The pulmonary circulation differs significantly in characteristics from the systemic circulation. The pulmonary circulation is a low-pressure, low-resistance, high-flow circulation: a blood flow of 5 L/min (i.e. 100% of cardiac output, CO) is achieved with a driving pressure (i.e. mean pulmonary artery pressure, MPAP) of only 15 mmHg.
The pulmonary circulation differs significantly in characteristics from the systemic circulation. The pulmonary circulation is a low-pressure, low-resistance, high-flow circulation: a blood flow of 5 L/min (i.e. 100% of cardiac output, CO) is achieved with a driving pressure (i.e. mean pulmonary artery pressure, MPAP) of only 15 mmHg. Important features of the anatomy of the pulmonary circulation are as follows:
As may be expected, there are two pulmonary arteries: one for each lung. In contrast, there are at least four pulmonary veins: two arising from each lung.
Initially, the arteries, veins and bronchi run in close proximity to each other, dividing at the same points. In the periphery of the lung, the vessels separate: the veins pass between lung lobules, whilst the arteries and bronchi travel together to the centre of the lobules.
The pulmonary capillaries are fragile and very narrow (diameter 7–10 µm), just wide enough for red blood cells to squeeze through.
The conducting airways of the lung also have their own blood supply, called the bronchial circulation. Some of the deoxygenated blood is carried away by the pulmonary veins, where it mixes with oxygenated blood and enters the systemic circulation. This is one of the causes of an anatomical shunt (see Chapter 14).
Historically during lung transplant surgery, despite the fact that the bronchial arteries normally receive 3–5% of CO, the donor bronchial artery was not re-anastomosed. Because most grafts survived despite the loss of the bronchial artery, the bronchial circulation had been considered to be unnecessary. However, a subset of lung transplant patients develop an ischaemic bronchiolitis obliterans, thought to be due to loss of the bronchial circulation. There is currently a trend towards direct bronchial artery revascularisation to overcome this problem.
Unlike the systemic circulation, the pulmonary circulation does not need to direct blood flow from one region to another, with the exception of the property of hypoxic pulmonary vasoconstriction (see later). Therefore, pulmonary arterial pressure only needs to be high enough to propel blood to the lung apices.
The pulmonary capillaries are unique in being almost entirely surrounded by alveolar gas. They are also very fragile, with little connective tissue, and are therefore prone to distension or collapse. Transudation of fluid from the pulmonary capillary to the alveolus (i.e. pulmonary oedema) is dependent in part1 on the transmural pressure, given by the difference in pressures between the capillary and the alveolus. As alveolar pressure is very low, having a low-pressure pulmonary capillary is essential if pulmonary oedema is to be avoided.
This is akin to Ohm’s law in an electrical circuit:
Potential difference = current × resistance, V = IR
Similarly, for the vascular system:
Pressure difference = total flow × vascular resistance
where MPAP (mmHg) is the mean pulmonary artery pressure, PCWP (mmHg) is the pulmonary capillary wedge pressure, MAP (mmHg) is the mean arterial pressure (MAP), SVR (dyn.s.cm−5) is the systemic vascular resistance, PVR (dyn.s.cm−5) is the pulmonary vascular resistance, CO (L/min) is the cardiac output and 80 is a constant related to unit conversion.
Calculating SVR using typical values (MAP = 100 mmHg, mean right atrial pressure is 2 mmHg and CO = 5 L/min):
Likewise, calculating PVR with typical values (MPAP = 15 mmHg, PCWP = 5 mmHg, CO = 5 L/min):