What causes Hypotension ? 

Conditions or factors that disrupt the body's ability to control blood pressure cause hypotension. The different types of hypotension have different causes.
                
Orthostatic Hypotension
                
Orthostatic hypotension has many causes. Sometimes two or more factors combine to cause this type of low blood pressure.
                
Dehydration (de-hi-DRA-shun) is the most common cause of orthostatic hypotension. Dehydration occurs if the body loses more water than it takes in.
                
You may become dehydrated if you don't drink enough fluids or if you sweat a lot during physical activity. Fever, vomiting, and severe diarrhea also can cause dehydration.
                
Orthostatic hypotension also may occur during pregnancy, but it usually goes away after birth.
                
Because an older body doesn't manage changes in blood pressure as well as a younger body, getting older also can lead to this type of hypotension.
                
Postprandial hypotension (a type of orthostatic hypotension) mostly affects older adults. Postprandial hypotension is a sudden drop in blood pressure after a meal.
                
Certain medical conditions can raise your risk of orthostatic hypotension, including:
                
Heart conditions, such as heart attack, heart valve disease, bradycardia (a very low heart rate), and heart failure. These conditions prevent the heart from pumping enough blood to the body.
                
Anemia.
                
Severe infections.
                
Endocrine conditions, such as thyroid disorders, Addison's disease, low blood sugar, and diabetes.
                
Central nervous system disorders, such as Parkinson's disease.
                
Pulmonary embolism.
                
Some medicines for high blood pressure and heart disease can raise your risk of orthostatic hypotension. These medicines include:
                
Diuretics, also called "water pills"
                
Calcium channel blockers
                
Angiotensin-converting enzyme (ACE) inhibitors
                
Angiotensin II receptor blockers
                
Nitrates
                
Beta blockers
                
Medicines for conditions such as anxiety, depression, erectile dysfunction, and central nervous system disorders also can increase your risk of orthostatic hypotension.
                
Other substances, when taken with high blood pressure medicines, also can lead to orthostatic hypotension. These substances include alcohol, barbiturates, and some prescription and over-the-counter medicines.
                
Finally, other factors or conditions that can trigger orthostatic hypotension include being out in the heat or being immobile for a long time. "Immobile" means you can't move around very much.
                
Neurally Mediated Hypotension
                
Neurally mediated hypotension (NMH) occurs when the brain and heart don't communicate with each other properly.
                
For example, when you stand for a long time, blood begins to pool in your legs. This causes your blood pressure to drop. In NMH, the body mistakenly tells the brain that blood pressure is high. In response, the brain slows the heart rate. This makes blood pressure drop even more, causing dizziness and other symptoms.
                
Severe Hypotension Linked to Shock
                
Many factors and conditions can cause severe hypotension linked to shock. Some of these factors also can cause orthostatic hypotension. In shock, though, blood pressure drops very low and doesn't return to normal on its own.
                
Shock is an emergency and must be treated right away. If a person has signs or symptoms of shock, call 911.
                
Some severe infections can cause shock. This is known as septic shock. It can occur if bacteria enter the bloodstream. The bacteria release a toxin (poison) that leads to a dangerous drop in blood pressure.
                
A severe loss of blood or fluids from the body also can cause shock. This is known as hypovolemic (HI-po-vo-LE-mik) shock. Hypovolemic shock can happen as a result of:
                
Major external bleeding (for example, from a severe cut or injury)
                
Major internal bleeding (for example, from a ruptured blood vessel or injury that causes bleeding inside the body)
                
Major loss of body fluids from severe burns
                
Severe swelling of the pancreas (an organ that produces enzymes and hormones, such as insulin)
                
Severe diarrhea
                
Severe kidney disease
                
Overuse of diuretics
                
A major decrease in the heart's ability to pump blood also can cause shock. This is known as cardiogenic (KAR-de-o-JEN-ik) shock.
                
A heart attack, pulmonary embolism, or an ongoing arrhythmia (ah-RITH-me-ah) that disrupts heart function can cause this type of shock.
                
A sudden and extreme relaxation of the arteries linked to a drop in blood pressure also can cause shock. This is known as vasodilatory (VA-so-DI-la-tory) shock. It can occur due to:
                
A severe head injury
                
A reaction to certain medicines
                
Liver failure
                
Poisoning
                
A severe allergic reaction (called anaphylactic (AN-a-fi-LAK-tik) shock)