Pulmonary and Systemic Circulation

Pulmonary Blood Flow

• Approximately equal to flow through the whole systemic circulation - 6L/min under resting conditions, 25L/min with severe exercise
• Receives a significant amount of blood from the bronchial arteries which arise from the arch of the aorta, a fraction of which passes through postcapillary anastomoses to join the pulmonary veins, constituting an admixture of venous blood with arterialised blood from alveolar capillary networks
• There are also pre-capillary anastomoses from the bronchial arteries to the pulmonary arteries, which have muscular walls and can increase blood flow to the lungs if required

• Vital in conditions like pulmonary artery stenosis or pulmonary embolism

• Pulmonary and systemic circulations must have very similar outputs to both sides, or one circulation would become rapidly overloaded

Pulmonary Blood Volume

• Change from supine to erect posture decreases pulmonary blood volume by ~1/3rd

• This is due to blood pooling in the systemic circulation resulting in a reduction of cardiac output but 1/3rd

• Also influenced by systemic vascular tone

Pulmonary Vascular Pressures

• Mean PA pressure is 15mmHg (25/8mmHg) - only needs to be enough to lift blood to the apex
• Mean systemic circulation pressures: Arterial: 90, Capillary: 10-30, Right Atrial: 2
• Mean pulmonary circulation pressures: Arterial: 17, Capillary: 9-13, Left Atrial: 2
• Although pulmonary arterial pressure is only ~1/6th of systemic arterial pressure, capillary and venous pressures are not greatly different between the two circulations
• Flow is more pulsatile than in systemic circuit - S:D ratio of 3:1, and there is less arteriole damping of arterial pressure waves, therefore pulmonary capillary blood flow is pulsatile
• Vessel walls much thinner with much less muscle
• Accepts entire CO at any given moment without much selective diversion of blood flow, except in hypoxia

• Reduced pressure drop along pulmonary arterioles mean there is less potential for active regulation of pulmonary blood flow
• Blood flow is markedly affected by gravity, with overperfusion of dependent parts of the lung fields

• Pulmonary vascular resistance is ~10% of systemic vascular resistance (as demonstrated by R=ΔP/Flow)

• Systemic pressures are compared directly to atmospheric pressures, however in the lung the relatively large extravascular (intrathoracic) pressure (which can significantly affect intravascular pressure, and pulmonary venous pressure (which can significantly affect the driving pressure of the pulmonary circulation) must be taken into account.
• Transmural pressure - the difference in pressure between the inside of a vessel and the tissue surrounding the vessel (commonly measured as the oesophageal pressure)
• Driving pressure - the difference in pressure between a point in the circulation and another point downstream

• In the lung the driving pressure is the difference between pulmonary artery pressure and left atrium pressure

Water Balance in the Lung

(not that relevant here, but unsure where else to put this)

• Only 0.3μm separates capillary blood from air in the lung, so it is critical to keep alveoli free of fluid
• Starling's Law:

• The force pushing fluid out of the capillary is the capillary hydrostatic pressure minus the hydrostatic pressure of the interstitial fluid - P_c - P_i
• The force pulling fluid back into the capillary is the colloid osmotic pressure of the proteins of the blood, minus that of the interstitial fluid - 'π_c - π_i - which depends on the reflection coefficient σ which measures the effectiveness of the capillary wall in preventing protein passage across it
• Therefore: 'net fluid out = K[(P_c - P_i) - σ(π_c - π_i)] (where K is a constant called the filtration coefficient

• In practice:

• Colloid osmotic pressure in the capillary is ~25-28mmHg, colloid osmotic pressure of the interstitial fluid is ~20mmHg
• Capillary hydrostatic pressure is about halfway between arterial and venous pressures, and higher at the top. Interstitial hydrostatic pressure difficult to measure but low.
• Therefore the net pressure is outward, causing a small lymph flow of ~20ml/hour under normal conditions

• Fluid leaks out into the interstitium of the alveolar wall and tracks through the interstitial space to the perivascular/peribronchial spaces within the lungs
• The pressure in the perivascular space is low, acting as a sump for fluid drainage. There are also extensive lymphatics to transport fluid away.
• In pulmonary oedema, the first step is interstitial oedema - peribronchial/perivascular engorgement. Lymph flow rate increases drastically in this state.
• When maximum drainage rates in the interstitial space is exceed and pressure rises too much, fluid starts moving into the alveolar spaces.
• Na/K ATPase in epithelial cells actively pumps fluid out of alveolar spaces
• Alveolar oedema is much more serious than interstitial oedema because it interferes with pulmonary gas exchange