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Questions and Answers
What is the average diameter of pulmonary capillaries?
What phenomenon occurs as red blood cells pass through pulmonary capillaries?
What begins to happen in smaller pulmonary arterial vessels?
What type of influences affect the pulmonary vascular smooth muscle?
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Which segment may be considered functional pulmonary capillaries?
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What is the primary function of the pulmonary system?
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How does the diffusion rate of gases depend on the conditions within the lungs?
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What occurs to the pulmonary vascular resistance as lung volume increases?
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What is the significance of hypoxic pulmonary vasoconstriction?
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Which factor primarily limits the diffusion of oxygen in the lungs?
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Which of the following correctly contrasts pulmonary and systemic circulation?
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What effect does elevated intravascular pressure have on pulmonary vascular resistance?
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What is the primary anatomical unit involved in gas exchange in the lungs?
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What is a primary consequence of diffusion limitation in the lungs?
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Under which condition are diffusion abnormalities most prevalent?
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Which circulation provides oxygenated blood to the lung structures?
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What percentage of the left ventricle's output constitutes bronchial blood flow?
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What is the primary method through which lung structures distal to terminal bronchioles receive oxygen?
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What happens to pulmonary blood flow during a pulmonary embolus?
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How does pulmonary blood flow compare to cardiac output at rest?
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What is one of the major roles of the bronchial circulation?
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What is a characteristic of the pulmonary circulation system?
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What anatomical feature connects bronchial capillaries and pulmonary capillaries?
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What is the primary function of the lymphatics in pulmonary circulation?
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Which of the following describes the thickness of the pulmonary arteries compared to systemic vessels?
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How long does it approximately take for a red blood cell to travel through the pulmonary circulation at resting cardiac outputs?
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What capacity for blood do pulmonary veins possess compared to systemic veins?
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Which of the following statements best describes the composition of blood in pulmonary circulation?
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What is the major outcome of the thinner and more distensible nature of pulmonary vessels?
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What is located in close proximity to the terminal bronchioles within the lymphatic system?
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What is the primary site for gas exchange in the lungs?
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What primarily regulates pulmonary blood flow?
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What does hypoxic vasoconstriction achieve in the lungs?
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What is the role of oxygen in the mechanism of hypoxic vasoconstriction?
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What happens to pulmonary vascular resistance during widespread lung diseases?
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What effect does normal alveolar oxygen pressure (PAO2) have on pulmonary arterioles?
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Which of the following equations represents pulmonary vascular resistance?
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How does hypoxic vasoconstriction protect lung function?
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Which vasoactive substance is primarily responsible for changes in pulmonary vascular resistance?
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Study Notes
Pulmonary Circulation Physiology
- The pulmonary system facilitates gas exchange from environmental air into the circulatory system.
- The pulmonary circulation moves deoxygenated blood to lungs for oxygenation then returns oxygenated blood to the heart.
- Lung lobes are made up of alveoli, which are small sacks of air. There are about 300 million alveoli in healthy lungs.
- Diffusion occurs at the surface of the alveoli where gas exchange between air and pulmonary arterioles occurs.
- Diffusion is a passive movement of molecules from an area of higher concentration to an area of lower concentration.
Diffusion (Part 1/4)
- Ventilation is the movement of air into and out of the lungs.
- Ventilation creates an environment where oxygen (O2) is in high concentration in the lung and carbon dioxide (CO2) is in lower concentration relative to pulmonary capillaries.
- Diffusion rate is also dependent on the solubility of a gas, gas density, and the available surface area for diffusion.
- CO2 is highly soluble in physiologic conditions making O2 the limiting factor.
- Total available surface area is a very important variable in pulmonary pathology.
Diffusion (Part 2/4)
- A malformation in any of the parameters relating to diffusion may lead to hypoxia.
- Diffusion limitation occurs when the movement of O2 from alveoli to pulmonary vasculature is impaired.
- Examples of limited diffusion disease include lung fibrosis and chronic obstructive pulmonary disease (COPD).
Diffusion (Part 3/4)
- Perfusion is the flow of blood through the lungs.
- Ventilation/perfusion ratio (V/Q) is a measure of the efficiency of gas exchange in the lungs.
- V/Q mismatching is often coexistent with diffusion abnormalities and is most prevalent under exercise conditions.
Diffusion (Part 4/4)
- At rest, blood flow through the lung arterioles is slow enough to allow for proper diffusion.
- Under exercise conditions, cardiac output increases and there is less time for oxygenation to occur in the lung, which can lead to transient hypoxia.
Bronchial Circulation
- The bronchial circulation is part of the systemic circulation and supplies oxygenated blood to the lungs.
- The bronchial arteries arise from the aorta or the intercostal arteries.
- Bronchial arteries supply blood to the tracheobronchial tree and structures of the lung down to the terminal bronchioles.
- Bronchial arteries also provide blood flow to hilar lymph nodes, visceral pleura, pulmonary arteries and veins, vagus, and esophagus.
Bronchial Circulation (continued)
- Lung structures distal to the terminal bronchioles receive O2 directly by diffusion from alveolar air and nutrients from pulmonary circulation.
- The bronchial circulation constitutes about 2% of the output of the left ventricle.
- Blood pressure in bronchial arteries is the same as in systemic arteries and much higher than in pulmonary arteries.
Pulmonary Circulation
- Pulmonary circulation carries deoxygenated blood to the lungs for oxygenation and returns oxygenated blood to the heart.
- The pulmonary blood flow undergoes gas exchange with alveolar air in pulmonary capillaries.
- Pulmonary blood flow is equal to approximately 100% of the output of the left ventricle.
- Pulmonary blood flow is equal to the cardiac output which is approximately 3.5 L/min/m2 of body surface area at rest.
Pulmonary Circulation (continued)
- The pulmonary circulation is a low-pressure system.
- The Pulmonary circulation is divided into three parts: arterial circuit, venous circuit, and lymphatics.
Arterial Circuit
- The arterial circuit begins with the main pulmonary artery arising from the right ventricle.
- The pulmonary arteries are thinner and larger in diameter than systemic arteries.
- These vessels are more distensible and compliant which means they can accommodate greater changes in blood volume.
Venous Circuit
- The venous circuit begins with venules which drain the capillaries and join to form smaller and larger veins.
- The main pulmonary veins drain into the left atrium.
- Pulmonary veins are also thinner and more distensible than systemic veins.
- Pulmonary veins accommodate more blood because of their greater compliance.
Lymphatics
- Lymphatics prevent the accumulation of tissue fluid around the pulmonary circulation.
- They drain the mediastinal lymphatics before emptying into the right lymphatic duct.
Blood Flow to the Lung
- Histologists have identified anastomoses between some bronchial capillaries and pulmonary capillaries.
- These connections probably play little role in a healthy person but may open in pathologic states.
Control of Pulmonary Vascular Smooth Muscle
- Pulmonary vascular smooth muscle is responsive to both neural and humoral influences that produce active alterations in pulmonary vascular resistance (PVR).
- Passive factors, such as gravity, can also alter PVR.
Regulation of Pulmonary Circulation
- Pulmonary blood flow is regulated primarily by altering the resistance of the arterioles (PVR).
- Such changes in resistance are accomplished by changes in the tone of arteriolar smooth muscle.
- Changes in arteriolar tone are mediated by local vasoactive substances, especially oxygen (O2).
Hypoxic Vasoconstriction
- Hypoxic vasoconstriction is the major factor regulating pulmonary blood flow.
- The mechanism of hypoxic pulmonary vasoconstriction is not completely understood.
- The response occurs locally, that is, only in the area of the alveolar hypoxia.
- Decreases in PAO2 produce pulmonary vasoconstriction.
Hypoxic Vasoconstriction (continued)
- Hypoxic vasoconstriction in the lungs is an adaptive mechanism that reduces pulmonary blood flow to poorly ventilated areas.
- Pulmonary blood flow is directed away from poorly ventilated regions toward well-ventilated regions where gas exchange is better.
Regulation of Pulmonary Circulation (continued)
- In certain types of lung disease, hypoxic vasoconstriction serves a protective role because blood can be redirected to alveoli that are well oxygenated.
- If lung disease is widespread, the compensatory mechanism fails and if there are insufficient areas of well-ventilated alveoli, hypoxemia will occur.
Mechanism of Hypoxic Vasoconstriction
- The mechanism involves a direct action of alveolar PO2 on vascular smooth muscle of pulmonary arterioles.
- When PAO2 is normal (at 100 mmHg), O2 diffuses from the alveoli into the nearby arteriolar smooth muscle cells, causing vaso-relaxation and dilatation to the arterioles.
- If PAO2 is low, the smooth muscle cells contract, causing vasoconstriction of the arterioles.
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Description
This quiz covers the physiological aspects of pulmonary circulation and the process of diffusion related to gas exchange in the lungs. You will learn about the structure of alveoli, the movement of air during ventilation, and the factors affecting diffusion rates. Test your understanding of how oxygen and carbon dioxide interact in the pulmonary system.
