Describe Gaseous exchange in the alveoli of the lungs

Lungs contain millions of alveoli which act as the gas exchange surface for the body, O2 is absorbed and Co2 is excreted by the body via the lungs which contain a large number of small 'air sacs' known as alveoli. Alveoli are arranged in bunches at the end of bronchioles and are surronded by a network of cappilaries ensuring a large blood supply, the large number of Alveoli maximises the surface avaliable for gasesous exchange. 

Oxygen diffuses out of the alveoli across its epithelium and the capillaries to enter the bloodstream (via binding with haemoglobin). Co2 diffuses into the alveoli from the blood via the capillary and alveolar endothelium and is excreted by the body when you exhale.