The Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 37

and conditioning of the air as well as filtering through mucociliary action to re-
move mucous secretions in an upward motion towards the oesophagus. In the distal
branches, the epithelium becomes cuboidal to allow for gas exchange. The cartilage
support around the trachea and early branches also changes, progressively diminish-
ing in order to maintain patency of the smaller airways. During gas exchange oxygen
is brought into the body and is exchanged with carbon dioxide that is produced from
cell metabolism. This occurs in the alveolar-capillary network which consists of a
dense mesh-like network of the respiratory bronchioles, the alveolar ducts, the alve-
oli, and the pulmonary capillary bed. At the gas exchange surface of the alveoli is
a lining that is 1-2

m thick where O 2 and CO 2 passively diffuse across and into
plasma and red blood cells. The diffusion occurs between the alveolar gas and blood
in the pulmonary capillaries within less than one second.

μ

2.5.3 Variation and Disease of the Tracheobronchial Tree

and Lung Airways

The cross-section of the trachea typically has a coronal-to-sagittal diameter ratio of
0.6:1.0, and narrowing of the coronal diameter producing a coronal/sagittal ratio
of < 0.6 is then termed a sabre sheath trachea and is seen in patients with chronic
obstructive pulmonary disease (Brant and Helms 2007). A slight tracheal deviation
to the right after entering the thorax can be a normal radiographic finding and in
some instances, the presence of the aortic arch can lead to the left lateral wall of the
distal trachea being indented by the transverse portion of the aortic arch. In younger
individuals the trachea is elastic and extensible, while in older people it is more rigid
or even sometimes os sified, so that it is less distensible (Franciscus and Long 1991b).
There are many reported diseases of the lung airways ( pulmonary disease ) ranging
from the common cold to life-threatening examples such as bacterial pneumonia or
cancer, and include:

• Chronic Obstructive Pulmonary Disease (COPD)—One of the most common

pulmonary diseases (e.g. bronchitis, emphysema and asthma), resulting in
the inflammation of the airways which in turn causes narrowing and obstruc-
tion of the airways, seriously affecting the capacity for normal respiratory

function.

• Restrictive lung disease ( interstitial lung disease )—is a disease of the lung

parenchyma (covering layer of the lungs), and the connective tissue that hold
the air sacs together. This results in a decreased ability to breathe in because of
incomplete lung expansion and increased lung stiffness.

• Respiratory tract infection—any infection that can affect any part of the respira-

tory system such as viral or bacterial. This is normally categorised as an upper
respiratory tract infection (nose, sinus, pharynx, larynx) or a lower respiratory
tract infection. The most common lower respiratory tract infection is pneumonia.

• Lung cancer—is a disease of uncontrolled cell growth in tissues of the lung. The

collection of these cells forms a tumour which is either malignant or benign. The