Page 3 of 6
AN24.1-6 | Lungs & Trachea — Part 2
Blood Supply of the Lungs (AN24.5)
The lung has a dual blood supply — two completely separate circulations:
Figure: Blood Supply of the Lungs (AN24.5)
Pulmonary vs Bronchial Circulation
| Feature | Pulmonary Circulation | Bronchial Circulation |
|---|---|---|
| Function | Gas exchange (functional) | Nutrition of lung tissue (nutritive) |
| Source | Right ventricle → pulmonary trunk | Thoracic aorta (left bronchial aa.) and aorta/intercostals (right bronchial a.) |
| Pressure | Low pressure (25/10 mmHg) | High pressure (systemic) |
| Flow volume | High flow (entire cardiac output) | Low flow (1-2% of CO) |
| Drainage | 4 pulmonary veins → left atrium | Bronchial veins → azygos system (right) or accessory hemiazygos (left); some drain into pulmonary veins |
| Distribution | Follows bronchial tree to alveolar capillaries | Supplies bronchial walls, visceral pleura, connective tissue down to terminal bronchioles |
1. Pulmonary Circulation (functional — gas exchange):
- Pulmonary arteries carry deoxygenated blood from the right ventricle to alveolar capillaries
- One pulmonary artery per lung divides to follow the bronchial tree
- Pulmonary veins (2 per lung, 4 total) carry oxygenated blood → left atrium
- The pulmonary circulation is a low-pressure, high-flow system (PA pressure ~25/10 mmHg)
- Cross-ref PY: Pulmonary arterial hypertension (PAH) — elevated PA pressure causes RV hypertrophy and failure
2. Bronchial Circulation (nutritive — supplies bronchial walls):
- Bronchial arteries — left side: 2 arteries from descending thoracic aorta; right side: 1 artery from a right intercostal or right superior intercostal artery
- Supply the bronchial walls, visceral pleura, and lung connective tissue down to the respiratory bronchioles
- Bronchial veins — only the larger bronchi drain via bronchial veins; smaller bronchi drain into pulmonary veins (physiological shunt — contributes to the normal 2–3% right-to-left shunt)
Pulmonary infarction:
- Despite dual supply, pulmonary embolism can cause infarction when bronchial supply is also compromised (e.g., pre-existing lung disease, congestive heart failure)
- Infarction: haemorrhagic wedge-shaped opacity at lung periphery (Hampton's hump on CXR)
SELF-CHECK
A. 2 (one from each lung)
B. 3 (two from right, one from left)
C. 4 (two from each lung)
D. 6 (three from right, three from left)
Reveal Answer
Answer: .
There are 4 pulmonary veins — superior and inferior from each lung — all draining into the left atrium. This is clinically important in atrial fibrillation ablation (pulmonary vein isolation).
Pleura and Pleural Recesses (AN24.6)
The Pleura:
Figure: Pleura and Pleural Recesses (AN24.6)
The pleura is a serous membrane consisting of two continuous layers:
- Visceral pleura — covers the lung surface, dips into the fissures. Supplied by bronchial vessels; no pain fibres
- Parietal pleura — lines the thoracic wall, diaphragm, and mediastinum. Has pain fibres (intercostal nerves + phrenic nerve for diaphragmatic pleura)
The pleural cavity between them contains 5–15 mL serous fluid (lubricant).
Pleural Recesses (potential spaces where parietal layers are in contact):
| Recess | Location | Clinical Importance |
|---|---|---|
| Costodiaphragmatic | Between costal and diaphragmatic pleura, at 8th rib MCL | Most dependent recess; first to fill with fluid |
| Costomediastinal | Anterior, between costal and mediastinal pleura | Smaller; less clinically significant |
Why recesses matter:
- Pleural effusion collects first in the costodiaphragmatic recess (most dependent)
- On PA CXR: blunting of the costophrenic angle = 200–300 mL of fluid
- Chest drain insertion site: 5th–6th intercostal space, midaxillary line (above the lower rib to avoid neurovascular bundle)
Surface markings of pleura and lung:
The pleura extends further than the lung edge:
- Anteriorly: right pleura meets midline at 4th costal cartilage → extends to 6th cartilage; left similar but deviates left at 4th (creating the cardiac notch)
- Inferiorly: Lung base at 6th rib MCL, 8th rib MAL, 10th rib paravertebral
- Pleura at 8th rib MCL, 10th rib MAL, 12th rib paravertebral
- The gap between lung and pleura = costodiaphragmatic recess (2 ribs' worth)
Fissures:
- Both lungs: oblique fissure (separates upper/lower → starts posteriorly T3–T4, ends anteriorly at 6th costal cartilage)
- Right lung only: horizontal fissure (separates upper from middle → from oblique fissure at midaxillary line → 4th costal cartilage anteriorly)
SELF-CHECK
A. 50 mL
B. 100 mL
C. 200–300 mL
D. 500 mL
Reveal Answer
Answer: .
The costophrenic angle blunts on an erect PA chest X-ray when approximately 200–300 mL of fluid has accumulated in the costodiaphragmatic recess.
CLINICAL PEARL
Safe triangle for chest drain insertion: The "triangle of safety" is bounded by:
- Anterior border of latissimus dorsi (posterior)
- Lateral border of pectoralis major (anterior)
- A line above the level of the nipple (inferior)
- Apex: axilla
This avoids the neurovascular bundle (under each rib) and breast tissue. Insert through the 5th–6th intercostal space, midaxillary line, above the upper border of the lower rib.