To determine whether it is blood in the mouth or blood in the lungs, you can look at both the condition of the blood and the accompanying symptoms. 1. The condition of the blood: If the blood is mixed with sputum, darker red in color and foamy, it is most likely to be bleeding from the lungs. If the bleeding is from the mouth, it is mostly fresh blood, which can be seen oozing out along the gingival sulcus, mostly after brushing teeth and eating. 2. Accompanying symptoms: Bleeding caused by lung diseases is often accompanied by systemic symptoms such as coughing and itching throat, chest tightness and shortness of breath, poor spirit, poor appetite and so on. Bleeding caused by oral periodontitis is manifested as edema of periodontal tissues, which can be accompanied by swelling and pain in the gums, loosening of the teeth, and in severe cases, there will be overflow of pus in the periodontium. Bleeding may have other reasons, it is recommended to go to the hospital in time, complete the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment.