Pulmonary circulation refers to the blood circulation between the heart and the lungs. The pulmonary arteries carry deoxygenated blood away from the heart into the lungs for oxygen renewal, and then the pulmonary veins take the oxygenated blood from the lungs back to the heart, and from there the oxygen is distrubuted throughout the entire body.
Two bronchi branch off from the trachea, commonly known as the windpipe, and one bronchus enters each lung. Inside the lungs the bronchi branch out into a further three, and from here into smaller structures known as bronchioles.
The lungs are divided into lobes which are marked by fissures. The left lung consists of two lobes, the superior and the inferior, and the right lung comprises of three, the superior, middle and the inferior.
Each lobe is made up of a vast number of lobules, and these consist of terminal and respiratory bronchioles, alveolar ducts and alveoli. It is within the alveoli, which are tiny air sacs, that the renewal of blood oxygen takes place.
The walls of the alveoli are surrounded by a network of capillaries which link together the arteries and the veins. The capillaries and blood vessels within the lungs are supported by connective tissue. When air is inhaled it travels along the trachea, into the bronchi and then entering into the bronchioles and finally reaching the alveoli where the oxygen from the air is transferred to the blood within the small blood vessels known as capillaries.
Both the alveoli and capillary walls consist of a very fine membrane of flattened epithelial cells, and it is across this membrane that the exchange of gases takes place. Gases diffuse from a higher to a lower concentration gradient, therefore the carbon dioxide crosses the capillary/alveoli membrane and enters into the alveoli because the concentration of carbon dioxide is higher in the blood within the capillaries. Oxygen enters the capillary blood from the alveoli in the same way because the oxygen concentration is greater within the alveoli. The carbon dioxide within the alveoli is then exhaled travelling back through the bronchioles, then into the bronchi, and on to the trachea and finally out through the mouth. The oxygenated blood travels along the capillary network entering into the venules; these are larger than capillaries but smaller than veins. The blood continues along the venules eventually reaching the pulmonary veins, which leave each lung and return oxygenated blood to the heart.
The oxygenated blood is then pumped out of the heart so as to distribute the oxygen, along with nutrients and various hormones, to the rest of the body. The delivery of oxygenated blood and the return of deoxygenated blood to the heart is referred to as the systemic circulatory system.