AS Biology: Human ventilation system

• Lungs are the gas exchange organs in humans and other mammals.
• Lungs are a pair of lobed structured made up of many highly branched tubules called bronchioles which  had tiny air sacs called alveoli.
• They fill most of the space inside the thorax which is bounded by the rib cage, sternum and muscular diaphragm.
• A system of tubes takes air into and out of the lungs.
• This consists of the nasal cavity were the air is filtered, warmed and moistened  continues across the pharynx to the trachea

The trachea

• The trachea is supported and prevented from collapsing by C-shaped ring of cartilage in its walls. The walls of the trachea are lined with ciliated epithelial cells and goblet cells.
• The goblet cells produce mucus to trap dirt particles and bacteria from the air breathed in. The cilia moves this mucus together with the trapped particles up to the throat. The mucus is then transported down to oesophagus to the stomach. 

Bronchi and bronchioles

• At the base the trachea divides into a left and right bronchus. These are similar in structure to the trachea. Both of the bronchi divide into smaller tubes, which continues to subdivide to eventually form narrow tube called bronchioles.
• Bronchioles have muscle in their walls, which allows them to constrict and control the flow of air in and out of the alveoli. The alveoli are well supplied with blood capillaries.
• Bronchioles are narrower than bronchi. Larger ones have some cartilage but smaller ones don't.
• The wall is mostly smooth muscles and elastic fibres, the smallest bronchioles have clusters of alveoli at their ends.

Tissue

• Trachea and bronchi have a similar structure, they differ only in size as the bronchi are narrower.
• Most of the wall contains c-shaped rings  in the cartilage.
• On the inside surface of the cartilage is a layer of glandular tissues, connective tissues, elastic fibres, small muscles and blood vessels.
• The inner lining is an epithelium layer that has two types of cells. Most cells have ciliated epithelium  among this are goblet cells. 

Cartilage

• Supports he trachea and bronchi - keeps them open. Prevents collapse during inhalation. C-shaped, flexible and allowed neck to move without constricting airways.

Smooth muscles

• Can contract and constrict the airway - this makes airway narrower which can restrict air flow - could be harmful if there's harmful substances in the air. It's not a voluntary act, some people have allergic reactions causing bronchioles to constrict making it difficult to breathe. (One of the cause: asthma)

Elastic fibres

• when smooth muscles contract and narrow the airways it cannot reverse the change - when it relaxes the elastic fibred recoils to their original shape and size
• Q.A. Antagonistic means that they work against each other. The smooth muscle contracts to narrow the lumen of the bronchioles. As this happens, the elastic fibres are deformed. When the muscle relaxes, the elastic fibres recoil against their original shape and extend the muscle fibres again.

Goblet cells

• These secrete mucucs
• Mucus traps tiny particles in the air e.g bacteria and ergo reduces chances of infections.

Ciliated epithelium

• Cilia move in a synchronised pattern to move mucus up the airway to the back of the throat. Mucus is swallowed and the acidity in the stomach kills the bacteria.