The Respiratory System

The Respiratory System

The second body system we will look at is the Respiratory system.

The diagram below shows the main features of the respiratory system:-


  1. Trachea - Also known as the windpipe, this muscular tube ringed with cartilage connects the nose and mouth to the lungs.

  2. Bronchi - Each person has two bronchus which connect each lung to the trachea.

  3. Bronchioles - Smaller than the bronchi, the bronchioles branch out smaller and smaller spreading out through the lungs. They connect the bronchi to the alveoli.

  4. Alveoli - Also known as air sacs, the alveoli are the site of gas exchange with the blood.

  5. Diaphragm - A large dome shaped muscle below the lungs. When a person breathes in, the diaphragm flattens, increasing the volume of the lungs, causing air to be drawn into them.

Inhalation and Exhalation

Even though this system is known as the respiratory system, the act of breathing in (inhaling) or breathing out (exhaling) is not actually Respiration. Respiration occurs within the cells of the body when Oxygen is used to generate Energy from food, and is a process that will be looked at later.

The function of the Respiratory system is therefore to provide Oxygen to the bloodstream to be used in Respiration. It is the process of breathing that allows this Oxygen to enter the body whilst getting rid of the waste gas Carbon Dioxide.

The diagram below shows the main muscles and bones involved with breathing:-

Image result for inhalation

To Inhale the following must happen:-

  1. The Intercostal muscles contract, expanding the rib-cage.

  2. The diaphragm contracts and flattens, increasing the volume of the chest.

  3. This increase in volume lowers the air pressure inside the lungs, and air is drawn in to equalise the pressure.

To Exhale the following must happen:-

  1. The Intercostal muscles relax, reducing the rib-cage size.

  2. The diaphragm relaxes, decreasing the volume of the chest.

This decrease in volume increases the air pressure inside the lungs, and air is forced out to equalise the pressure.

Alveoli and Gas Exchange

As stated above, it is within the alveoli that gas exchange with the blood occurs.

The diagram above shows the detailed structure of the alveoli.

The alveoli are very small air-filled sacs that form the final part of the Respiratory system. The alveoli have several adaptations that allow gas exchange to happen easily:-

  1. The alveoli have a folded structure, to give them a large surface area for gas exchange to occur over.

  2. The walls of the alveoli are only 1 cell thick, so the gas only has to diffuse a short distance.

  3. The alveoli are surrounded by capillaries, ensuring a good blood supply.

Gas Exchange

The function of the Respiratory system is to exchange gases with the blood. Gases will always move from regions of high concentration to low concentration by diffusion.

The diagram below shows how the two gases involved in Respiration (Oxygen and Carbon Dioxide) are exchanged within the alveoli:-

Oxygen - The concentration of Oxygen is greater in the alveoli, and so Oxygen diffuses from the alveoli into the blood.

Carbon Dioxide - The concentration of Carbon Dioxide is greater in the blood, and so the Carbon Dioxide diffuses from the blood into the alveoli.

As can be seen above, the Oxygen will diffuse into the blood as long as the concentration of the blood Oxygen is less than the concentration of Oxygen in the alveoli. This means that only some of the Oxygen diffuses into the blood. This is why Mouth to Mouth Resuscitation can be used to save lives, as the exhaled air still has Oxygen within it.

The table below shows the Percentage of Gases entering and leaving the Respiratory system:-

Peak Flow and Vital Capacity

The amount of gas exchange that can occur depends on how much air can be taken in with each breath, as well as how often a person breathes.

This is why during exercise both the volume of air inhaled and the rate of breathing increase to cope with the demand for Oxygen.

There are two measurements that can be taken to measure the effect of exercise on breathing:-

  1. Peak Flow - The maximum speed of air that a person can force out of their Lungs.

  2. Vital Capacity - The maximum volume of air that a person can breathe out in one breath.

The diagram below shows a Peak Flow meter, a device used to measure Peak Flow:-

In the peak flow meter, the higher the speed of air being blown through the mouthpiece, the further up the scale the pointer is moved.

The image below shows a Vital Capacity test kit. The kit consists of a long plastic bag with a mouthpiece. By breathing out into the bag, the volume of air exhaled (the vital capacity) can be found:-

Case Study - Asthma

Asthma is a very common lung disease that normally starts during childhood. People with asthma will lead generally normal lives, but at certain times will suffer asthma attacks.

The diagram below shows the changes which occur during an asthma attack:-

During an asthma attack, a person's bronchioles become inflamed and swell up. This reduces the size of the airway, which can become blocked with mucus, causing the person to struggle to breathe. In severe cases, this can be fatal.

Asthma is incurable, but asthma attacks can be treated by the use of an inhaler containing medication that causes the muscles in the bronchioles to relax, opening up the airway.

The Respiratory System and Smoking

Smoking of different substance has been a part of human culture for thousands of years. The smoking of hallucinogenic plants was used by religious leaders (such as Shamans or Mystics) to have religious experiences or to talk to spirits as long ago as ~5000 BCE.

The smoking of the leaves of the Tobacco plant (the main component of cigarettes) originated in North America in the native population, and was brought back to Europe by the first european explorers who explored the New World in ~1560s.

The smoking of tobacco quickly became a huge industry for the settlers in the new world, leading in part to the rise of slavery within the colonies in order to keep up with demand from Europe.

It wasn't until the early 20th century that the severe health risks caused by smoking tobacco became apparent. Over the last 60 years, there has been large decrease in people smoking in the U.K. due to this.

The graph below shows the percentage of the population of Great Britain which smoke over the last 50 years:-

Image result for number of smokers

As can be seen from the above graph in the 1940's over 60% of the male population smoked, whereas by 2009 that had dropped to ~20%. This reduction has been caused by people being much more aware of the risks of smoking on their health.

What's in a Cigarette?

The main chemical most people are aware of in cigarette smoke is Nicotine. However, there are over 4,000 other chemicals in cigarette smoke.

Nicotine is the main chemical within cigarettes that causes addiction. Nicotine is addictive because it stimulates the reward centres of the brain into releasing chemicals including Serotonin, a chemical which is linked to happiness and pleasure.

The diagram below shows some of the chemicals found within cigarette smoke:-

Image result for chemicals cigarette

Smoking and Health

As can be seen above, for the last ~60 years there has been a large decrease in smoking rates due to a better understanding of the health effects of smoking.

The diagram below shows some of the effects of smoking on the human body:-