The alveolar-capillary network, formed by the branching of pulmonary arteries, consists of a dense mesh-like arrangement of capillaries surrounding alveoli. This network, comprising thin epithelial cells of the alveolus and endothelial cells of the vessels, along with their supportive matrix, possesses an alveolar surface area equivalent to a tennis court (approximately 85 m2). Acting as the sole barrier between gas in the airway and blood in the capillary, this network's structural matrix and tissue components play a crucial role. The barrier, measuring 1 to 2 µm in thickness, encompasses type I alveolar epithelial cells, capillary endothelial cells, and their respective basement membranes, positioned back to back. Primarily encompassed by air, this alveolar-capillary network provides an ideal environment for gas exchange. Within a timeframe of less than 1 second, red blood cells pass through the capillary component of this network in a single file, facilitating the exchange of CO2 and O2 gases.

Alongside facilitating gas exchange, the alveolar-capillary network also plays a role in regulating lung fluid levels. At the pulmonary capillary level, the balance between hydrostatic and oncotic pressure across the capillary wall results in a minor net movement of fluid from the vessels into the interstitial space. Subsequently, the lymphatic system removes this fluid from the lung interstitium, which then enters the circulation through the vena cava in the lung hilus region. In healthy adults, approximately 30 mL of fluid per hour is returned to the circulation via this route.

Distinct from the pulmonary circulation, the bronchial circulation serves as a separate system in the lung. It provides systemic arterial blood to various structures, including the trachea, upper airways, surface secretory cells, glands, nerves, visceral pleural surfaces, lymph nodes, pulmonary arteries, and pulmonary veins. Similar in structure to the systemic circulatory system, the bronchial circulation perfuses the upper respiratory tract but does not reach the respiratory bronchioles or alveoli. Venous blood from the capillaries of the bronchial circulation flows either through true bronchial veins or bronchopulmonary veins towards the heart. True bronchial veins exist in the lung hilus region, and blood enters the right atrium after flowing into the azygos, hemiazygos, or intercostal veins. Bronchopulmonary veins, formed through a network of tributaries from bronchial and pulmonary circulatory vessels, anastomose to create vessels with mixed blood from both circulatory systems. These anastomosed vessels then return blood to the left atrium through pulmonary veins. The pulmonary veins and this anastomosis route account for approximately two-thirds of the total bronchial circulation returned to the heart.

Normally, the bronchial circulation receives only around 1% of total cardiac output, whereas the pulmonary circulation receives nearly 100%. However, in conditions such as cystic fibrosis, the bronchial arteries, which typically receive only 1% to 2% of cardiac output, experience an increase in size (hypertrophy) and can receive as much as 10% to 20% of the cardiac output. Bacterial infection-induced erosion of inflamed tissue into these vessels contributes to the occurrence of hemoptysis (coughing up blood) in this disease.

Breathing is an automatic process regulated by the central nervous system (CNS). The lungs are innervated by the autonomic nervous system of the peripheral nervous system (PNS), which operates under the control of the CNS. The autonomic nervous system comprises four distinct components: parasympathetic, sympathetic, nonadrenergic noncholinergic inhibitory, and nonadrenergic noncholinergic stimulatory.
The sequential branching of the pulmonary arteries culminates in a dense mesh-like network of capillaries that surround alveoli. This alveolar-capillary network is composed of thin epithelial cells of the alveolus and endothelial cells of the vessels and their supportive matrix, and it has an alveolar surface area of approximately 85 m2 (the approximate size of a tennis court). The structural matrix and the tissue components of this alveolar-capillary network provide the only barrier between gas in the airway and blood in the capillary. The cells of this barrier, which is 1 to 2 µm thick, include type I alveolar epithelial cells, capillary endothelial cells, and their respective basement membranes, which are back to back (see
Fig. 20.6B ). Surrounded mostly by air, this alveolar-capillary network is an ideal environment for gas exchange. Red blood cells pass through the capillary component of this network in single file in less than 1 second, which is sufficient time for CO2 and O2 gas exchange.
In addition to gas exchange, the alveolar-capillary network regulates the amount of fluid within the lung. At the pulmonary capillary level, the balance between hydrostatic and oncotic pressure across the wall of the capillary results in a small net movement of fluid out of the vessels into the interstitial space. The fluid is then removed from the lung interstitium by the lymphatic system and enters the circulation via the vena cava in the area of the lung hilus. In normal adults, an average of 30 mL of fluid per hour is returned to the circulation via this route.
The bronchial circulation is a distinct system, separate from the pulmonary circulation in the lung, that provides systemic arterial blood to the trachea, upper airways, surface secretory cells, glands, nerves, visceral pleural surfaces, lymph nodes, pulmonary arteries, and pulmonary veins. The bronchial circulation is similar in structure to the systemic circulatory system and perfuses the upper respiratory tract; it does not reach the terminal or respiratory bronchioles or the
CHAPTER 20 Introduction to the Respiratory System alveoli. Venous blood from the capillaries of the bronchial circulation flows to the heart through either true bronchial veins or bronchopulmonary veins. True bronchial veins are present in the region of the lung hilus, and blood flows into the azygos, hemiazygos, or intercostal veins before entering the right atrium. The bronchopulmonary veins are formed through a network of tributaries from the bronchial and pulmonary circulatory vessels that anastomose and form vessels with an admixture of blood from both circulatory systems. Blood from these anastomosed vessels returns to the left atrium through pulmonary veins. Approximately two thirds of the total bronchial circulation is returned to the heart via the pulmonary veins and this anastomosis route.
The bronchial circulation receives only approximately 1% of total cardiac output; in comparison, the pulmonary circulation receives almost 100%. In the presence of diseases such as cystic fibrosis, the bronchial arteries, which normally receive only 1% to 2% of cardiac output, increase in size (hypertrophy) and receive as much as 10% to 20% of the cardiac output. The erosion of inflamed tissue into these vessels as a result of bacterial infection is responsible for the hemoptysis (coughing up blood) that can occur in this disease.
Breathing is automatic and under control of the central nervous system (CNS). The lungs are innervated by the autonomic nervous system of the peripheral nervous system (PNS), which is under CNS control (
Fig. 20.8).
The autonomic nervous system has four distinct components: parasympathetic, sympathetic, nonadrenergic noncholinergic inhibitory, and nonadrenergic noncholinergic stimulatory.