Anatomy of the Pulmonary Arteries and Veins
The vascular system of the lungs originates from the pulmonary trunk (truncus pulmonalis), which arises from the right ventricle. At the level of the fifth thoracic vertebra (Th5), the trunk divides into the right and left pulmonary arteries.
- The left pulmonary artery arches over the left main bronchus. It either immediately continues as the interlobar artery or divides into ascending and descending branches.
- The right pulmonary artery passes beneath the aortic arch and anterior to the right main bronchus, dividing into ascending and interlobular branches.
Arterial branching strictly follows the bronchial tree, supplying each segment. The parenchyma also contains accessory branches that do not run alongside bronchi. The normal diameter of the pulmonary trunk in adults reaches 29–33 mm (depending on body surface area). For pathological screening, the pulmonary artery-to-aorta ratio on CT is used: a value greater than 1 or 1.1 indicates possible pulmonary hypertension.
Oxygenated blood drains into the left atrium via the pulmonary veins (typically two on each side: superior and inferior). Unlike the arteries, they run independently of the bronchial tree.
Bronchial Blood Flow
Although bronchial arteries (arteriae bronchiales) account for only about 1% of total cardiac output, they are critical for nourishing the bronchial tree, visceral pleura, and lung parenchyma.
Normally (in more than 70% of people), 2 to 4 bronchial arteries arise from the descending thoracic aorta at the T5–T6 level. Small peripheral branches form anastomoses with the pulmonary artery. Blood drains via the azygos vein on the right, the hemiazygos or accessory hemiazygos vein on the left, or directly into the pulmonary veins.
Hemodynamic Physiology
The lesser circulation is a low-pressure system. Pressure in the pulmonary capillaries and veins is approximately equal to left atrial pressure (7–12 mmHg). At rest, only one-third of the capillaries are perfused; the rest are recruited during stress.
Key physiological phenomena:
- Euler-Liljestrand reflex. Hypoxia triggers local vasoconstriction of pulmonary arterioles. This redirects blood to better-ventilated areas, eliminating ventilation-perfusion mismatch.
- West zones. Blood flow depends on gravity. In the upright position, the basal regions (Zone III) are maximally perfused because intravascular pressure exceeds alveolar pressure. Conversely, in the apical region (Zone I), alveolar pressure may compress the capillaries.
- Fluid balance. Hydrostatic pressure pushes fluid into the interstitium, while oncotic pressure retains it. Excess fluid is drained by the lymphatic system, whose capacity can increase tenfold in heart failure.
Pulmonary Venous Hypertension
In left heart pathologies (e.g., left ventricular failure), pulmonary venous (PV) pressure rises, which retrogradely increases capillary pressure. Severity is assessed by pulmonary artery wedge pressure (PAWP).
There are three degrees of severity, which are clearly visible on chest X-rays:
- Grade 1: Vascular redistribution (12–20 mmHg). Upper lobe veins dilate and become larger than lower lobe veins. This phenomenon is only noticeable in the upright position.
- Grade 2: Interstitial edema (20–25 mmHg). Fluid accumulates in the interstitium. Peribronchial cuffing and Kerley lines appear. Kerley B lines (short, peripheral) and A lines (long, central-directed) indicate thickening of the interlobular septa.
- Grade 3: Alveolar edema (> 25–30 mmHg). The lymphatic system fails to cope, fluid fills the alveoli, severely impairing gas exchange.
It is important to note that with acute pressure spikes, edema can develop immediately, bypassing the venous distension phase characteristic of chronic progression.
Briefly: answers to topical questions
What is the difference between Kerley A and Kerley B lines?
Kerley A and Kerley B lines differ in their anatomical substrate, dimensions, and location within the lung tissue.
| Sign | Kerley A lines (Kerley A lines) | Kerley B lines (Kerley B lines) |
|---|---|---|
| Substrate | Thickened deep septa | Thickened interlobular septa |
| Dimensions | Length up to 80–100 mm | Length 30–60 mm, width 1–2 mm |
| Localization | Inner two-thirds of the lung | Subpleural regions (especially the basal parts of the right lower lobe) |
| Direction | In various directions, usually medially toward the hilum (hilum) | Perpendicular to the pleural surface |
| Shape | Sometimes curved (angulated) | Linear opacities |
Class and exam questions
Why do pulmonary vessels constrict rather than dilate during hypoxia?
This is the protective Euler-Liljestrand reflex. Vasoconstriction in poorly ventilated areas redirects blood to healthy alveoli for efficient gas exchange.
What are Kerley lines on a chest X-ray?
This is a radiological sign of interstitial edema. They represent interlobular septa thickened by fluid accumulation.
Why is the pulmonary artery-to-aorta ratio measured?
This CT index is used to screen for pulmonary hypertension. Normally it is less than 1; a value >1 indicates dilation of the pulmonary trunk.
What else to study on this topic
- Anatomical variability of segmental pulmonary arteries
- Differential diagnosis of persistent septal lines (Kerley lines)
- Measurement of pulmonary artery wedge pressure (Swan-Ganz catheter)
- Impact of chronic PE on the development of right ventricular failure
- Significance of pulmonary venous anatomy in atrial fibrillation ablation planning
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