The pulmonary circulation is a closed system within the body. It works opposite from the artieries and veins. Our first thoughts are oh no, that cannot be so, arteries carry only fresh oxygenated blood and veins are the passageways of blood devoid of most of its oxygen supply. That is true of the systemic circulation but is not true for the heart lung circuit. Here a reversal takes place. The pulmonary arteries carry oxygen poor blood to the lungs for a fresh load of oxygen and after the capillary gas exchange – oxygen and carbon dioxide – veins carry the blood back to the heart where begins its journey throughout the system.
Lets trace the pulmonary veins and arteries around their loops to get a better picture of their activity: From two large veins, the superior vena cava that drains the upper section of the body and the inferior vena cava that drains the lower section of the body, blood, devoid of oxygen and carrying waste in the form of carbon dioxide enters the right atrium. The right atrium filled with blood contracts forcing open the one way valve open permitting the blood to enter the right ventricle. The right ventricle contracts forcing the pulmonary valve to open allowing blood to enter the pulmonary artery leading to the lungs.
Note: In the description above of the detour of the waste flow of blood from the body the arteries are the main conduits as are the arteries in the systemic circulation. It is in function and not content that the arteries are the same through out the body. The used blood returning to the heart has need for a sturdier vessel to propel the blood into the pulmonary circuit, the artery. In this sense also, we see that, in essence, the pulmonary circuit mimics the overall systemic circulation. It is only in the type of cargo that the arteries and veins in the pulmonary system differ.
After the capillary exchange of gases, fresh blood enters the pulmonary veins and returns to the left atrium of the heart. From here through the one way valve into the left ventricle and then the oxygenated blood becomes part of the systemic system and delivers blood and nutrients to the whole body. Veins are lesser of the two vessels and are always concerned with slightly less pressure than the arteries. Fresh blood on its return from the lungs to the heart uses the less pressurized veins. From the left atrium through the one way valve – mitral – blood enters the left ventricle and from there through the aortic valve into the aorta for another journey.
Pulmonary vessels and blood vessels concern themselves only with the lungs but the overall effect, the oxygenation of blood, last throughout the completed systemic circulation. After this special routing the blood enters the largest artery, the aorta, and travels to the organs, muscles and tissues making sure every inch of territory is covered. To accomplish this feat it makes inroads into, over, under and through coronary muscles, abdominal portals and endocrine sanctuaries with its fresh supply of oxygen and specially labeled nutrients. This is to make sure every bit of the vast territory of the body is serviced.
The word pulmonary has everything to do with the lungs. When the lungs are defective and are not capable of producing the right amount of oxygen to the blood then the whole breathing apparatus is not up to par. When this happens the system manager sends out alerts for more oxygen, breathing is sped up. Disease or lack of disease often is the direct result of adequate lung capacity.
The lungs are two delicate organs about ten or 12 inches long composed of lobes whose function is breathing in and breathing out. When we breathe in- inhale – oxygenated air enters our lungs, when we breathe out – exhale – we send out carbon dioxide. These organs are attached to two bronchial tubes, one left and one right. The bronchial tubes further subdivide into bronchioles until all areas of the lungs are connected to the lobes of the lungs. The endings of these bronchioles contain millions of tiny little alveoli who wrestle with the exchange of oxygen and carbon dioxide.
The mechanism of breathing is by means of the diaphragm, the largest muscle in the body. It moves and widens out when we breathe in and relaxes when we exhale. It also separates the lungs from the lower organs such as liver, stomach, spleen and others. When it is not working properly breathing will likewise not be up to par and our breaths may be shallow and less effective. For more information on how the lungs work, seek the knowledgeable American Thoracic Society.