Respiratory Physiology Part 1

RESPIRATORY PHYSIOLOGY

Written by – Dr Madhuri Anupindi

Describe the anatomy of the trachea and bronchial tree to the level of the segmental bronchi

The trachea is a fibrocartilaginous tube which typically has an inner diameter of about 2.5cm and a length of about 10 – 16cm. It begins at the lowest border of the larynx around the level of C6, and ends by bifurcating into the right and left main bronchi around T5-7. The trachea is D shaped in cross section and consists of curved cartilage rings anteriorly and a flat, longitudinal muscle called the trachealis posteriorly. The top of the trachea contains the cricoid cartilage which is the only complete ring of cartilage in the trachea. Extending downwards throughout the length of the trachea are between 15 – 20 C-shaped cartilaginous rings which are composed mainly of collagen. The trachealis muscle joins the ends of the C-shaped cartilage rings and the oesophagus lies posteriorly to the trachea. The trachea is supplied by the inferior thyroid and bronchial arteries and innervated by the vagus nerve and T2-T6 sympathetic chain. Itis lined with pseudostratified columnar ciliated epithelium and goblet cells which produce mucus. The cilia and mucus form the mucociliary escalator which trap inhaled foreign particles, then sweep these substances up to the pharynx where it can be swallowed into the stomach or coughed out.

The bronchial tree consists of 23 divisions, the first 16 of which are conducting zones as they do not participate in gas exchange, and the remaining 7 divisions comprising the respiratory zone. The bronchi have a similar structure to the trachea except the main bronchi have cartilage rings that completely encircle the lumen while the lobar and segmental bronchi have incomplete rings of cartilage. The blood supply to the bronchi are via bronchial arteries and venous drainage is via the hemiazygos and azygos veins.

The first division of the bronchial tree is the right and left main bronchus. At this bifurcation there is a ridge of cartilage called the carina which runs anteroposteriorly. The right main bronchus is about 2.5cm long and is wider, and more vertical compared to the left. The azygos vein arches over the right main bronchus from behind it. The right pulmonary artery initially lies below the right main bronchi and then later anteriorly to it. The right main bronchus has 3 subdivisions, known as secondary or lobar bronchi. The first is the right upper lobe bronchus, which arises from the superolateral wall, and branches into segmental or tertiary bronchi which are the apical, anterior and posterior bronchi. The right bronchus intermedius runs distal to the right upper lobe bifurcation and divides into the right middle lobe and right lower lobe bronchi. The right middle lobe bronchus traverses the right hilum into the right middle lobe and divides into branches to the medial and lateral segments. The right lower lobe bronchus arises as the continuation of the bronchus intermedius. It divides immediately into an apical segmental bronchus, and four basal segments which are the medial, lateral, anterior and posterior.

The left main bronchus is about 5cm long. It passes inferiorly to the arch of the aorta, and anteriorly to the thoracic aorta and oesophagus. The lobar branches are the left upper lobe and left lower lobe. The left upper lobe bronchus divides into the superior division and the lingular division. The superior division gives rise to the segmental apicoposterior and anterior bronchi. The lingular bronchus divides into the superior and inferior bronchi. The left lower lobe bronchus divides into bronchi to the apical, anterior basal, lateral basal and posterior basal segments.

Outline the anatomy of the diaphragm and its function in respiration

The diaphragm is a large membranous, dome-shaped skeletal muscle which separates the chest cavity from the abdominal cavity. It is the most important muscle of inspiration and has motor innervation by C3-5 via the phrenic nerve. The sensory innervation of the peripheries is via the 6^th-11^th intercostal nerves. The blood supply is via the subcostal and lowest 5 intercostal arteries and the inferior and superior phrenic arteries.

The diaphragm is comprised of peripheral muscle which inserts into a central tendon that ascends to fuse with the inferior surface of the pericardium. On either side of the pericardium is the right and left domes of the diaphragm, the right one lying slightly higher due to the liver. The peripheral muscle of the diaphragm originates from three areas:

• Sternal part: from the posterior aspect of the xiphoid process
• Costal part: internal surfaces of the lower costal cartilages and ribs 7 – 12
• Lumbar part: medial and lateral arcuate ligaments, bodies of vertebra L1-3, and the anterior longitudinal ligament. The parts that arise from the vertebra are tendinous and known as the right crus which arises from L1-3 and their intervertebral discs, and the left crus which arises from L1-2 and their intervertebral discs. Some of the fibres from the right crus surround the oesophageal opening and act as a physiological sphincter.

The diaphragm has three major openings through which structures can pass from the chest to the abdomen. These are:

• The caval hiatus: at T8 through which the IVC and terminal branches of the right phrenic nerve passes
• Oesophageal hiatus: at T10, through which the oesophagus, right and left vagus nerves, and the oesophageal branches of the left gastric artery/vein passes
• Aortic hiatus: at T12, through which the aorta, thoracic duct, and azygous vein pass.

The diaphragm contracts during inspiration which causes it to flatten, resulting in an increased volume of the thoracic cavity. This decreases intrapulmonary pressure and helps air to flow into the lungs. During normal breathing, the diaphragm descends about 1.5cm but with forced inspiration, it can descend up to 10cm. The descent of the diaphragm also increases intra-abdominal pressure, and the combination of raised intra-abdominal pressure and reduced intrathoracic pressure helps to increase venous return to the heart. When the diaphragm relaxes, the intrathoracic volume decreases intrapulmonary pressure increases and this aids in expiration. Furthermore, diaphragm contraction assists in the contraction of muscles of the anterior abdominal wall to raise intra-abdominal pressure during processes such as vomiting, defecation and childbirth.

Briefly compare the pulmonary and bronchial arteries

The lung has a dual blood supply, comprised of the pulmonary and the bronchial arteries. The pulmonary arteries are low pressure, an elastic system with much thinner walls compared to the systemic arteries. They carry deoxygenated blood from the right ventricle to be oxygenated in the lungs. They supply 99% of blood flow to the lungs and participate in a gas exchange at the alveolar-capillary membrane. The bronchial arteries carry only about 1% of total cardiac output and carry high-pressure oxygenated blood to the supporting structures of the lungs including the conducting airways down to the level of the terminal bronchioles, nerves, lymph nodes, visceral pleura, and the walls of the large pulmonary vessels. They do not participate in gas exchange. The pulmonary arteries have pressures of about 25/8mmHg while the bronchial arteries have systemic blood pressures, so about 120/80mmHg.

The bronchial arteries arise between T3-T8, and there are usually 2 left and one right, but sometimes there are also additional smaller bronchial arteries arising from the descending thoracic aorta. The left bronchial arteries arise directly from the anteromedial thoracic artery. The superior left bronchial artery arises near the level of the aortic arch, lateral to the carina and posterior to the left main bronchus. The inferior left bronchial artery arises from the aorta parallel to the superior artery but inferior to the left main bronchus. The right bronchial artery has a common origin with a posterior intercostal artery and arises from the right anteromedial aspect of the thoracic aorta.

The pulmonary trunk arises from the base of the right ventricle, passes behind the ascending aorta and splits into the left and right pulmonary arteries at the level of T5-6. The pulmonary trunk is covered by pericardium and lies with the ascending aorta. It contains the pulmonary valve which allows blood to flow from the right ventricle to the pulmonary trunk. The cusps of the valve form mild dilatations called sinuses. The left pulmonary artery is a direct continuation of the pulmonary trunk and is shorter than the right. It arches over the left main bronchus before reaching the left lung. The right pulmonary artery arises at about 90 degrees off the pulmonary trunk and runs underneath the aortic arch, then behind the SVC to the right lung.

Define lung compliance

Lung compliance is the change in lung volume per unit change in transmural pressure gradient, that is the pressure difference between the alveolus and pleural space. It is normally expressed in ml/cmh20. Lung compliance can be described as static or dynamic. Static compliance the change in volume divided by change in pressure, measured in the absence of gas flow, therefore it is not affected by resistance. Dynamic lung compliance is compliance during respiration or in the presence of gas flow. It is always lower than static compliance as it incorporates resistance to airflow. Specific compliance is the compliance normalised for lung volume which is calculated by static lung compliance divided by functional residual capacity. This allows us to compare the compliance of the lungs of different volumes. A normal value is about 0.05ml/cmH20.

Briefly outline the factors that affect lung compliance

• Surfactant
  • Decreases surface tension at the alveolar air-water interface and prevent small alveoli from collapsing
  • This increases lung compliance
• Lung volume
  • Compliance greatest around FRC and reduced at high and low lung volumes
  • Decreased lung volume such as with collapse, consolidation, atelectasis or lung resection will decrease lung compliance
  • Maybe negated by calculating specific compliance
• Posture
  • Compliance is highest in an upright position, lower in a supine position and lowest in a lateral position. Prone positioning results in increased lung compliance compared to supine
• Pulmonary blood volume
  • Pulmonary venous congestion decreases compliance
• Airway resistance
  • Increased airway resistance decreases dynamic lung compliance
• Increased airflow
  • Increased airflow such as with increased respiratory rates decreases dynamic lung compliance
• Lung elastic recoil
  • Decreased elastic recoil such as in emphysema causes increased lung compliance
  • Increased elastic recoil such as in pulmonary fibrosis results in decreased lung compliance

What are the factors that affect airway resistance?

Resistance is the change in pressure divided by flow and the normal airway resistance is 0.5 – 2cmH20/L/sec. The main determinants of airway resistance are the size of the airway and the properties of gas flow. We can think of this in terms of re-arranging the Hagen-Poiseuille equation where resistance then equals (8 x viscosity x length) divided by pi x radius^4. Caveats to this, is that the Hagen-Poiseuille is reliant on laminar flow, which is not always the case in the airways. Determinants of airway resistance are:

• Airway radius
  • A reduction in radius size will increase the resistance by a factor of 4. Changes in airway radius can be due to:
    • Changes in bronchial smooth muscle tone e.g. increased tone with bronchospasm and decreased tone with bronchodilators
    • A decrease in intra-luminal area e.g. oedema or secretions
    • Mechanical obstruction or compression e.g. extrinsic compression by a tumour
• Nature of gas flow
  • This is defined by Reynolds number which is equal to diameter x velocity x gas density all divided by the viscosity of the gas. <2000 is generally laminar flow, 2000 – 4000 is transitional and > 4000 is usually turbulent.
  • Laminar flow involves an orderly, concentric distribution of air particles which moves through a tube in parallel layers, the central of which flow with the greatest velocity. Resistance is low and constant and flow is proportional to driving pressure with laminar flow.
  • Turbulent flow refers to air is flowing in a disorganized manner, at different speeds and pressures. Turbulent flow is proportional to the square root of driving pressure. Therefore, to double the flow, the pressure across the airway must be quadrupled. Turbulent flow occurs when the air is at high velocity, in larger diameter airways and when the density is increased. Airway resistance increases exponentially when the flow is turbulent.
• Composition of gas
  • Decreased gas density decreases airway resistance as per Reynold’s number. This is why helium, as it is less dense than oxygen, may reduce airway resistance by increasing the likelihood of laminar flow
  • Viscosity: increased viscosity will increase resistance as per Hagen-Poiseuille’s law but also decreases Reynold’s number, therefore, increasing the likelihood of laminar flow. In practice, gas viscosity is relatively less important as they do not vary greatly, and the density is more important for airway resistance.
• Lung volume and location
  • Lung expansion causes the airways to also expand and decreases airway resistance
  • Resistance is usually maximal in medium-sized airways and low in small airways as airflow occurs in series