--- title: "Pulmonary Venules | Anatomy and Physiology I" description: "Pulmonary venules are tiny lung vessels that drain blood from alveolar capillaries into pulmonary veins, finishing gas exchange in Anatomy and Physiology I." canonical: "https://fiveable.me/anatomy-physiology/key-terms/pulmonary-venules" type: "key-term" subject: "Anatomy and Physiology I" unit: "Unit 22" --- # Pulmonary Venules | Anatomy and Physiology I ## Definition Pulmonary venules are small vessels in the lungs that collect blood from alveolar capillaries and drain it into the pulmonary veins. In Anatomy and Physiology I, they are part of the pathway that returns oxygenated blood to the heart. ## What It Is Pulmonary venules are the small vessels in the lungs that pick up blood after it has passed through the alveolar capillary network. In Anatomy and Physiology I, they sit on the venous side of pulmonary circulation, between the capillaries where gas exchange happens and the larger pulmonary veins that carry blood back to the heart. A useful way to picture them is as the first drainage vessels after the exchange surface. Blood leaves the alveolar capillaries, which are wrapped around the alveoli, and enters the venules before collecting into the pulmonary veins. That means pulmonary venules are part of the return path that sends blood toward the left atrium, not the path that brings blood to the lungs. Their walls are thin, like other small vessels, so they do not block flow or add much resistance. Even though gas exchange mostly happens across the alveolar-capillary membrane, the venules are still part of the same low-pressure system that supports efficient movement of blood through the lungs. This low-pressure flow matters because the pulmonary circuit is designed for exchange, not for pushing blood through a high-resistance network. A common point of confusion is the direction of oxygenation. By the time blood enters the pulmonary venules, it has usually already picked up oxygen and released carbon dioxide in the capillaries. So pulmonary venules do not mainly perform exchange themselves, but they carry the blood immediately after exchange and help maintain smooth drainage from the lung tissue. You can think of them as a transition vessel. They connect microscopic exchange regions to the larger vessels that return blood to the heart, which makes them part of the bigger story of how the respiratory and cardiovascular systems work together to keep blood oxygenated. ## Why It Matters Pulmonary venules show up any time you trace blood flow through the lungs from the right side of the heart to the left side. If you can identify them, you can explain the full sequence of pulmonary circulation instead of stopping at the alveoli. They also help you keep the direction of gas exchange straight. Oxygen enters the blood at the alveolar capillaries, then the blood moves into the venules, then into the pulmonary veins, and finally back to the left atrium. That order matters on diagrams, labeling questions, and short-answer prompts that ask you to describe the route of blood through the respiratory system. The term also comes up when a question asks what happens if lung drainage or pressure changes are abnormal. Because pulmonary venules are part of the low-pressure return pathway, blockage or backup in this region can affect how well blood leaves the lungs and can interfere with efficient oxygenation. In a lab or visual identification task, you use the term to distinguish a venule from an arteriole or a capillary. That means looking at vessel size, position in the circulation, and whether blood is entering or leaving the exchange area. ## Connections ### Alveolar Capillaries Alveolar capillaries are the tiny vessels where gas exchange actually happens. Blood in these capillaries gives up carbon dioxide and takes on oxygen before draining into pulmonary venules, so they are the immediate upstream connection. ### Pulmonary Veins Pulmonary venules feed into the pulmonary veins. The venules are the smaller collecting vessels, while the pulmonary veins are the larger vessels that carry oxygenated blood back to the left atrium. ### Deoxygenated Blood Deoxygenated blood is the blood entering the lungs from the right side of the heart. It reaches the alveolar capillaries first, then passes into pulmonary venules after gas exchange has shifted it toward an oxygenated state. ### [Carbonic Anhydrase](/anatomy-physiology/key-terms/carbonic-anhydrase) Carbonic anhydrase helps convert carbon dioxide and bicarbonate forms inside red blood cells, which supports CO2 unloading in the lungs. That chemistry happens in the exchange region before blood drains into pulmonary venules. ## On the AP Exam A quiz question might ask you to label a lung blood vessel on a diagram or trace the path of blood after gas exchange. Pulmonary venules are the correct answer when the vessel is carrying blood away from the alveolar capillaries and toward the pulmonary veins. If you get a sequence question, place them after the capillaries and before the pulmonary veins. If you are given a case about impaired lung drainage or elevated pulmonary pressure, you may need to explain that venules are part of the low-pressure return side of the pulmonary circuit, so backup here can disrupt normal flow through the lungs. On an image or flow chart, the safest move is to identify what comes before and after the venule. If it receives blood from capillaries, it is a venule. If it is larger and heading back to the heart, it is a vein. That distinction shows up often in anatomy labeling and blood flow questions. ## Pulmonary Venules vs Pulmonary Veins Pulmonary venules and pulmonary veins both carry blood back toward the heart, but they are not the same size or location. Pulmonary venules are the smaller vessels that drain the alveolar capillaries, while pulmonary veins are the larger vessels that collect that blood and deliver it to the left atrium. ## Key Takeaways - Pulmonary venules are small lung vessels that drain blood from alveolar capillaries into the pulmonary veins. - They sit on the return side of pulmonary circulation, after gas exchange has already happened in the capillaries. - Their job is drainage and transport, not the main exchange of oxygen and carbon dioxide. - Knowing their place in the blood flow sequence helps you label diagrams and answer pathway questions correctly. - If pulmonary venous flow is backed up, the lungs can have trouble moving blood efficiently through the exchange system. ## FAQs ### What is pulmonary venules in Anatomy and Physiology I? Pulmonary venules are tiny vessels in the lungs that collect blood from alveolar capillaries and move it into the pulmonary veins. They are part of pulmonary circulation, so they help return blood toward the left side of the heart after gas exchange. ### Are pulmonary venules where gas exchange happens? Not mainly. Gas exchange happens across the alveolar-capillary membrane, especially in the capillaries wrapped around the alveoli. Pulmonary venules carry the blood after that exchange step and help drain it out of the lung tissue. ### How are pulmonary venules different from pulmonary veins? Pulmonary venules are smaller collecting vessels, and pulmonary veins are larger vessels that carry the blood to the left atrium. Think of the venules as the first drainage channels after the capillaries, and the veins as the bigger return route. ### Where do pulmonary venules fit in the path of blood flow through the lungs? Blood flows from the right ventricle to the pulmonary arteries, then to arterioles and alveolar capillaries. After gas exchange, it moves into pulmonary venules, then pulmonary veins, and finally back to the left atrium. ## Related Study Guides - [22.2 The Lungs ](/anatomy-physiology/unit-22/2-lungs/study-guide/IRaA60K8hYqmoOGP) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/anatomy-physiology/key-terms/pulmonary-venules#resource","name":"Pulmonary Venules | Anatomy and Physiology I","url":"https://fiveable.me/anatomy-physiology/key-terms/pulmonary-venules","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/anatomy-physiology/key-terms/pulmonary-venules#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:20:41.765Z","isPartOf":{"@type":"Collection","name":"Anatomy and Physiology I Key Terms","url":"https://fiveable.me/anatomy-physiology/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/anatomy-physiology/key-terms/pulmonary-venules#term","name":"Pulmonary Venules","description":"Pulmonary venules are small vessels in the lungs that collect blood from alveolar capillaries and drain it into the pulmonary veins. 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