (singular = cilium). These cilia are surrounded and covered by a viscous slime, that some particles (e.g. pollen) stick to. The cilia constantly move in the same pattern: a swift brush in the direction of the mouth and a slow brush in the opposite direction. This pushes the slime (with whatever particles have been caught) up to a point from where you can remove it by coughing / hawking.

Cystic fibrosis is a genetic disease in which the cells producing the slime are unable to do so properly. The slime becomes too thick and the cilia can't shove it upwards. This means that the airways are not as well protected and patients experience multiple lung infections (among other things). The disease also affects other organs (such as kidneys, intestines and pancreas).

Lungs

The bronchi, bronchioli and alveoli are all part of the lungs. Here I will describe physical characteristics of the lungs (e.g. shape and location).

The lungs are shaped like two rounded triangles and are located in the chest (thorax) on each side of the heart.

The uppermost, pointed part (apex) is situated behind the collar bone (clavicula) and the bottom (basis) rests on the diaphragm (see below). The front, back and outer sides of the lungs follow the shape of the rib cage. The inner sides have a close relation to the structures between the lungs (heart, aorta, other large blood vessels). The right lung is a little larger that the left.

The size and shape of the lungs vary according to shape of chest, height and other factors. Their colour changes with age; a newly born child will have light red lungs, and adult will have darker red lungs. The colour comes in part from the huge amount of blood that can be found in the lungs' blood vessels. Smoking and breathing traffic smoke can cause coal to deposit in and on the lungs giving them a darker and even grey / black colour.

Surrounding each lung is the pleura. This consists of two thin sheets with an extremely thin layer of water between them. Without getting into too much detail, the water makes the two pleura-sheets stick together while still allowing them to slide on each other.

So by the function of the pleura and water, the lungs are "glued" to the inside of the chest wall.

The diaphragm (diphragma) is a flat muscle, shaped like a dome. It has its origin at the ribs, spine and chest bone (sternum). The muscle does not insert on a bone, as you may be used to. This muscle inserts into a tendon-like centre. When the diaphragma contracts, the dome flattens, expanding the lungs, which are then filled with air. When relaxing, the elasticity of the lungs pushes the air out of the lungs and draws the diaphragma back into the dome-shape.

Chronic Obstructive Pulmonary Disease (COPD) is related to asthma in that the patient cannot empty his / her lungs of air. Unlike asthma COPD does not come in attacks, but affects breathing all the time. The disease is caused by destruction of the elastic fibres in the lungs. One cause of this destruction is smoking.

Function

As mentioned, the lungs are responsible for the exchange of gasses between the blood and the air. The gasses pass from high pressure (when talking gasses, we talk pressure rather than concentration) to low pressure. This is what is called diffusion; a particle (e.g. oxygen) moving from high to low concentration / pressure with no help.

The exchange of gasses happens in the alveoli. At this particular location in the lungs the blood and air gets very close to each other. The only thing separating the two is a thin barrier made of two layers: The alveolus wall (one cell thick) and the capillary wall (smallest blood vessels; one cell thick). So as blood with little oxygen passes the oxygen-rich alveolus, the oxygen will travel into the blood stream (and bind to haemoglobin in the red blood cells). In the blood, the pressure of CO2 (and other gasses) is higher than in the air. Therefore these excess gasses move to the air and are removed when the lungs are emptied.

The lungs of a healthy adult can contain approximately six times the amount of oxygen the body needs (generally speaking; when talking biology, everything varies).

The capacity and function of the lungs drops naturally with age.

Also, you can't empty your lungs completely. Even when you try forcing it all out, some air will stay inside the airways.

Sources:

- My education (medicine)

- Some anatomy books