Pulmonary Circulation and Gas Exchange

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Questions and Answers

Explain the physiological significance of low pulmonary vascular resistance (PVR) for gas exchange in the lungs.

Low PVR allows for a decrease in blood flow velocity, providing more time for gas exchange. Increased capillary distension due to lower pressure also increases surface area for efficient gas exchange.

Describe how the pulmonary circulation adapts to an increase in cardiac output during vigorous exercise.

Despite the increased blood flow and pressure, pulmonary vascular resistance decreases, reducing the right ventricular workload and preventing pulmonary edema.

What is the effect of parasympathetic nervous system stimulation on pulmonary vascular resistance, and why does this occur?

Parasympathetic stimulation increases PVR via vasoconstriction, likely due to the release of neurotransmitters like acetylcholine, although this effect is less pronounced than sympathetic effects.

Explain the mechanism of hypoxic vasoconstriction in the pulmonary circulation and its significance.

Hypoxia causes vasoconstriction in the pulmonary arteries, diverting blood flow away from poorly ventilated areas to better-ventilated areas, maximizing gas exchange efficiency. Signup and view all the answers

How does the response to low oxygen levels in the pulmonary circulation differ from that in systemic circulation?

In the pulmonary circulation, low oxygen triggers vasoconstriction, while in systemic circulation, it induces vasodilation, reflecting the differing physiological requirements of each system. Signup and view all the answers

Name two potent vasoconstrictors and two vasodilators that influence pulmonary vascular resistance, and briefly explain their effects.

Vasoconstrictors: Serotonin and thromboxane A2 increase PVR by constricting smooth muscle. Vasodilators: Acetylcholine and prostacyclin relax smooth muscle, leading to decreased PVR. Signup and view all the answers

How does gravity impact pulmonary vascular pressure, and what are the implications for blood flow distribution in the lungs?

Gravity causes higher hydrostatic pressure in the lower lung regions, leading to greater pulmonary vascular pressure than in the upper regions. This uneven distribution of pressure affects blood flow. Signup and view all the answers

Why is it important to understand the factors affecting pulmonary blood flow when considering the impact of diseases on the lungs?

Understanding pulmonary blood flow regulation is crucial for comprehending how diseases like pulmonary hypertension, pulmonary edema, and respiratory distress syndrome can disrupt normal lung function. Signup and view all the answers

What is the primary function of the bronchial artery?

To supply arterial blood to the bronchi and supporting lung structures. Signup and view all the answers

What percentage of cardiac output is typically supplied by the bronchial artery?

1-2% of cardiac output. Signup and view all the answers

Where does the venous blood from the bronchial circulation primarily drain?

Into the pulmonary veins, mixing with oxygenated blood. Signup and view all the answers

What arteries do the bronchial veins drain into on the right side of the body?

The azygos vein. Signup and view all the answers

How does the blood from the distal portion of bronchial circulation differ from proximal drainage?

It drains directly into tributaries of pulmonary veins rather than bronchial veins. Signup and view all the answers

What physiological occurrence results from the mixing of venous and arterial blood in the lungs?

Physiological shunt. Signup and view all the answers

Which structures does the bronchial artery supply besides the bronchi?

Connective tissue, lung stroma, visceral pleura, and pulmonary lymph nodes. Signup and view all the answers

What type of blood does the bronchial artery carry?

Oxygenated blood. Signup and view all the answers

What is the relationship between the volume of the pulmonary capillary and the stroke volume of the right ventricle?

The volume of the pulmonary capillary is approximately equal to the stroke volume of the right ventricle. Signup and view all the answers

How can blood volume in the lungs change in response to systemic hemorrhage?

Blood volume in the lungs can vary from one half normal to twice its volume, compensating for blood loss by shifting blood to the systemic vessels. Signup and view all the answers

What is the primary function of pulmonary lymphatics?

The primary functions are to remove particulate matter entering the alveoli and to prevent edema by managing plasma protein leakage from lung capillaries. Signup and view all the answers

Compare the distensibility of pulmonary blood vessels to systemic blood vessels.

Pulmonary blood vessels are more distensible than systemic blood vessels, allowing for lower pulmonary blood pressure. Signup and view all the answers

What are the mean pressures in the pulmonary artery and systemic artery?

The mean pressure in the pulmonary artery is 15 mmHg, and in the systemic artery (aorta), it is 100 mmHg. Signup and view all the answers

State the systolic and diastolic pulmonary arterial pressures.

The systolic pulmonary arterial pressure is 25 mmHg, and the diastolic pulmonary arterial pressure is 8 mmHg. Signup and view all the answers

What is the mean pressure in the left atrium and major pulmonary veins?

The mean pressure in the left atrium and major pulmonary veins averages around 2 mmHg, typically ranging from 1 to 5 mmHg. Signup and view all the answers

Explain the significance of the driving pressure in both pulmonary and systemic circulation.

The driving pressure in pulmonary circulation is 10 mmHg, while in systemic circulation, it is 98 mmHg, highlighting the disparity in pressure needed to move blood through each system. Signup and view all the answers

What is the primary physiological effect of administering diuretics in cases of pulmonary edema?

Diuretics reduce pulmonary capillary hydrostatic pressure by promoting fluid loss, thus alleviating edema. Signup and view all the answers

How does the composition of fresh water aspirated during drowning lead to different physiological effects compared to saltwater?

Fresh water causes low hydrostatic pressure and hemolysis of red blood cells, while saltwater's high Na+ and Cl- concentration leads to pulmonary edema and asphyxia. Signup and view all the answers

What role does pleural fluid play in maintaining lung expansion?

Pleural fluid creates negative pressure, which keeps the lungs pulled against the chest cavity, preventing collapse. Signup and view all the answers

What is a likely cause of pleural effusion related to cardiac failure?

Cardiac failure leads to high peripheral and pulmonary capillary pressure, causing fluid accumulation in the pleural space. Signup and view all the answers

Describe the connection between hyperkalaemia, hyponatremia, and drowning in fresh water.

Fresh water drowning can lead to hemolysis of red blood cells, resulting in hyperkalaemia and hyponatremia, which may trigger ventricular fibrillation. Signup and view all the answers

What occurs in Zone 2 of the lungs regarding pulmonary blood flow?

In Zone 2, blood flow is intermittent, related to the phases of inhalation and exhalation. Signup and view all the answers

Describe the average ventilation/perfusion (V/Q) ratio at rest.

The average V/Q ratio at rest is 0.8, with ventilation at 4.2 mL and perfusion at 5.5 mL. Signup and view all the answers

What happens to the PO2 and PCO2 in the alveoli if ventilation exceeds perfusion?

If ventilation exceeds perfusion, the PO2 in the alveoli increases and the PCO2 decreases. Signup and view all the answers

How does the supine position affect pulmonary blood flow in the lungs?

In the supine position, the whole lungs act as Zone 3, allowing for blood flow to all parts of the lungs. Signup and view all the answers

What is the significance of the arterial to venous pressure gradient in pulmonary blood flow?

The arterial to venous pressure gradient determines the rate of blood flow, which is influenced by alveolar pressure. Signup and view all the answers

What occurs in the pulmonary circulation when there is left-sided heart failure?

Left-sided heart failure leads to increased left atrial pressure, causing pulmonary venules to expand and allowing continuous blood flow despite pressure changes. Signup and view all the answers

Why is pulmonary capillary pressure lower than systemic capillary pressure?

Pulmonary capillary pressure is lower (about 7 mmHg) compared to systemic capillary pressure (about 17 mmHg) to facilitate efficient gas exchange. Signup and view all the answers

What happens to the perfusion and ventilation in the alveoli if perfusion exceeds ventilation?

If perfusion exceeds ventilation, PCO2 in the alveoli increases while PO2 decreases. Signup and view all the answers

What role does pulmonary interstitial negative pressure play in keeping the alveoli dry?

Pulmonary interstitial negative pressure helps prevent fluid accumulation in the alveoli by promoting fluid absorption into the pulmonary capillaries and lymphatics. Signup and view all the answers

How does increased lymphatic drainage contribute to fluid management in the lungs?

Increased lymphatic drainage removes excess fluid from the lung interstitium, preventing pulmonary edema by maintaining a normal interstitial fluid balance. Signup and view all the answers

What causes pulmonary edema according to the mechanisms described?

Pulmonary edema can result from increased capillary hydrostatic pressure, decreased oncotic pressure, or increased capillary permeability. Signup and view all the answers

Describe how high interstitial compliance affects fluid dynamics in the lungs.

High interstitial compliance enables fluid to accumulate in the interstitium without a significant rise in hydrostatic pressure, alleviating immediate edema risks. Signup and view all the answers

What happens to interstitial oncotic pressure when lymph flow drains albumin from pulmonary capillaries?

When lymph flow drains leaked albumin, the interstitial oncotic pressure decreases, which can assist in preventing fluid accumulation. Signup and view all the answers

Why can alveolar walls become easily ruptured by positive interstitial pressure?

The alveolar walls are relatively thin and weak; positive interstitial pressure can easily surpass their structural integrity and cause them to rupture. Signup and view all the answers

What are two physiological consequences of increased alveolar space tension?

Increased alveolar space tension can lead to impaired gas exchange and can contribute to the risk of pulmonary edema. Signup and view all the answers

How does the leaky nature of pulmonary capillaries affect interstitial oncotic pressure?

The leaky pulmonary capillaries allow proteins to pass into the interstitial space, which increases interstitial oncotic pressure and influences fluid dynamics. Signup and view all the answers

Study Notes

Pulmonary Circulation and Ventilation and Perfusion

Pulmonary circulation involves the same blood volume as systemic circulation.
Pulmonary vascular capacity is lower than systemic, but the pulmonary vascular bed regulates cardiac output to prevent excessive pressure.
Pulmonary vascular resistance is one-tenth the systemic vascular resistance.
Total blood volume in lungs is 500mL, which is 10% of overall body blood volume.

Learning Outcomes

Discuss the physiology of bronchial circulation.
Understand the significance of the ventilation-perfusion ratio.
Explain the variations in ventilation-perfusion ratio between lung apex and base.
Explain the filtration mechanisms in pulmonary capillaries and the genesis of pulmonary edema.

Blood Circulation

Blood pumped into pulmonary circulation equals blood pumped into systemic circulation.
Pulmonary vascular bed regulates cardiac output, ensuring less pressure compared to systemic circulation.
Systemic blood pressure is high due to systemic vessel resistance, while pulmonary pressure is low due to low resistance in pulmonary vessels.
Pulmonary circulation resistance is one-tenth that of systemic circulation.

Blood Supply to the Lungs

Lungs receive blood via pulmonary arteries and bronchial arteries.
Pulmonary arteries deliver blood to the lungs for gas exchange.
Bronchial arteries supply blood to lung tissue structures and are a branch off of the descending thoracic aorta.
Pulmonary blood vessel volume in the lung accounts for ~40% of the total lung weight.
Blood is distributed segmentally from the pulmonary arteries into the bronchi

Bronchial Artery

Arise from the descending thoracic aorta.
Supplies bronchi, connective tissue, and other lung structures.
Drains into pulmonary veins or drains into the azygos vein (right side) or hemiazygos/superior intercostal veins (left side).

Blood Supply to the Lungs (cont.)

Deoxygenated blood from the pulmonary artery delivers the required blood flow to all parts of the lung for gas exchange.
Oxygenated blood is collected and delivered to the left atrium via one pulmonary vein from each lung.

Physiological Shunt

Diversion where venous blood mixes with arterial blood.
Has two components:
- Flow of deoxygenated blood from bronchial circulation into pulmonary veins (without oxygenation) - normal part of pulmonary circulation (1-2% of CO).
- Flow of deoxygenated blood from Thebesian veins directly into cardiac chambers.

Physiological Features of Pulmonary Blood Vessels

Pulmonary artery and large branches are 30% thinner than the aorta wall.
Small arterial vessels have little muscle in their walls.
Post-capillary vessels have smooth muscle.
Pulmonary vascular bed enables compliance to accommodate large stroke volumes.
Large numbers of capillaries provide ample surface area.
Each alveolus is surrounded by a capillary network.

Function of Pulmonary Circulation

Gas exchange: Blood is brought from systemic circulation to contact with alveoli. Oxygenated blood is then collected and returned via pulmonary veins.
Filtering: Pulmonary vessels filter thrombi and emboli originating from the venous compartment and right side of the heart. Removing clots or emboli from blood helps prevent them from reaching coronary or cerebral vessels.
Metabolic regulation: Pulmonary vessels metabolize vasoactive hormones via ACE. They control blood vessel tone through inactivation of bradykinin, serotonin, prostaglandins (E1, E2, E2a), and norepinephrine.
Blood reservoir: The lungs hold about 500 mL of blood, which is approximately equal to the right ventricle stroke volume. This acts as a reservoir, compensating for blood loss elsewhere in the body.

Pulmonary Lymphatics

Lymphatic vessels are present in supporting structures beginning in tissues around terminal bronchioles.
Lymphatic vessels drain into the right thoracic duct (hilum of lungs).
Functions:
- Remove particulate matter from alveoli.
- Remove leaked plasma proteins from lung capillaries. (important for maintaining a healthy interstitial area). This helps prevent edema (fluid buildup in air sacs).

Pulmonary Blood Pressure

Pulmonary blood vessels are more distensible than systemic vessels.
Thus, pulmonary blood pressure is lower than systemic pressure.

Pressure in the Pulmonary Circulation

Pulmonary artery pressure: 15 mmHg
Systemic artery pressure: 100mmHg
Other values for pressures at each point in the pulmonary circulation are provided in the text.
This section presents a comparison of pressures in pulmonary and systemic circulation, including pressures in arteries, capillaries, and veins for both circulations.

Left Atrial and Pulmonary Venous Pressure

Mean left atrium pressure and major pulmonary veins approximately 2 mmHg (range 1-5 mmHg)

Pressure Differences in Lung Vessels

Pressure is highest at the pulmonary artery level, then gradually drops to the lowest pressure at the left atrium level as the blood makes its way through the capillaries.

Shift of the Blood Indicates Cardiac Pathology

Left-sided heart failure or increased mitral valve resistance can lead to increased pulmonary vascular pressure.

Distribution of Blood Flow Through the Lungs

Pulmonary capillaries, which act as a "sheet" of blood flow efficiently across alveolar surfaces, create less resistance to blood flow.
Systemic capillaries have a tighter structure and represent a high resistance to blood flow.

Special Feature of Pulmonary Circulation

Pulmonary circulation has lower pressure (15 mmHg) and low resistance compared to systemic circulation.
Pulmonary vessels(arteries, veins, and branches) are compliant due to thin walls and less smooth muscle.
The lungs' large volume capacity allows them to continuously receive all cardiac output and accommodate variations in blood volume associated with posture changes such as standing and lying down).

Factors Affecting Pulmonary Blood Flow

Cardiac Output: This is directly proportional and regulated by factors such as venous return, force of contraction, rate of contraction, and peripheral vascular resistance.
Vascular Resistance: Affected by respiratory phases; inspiration (increased flow) and expiration (reduced flow). Exercise leads to lowered vascular resistance and increased flow. Variations in oxygen and carbon dioxide levels also impact vascular resistance.
Nervous Factors: Parasympathetic stimulation increases vascular resistance, while sympathetic stimulation decreases it.
Chemical Factors: Hypoxia (low oxygen) causes vasoconstriction in pulmonary vessels, while elevated oxygen levels cause vasodilation. Hormones, including serotonin, norepinephrine, histamine, thromboxane A2, leukotrienes, adenosine, acetylcholine, prostacyclin (PG-12), and isoproterenol, also affect blood flow and resistance.
Gravity/Hydrostatic Pressure: Blood flow is highest at the base and lowest at the apex of the lungs due to gravity. Alveolar pressure, blood flow, and pressure differentials vary throughout the lungs. This is grouped into 3 zones. The sections provide a visual analogy for understanding
Arterial to Venous Pressure Gradient: Arterial-venous pressure differences regulate blood flow determined by alveolar pressures throughout the lungs.

Abnormalities of Pulmonary Circulation

High left atrial pressure (left-sided heart failure) causes small pressure changes affecting pulmonary circulation due to pulmonary venule expansion and opening, allowing for continued blood flow.
High left atrial pressure exceeding 7-8 mmHg causes pulmonary edema.

Capillary Exchange of Fluid and Pulmonary Interstitial Fluid Dynamics

Pulmonary capillaries have relatively low pressure compared to systemic capillaries - 7 mmHg vs 17 mmHg.
Interstitial pressure in the lungs is more negative than peripheral tissues, favoring fluid movement into the capillaries.
Pulmonary capillaries are more permeable to proteins, increasing interstitial oncotic pressure.
Alveolar walls are thin and weak, potentially susceptible to rupture with high interstitial pressure. Fluid is subsequently moved to the alveoli, facilitating lung drainage via lymphatic system.

Forces Tending to Cause Fluid Movement

Capillary pressure, interstitial fluid colloid osmotic pressure, and negative interstitial fluid pressure contribute to fluid movement outward.
Plasma osmotic pressure and inward interstitial forces (of equal magnitude to outward forces) work toward fluid absorption.
Net fluid filtration pressure is typically positive, leading to fluid outward from capillaries into the pulmonary interstitial space.

Pulmonary Exchange of Fluid

Negative interstitial pressure keeps alveoli dry by preventing fluid accumulation in the interstitial spaces.
Small openings between alveolar epithelial cells allow passage of water and electrolytes, but not large proteins. Pulmonary lymphatics help to prevent fluid buildup in the lungs.

Principle of Lungs Being Kept Dry

Lymphatic drainage is crucial in maintaining oedema-free pulmonary interstitium.
High interstitial compliance and lowered oncotic pressure help fluid absorption, which together limit oedema development.

Pulmonary Oedema Causes and Treatment

Causes include: increased capillary hydrostatic pressure (e.g., left-sided heart failure, mitral valve disease); increased alveolar space tension; decreased oncotic pressure; and increased capillary permeability (e.g., inflammatory reactions, toxic exposures).
Treatment aims to reduce pulmonary capillary hydrostatic pressure (e.g., diuretics, digitalis).

Drowning

Fresh water drowning: death from ventricular fibrillation due to aspirated water entering alveoli and causing low hydrostatic pressure and high oncotic pressure in systemic circulation.
Saltwater drowning: death from asphyxia (inadequate breathing) due to hypertonic water content causing pulmonary edema and aspiration (inhalation) of water.

Negative Pressure in Pleural Fluid

Negative pleural pressure is maintained by the pumping activity of the lymphatic system (vital for keeping lungs expanded).
The thin layer of mucoid fluid in the pleura acts as lubrication. Prevents the lungs from sticking to the chest cavity during inhalation).

Pleural Effusion

Collection of excessive fluid in the pleural space, a condition known as pleural effusion.
Causes include: lymphatic drainage blockage, high peripheral/pulmonary capillary pressure, low oncotic pressure, or inflammation/breakdown of capillary membranes.

Ventilation/ Perfusion Ratio

V/Q ratio varies throughout the lungs, with the highest at the base and lowest at the apex, due to gravity and positioning.
Average V/Q = 0.8 (4.2mL /5.5mL)
Normal ventilation > perfusion (V/Q >1): higher oxygen in alveoli.
Normal perfusion > ventilation (V/Q <1 ): higher CO2 in alveoli.

What about the Supine Position?

Lungs become zone 3 in this position: uniform blood flow throughout the entirety of the lungs, without the influence of gravity.

Arterial to Venous Pressure Gradient

Pressure difference between arterial and venous blood in different pulmonary zones determines blood flow rate. Alveolar pressure influences this gradient.

Additional Note:

The factors determining blood flow are complex and interconnected, requiring a detailed comprehension of the underlying mechanisms for a full grasp of the topic.

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Description

This quiz explores the physiological significance of low pulmonary vascular resistance (PVR) for gas exchange in the lungs. It covers adaptations of pulmonary circulation during vigorous exercise, the effects of the parasympathetic nervous system, hypoxic vasoconstriction mechanisms, and the implications of gravity on pulmonary blood flow. Understanding these factors is crucial for evaluating the impacts of lung diseases.