Pulmonary artery
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. Structure. The pulmonary arteries are blood vessels that carry systemic venous blood returning to the right side of the heart through to the microcirculation of the lungs. Unlike in other organs where arteries supply oxygenated blood, the blood carried by the pulmonary  arteries is deoxygenated, as it is venous blood returning to the heart. The main pulmonary arteries emerge from the right side of the heart, and then split into smaller arteries that progressively divide and become arterioles, eventually narrowing into the capillary microcirculation of the lungs where gas exchange occurs. Pulmonary trunk. In order of blood flow, the pulmonary arteries start as the pulmonary trunk that leaves the fibrous pericardium (parietal pericardium) of the ventricular outflow tract of right ventricle (also known as infundibulum or conus arteriosus. The outflow track runs superiorly and to the left, posterior to the pulmonary valve . The pulmonary trunk bifurcates into right and left pulmonary arteries below the arch of aorta and in front of the left main bronchus. Pulmonary trunk is short and wide—approximately in length and in diameter. The pulmonary trunk splits into the right and the left main pulmonary artery. The left main pulmonary artery is shorter than the right, passes behind and downwards the descending aorta and above the left main 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. The right  pulmonary artery pass across the midline of the body, below the carina of trachea, and comes in front of the right main bronchus. Branches. The left main pulmonary artery then divides into two lobar arteries, one for each lobe of the left lung. At the right root of the lung, it bifurcates into artery that supplies the right upper lobe of the lung, in front of the right upper lobe bronchus, and interlobar artery that supplies the right middle and inferior lobes of the lung, running together with bronchus intermedius. The right and left main pulmonary (lungs) arteries give off  branches that supplies the corresponding lung lobes. In such cases it is termed lobar arteries. The lobar arteries branch into segmental arteries (roughly 1 for each segment). Segmental arteries run together with segmental bronchi, at the at posterolateral surfaces of the bronchi. These in turn branch into subsegmental pulmonary arteries. These eventually form intralobular arteries. The pulmonary arteries supply the alveoli of the lungs. In contrast, bronchial arteries, that has different origins, supply the bronchi of the lungs. Development. The pulmonary arteries originate from the truncus arteriosus and the sixth pharyngeal arch. The truncus arteriosus 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. During early development, the ductus arteriosus connects the pulmonary trunk and the aortic arch, allowing blood to bypass the lungs. Function. 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. In contrast to the "pulmonary arteries", the bronchial arteries supply nutrition to the lungs themselves. Pressure. 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. 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, mitral valve stenosis, and other conditions, such as sickle cell disease. Clinical significance. 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. 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. In Chest X-rays, diameter of descending pulmonary artery more than 16 mm indicates 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. Most recently, computational fluid based tools (non-invasive) have been proposed to be at par with  the current clinical tests (invasive) of pulmonary hypertension. 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. 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". Several animal models have been utilized for investigating pulmonary artery related pathologies. Porcine model of pulmonary artery is the most frequently  used and it was recently found that their mechanical properties vary with every subsequent branching.