Pulmonary circulation refers to the circulation of blood between the heart and lungs. Deoxygenated blood is pumped from the heart to the lungs where a gaseous exchange between carbon dioxide and oxygen takes place, and then the oxygenated blood is carried back to the heart. The blood circulation that occurs throughout the rest of the body is known as systemic circulation.
The interior of the heart is divided into four chambers, the two upper chambers are referred to as the atria, and the lower chambers are the ventricles. The two sides of the heart are divided by a septum with one atrium and one ventricle situated on each side. There are four valves that control one way blood flow through the heart, the pulmonary valve and the tricuspid, also known as the right atrioventricular valve are located on the right. On the left are the aortic valve and the mitral, also referred to as the left atrioventricular valve.
Deoxygenated blood exits the heart from the right ventricle which contracts and pushes the blood into the pulmonary valve and then out into the pulmonary artery, this then divides into two to become the left and right pulmonary arteries.
The left pulmonary artery enters the left lung where it again divides into two and each artery transports blood to one of the two lobes of the lung.
The right pulmonary artery also divides into two as it enters the right lung which consists of three lobes. One artery takes blood into the upper lobe and the other to the middle and lower lobes.
The arteries then form into much smaller structures known as arterioles. These then continue to narrow down and branch off into capillaries; it is the network of capillaries that link the arteries and veins, and this network is supported by connective tissue. The capillaries surround the alveoli; these are the tiny air sacs within the lungs, and both the alveoli and capillary walls consist of a very fine membrane of flattened epithelial cells, and it is across this membrane that the gaseous exchange takes place.
Gases diffuse from a higher to a lower concentration gradient, and the concentration of carbon dioxide is higher in the blood within the capillaries, therefore the carbon dioxide crosses the capillary membranes and through the membranes of the alveoli. Oxygen enters the capillary blood from the alveoli in the same way because the oxygen concentration is greater within the alveoli. The oxygenated blood then travels along the capillary network into the venules which are larger than capillaries but smaller than veins. The venules then merge into the pulmonary veins which leave each lung and carry the oxygenated blood to the heart.
Oxygenated blood enters the heart at the left atrium from the pulmonary veins. The left atrium then contracts and propels the blood through the mitral (left atrioventricular) valve into the left ventricle, the ventricle then contracts forcing the blood through the aortic valve and out into the aorta. It is here that pulmonary circulation ends and systemic circulation begins, the oxygenated blood circulates throughout the rest of the body delivering the oxygen and other necessary substances into the cells.