Anterior (frontal) view of the opened heart. White arrows indicate normal blood flow. (Pulmonary artery labeled at upper right.)
A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the main pulmonary artery or pulmonary trunk from the heart, and the smallest ones are the arterioles, which lead to the capillaries that surround the pulmonary alveoli.
The pulmonary arteries are blood vessels that carry blood from the right side of the heart through to the capillaries of the lungs. The blood that is carried is, unlike other arteries, without oxygen ("deoxygenated"). The main pulmonary arteries emerge from the right side of the heart, and these split into smaller arteries that progressively divide and become smaller until they become arterioles and eventually capillaries.
In order of blood flow, the pulmonary arteries start as the pulmonary trunk or main pulmonary artery. The main pulmonary artery begins at the base of the right ventricle. It is short and wide--approximately 5 centimetres (2.0 in) in length and 3 centimetres (1.2 in) in diameter.
The main pulmonary artery splits into the right and the left main pulmonary artery. The left main pulmonary artery is shorter and somewhat smaller than the right, passes horizontally in front of the descending aorta and left bronchus to the root of the left lung. Above, the left main pulmonary artery is connected to the concavity of the proximal descending aorta by the ligamentum arteriosum.
It then divides into two lobar arteries, one for each lobe of the left lung. The right main pulmonary artery follows a longer and more horizontal course as it crosses the mediastinum. It passes underneath the aortic arch, behind the ascending aorta, and in front of the descending aorta. It courses posterior to the superior vena cava and in front of the right bronchus. Upon reaching the hilum of the right lung the right main pulmonary artery divides into two branches:
The right and left main pulmonary arteries give off branches that roughly correspond to the lung lobes and can in such cases be termed lobar arteries. The lobar arteries branch into segmental arteries (roughly 1 for each lobe segment), which in turn branch into subsegmental pulmonary arteries. These eventually form intralobular arteries.
The pulmonary arteries originate from the truncus arteriosus and the sixth pharyngeal arch. The truncus arteriosis is a structure that forms during the development of the heart as a successor to the conus arteriosus.
By the third week of development, the endocardial tubes have developed a swelling in the part closest to the heart. The swelling is known as the bulbus cordis and the upper part of this swelling develops into the truncus arteriosus  The structure is ultimately mesodermal in origin. During development of the heart, the heart tissues undergo folding, and the truncus arteriosus is exposed to what will eventually be both the left and right ventricles. As a septum develops between the two ventricles of the heart, two bulges form on either side of the truncus arteriosus. These progressively enlarge until the trunk splits into the aorta and pulmonary arteries. 
The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. The blood here passes through capillaries adjacent to alveoli and becomes oxygenated as part of the process of respiration.
The pulmonary artery pressure (PA pressure) is a measure of the blood pressure found in the main pulmonary artery. This is measured by inserting a catheter into the main pulmonary artery.:190-191 The mean pressure is typically 9 - 18 mmHg, and the wedge pressure measured in the left atrium may be 6-12mmHg. The wedge pressure may be elevated in left heart failure,:190-191mitral valve stenosis, and other conditions, such as sickle cell disease.
The pulmonary artery is relevant in a number of clinical states. Pulmonary hypertension is used to describe an increase in the pressure of the pulmonary artery, and may be defined as a mean pulmonary artery pressure of greater than 25mmHg.:720 As can be measured on a CT scan, a diameter of more than 29 mm diameter is often used as a cut-off to indicate pulmonary hypertension. This may occur as a result of heart problems such as heart failure, lung or airway disease such as COPD or scleroderma, or thromboembolic disease such as pulmonary embolism or emboli seen in sickle cell anaemia.:720-721
Pulmonary embolism refers to an embolus that lodges in the pulmonary circulation. This may arise from a deep venous thrombosis, especially after a period of immobility. A pulmonary embolus is a common cause of death in patients with cancer and stroke.:720-721 A large pulmonary embolus that becomes lodged in the bifurcation of the pulmonary trunk with extensions into both the left and right main pulmonary arteries is called a saddle embolus.
CT scan of a normal lung, with different levels of pulmonary arteries.