Perfusion of the lungs & the alveolar gas-blood interface L22 Flashcards
Perfusion
The process by which blood is delivered to the tissues and organs of the body, providing them with essential nutrients and oxygen while removing waste products like carbon dioxide. In the context of the lungs, pulmonary perfusion refers to the flow of blood through the pulmonary capillaries surrounding the alveoli, where gas exchange occurs.
Pulmonary Circulation pressure
Pressure: The pulmonary artery pressure is shown as 22/10 mmHg with a mean pressure of 14 mmHg.
22 mmHg is the systolic pressure (SP) when the heart contracts and pumps blood into the pulmonary arteries.
10 mmHg is the diastolic pressure (DP) when the heart is relaxed, and blood is returning to the heart.
The pulmonary circulation operates under lower pressure because its main job is to oxygenate the blood, requiring only a short distance of blood flow between the heart and lungs.
Pulmonary Circulation (right side of the heart): Blood from the body returns to the right atrium, moves into the right ventricle, and is pumped through the pulmonary arteries to the lungs for oxygenation.
Systemic Circulation:
Pressure: The systemic circulation pressure is much higher at 120/80 mmHg with a mean pressure of 93 mmHg.
120 mmHg is the systolic pressure, the peak pressure during heart contraction when blood is pumped into the systemic arteries.
80 mmHg is the diastolic pressure, the lowest pressure between heartbeats when the heart is refilling with blood.
The higher pressure in systemic circulation is necessary to ensure blood can reach all parts of the body, from head to toes.
Systemic Circulation (left side of the heart): Oxygenated blood from the lungs returns to the left atrium, moves into the left ventricle, and is pumped out through the aorta to supply the body with oxygen and nutrients
Mean Arterial Pressure (MAP)
Mean Arterial Pressure (MAP): A crucial measure of overall blood flow and pressure, calculated as:
Two circulations to the lungs: 1. Pulmonary Circulation
- Pulmonary Circulation:
Function: The primary purpose of the pulmonary circulation is gas exchange.
Pathway:
Deoxygenated blood from the right heart (right ventricle) is pumped into the pulmonary arteries.
This blood travels to the alveolar capillaries surrounding the alveoli (tiny air sacs in the lungs).
In the alveoli, oxygen is absorbed into the blood, and carbon dioxide is released from the blood into the lungs to be exhaled (ventilation).
Oxygenated blood then returns to the left heart (left atrium) via the pulmonary veins, from where it is pumped to the systemic circulation to supply the body with oxygen.
Role: Pulmonary circulation is essential for oxygenating the blood and removing carbon dioxide, supporting respiration.
Two circulations to the lungs: 2. Bronchial Circulation
Function: This circulation supplies oxygenated blood to the lung tissue itself, particularly the tracheobronchial tree (the airways including the bronchi and bronchioles).
Pathway:
Oxygenated blood from the aorta flows through the bronchial arteries to nourish the tissues of the lungs, including the bronchi and connective tissues.
The deoxygenated blood from this circulation drains either into the pulmonary veins or back into the systemic veins. This is where the anatomical shunt comes in.
Anatomical Shunt: Some deoxygenated blood from the bronchial circulation mixes with oxygenated blood in the pulmonary veins, slightly lowering the overall oxygen content of the blood returning to the heart. This mixing is normal and is referred to as a physiological shunt.
Pulmonary pressure profile
Pulmonary Artery Pressure:
The graph begins with a systolic pressure of 22 mmHg and a diastolic pressure of 8 mmHg in the pulmonary artery.
The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs.
The mean pulmonary artery pressure is indicated to be 14 mmHg, a value that averages systolic and diastolic pressures.
Pulmonary Capillaries:
As the blood moves through the pulmonary circulation, pressure drops significantly, especially as it passes through the pulmonary capillaries, where gas exchange occurs.
This pressure drop is important because the capillaries must allow enough time for oxygen to diffuse into the blood and carbon dioxide to diffuse out.
Left Atrium:
By the time the blood reaches the left atrium (the chamber that receives oxygenated blood from the lungs), the pressure has dropped to about 5 mmHg or lower.
This low pressure is necessary to maintain proper flow and avoid fluid buildup in the lungs.
Systolic and Diastolic Oscillations
The systolic-diastolic oscillations (wave-like curves) seen in the graph represent the fluctuations in pressure during the heart’s contraction and relaxation phases.
The systolic pressure represents the peak during heart contraction (when the blood is actively pumped), while diastolic pressure is the lowest point during heart relaxation.
Sheet blood flow around alveoli
Key Features:
Arteriole (left side, purple):
Arterioles carry deoxygenated blood from the pulmonary arteries to the alveolar capillaries.
The blood entering this structure has low oxygen and high carbon dioxide levels, coming from the body through the pulmonary circulation.
Venule (right side, red):
Venules carry oxygenated blood away from the alveolar capillaries to the pulmonary veins.
After gas exchange, this blood is rich in oxygen and low in carbon dioxide, ready to be sent to the left side of the heart and then the body.
Posts (representing the alveoli):
The alveoli are represented by the “posts” at the top of the diagram. These are the air sacs where oxygen from inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses into the alveoli to be exhaled.
Interstitial Space:
The interstitium is the space between the alveoli and the blood vessels (capillaries) where gas exchange takes place.
The thickness of the interstitial space (denoted by h x) is critical, as the thinner it is, the more efficient the gas exchange.
Direction of Blood Flow:
Blood flows from the arteriole (left) through the capillary sheet around the alveoli (middle), and then into the venule (right).
This flow pattern is crucial for ensuring that deoxygenated blood is exposed to the alveoli for oxygenation.
Physical factors affecting blood flow
As pressure in the pulmonary arteries increases, resistance decreases. This inverse relationship ensures that the lungs can accommodate higher blood flow without increasing pressure too much, which would otherwise lead to fluid accumulation or oedema.
Distension and Recruitment:
Two mechanisms help manage increased blood flow:
Distension: At higher pulmonary pressures, the pulmonary capillaries expand or distend. This increases their capacity, reducing resistance to blood flow.
Recruitment: When blood flow increases, previously closed or underused capillaries are opened (recruited), allowing more capillaries to carry blood. This also lowers overall pulmonary resistance.
Prevents Oedema:
Oedema occurs when excess fluid accumulates in the lungs, impairing gas exchange. By reducing pulmonary resistance via recruitment and distension, the lungs can handle increased blood flow without raising too much pressure, which helps prevent oedema.
This is crucial for maintaining the integrity of the blood-gas barrier in the lungs, ensuring that the delicate balance of pressures allows efficient gas exchange while avoiding fluid leakage into the alveoli.
Process of pulmonary hypoxic vasoconstriction
The chemical control of pulmonary blood vessels in response to low oxygen levels in the lungs.
Pulmonary Artery and Vein:
The pulmonary artery delivers deoxygenated blood from the right heart to the lungs.
The pulmonary vein returns oxygenated blood to the left atrium of the heart after gas exchange has occurred in the lungs.
Alveoli and Airway:
The alveoli (air sacs) are where gas exchange happens. Oxygen from the air enters the blood, and carbon dioxide from the blood is expelled into the alveoli to be exhaled.
The diagram shows alveolar oxygen pressure, denoted as Pₒ₂ (partial pressure of oxygen), which is critical for proper gas exchange.
Pulmonary Hypoxic Vasoconstriction:
When an alveolus has low oxygen levels (hypoxia), the nearby pulmonary arteries constrict (narrow). This process is called regional vasoconstriction.
Pulmonary hypoxic vasoconstriction is the body’s response to divert blood away from poorly ventilated or damaged alveoli (low oxygen areas) to better-ventilated areas where gas exchange can occur more effectively.
This helps optimize oxygen uptake by ensuring blood flows to areas of the lung where oxygen is plentiful.
Purpose:
Prevents Mismatch: Pulmonary hypoxic vasoconstriction prevents a ventilation-perfusion mismatch, where blood flows to areas of the lung that are not well-oxygenated. This process improves the efficiency of gas exchange.
Adaptive Response: It is an important adaptive mechanism, especially in conditions like lung disease or high altitudes, where some areas of the lung may have reduced oxygen availability.
Summary:
Pulmonary hypoxic vasoconstriction is a regulatory mechanism where blood vessels around poorly ventilated alveoli constrict in response to low oxygen levels. This redirects blood to well-ventilated areas, ensuring efficient oxygen uptake and preventing wasted blood flow to areas where oxygen cannot be absorbed effectively.
The un-even blood flow in the lungs
Uneven Distribution of Blood Flow:
The Y-axis represents blood flow per unit volume, and the X-axis represents the distance up the lung (from bottom to top).
Blood flow is highest at the base (bottom) of the lungs and progressively decreases as you move toward the apex (top).
This variation in blood flow is due to the effects of gravity on the pulmonary circulation.
Explanation:
Gravity plays a key role in pulmonary blood flow distribution. When standing or sitting, the lower parts of the lung receive more blood due to gravitational forces pulling blood downward.
Blood vessels are more distended at the base of the lungs, allowing for greater blood flow. This is reflected in the higher measurements at the bottom of the lung.
In contrast, at the apex of the lungs, blood vessels are less distended, and there is less perfusion due to lower pressure. This results in reduced blood flow at the top of the lung.
Three Zones of Pulmonary Blood Flow:
Zone 1 (Top of the Lung):
Pressures: PA > Pa > Pv
In this region, alveolar pressure (PA) is higher than both arterial (Pa) and venous (Pv) pressures. This means that the pressure inside the alveoli can compress the capillaries, reducing or even stopping blood flow in these vessels.
Flow: Very little or no blood flow occurs in Zone 1 because the alveolar pressure essentially “pinches” the capillaries closed.
Zone 2 (Middle of the Lung):
Pressures: Pa > PA > Pv
In Zone 2, arterial pressure (Pa) is greater than alveolar pressure (PA), which allows blood flow to occur. However, alveolar pressure is still higher than venous pressure (Pv), meaning that the amount of blood flow is determined by the difference between arterial and alveolar pressures.
Flow: Blood flow is moderate in this region, but the alveolar pressure still has some effect on the flow.
Zone 3 (Bottom of the Lung):
Pressures: Pa > Pv > PA
In this zone, both arterial (Pa) and venous pressures (Pv) are greater than alveolar pressure (PA). This means that the blood vessels are fully open, and blood flow is maximized.
Flow: This is where the highest blood flow occurs because the capillaries are not compressed, and the pressure gradient between arterial and venous pressures is driving the flow.
Gravity’s Effect:
Gravity causes blood to pool more toward the bottom of the lungs, increasing blood flow in Zone 3 and decreasing flow in Zone 1.
As seen on the right side of the diagram, blood flow increases from the top to the bottom of the lung, with Zone 3 having the greatest blood flow due to gravity’s effect and the pressure gradient.
Ventilation-Perfusion (V/Q) Ratio
A ratio which compares the amount of air reaching the alveoli (ventilation) to the amount of blood reaching the alveoli (perfusion).
Disease states
Pulmonary Hypertension:
Hypoxia (low oxygen levels) leads to vasoconstriction (narrowing of blood vessels), specifically in the pulmonary arteries.
This increases the pressure in the pulmonary circulation and forces the right ventricle to work harder to pump blood through the lungs.
Over time, this can cause right heart failure as the right side of the heart becomes strained and weakened.
Pulmonary Edema:
Caused primarily by left heart failure, where the left ventricle cannot efficiently pump blood to the body.
This failure causes a backup of blood in the lungs, leading to fluid accumulation in the lung tissue (edema).
The resultant systemic hypoxia (low oxygen throughout the body) causes breathlessness or dyspnea.
Factors regulating movement of gas across the respiratory surface
Area, thickness of tissue, partial pressure differential across tissue, solubility of gas in blood, molecular weight of gas
How does thickness of tissue regulate movement of gas exchange?
A: Surfactant – A substance that reduces surface tension in the alveoli, preventing collapse and aiding in gas exchange.
B: Epithelium – The thin layer of cells that line the alveoli, allowing oxygen to pass from the alveoli into the bloodstream.
C: Interstitium – A thin layer of tissue between the alveolar epithelium and the capillaries, containing connective tissue and other structural elements.
D: Basement membrane – A thin layer that supports the epithelial and endothelial cells and helps hold the structure together.
E: Endothelial cell – These cells line the blood capillaries where gas exchange takes place between the air and the blood.
Important point:
The thickness of the alveolar-capillary barrier is only about 0.5 micrometers (µm), which allows for efficient diffusion of gases like oxygen and carbon dioxide between the air in the alveoli and the blood in the capillaries.
Partial pressure differential across tissue
Oxygen (O₂) partial pressure differential:
In alveoli (air): The partial pressure of oxygen is around 100 mmHg.
In blood: The partial pressure of oxygen is around 40 mmHg.
This creates a large driving force for oxygen to move from the alveoli into the blood. The driving force is described as 10x bigger than that for carbon dioxide.
Carbon dioxide (CO₂) partial pressure differential:
In alveoli (air): The partial pressure of CO₂ is around 40 mmHg.
In blood: The partial pressure of CO₂ is around 46 mmHg.
This generates a smaller driving force for carbon dioxide to move from the blood into the alveoli for expiration.
Despite the smaller gradient, CO₂ diffuses easily because it is more soluble in blood compared to O₂. This balance in diffusion rates ensures that both oxygen enters the bloodstream efficiently, and carbon dioxide is effectively removed, maintaining proper gas exchange and respiration.
