The function of the respiratory system in humans is to supply the lungs with air and to facilitate the diffusion of oxygen into the blood stream and also remove carbon dioxide from the blood so it can be exhaled and removed from the body before it becomes toxic. The respiratory system consists of two parts, the upper and lower tracts, the upper respiratory tract includes the nose, mouth, nasal cavity, the pharynx and larynx, each part plays an important function to aid the flow of air into the body (Mannheim 2017). Figure 1: shows the main organs required for respirationThe mouth and nose take in air as we breathe in, its passes the nasal cavity where the air is warmed up and cleaned by tiny cilia removing particles before, the air is then passed along to the pharynx, this acts as a passageway for food on the way to the stomach and air on the way to the lungs (Richardson 2006).The larynx separates the digestive system and the respiratory system and is vital for both functions, it keeps food and drink out of the airways during swallowing and during coughing and breathing, it manages airflow.The lower respiratory tract includes the trachea, bronchi, bronchioles and the alveoli. The bronchus separates into secondary and tertiary bronchi which branch into smaller airways called bronchioles, they have small balloon shaped air sacs on the end called alveoli which are bunched together in clusters. They are shaped like grapes to increase surface area which increases the rate of diffusion The alveoli are only one cell thick which allows for diffusion to happen easily, oxygen is able to pass quickly into the blood in the capillaries and in return, carbon dioxide can pass from the blood to the alveoli so it can be exhaled (lechtzin, 2018).Blood containing a higher level of carbon dioxide arrives at the alveoli which is produced during cellular respiration, the air contained within the alveoli has a much lower concentration of carbon dioxide which causes a concentration gradient, this allows for the carbon dioxide to diffuse from the blood into the alveolar air. Oxygen is able to diffuse easily into the blood stream where it binds to red blood cells creating oxyhemoglobin.Figure 2 shows the circulatory system including the gas exchange in humans.The circulatory system is made up of three systems that work together, cardiovascular which is the heart, pulmonary which is the lungs and systemic which consists of arteries, veins, coronary and portal vessels (Toro, 2013). The system controls nutrients, the flow of blood, hormones to and from cells as well as oxygen and other gasses.Pulmonary circulation is a circuit through the lungs in which blood is oxygenated, it joins systemic circulation providing oxygenated blood to the rest of the body. Blood which is low in oxygen enters the right atrium of the heart and into the left ventricle, it is pumped to the lungs where carbon dioxide is released from the blood and oxygen can be absorbed which can then be pumped back tor the heart along the pulmonary vein (Zimmermann, 2016).