Human Gas Exchange

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Structure

Human gas exchange takes place in specialised organs called the lungs. The lungs are specialised for the quick exchange of oxygen and carbon dioxide with the bloodstream.

Structure

Trachea

The trachea is the entrance to the human gas exchange system.
When you breathe in, air flows through the trachea.
Ridges of cartilage surround the front of the trachea to provide protection and structure.
- There is no cartilage at the back of the trachea so that the oesophagus (the tube where food travels) is not constricted.

Bronchi & bronchioles

The trachea divides into two bronchi.
Air flows along each bronchus to a lung.
The bronchi are made from cartilage and smooth muscle.
Each bronchus divides into many smaller bronchioles.
- The many bronchioles branch throughout the the lungs into small air-sacs called alveoli.

Alveoli

The alveoli are sacs that fill with air when you breathe in.
Oxygen in the alveoli diffuses into the bloodstream and carbon dioxide in the bloodstream diffuses into the alveoli.
There are millions of alveoli in the lungs
The alveoli provide a large surface area for gas exchange.

Control of ventilation

Ventilation is controlled by the ribcage, intercostal muscles and the diaphragm.
When you breathe in, these structures move to allow the lungs to fill with air.
When you breathe out, these structures move to allow air to leave the lungs.

Columnar epithelium

The trachea, bronchi, and large bronchioles are lined with columnar ciliated epithelium.
Goblet cells are the mucus-secreting cells of ciliated epithelium. This mucus traps dust, particles, and pathogens.
Ciliated cells are columnar in shape. They have multiple, thin projections from their surface ('cilia").
- Cilia contract and "waft" the mucus produced by goblet cells up the trachea to the back of the throat, where it is swallowed.
Any swallowed pathogens are destroyed by stomach acid.

Alveoli

Alveoli are the millions of air sacs within the lungs where gases are exchanged with the bloodstream. They provide a large surface area for gas exchange.

Capillaries

Each alveolus is surrounded by a network of capillaries.
The many capillaries provide a large surface area for gas exchange between the alveoli and the bloodstream.

Alveolar epithelium

The alveolar epithelium is made up of a single layer of squamous epithelial cells that line the walls of the alveoli.
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus.
In the alveoli, the squamous cells are arranged in a single layer to facilitate gas diffusion.
- The squamous epithelium provides a very short diffusion distance from the alveoli to the capillaries which maximises the rate of gas exchange.

Concentration gradient

The capillaries supply carbon dioxide to the alveoli and oxygen is rapidly carried away from the alveoli.
The quick transport of gases in the bloodstream maintains a steep concentration gradient of oxygen and carbon dioxide.
- The steep concentration gradient allows quick diffusion of gases into and out of the bloodstream.

Smoking and the Respiratory System

Smoking can causes a range of very serious diseases. Cigarette smoke contains harmful chemical such as nicotine, carbon monoxide and carcinogens (cancer-causing chemicals).

Bronchitis

When smoke is breathed in, it damages the cilia cells of the airways.
This means the cilia can't waft away the mucus produced by goblet cells, and the mucus builds up.
This can leads to bronchitis, where the airways are partly blocked with mucus.

Emphysema

Smoking damages the elastic fibres in the alveoli. This prevents elastic recoil and causes overinnflation of the lungs.
This can cause emphysema, where gas exchange can't happen efficiently, causing shortness of breath.
Together bronchitis and emphysema form a condition called chronic obtrusive pulmonary disease (COPD).
- Symptoms include a cough, breathlessness, and wheezing.

Cancers

The carcinogens in the tar in cigarettes can cause tumours to form in the airways.
- Smoking can cause mouth, throat and lung cancers.

Heart disease and strokes

Nicotine is very addictive and puts strain on the heart.
- Nicotine acts as a stimulant and causes the heart to beat faster at with greater force. This can increase blood pressure.
Carbon monoxide reduces the ability of red blood cells to carry oxygen, which can also put strain on the heart. Carbon monoxide binds to haemaglobin with 200x the affinity of oxygen.
Both of these substances can cause heart disease and strokes.