Pulmonary circulation as the name implies (pulmonary=lungs; circulation=exchange) circulate blood to and from the lungs. It is one of the two parts of the cardiovascular system, where systemic circulation being the other. Cardiovascular system is the blood supply of the entire body, being formed of heart and vessels. Heart pumps blood and the vessels-named arteries, veins and capillaries-carry the blood throughout the body.
The heart-though it is usually considered to be a single pump-is actually made up of two separate pumps. The right side pumps blood to the lungs and is part of the pulmonary circulation. Left side pumps blood to the rest of the body and is part of the systemic circulation. There is a tough septum separating the two sides of the heart. Left heart is made up of left atrium and left ventricle while the right heart is made up of right atrium and right ventricle.
A great deal of credit in explaining the circulatory system goes to William Harvey. But it was the systemic circulation that William Harvey did explained. Though considered lesser to the systemic circulation, pulmonary circulation had already been explained by that time by Ibn al-Nafis and Michael Servetus. Ibn al-Nafis was the first to explain the circulatory system in a great detail and therefore is considered to be the ‘father of circulatory physiology’. Michael Servetus was the first European to explain the pulmonary circulation since Ibn al-Nafis was actually an Arabian.
The pulmonary circulation is a continuous cyclic process. There’s no starting point or an end point to it. However it is common practice to start the explanation with the right atrium. Right atrium first gets filled with deoxygenated blood. This blood comes from the systemic circulation through venous system and ultimately delivers by superior and inferior vena cava. Superior vena cava carries the deoxygenated blood from the upper part of the body while inferior vena cava carries that of the lower part of the body.
At right atrium the blood first gets filled there as the valve-the tricuspid or right atrioventricular valve-remains closed. This valve connects the right atrium to the right ventricle which is situated below the atrium; when the right atrium starts to contract due to the impulses generated at the sino atrial node (SA node), the pressure inside the right atrium increases. At a certain point this reaches a maximum where the increased pressure opens the tricuspid valve towards the ventricle. The blood that was filling in the right atrium now moves to the right ventricle through the newly opened right ventricle.
In the mean time the impulses generated at the SA node reaches atrioventricular node (AV node). Then they cross over to the ventricular myocardium through the bundle of His. Bundle of His transfer the impulses to the Purkinje fibers, which ultimately make the ventricles contract. With the contraction of the right ventricle the tricuspid valve closes at a certain point. This prevents the blood regurgitating back into the right atrium and in that way the tricuspid valve acts as a one way valve. The further increase in the pressure causes a different valve to open, a semi lunar valve named pulmonary valve. This opens into the root of the trunk of the pulmonary artery forcing the blood into it.
The pulmonary artery carries the blood to the lungs. It is different to the rest of the arteries in the body as pulmonary artery carries deoxygenated blood while all the other arteries carry oxygenated blood. Pulmonary artery bifurcates into two arteries named right and left pulmonary arteries which enter the right and left lungs respectively.
The lungs are light weight, spongy organs which occupy most of the thoracic cavity. They are located inside two pleurae named visceral pleura and parietal pleura. The lungs have lobes where right lung has three and left lung two. The lobes are again divided into bronchioles. Each bronchiole is a separate compartment with its own blood vessels and air ways. However the ultimate functional and anatomical unit of the lung is an air filled sac named alveolus.
The function of the lung is to exchange gasses. The deoxygenated blood carried in by the pulmonary artery is rich in carbon dioxide. The air inside the alveoli is rich in oxygen. The alveoli are heavily surrounded by capillaries where extensive exchange takes place between blood and air. Carbon dioxide leaves blood inside the capillaries and enters the alveolus. The oxygen defuses into blood capillaries from the alveoli.
The next pace of the pulmonary circulation is to carry the oxygenated blood back to the heart. This is done by the pulmonary veins. Pulmonary veins too are different to the rest of the veins where they carry oxygenated blood where rest of the veins carry deoxygenated blood. There are four pulmonary veins opening into the back of the left atrium. The left atrium is the starting point for the systemic circulation where the oxygenated blood moves to the left ventricle to be sent to the rest of the body through Aorta.