6. Trachea, Bronchi, Bronchioles and Alveoli

what is the trachea?

single widest of the conducting airways, however it has the smallest total cross-sectional area and therefore is responsible for most of the airway resistance

what prevents trachea collapse on inspiration?

the surrounding and supporting rings of hyaline cartilage

what does the trachea bifurcate into?

two primary bronchi which enter the lung. These form the trunks of each of the lungs and go on to further divide (up to 11 times) into smaller and smaller bronchi. These conducting tubes are also supported by rings of cartilage

what do the first 17 generations of airway form?

the conducting zone of the lower respiratory system

when can bronchioles be found?

At about the 12th division of the bronchi

what are bronchioles?

small, collapsible passageways with smooth muscle walls

when do bronchioles become respiratory bronchioles?

after further branching up to 11 times

what are respiratory bronchioles?

the transition between the conducting airways and the gas exchanging tissues of the lung

where do respiratory bronchioles terminate?

at the alveolar ducts which lead to the alveoli

what forms the respiratory zone of the lower respiratory system?

structures from the respiratory bronchioles to the alveoli

what are alveoli?

the primary site of gas exchange. They are small outpockets which greatly increase the surface area for gas exchange

how are alveoli arranged?

in lung lobules which are a cluster of alveoli surrounded by elastic fibres and a network of capillaries

what are lung lobules?

a cluster of alveoli supplied by a single respiratory bronchiole, surrounded by the connective tissue of the lung

what is emphysema caused by?

largely associated with smoking. It is caused by gradual damage to the alveoli, usually by irritants or infection, which makes them loose their natural elasticity

what is emphysema?

the lungs are unable to expel all the air on expiration and leaves them feeling continually over inflated and the patient struggles to force air in and out of their lungs

what can emphysema lead to?

a state of chronic hypoxia causing fatigue and weight loss. It may only start off as breathlessness however it may lead to the patient being housebound and reliant on oxygen supplies

how can emphysema be complicated?

by combination with chronic bronchitis, where the lungs are unable to clear mucus (due to damage to the respiratory epithelium, often by smoking) which becomes infected.

This combination is called Chronic Obstructive Pulmonary Disease (COPD)

what is asthma?

a patient experiences difficulty breathing as the conducting airways become restricted as the diameter reduces

what is the current theory of asthma?

immunological disease caused by immune system over-reacting to an antigen that would normally be over looked

what is status asthmaticus?

an acute asthma attack, triggered by an irritant

what are the treatments for an acute asthma attack?

bronchodilators such as salbutamol (ventolin) are used. Prophylactic inhaled steroids such as beclomethasone dipropionate (BDP) helps reduce long-term airway damage

what are cavities lined with?

pseudostratified columnar epithelium

what do columnar cells have?

numerous cilia on their surface

what does the epithelium contain?

mucous secreting goblet cells

what is respiratory type epithelium?

type of epithelium (pseudostratified columnar, with cilia and goblet cells) present throughout upper respiratory system

what is the bronchial tree lined with?

respiratory type epithelium in the larger bronchi

what happens to the epithelium in more peripheral branches?

becomes thinner though cells are still ciliated columnar with goblet cells

what are bronchioles lined by?

simple (not pseudostratified) ciliated columnar epithelium and goblet cells are sparse

what is the site of gaseous exchange?

distal respiratory tract

what is lined with cuboidal ciliated epithelium?

The most proximal components are the respiratory bronchioles

what is lined with flattened epithelium?

The alveolar ducts (passageways formed mostly from the openings to alveoli)

what do alveoli consist of?

Type I and II pneumocytes that lie on the basement membrane and also of alveolar macrophages.

what does the alveolar wall contain?

considerable amounts of elastin

what does elastin do?

allows the lungs to stretch to accommodate inspired air, it recoils to its original shape, allowing air to be expelled and it acts as a support, attaching airways with no cartilage firmly to the connective tissue of the lung

what are type 1 pheumocytes?

very thin cells which allow gaseous diffusion. They represent about 40% of the number of alveolar cells but 90% of the surface area lining the alveoli.

what is the structure of type 1 pneumocytes?

flattened cells with flattened nuclei and are joined together by tight junctions; have few organelles and provide a very thin covering over the basement membrane; the thinness of which contributes to the efficiency of gaseous exchange.

what are type 2 pneumocytes?

secrete surfactant. They represent 60% of the number of alveolar cells but only 5-10% of the surface area lining the alveoli

what is the structure of type 2 pneumocytes?

cells are rounded in shape and contain numerous mitochondria

what is surfactant?

acts as a detergent, reducing alveolar surface tension, preventing collapse of the alveoli during expiration and facilitating inspirational expansion

what can type 2 pneumocytes act as?

precursor stem cells for type I pneumocytes

what do alveolar macrophages do?

‘patrol’ the alveolar air spaces and the interalveolar septa (the walls of the alveoli) and can pass freely between the two.

They phagocytose inhaled debris and pathogens.

what happens after phagocytosis?

they may pass into the lymphatic system or adhere to mucous-coated ciliated epithelium and continue up the mucociliary escalator to the trachea where they are cleared by coughing