Upper airway

As discussed in Chapter 9, the function of the nose is to provide humidification, filtration and warming of the inspired air. Although the nose is designed as the primary route of entry for gases, most people also breathe through the mouth, particularly during stress. In this case many of the nasal defensive pathways are lost. Hence the lung also has to have mechanisms to condition the air and remove foreign particles.

The nasopharynx is primarily a passageway from the nose to the oral pharynx for air and secretions. It is also connected to the tympanic cavity of the middle ear through the auditory tubes that open on both lateral walls. The act of swallowing briefly opens the normally collapsed auditory tubes and allows the middle ears to be aerated and pressure differences to be equalized. In the posterior wall of the nasopharynx is located a lymphatic organ, the pharyngeal tonsil.

The larynx is an organ of complex structure that serves a dual function: as a controlling air canal to the lungs and as the organ of phonation. Below the larynx lies the trachea, a tube about 10 to 12 centimetres long and two centimetres wide. Its wall is stiffened by 16 to 20 characteristic horseshoe-shaped, incomplete cartilage rings that open toward the back and are embedded in a dense connective tissue. The dorsal wall contains a strong layer of transverse smooth muscle fibres that span the gap of the cartilage. In an adult, the trachea is 11 to 13 cm in length and 1.5 to 2.5 cm in diameter, and bifurcates to form the right and left main bronchi (Figure 10.1). The right bronchus divides into the upper, middle and lower branches whilst the left bronchus divides only into an upper and a lower branch. These lobar bronchi give rise to branches called 'segmental bronchi'. The branching angle of 37° appears to be optimal to ensure smooth airflow.

Structure of the tracheo-bronchial tree

The hierarchy of the dividing airways, and partly the blood vessels, in the lung largely determines the internal lung structure. Functionally the system can be subdivided into three zones, a proximal, purely conducting zone, a peripheral, purely gas-exchanging zone, and a transitional zone in between. Morphologically the relatively thick-walled, purely air-conducting tubes can be distinguished from those branches of the airway tree structurally designed to permit gas exchange.

Every branching of the tracheo-bronchial tree produces a new 'generation' of tubes, the total cross-sectional area increasing with each generation (Figure 10.2). The main bronchi are known as the first generation, the second and third generations are the lobar and segmental bronchi respectively. The fourth to ninth generations are the small bronchi and in these segmental bronchi the diameters decrease from approximately 4 to 1 mm. At a diameter of less than 1 mm they lie inside the lobules and are then correctly termed bronchioles. Here the function of the tissue changes from conducting airway to gas

Carina

Carina

Right Lung Showing Airways

Figure 10.1 Structure of the lungs

Left Pulmonary

Right Lung Showing Airways

Left Lung Showing Pulmonary Blood Vessels

Figure 10.1 Structure of the lungs

Figure 10.2 The tree structure of the lung

exchange. Although the decrease in diameter minimizes the deadspace, it is associated with a large increase in resistance to flow. For example, a 10% reduction in diameter is associated with an increase in airway resistance of more than 50%. Thus the airways should be as wide as possible to minimize resistance, but this will of course increase deadspace. In the body these opposing factors are balanced by finely tuned physiological control which is easily disturbed by lung pathologies such as asthma and bronchitis.

The respiratory gases diffuse from air to blood, and vice versa, through the 140 square metres of internal surface area of the tissue compartment. The gas-exchange tissue proper is called the pulmonary parenchyma, while the supplying structures, conductive airways, lymphatics, and non-capillary blood vessels belong to the non-parenchyma. The parenchyma of the lung consists of approximately 130,000 lobules, each with a diameter of about 3.5 mm and containing around 2,200 alveoli. It is believed that each lobule is supplied by a single pulmonary arteriole. The terminal bronchioles branch into approximately 14 respiratory bronchioles, each of which branches further into the alveolar ducts. The ducts carry 3 or 4 spherical atria that lead to the alveolar sacs supplying 15-20 alveoli (Figure 10.3). Additional alveoli arise directly from the walls of the alveolar ducts, and these are responsible for approximately 35% of the total gas exchange. It has been estimated that there are 300 million alveoli in an adult human lung. The volume of an alveolus changes with the degree of inflation, but assuming 75% lung inflation, the diameter of an alveolus is thought to be between 250 and 290 pm. It is estimated that each alveolus has a volume of 1.05x10- 5ml, with an air-tissue interface of 27x10-4cm2. For these calculations, it was assumed that the lung had a total air volume of 4.8 L, a total respiratory zone volume of 3.15 L and a total alveolar air-tissue interface of 81 m2.

Figure 10.3 Structure and perfusion of the alveoli

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Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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