<P> The lungs have a dual blood supply provided by a bronchial and a pulmonary circulation . The bronchial circulation supplies oxygenated blood to the airways of the lungs, through the bronchial arteries that leave the aorta . There are usually three arteries, two to the left lung and one to the right, and they branch alongside the bronchi and bronchioles . The pulmonary circulation carries deoxygenated blood from the heart to the lungs and returns the oxygenated blood to the heart to supply the rest of the body . </P> <P> The blood volume of the lungs, is about 450 millilitres on average, about 9 per cent of the total blood volume of the entire circulatory system . This quantity can easily fluctuate from between one - half and twice the normal volume . </P> <P> The lungs are supplied by nerves of the autonomic nervous system . Input from the parasympathetic nervous system occurs via the vagus nerve . When stimulated by acetylcholine, this causes constriction of the smooth muscle lining the bronchus and bronchioles, and increases the secretions from glands . The lungs also have a sympathetic tone from norepinephrine acting on the beta 2 receptors in the respiratory tract, which causes bronchodilation . </P> <P> The action of breathing takes place because of nerve signals sent by the respiratory centres in the brainstem, along the phrenic nerve to the diaphragm . </P>

Where is the apices of the lung located