24. Pulmonary circulation, pulmonary hypertension
Logically, all the blood that enters the systemic circulation must also enter the pulmonary circulation. However, at any point in time the volume of blood that is present in the pulmonary vessels is very low compared to the stored blood volume in the systemic circulation. In physiological cases 6-9% of the total blood volume is stored in the pulmonary circulation. In pulmonary congestion will the number go up, perhaps all the way to 20%!
The pulmonary circulation doesn’t supply the lung parenchyme with blood. This blood supply comes from the bronchial circulation which arises from the thoracic aorta.
The pressure in the pulmonary artery is 25/15 mmHg, which is much less than the pressure in the systemic circulation (120/80). The cardiac output of the right ventricle is equal to the left ventricle, but how can this be explained when the pressures inside the pulmonary and systemic circulations are so different?
It is explained by how the pulmonary vascular resistance (PVR) is much lower than the total peripheral resistance of the systemic circulation. In fact, the PVR is 5 times lower than the TPR! (MAP of the systemic circulation is 93 while MAP of the pulmonary circulation is 18, which is 5 times lower)
The TPR is mostly determined by the diameter of the arterioles, however the pulmonary resistance is determined by the arteries, arterioles, veins and capillaries. Constriction of any of these vessels will lead to pulmonary hypertension.
In rest the time that blood and alveolar air is in contact is 0.7-0.8 seconds. In exercise the velocity of the pulmonary circulation increases, so this contact time decreases to 0.3 seconds, which is still sufficient for diffusion.
Another important measure of the pulmonary circulation is the pulmonary wedge pressure (PWP). This pressure is measured in a special way (not important) and correlates with the pressure inside the left atrium. The normal range is 5 – 12 mmHg. When PWP is high, the left atrial pressure is high, which is a sign of pulmonary congestion.
Regulation of pulmonary circulation
Local hypoxia of the lungs causes local vasoconstriction in the pulmonary circulation. This contrasts with the systemic circulation, where hypoxia triggers vasodilation. This is beneficial because if a part of the lung doesn’t get enough air, the alveoli aren’t optimally perfused, so gas exchange doesn’t occur optimally there. Therefore, it’s better to restrict the blood that travels to this area and instead redirect this blood to other parts of the lung where alveolar ventilation is still good. The disadvantage of this is that a widespread hypoxia of the lungs will cause widespread pulmonary vasoconstriction, leading to pulmonary hypertension.
Sympathetic activation, and especially circulating catecholamines cause vasoconstriction of the pulmonary circulation, therefore shortening the time that blood spends inside the pulmonary circulation and instead increases the filling of the left heart. Parasympathetic activation causes vasodilation.
Pulmonary hypertension
Pulmonary hypertension is defined as when the mean arterial pressure in the pulmonary artery is above 25 mmHg. It can have either primary or secondary causes, however the primary causes are rare.
Pulmonary hypertension is classified into five types according to their cause:
- Group 1 – Pulmonary arterial hypertension
- Idiopathic pulmonary arterial hypertension
- Hereditary pulmonary arterial hypertension
- Group 2 – Pulmonary hypertension due to left heart disease
- Left-sided heart failure
- Valvular heart disease
- Left-to-right shunt
- Group 3 – Chronic lung diseases
- COPD
- Obstructive sleep apnoea
- Interstitial lung disease
- Sarcoidosis
- Silicosis
- Pulmonary fibrosis
- Group 4 – Chronic thromboembolic occlusion of pulmonary vessels
- Multiple thromboemboli in the pulmonary circulation
- Group 5 – Unknown causes
The types of pulmonary hypertension are classified based on whether there is pulmonary congestion or not. This is determined by the pulmonary wedge pressure (PWP).
When the pulmonary wedge pressure is normal in a pulmonary hypertension, we say that it is the precapillary type. Pulmonary hypertension due to vasoconstriction or thromboembolic causes are precapillary hypertensions.
When the pulmonary wedge pressure is increased (>15 mmHg), the hypertension is of postcapillary type. This type occurs when the congestion occurs due to congestion from the left ventricle.
Consequences
When the pressure inside the pulmonary circulation is high the right ventricle must work harder to pump blood into it. The right ventricle will eventually dilate and fail. This disorder is called cor pulmonale.
Because of the increased pressure inside the pulmonary circulation can vascular damage occur. This damage can be the beginning for atherosclerosis to develop there. Atherosclerosis is usually not found in the pulmonary circulation unless there is vascular damage first.