With respect to blood circulation, the lung is a complex organ. It has two distinct though not completely separate vascular systems: a low-pressure pulmonary system and a high-pressure bronchial system. The pulmonary (or lesser) circulation is responsible for supplying oxygen to the tissues of the body. Blood, low in oxygen content but laden with carbon dioxide, is carried from the right heart through the pulmonary arteries to the lungs. On each side, the pulmonary artery enters the lung in the company of the stem bronchus and then divides rapidly, following relatively closely the course of the dividing airway tree. After numerous divisions, small arteries accompany the alveolar ducts and split up into the alveolar capillary networks. Because intravascular pressure determines the arterial wall structure, the pulmonary arteries, which have on average a pressure five times lower than systemic arteries, are much flimsier than systemic arteries of corresponding size. The oxygenated blood from the capillaries is collected by venules and drained into small veins. These do not accompany the airways and arteries but run separately in narrow strips of connective tissue delimiting small lobules. The interlobular veins then converge on the intersegmental septa. Finally, near the hilum the veins merge into large venous vessels that follow the course of the bronchi. Generally, four pulmonary veins drain blood from the lung and deliver it to the left atrium of the heart.

The bronchial circulation has a nutritional function for the walls of the larger airways and pulmonary vessels. The bronchial arteries originate from the aorta or from an intercostal artery. They are small vessels and generally do not reach as far into the periphery as the conducting airways. With a few exceptions, they end several generations short of the terminal bronchioles. They split up into capillaries surrounding the walls of bronchi and vessels and also supply adjacent airspaces. Most of their blood is naturally collected by pulmonary veins. Small bronchial veins exist, however; they originate from the peribronchial venous plexuses and drain the blood through the hilum into the azygos and hemiazygos veins of the posterior thoracic wall.

The lymph is drained from the lung through two distinct but interconnected sets of lymphatic vessels. The superficial, subpleural lymphatic network collects the lymph from the peripheral mantle of lung tissue and drains it partly along the veins toward the hilum. The deep lymphatic system originates around the conductive airways and arteries and converges into vessels that mostly follow the bronchi and arterial vessels into the mediastinum.

Within the lung and the mediastinum, lymph nodes exert their filtering action on the lymph before it is returned into the blood through the major lymphatic vessels, called bronchomediastinal trunks. Lymph drainage paths from the lung are complex. The precise knowledge of their course is clinically relevant, because malignant tumours of the lung spread via the lymphatics.

The pleurae, the airways, and the vessels are innervated by afferent and efferent fibres of the autonomic nervous system. Parasympathetic nerve fibres from the vagus nerve (10th cranial nerve) and sympathetic branches of the sympathetic nerve trunk meet around the stem bronchi to form the pulmonary autonomic nerve plexus, which penetrates into the lung along the bronchial and vascular walls. The sympathetic fibres mediate a vasoconstrictive action in the pulmonary vascular bed and a secretomotor activity in the bronchial glands. The parasympathetic fibres stimulate bronchial constriction. Afferent fibres to the vagus nerve transmit information from stretch receptors, and those to the sympathetic centres carry sensory information (e.g., pain) from the bronchial mucosa.

Lung development

After early embryogenesis, during which the lung primordium is laid down, the developing human lung undergoes four consecutive stages of development, ending after birth. The names of the stages describe the actual morphology of the prospective airways. The pseudoglandular stage exists from five to 17 weeks; the canalicular stage, from 16 to 26 weeks; the saccular stage, from 24 to 38 weeks; and finally the alveolar stage, from 36 weeks of fetal age to about 11/2 to two years after birth.

The lung appears around the 26th day of intrauterine life as a ventral bud of the prospective esophagus. The bud separates distally from the gut, divides, and starts to grow into the surrounding mesenchyme. The epithelial components of the lung are thus derived from the gut (i.e., they are of endodermal origin), and the surrounding tissues and the blood vessels are derivatives of the mesoderm.

Following rapid successive dichotomous divisions, the lung begins to look like a gland, giving the first stage of development (pseudoglandular) its name. At the same time the vascular connections also develop and form a capillary plexus around the lung tubules. Toward week 17, all the conducting airways of the lung are preformed, and it is assumed that, at the outermost periphery, the tips of the tubules represent the first structures of the prospective gas-exchange region.

During the canalicular stage, the future lung periphery develops further. The prospective airspaces enlarge at the expense of the intervening mesenchyme, and their cuboidal epithelium differentiates into type I and type II epithelial cells or pneumocytes. Toward the end of this stage, areas with a thin prospective air–blood barrier have developed, and surfactant production has started. These structural and functional developments give a prematurely born fetus a small chance to survive at this stage.

During the saccular stage, further generations of airways are formed. The tremendous expansion of the prospective respiratory airspaces causes the formation of saccules and a marked decrease in the interstitial tissue mass. The lung looks more and more “aerated,” although it is filled with fluid originating from the lungs and from the amniotic fluid surrounding the fetus. Some weeks before birth, alveolar formation begins by a septation process that subdivides the saccules into alveoli. At this stage of lung development, the infant is born.

At birth the intrapulmonary fluid is rapidly evacuated and the lung fills with air with the first breaths. Simultaneously, the pulmonary circulation, which before was practically bypassed and very little perfused, opens up to accept the full cardiac output.

The newborn lung is far from being a miniaturized version of the adult lung. It has only about 20 million to 50 million alveoli, just a small percentage of the full adult complement. Therefore, alveolar formation is completed in the early postnatal period. Although it was previously thought that alveolar formation could continue to age eight and beyond, it is now accepted that the bulk of alveolar formation is concluded much earlier, probably before age two. Even with complete alveolar formation, the lung is not yet mature. The newly formed interalveolar septa still contain a double capillary network instead of the single one of the adult lungs. This means that the pulmonary capillary bed must be completely reorganized during and after alveolar formation; it has to mature. Only after full microvascular maturation, which is terminated sometime between ages two and five, is the lung development completed, and the lung can enter a phase of normal growth.