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Respiratory system

The respiratory system is used to exchange gases between an organism and its environment.

The body takes in oxygen and gets rid of carbon dioxide. This is a form of gas exchange. It occurs in the lungs.

Only multicellular animals have a respiratory system. However, some small animals such as the sea spider do not! Gases simply diffuse into the body from the surroundings.

In humans, gas exchange can be seen indirectly from the change in composition of inhaled and exhaled air:

Gas Inhaled air Exhaled air
Oxygen 21% 16%
Carbon dioxide 0.03% 4.5%

For efficient gas exchange, the lungs need a regular supply of fresh air. This maintains a diffusion gradient between the air and the blood.

Important! Do not confuse the respiratory system with respiration. Respiration is the process within cells that uses oxygen to provide energy. The respiratory system (combined with the circulatory system) delivers oxygen needed for respiration to cells and removes carbon dioxide.

Heat is lost from the body in exhaled air.
Heat is lost from the body in exhaled air.

The lungs are the site of gas exchange between the blood and the environment. They are connected to the environment through a series of hollow tubes:

  1. Air travels from the mouth to the lungs through the trachea .
  2. The larynx (or voice box) sits above the trachea. It helps prevent ingested material from entering the trachea.

  3. The trachea splits into two bronchi (singular bronchus).
  4. The bronchi branch into a large network of bronchioles .

The lungs contain roughly 1,500 miles of airways!

The larynx prevents food from entering the lungs
The larynx prevents food from entering the lungs

Cilia are tiny hair-like structures covering the walls of the trachea. Cilia transport mucus away from the lungs. Mucus traps dust and microbes that can damage the lungs.

An alveolus (plural alveoli) is a small air sac within the lungs where gases are exchanged between blood and air. Alveoli form bunches at the end of bronchioles.

Alveoli are well adapted for gas exchange:

  • Large blood supply from capillary network that surrounds alveolus.
  • Short diffusion path: the walls of the alveoli and capillaries are only one cell thick.
  • Moist alveolar walls increase efficiency of diffusion.
  • Folded structure increases surface area for gas exchange.

The lungs contain roughly 400 million alveoli, creating a huge surface area for gas exchange.

The total surface area of the lungs is roughly the size of half a tennis court!

 Alveoli are bunched at the terminal ends of bronchioles.
Alveoli are bunched at the terminal ends of bronchioles.

Carbon dioxide is removed from the body by diffusing from the blood plasma to the alveoli. It is then breathed out.

Carbon dioxide is transported in the blood in three forms.

  • Roughly 10% is dissolved in blood plasma.
  • Roughly 20% is bound to haemoglobin in red blood cells.
  • Roughly 70% is carried as hydrogen carbonate ions ($$\ce{HCO3-}$$) by plasma and red blood cells.

Red blood cells contain an enzyme called carbonic anhydrase. This enzyme switches carbon dioxide between two forms. It drives the reaction in both directions. $$$\ce{CO2 + H2O <=> H2CO3}$$$ $$$\text{carbon dioxide + water $\rightleftharpoons$ hydrogen carbonate}$$$

Important! Hydrogen carbonate and hydrogen carbonate ions are essentially different forms of carbon dioxide. Hydrogen carbonate ions are the dissolved from of hydrogen carbonate.

When red blood cells reach the lungs, carbonic anhydrase converts the hydrogen carbonate back into $$\ce{CO2}$$ and water. $$\ce{CO2}$$ then diffuses into the blood plasma and then the alveoli.

Blood plasma does not contain the carbonic anhydrase enzyme.
Blood plasma does not contain the carbonic anhydrase enzyme.

Breathing is the series of muscular contractions that moves air in and out of the lungs.

The ribcage provides protection for the lungs and also assists in breathing.
The ribcage provides protection for the lungs and also assists in breathing.

The lungs are housed by the ribcage.

There are two sets of muscles involved in breathing:

  • Intercostal muscles: muscles between the ribs.
  • Diaphragm: sheet of muscle at the bottom of the ribcage.

The diaphragm separates the thorax (ribcage, heart and lungs) from the organs below.

Breathing is NOT the same as respiration. Respiration is a chemical process within cells, whereas breathing is a mechanical process involving the lungs.

Muscular contractions during breathing create pressure changes in the lungs that drive the movement of air in and out of the lungs.

There are two stages of breathing:

Inspiration (breathing in) is an active process:

  • Intercostal muscles contract, raising rib cage.
  • Diaphragm contracts, moving downwards.

This increases the volume and decreases the pressure in the thorax. Air rushes into the lungs to counter this change.

Expiration (breathing out) is a passive process:

  • Intercostal muscles relax, lowering rib cage.
  • Diaphragm relaxes, moving upwards.

This decreases the volume and increases the pressure in the thorax. Air rushes out of the lungs to counter this change.

The diagram illustrates the changes that occur during breathing.
The diagram illustrates the changes that occur during breathing.

Tobacco smoke contains toxins that can damage the respiratory system.

Tobacco smoke contains three major toxins:

  • Nicotine: The addictive substance in tobacco. Increases blood pressure and heart rate.
  • These effects are due to an increased release of adrenaline by the body.

  • Carbon monoxide: Irreversibly binds to haemoglobin, preventing oxygen transport. This reduces gas exchange in the lungs.
  • Tar: Paralyses cilia in the air passages. Weakens alveolar structure, causing collapse and decreased surface area.
  • Tar also contains carcinogens (cancer-causing substances).

Tobacco kills nearly 6 million people each year. Of these, 600,000 die from second-hand smoke (being exposed to someone else's smoke).

The lung from the smoker is blackened with tar from tobacco smoke.
The lung from the smoker is blackened with tar from tobacco smoke.

Smoking can lead to many different forms of disease.

Disease Explanation
Chronic bronchitis
  • Tar prevents cilia from clearing airways
  • Persistent cough to compensate
  • Build-up of mucus and irritants in bronchi
  • Inflammation and infection of bronchi
Emphysema
  • Tar blocks bronchioles
  • Alveoli lose elasticity and collapse
  • Decreased oxygenated blood supply
  • Less energy, reduced fitness, wheezing
Heart disease and stroke
  • Nicotine increases blood pressure
  • Damage to arteries and fatty deposits build up in blood vessels
  • Increased risk of blood clotting
Cancer
  • Tar contains carcinogens
  • Cancer can begin in the mouth, throat, nose or lungs.

Despite these deadly risk factors, more than one billion people smoke worldwide.