The lining of the nose has many tiny blood vessels. Those blood vessels lie close to the surface and are easily irritated.

The two most common causes of nosebleeds are:

1. Dry air. When nasal membranes dry out, they're more likely to bleed.
2. Nose picking.

Other causes of nosebleeds include:

1. Accidents or other injury to the nose.
2. Acute sinusitis
3. Allergies
4. Aspirin use.
5. Bleeding conditions, such as hemophilia.
6. Blood thinners, called anticoagulants, such as warfarin and heparin.
7. Chemicals that can irritate the inside of the nose, such as ammonia.
8. Chronic sinusitis
9. Cocaine use.
10. Common cold
11. Deviated septum
12. Nasal sprays, such as those used to treat allergies, if they're used often.
13. Nonallergic rhinitis
14. Object in the nose.

Less common causes of nosebleeds include:

1. Alcohol use.
2. Hereditary hemorrhagic telangiectasia
3. Immune thrombocytopenia (ITP)
4. Leukemia
5. Nasal and paranasal tumors
6. Nasal polyps
7. Nasal surgery.

In general, nosebleeds are not a symptom of or caused by high blood pressure.