There are many reasons for the appearance of bloodstains on the inner side of the upper arm, which may be caused by exposed capillaries, broken collagen fibers, abnormal coagulation function, trauma and other factors. 1. Capillary exposure: when the skin becomes thin or white, the red lines on the inner arm may be exposed capillaries, which are not actually blood marks. 2. Collagen fiber breakage: part of the obese people or pregnant women with broken collagen fibers in the skin, there will be corresponding red blood marks. 3. Abnormal coagulation function: When some blood system diseases lead to abnormal coagulation function, a slight bump may lead to obvious bleeding spots or blood marks on the skin. 4. Trauma: Some external blows and knocks may also cause blood marks on the inner side of the arm. When blood marks appear on the inner side of the arm and the cause cannot be determined, it is recommended to consult a doctor in time for a comprehensive examination and related treatment.