Pulmonary Circulation Quiz

Pulmonary Circulation Physiology

• Pulmonary circulation facilitates gas exchange between the environment and the circulatory system
• Oxygen (O2) is inhaled, diffuses into the blood, enters systemic circulation, and produces ATP for cellular energy
• Carbon dioxide (CO2) and metabolic byproducts are exhaled
• Lungs contain alveoli (air sacs), approximately 300 million in healthy lungs
• Gas diffusion occurs at the alveoli surface into pulmonary arterioles

Learning Objectives

• Understand bronchial and pulmonary circulation, anatomy, and physiology
• Compare and contrast pulmonary and systemic circulation
• Describe effects of lung volume on pulmonary vascular resistance
• Explain effects of elevated intravascular pressures on pulmonary vascular resistance
• Identify neural and humoral factors that influence pulmonary vascular resistance
• Describe interrelationships of alveolar, pulmonary arterial, and pulmonary venous pressure, and their effects on pulmonary blood flow
• Describe hypoxic pulmonary vasoconstriction and its role in localized and widespread alveolar hypoxia

Diffusion

• Diffusion is passive movement from higher to lower concentration
• Ventilation moves air into and out of the lungs
• Oxygen is high in concentration in the lung and lower in pulmonary capillaries
• Carbon dioxide is lower in concentration in the lung and higher in pulmonary capillaries
• Diffusion rate depends on gas solubility, density, and available surface area in the lung

Diffusion (part 2/4)

• Carbon dioxide is highly soluble, so oxygen is the primary limiting factor in diffusion
• Total available surface area is a key factor in pulmonary pathology
• Reduced alveolar surface area relative to arteriolar perfusion decreases oxygen diffusion into the blood, potentially causing hypoxia

Diffusion (part 3/4)

• Diffusion limitation occurs when oxygen movement from alveoli to pulmonary blood vessels is impaired
• Lung fibrosis and parenchymal destruction reduce alveolar surface area
• Perfusion is blood flow through the lungs
• Ventilation/perfusion ratio (V/Q) measures gas exchange efficiency in the lungs
• V/Q mismatching often coexists with diffusion abnormalities, prevalent during exercise

Diffusion (part 4/4)

• Resting blood flow in lung arterioles is slow, allowing sufficient diffusion time
• Exercise increases cardiac output, reducing oxygenation time and possibly causing transient hypoxia
• Lung fibrosis and COPD are examples of limited diffusion diseases

Pulmonary Circulation

• The lung receives blood flow via bronchial and pulmonary circulation
• Bronchial blood flow is part of systemic circulation, supplying oxygenated blood to the lungs
• Pulmonary blood flow carries deoxygenated blood to the lungs for oxygenation and returns oxygenated blood to the heart

Bronchial Circulation

• Bronchial arteries arise from the aorta, or intercostal arteries
• They supply arterial blood to the tracheobronchial tree and terminal bronchioles
• They also supply blood to hilar lymph nodes, visceral pleura, pulmonary vessels, vagus, and esophagus
• Lung tissues distal to terminal bronchioles receive oxygen directly through diffusion from alveolar air and nutrients from mixed venous blood in pulmonary circulation
• Bronchial blood flow accounts for about 2% of the left ventricle output
• Bronchial artery pressure is similar to other systemic arteries, but higher than pulmonary artery pressure

Blood Flow to the Lung

• Histologists have identified anastomoses between bronchial and pulmonary capillaries and arteries
• These connections are generally minor in healthy individuals but can be important in pathological conditions
• Blood flow to a portion of the lung can be impacted by either bronchial or pulmonary occlusion, with bronchial flow to that area increasing

Pulmonary Circulation (Part 2)

• Pulmonary blood flow undergoes gas exchange within pulmonary capillaries
• Blood flow equals approximately 100% of left ventricular output
• Approximately 3.5 L/min/m² of blood flow at rest with 280 billion pulmonary capillaries supplying ~300 million alveoli
• One arteriole and venule supply and drain a pulmonary lobule

Pulmonary Circulation: A Low-Pressure System

• Diagram illustrating the pulmonary circulation and its low-pressure system flow

Pulmonary Circulation Divisions

• Pulmonary circulation is divided into 3 parts: arterial circuit, venous circuit, and lymphatics

Arterial Circuit

• Originates from the main pulmonary artery, arising from the right ventricle
• It runs a short course (~5cm), then divides into the right and left main branches further dividing into smaller arteries, arterioles, and capillaries
• The vessels are thinner than systemic vessels (1/3 thickness) and have a larger diameter, making them more distensible and compliant

Venous Circuit

• Venules drain capillaries forming smaller veins ultimately connecting to the main pulmonary veins, draining into the left atrium
• Pulmonary veins are also thinner and more distensible than systemic veins, accommodating more blood due to greater compliance

Lymphatics

• Lymphatics are crucial in maintaining a dry alveolar membrane, preventing the accumulation of tissue fluid around the pulmonary circulation
• They are located near terminal bronchioles
• They drain mediastinal lymphatics and ultimately empty into the right lymphatic duct

Pulmonary Veins and Arteries

• Diagram showing the pulmonary veins and arteries within the lungs

Pulmonary Vascular Resistance

• PVR is calculated as (MPAP-MLAP)/PBF, which is equal to the cardiac output, reflecting the resistance to flow in the pulmonary vascular system

Pulmonary Vascular Resistance (Part 2)

• Pulmonary blood flow is pulsatile and can be turbulent
• Pulmonary circulation is distensible and compressible
• Complex branching structure of the pulmonary circulation impacting the pressure gradient
• Mean left atrial pressure might not accurately represent downstream pressure in various lung conditions

Pulmonary Vascular Resistance (Part 3)

• Right and left ventricular outputs must be approximately equal for prolonged stability preventing fluid buildup in the lung or periphery
• Given a drop in pressure across systemic and pulmonary circulation of about 98 mmHg vs 10 mmHg for the respective systems, pulmonary vascular resistance is ~1/10th of systemic vascular resistance

Pulmonary Vascular Resistance (Part 4)

• Low resistance in pulmonary circulation relates to thinner walls with less vascular smooth muscle, offering greater distensibility than systemic circulation

Distribution of Pulmonary Vascular Resistance

• Pressure distribution of the pulmonary vasculature is evenly distributed throughout the pulmonary arteries, capillaries, and veins
• Pulmonary vasculature differs from systemic vasculature by uneven pressure distribution across the lung areas

Distribution of Pulmonary Blood Flow

• Blood flow in the lungs is unevenly distributed and influenced by gravity
• Supine position results in a more uniform distribution
• Upright positions exhibit the lowest flow at the apex and highest at the base due to gravitational influence on hydrostatic blood pressure

Zone 1 Distribution of Pulmonary Blood Flow

• Gravity-dependent arterial pressure can be lower than alveolar pressure at the lung apex
• This compression can cause capillary closure, although perfusion still occurs at low flows

Zone 2 Distribution of Pulmonary Blood Flow

• Higher arterial pressure than alveolar pressure due to gravity's effect, with blood flow driven by the difference between arterial pressure and alveolar pressure, not arterial vs venous, unlike systemic blood flow
• Capillary compression isn't a concern

Zone 3 Distribution of Pulmonary Blood Flow

• Gravity-dependent increased arterial and venous pressure, and both are higher than alveolar pressure
• Blood flow is driven by the difference between arterial and venous pressure

Common Pathologies of the Pulmonary Circulation

• Pulmonary edema: any disturbance in the pulmonary circulation causing fluid accumulation in alveoli and respiratory distress; causes can be cardiogenic or non-cardiogenic
• Pulmonary embolism: dislodged clot causing ischaemia and potential lung parenchyma infarction due to impaired gas exchange; common origin is a deep vein thrombosis
• Pulmonary hypertension: elevation of MPAP above 25mmHg, leading to impaired gas exchange and exertional dyspnea
• Pleural effusion: accumulation of fluid in the pleural space leading to pleuritic chest pain and respiratory distress

Conclusion

• Compared to systemic arteries, pulmonary arteries have significantly less vascular smooth muscle, resulting in lower resistance to blood flow due to the lower vascular transmural pressure gradient
• Passive factors play a more prominent role in pulmonary vascular resistance than neural and humoral factors in contrast to those in systemic circulation
• PVR typically decreases in relation to increase in pulmonary blood flow, pulmonary artery pressure, left atrial pressure, and pulmonary capillary blood volume due to vessel distention
• Blood flow in lower lung regions is typically more ample than in the upper regions
• Alveolar hypoxia or hypercapnia causes constriction of pre-capillary vessels diverting blood away from poorly ventilated regions of the lungs

Recommended Further Literature

• "Physiology" by Linda S. Costanzo (6th Edition) is recommended for further study