How the pulmonary circulation works
The pulmonary circulation, together with the great or systemic circulation, forms the human circulatory system. It begins in the right heart: the blood, which is low in oxygen and loaded with carbon dioxide, coming from the body is pumped via the right atrium and the right ventricle into the truncus pulmonalis (pulmonary trunk or pulmonary artery). This splits into the right and left pulmonary arteries, which branch into thinner and thinner vessels until they become capillaries. These delicate blood vessels surround the more than 100 million alveoli (pulmonary alveoli) filled with breathable air like a network. This is where gas exchange takes place: Carbon dioxide is released from the blood into the alveoli through the thin wall between the alveoli and the capillaries and then exhaled via the breath.
Conversely, the oxygen taken in with the breath passes from the alveoli into the bloodstream and is bound to the red blood pigment (hemoglobin) for further transport. The now oxygenated blood then travels back to the heart via several pulmonary veins, to the left atrium and on to the left ventricle. From here, it is pumped into the aorta and onward throughout the body (systemic circulation or great circulation).
Part of the low-pressure system
The fetus does not yet have a pulmonary circulation
In the unborn child there is a special feature: the blood of the fetus is not oxygenated in the lungs, but in the maternal placenta (which the child does not yet breathe). For this purpose, there is a bypass of the pulmonary circulation via the ductus arteriosus Botalli, a direct connection of the truncus pulmonalis with the aorta. In the heart itself, there is also an opening between the right and left atria (foramen ovale) through which blood bypasses the pulmonary circulation via the umbilical vein.
Problems in the pulmonary circulation
In pulmonary hypertension, blood pressure in the pulmonary circulation is chronically elevated (pulmonary hypertension). The right heart must use increased force to pump blood into the pulmonary circulation, which backs up due to increased resistance in the pulmonary vessels. The excessive stress on the right ventricle leads to wall thickening (hypertrophy) and/or dilatation – cor pulmonale (pulmonary heart) develops.
Pulmonary hypertension is usually caused by other conditions such as chronic heart disease (such as left heart failure) or lung disease (such as COPD, pulmonary embolism, pulmonary fibrosis). Very rarely, chronic elevation of blood pressure in the pulmonary circulation occurs as an independent disease.
