The lungs, which are situated in the chest with the heart sitting between the right and the left lung lobe, usually get their blood supply from 2 different sources.
Most of the blood (95%) comes from the low-pressure pulmonary arteries and ends up in the pulmonary capillary bed, where gas is exchanged. A small portion (about 5%) of the blood supply circulates via high-pressure bronchial arteries, which come from the aorta and supply the structures of the major airways with blood.
In most cases of haemoptysis the blood originates from the pulmonary capillary bed (low pressure) and only in more rare cases (e.g. due to trauma or injury) from the high-pressure bronchial arteries.
If large volumes of blood enter the airway there is a risk of drowning and massive haemoptysis may result in severe anaemia, both of which are life threatening.
The reasons for haemoptysis can vary widely, common causes of haemoptysis include: