The pulmonary vasculature is a network of blood vessels in the lungs, a specialized circuit. It facilitates blood flow between heart and lungs, ensuring oxygen delivery and waste gas expulsion. It operates under distinct conditions, differing from systemic circulation.
Components of the Pulmonary Vasculature
Pulmonary vasculature includes arteries, capillaries, and veins. Pulmonary arteries originate from the right ventricle, carrying deoxygenated blood to lungs. They branch extensively, transitioning from large, elastic to smaller, muscular arteries in lung tissue.
Smaller arteries divide into arterioles, leading into pulmonary capillaries. Microscopic capillaries form dense mesh around alveoli. Their thin walls (often one cell thick) allow efficient exchange, maximizing surface area for gas transfer.
After gas exchange, capillaries form venules, merging into larger pulmonary veins. They collect oxygenated blood from lungs. These veins return oxygen-rich blood to left atrium, pumping it to the body.
How Pulmonary Blood Vessels Work
Pulmonary circulatory system is low-pressure compared to systemic circulation. Pulmonary artery pressure (15-30/4-12 mmHg) is lower than aortic pressures. This minimizes capillary strain, protecting them and preventing fluid leakage.
Gas exchange, primary function, occurs across the alveolar-capillary membrane. Oxygen from inhaled air diffuses from alveoli, through thin capillary walls, into red blood cells. Simultaneously, carbon dioxide diffuses from red blood cells into alveoli for exhalation. This exchange is facilitated by the vast surface area of millions of alveoli and their capillary networks (70-100 sq. meters).
Pulmonary blood flow is regulated to optimize gas exchange via hypoxic pulmonary vasoconstriction. When lung oxygen levels decrease, local pulmonary arterioles constrict, diverting blood from poorly ventilated areas. This directs blood to well-oxygenated lung regions, maintaining efficient oxygen uptake. This contrasts with systemic vessels, which dilate with low oxygen.
Pulmonary vascular resistance (PVR), resistance to blood flow, is low. PVR is influenced by vessel caliber, changing with oxygen, nitric oxide, and local signals. Narrowing or stiffening vessels increases PVR, impacting the heart’s pumping ability. Beyond gas exchange, it filters small clots or air bubbles before reaching systemic circulation. It also serves as a blood reservoir (about 9% of total blood volume), mobilizable as needed.
Conditions Affecting Pulmonary Blood Vessels
Conditions can disrupt pulmonary vasculature, leading to health challenges like pulmonary hypertension (PH), high blood pressure in lung arteries. This forces right heart to work harder, potentially weakening and enlarging it. PH can arise from left-sided heart failure, chronic lung conditions (e.g., emphysema, pulmonary fibrosis), or be idiopathic.
Increased PH pressure leads to symptoms like shortness of breath, fatigue, and chest pain during activity. Over time, sustained high pressure thickens and stiffens pulmonary artery walls, increasing blood flow resistance. This remodeling makes the condition difficult to manage, impacting quality of life. Understanding PH type guides management.
Pulmonary embolism (PE) occurs when a blood clot, often from a deep leg vein, lodges in a pulmonary artery. This blocks blood flow to a lung portion, preventing gas exchange. Large or multiple emboli strain right heart, impairing oxygen delivery and potentially leading to life-threatening consequences.
Pulmonary edema is excess fluid in lung air sacs and surrounding tissue, often due to pulmonary vessel or heart issues. This fluid impairs gas exchange, making breathing difficult. It often results from conditions increasing pressure within pulmonary capillaries, forcing fluid into lung tissue. Heart failure is a common cause, as a weakened heart may not effectively pump blood from lungs, leading to pressure backup in pulmonary circulation.
References
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Klabunde, Richard E. “Cardiovascular Physiology Concepts.” Lippincott Williams & Wilkins, 2012.
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Konstantinides, Stavros V., et al. “2019 ESC Guidelines for the diagnosis and management of acute pulmonary embolism developed in collaboration with the European Respiratory Society (ERS): The Task Force for the diagnosis and management of acute pulmonary embolism of the European Society of Cardiology (ESC) and the European Respiratory Society (ERS).” European Heart Journal 40.32 (2019): 2915-2960.
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