The lungs' primary function is gas exchange, which necessitates the use of blood from the pulmonary circulation. This blood supply carries deoxygenated blood to the lungs, where erythrocytes, or red blood cells, pick up oxygen and transfer it to tissues throughout the body. The pulmonary artery is a branch of the pulmonary trunk that leads to the alveoli, carrying deoxygenated arterial blood. As it travels the bronchi, the pulmonary artery forks many times, with each branch becoming smaller in diameter. One lung lobule is supplied and drained by a single arteriole and its associated venule. The pulmonary arteries become the pulmonary capillary network when they approach the alveoli.The capillary network of the lungs is made up of small capillaries with very thin walls and no smooth muscle fibres. The capillaries branch out and follow the bronchioles and alveolar structure. The capillary wall meets the alveolar wall at this point, forming the respiratory membrane. After becoming oxygenated, blood drains from the alveoli through the several pulmonary veins, which exit the lungs through the hilum.