Pulmonary circulation includes the vast network of arteries, veins, and lymphatics that function to exchange blood and other tissue fluids between the heart, the lungs, and back. They are designed to perform certain specific functions that are unique to the pulmonary circulation, like exchanging gases in the lungs and acting as a reservoir for the storage of blood amongst others. The pulmonary circulation is divided into the following components:
It is appropriate to mention that a similar system of lymphatics and vessels exists between the parietal and visceral pleurae, draining the pleural fluid which plays an important role in providing a viscous medium for expansion of lungs during their respiratory excursion. The large negative pleural pressure ( approximately -4 to -7 mmHg) exists because of an efficient efferent venous and lymphatic system that keeps the alveoli closely tethered to the visceral pleura and prevents them from collapsing inwards.
In addition, the lung parenchyma receives oxygenated blood via the bronchial vessels (accounting for about 1% to 2 % of the cardiac output) arising from the left ventricle which ultimately drains into the left atrium after forming the bronchial veins. Hence, it is important to note that the cardiac output of the left ventricle is approximately 1% to 2 % greater than that of the right ventricle.
Pulmonary circulation is essential for the body to ensure a continuous supply of oxygenated blood. Any compromise can have grave consequences and lead to tissue dysfunction secondary to hypoxia.
Some of the common pathologies of the pulmonary circuit include but are not limited to the following:
Pulmonary edema: Any disturbance in the starling forces operating in the pulmonary circulation can lead to an accumulation of fluid in the alveoli, impairing gas exchange and causing respiratory distress.
Pulmonary embolism: A dislodged clot from a distant source (most commonly a deep venous thrombus) can embolize to the pulmonary circuit and lead to ischemia and, if prolonged, infarction of the lung parenchyma as well as severely impaired gaseous exchange.
Pulmonary hypertension: An increase in the mean pulmonary artery pressure beyond 25 mmHg is known as pulmonary arterial hypertension. It leads to impaired gas exchange and commonly manifests as exertional dyspnea. If prolonged, it can lead to right ventricular stain and right heart failure, a phenomenon known as cor-pulmonale.
Pleural effusion: A disturbance in the starling forces of the pleural circulation can lead to accumulation of fluid in the pleural space, a phenomenon known as pleural effusion. This manifests as pleuritic chest pain and respiratory distress.
The fetal circulation begins to form as early as 15 days after conception in the form of immature placental vessels and slowly grows to form a fully functional four-chambered heart, beating separately from the maternal circulation by the fourth week of gestation.
The growing fetus gets its nutrients and excretes its metabolic waste products via the placental vessels that connect the umbilical veins, which in turn drain into the inferior vena cava and then into the right atrium. The fetal circulation is designed to shunt blood across the liver and lungs during fetal life via the ductus venosus, foramen ovale, and the ductus arteriosus. The blood from the right atrium thus eventually makes its way to the systemic circulation without actually reaching the lungs. The pulmonary vessels remain closed under high pressure, and it is only after birth as the newborn takes its first breaths that the pulmonary artery pressure falls, the shunts existing in the fetal life close, and blood begins to enter the lungs for exchange of gases for the fetus is no longer dependent on the placental circulation. A failure in this process sometimes leads to persistent pulmonary hypertension, causing respiratory distress in the newborn. The condition requires multi-disciplinary management using supplemental breathing, artificial surfactant, and vaso-dilators to lower the pulmonary artery pressure.
Pulmonary circulation is involved in many essential functions. The primary function is the exchange of gases across the alveolar membrane which ultimately supplies oxygenated blood to the rest of the body and is also the chief mechanism by which the body eliminates carbon dioxide. Bronchial circulation provides oxygenated blood to be consumed by the lung parenchyma to carry out its own metabolic functions. The low-pressure venous system and an intricate system of lymphatics ensure that there is no build-up of edema fluid in the lungs.
The understanding of the pressure gradients across the pulmonary circuit is important to realize the fact that minor derangements in these pressures can lead to adverse outcomes like pulmonary edema and respiratory shunts.
The pressure gradients can be summarized as follows:
Chamber/vessel/pressure gradient (systolic/diastolic) (in mmHg)
It is important to note that the low pressure in the pulmonary capillaries allows for easy exchange of gases in the lung alveoli. Also, the pressure in the left atrium is difficult to measure directly, and a surrogate pressure, known as the pulmonary capillary wedge pressure, is often used.
At the time of exercise, there is a markedly increased blood flow which is accommodated by the pulmonary circulation in the following ways:
Zones of Pulmonary Blood Flow
A hydrostatic pressure gradient exists by virtue of gravity, from the apex of the lung to the base. This is about 23 mmHg (distributed as -15 mmHg from the level of the heart to the apex of the lung and +8 mmHg from the level of the heart to the base of the lung). This results in a 5-fold greater blood flow at the base of the lung as compared to the apex of the lung. Three zones of pulmonary blood flow can be delineated based on the pulmonary capillary pressure (Pcp)and the pulmonary alveolar air pressure (Ppac).
Zone 1: The Pcp is always less than the Ppac here, and there is no blood flow in the pulmonary capillary bed during any phase of the cardiac cycle. Zone 1 circuits are not seen in the normal lung and are only seen in certain conditions, such as after massive blood loss or if a person is breathing against a positive airway pressure.
Zone 2: Here the Pcp rises above the Ppac only during systolic blood flow, and so gas exchange occurs only during systole and not in diastole. Zone 2 blood flow is seen at the apices of the normal lung.
Zone 3: Here the Pcp remains greater than the Ppac in all phases of the cardiac cycle, allowing for an efficient exchange of gases. At the time of exercise, the pulmonary blood flow increases and all parts of the lung receive zone 3 blood flow.
This is a peptide released by the left ventricular myocardium in response to elevated filling pressures and raised blood volumes and is a very sensitive and specific marker of the cardiogenic cause of pulmonary edema. High levels have been shown to strongly suggest a cardiogenic cause while low levels have been used to successfully rule out a cardiogenic cause; however intermediate values have little significance and require further investigation.
This is an invasive method of inserting a catheter from a peripheral access site to reach the pulmonary artery and ultimately evaluate the pulmonary capillary wedge pressure. A value greater than 18 mmHg has been shown to be highly suggestive of a cardiogenic cause of pulmonary edema (corresponding to raised left atrial pressures).
Pulmonary edema is an accumulation of free fluid in the alveoli resulting in a decrease in the capacitance of the parenchyma and impairing gas exchange across the alveolar membrane. Acute onset pulmonary edema can lead to severe respiratory distress and death in 20 to 30 minutes.
To understand the pathophysiology of pulmonary edema, it is essential to understand the starling forces operating to maintain a homeostatic flow across the pulmonary capillary bed.
Outward Driving Force
7 mmHg (capillary hydrostatic pressure) + 8 mmHg (negative interstitial fluid pressure) + 14 mmHg (interstitial colloid osmotic pressure) = 29 mmHg
Inward Driving Force
28 mmHg (plasma colloid osmotic pressure)
Therefore, the net pressure of +1 mm Hg drives fluid out of the pulmonary capillaries and is taken away by an efficient network of pulmonary venules and lymphatics.
An imbalance in these forces in the form of raised pulmonary hydrostatic pressure (for diastolic: left ventricular failure), decreased plasma osmotic pressure (e.g., protein-losing enteropathy) or increased capillary membrane permeability (e.g., infections like pneumonia, inhalation of toxic gases like carbon monoxide) leads to pulmonary edema.
Pulmonary Edema Protection Factor
In accordance to the starling forces, excess edema fluid will accumulate when the interstitial tissue fluid overwhelms the capillary osmotic pressure (=28 mmHg).
Since the baseline left atrial pressure (and hence the pulmonary capillary wedge pressure) is 7 mmHg, this gives a protective factor of 21 mmHg, i.e., the left atrial pressure can rise by an additional 21 mmHg before pulmonary edema develops. However, beyond this value, the rate of accumulation of fluid is rapid, and pressures beyond 30 mmHg can lead to death due to pulmonary edema in 20 to 30 minutes. However, in long-standing cases like chronic mitral stenosis, the pulmonary capillary wedge pressure may be elevated to values up to 40 mmHg before edema starts to develop.
Acute pulmonary edema can have a cardiogenic or non-cardiogenic origin. The differentiation can be done clinically based on the clinical settings in which it arises. Cardiogenic edema is commonly preceded by an acute coronary event and is usually associated with elevated left ventricular filling pressures. Non-cardiogenic causes are commonly included in the umbrella term of acute respiratory distress syndrome (ARDS) which is associated with wide-spread systemic inflammation and release of cytokines causing increased permeability of the pulmonary alveolar capillaries and causing an exudative edema as compared to a transudative edema as seen in acute heart failure. ARDS is commonly seen in settings of systemic sepsis, burns, or massive blood transfusions.
Patients commonly present with tachypnea and chest pain. An arterial blood gas analysis shows respiratory alkalosis and hypoxemia. Chest X-ray changes suggestive of bilateral infiltrates are an early and hallmark finding in ARDS and are evident in the first 24-hours of presentation. Findings in cardiogenic pulmonary edema evolve over 2 to 3 days and show considerable overlap with that of ARDS. N-pro BNP levels may be used to differentiate between the two etiologies if there is uncertainty.
Management of pulmonary edema depends a lot on the etiology. Ventilation support forms an essential cornerstone of all cases and a target saturation of 88% to 92%, and a Po2 level of 60 to 80 mm Hg is a modest approach for management of hypoxia. Use of low tidal volume and judicious use of positive end-expiratory pressure ventilation has been shown to significantly improve mortality in ARDS.
Further management of ARDS relies on early diagnosis and management of underlying cause of systemic inflammation such as antibiotics for infections or conservative fluid resuscitation in case of burns and acute pancreatitis. Management of cardiogenic edema relies on the early mobilization of fluid and reduction of left ventricular fluid overload using diuretics and vasodilators in addition to treating the cause of decompensated heart failure.