What are the effects ol low pressure on the human body?

I assume you mean low blood pressure.
For some people, low blood pressure can signal an underlying problem, especially when it drops suddenly or is accompanied by signs and symptoms such as:

• Dizziness or lightheadedness
• Fainting (syncope)
• Lack of concentration
• Blurred vision
• Nausea
• Cold, clammy, pale skin
• Rapid, shallow breathing
• Fatigue
• Depression
• Thirst
• If you mean low
  atmospheric pressure
• When the atmospheric pressure drops, tissues expand. The expansion of tissues in and surrounding joints aggravates the nerves, causing pain. Thus, patients living in geographical locations with a higher atmospheric pressure tend to experience a greater severity of osteoarthritis (pain in the joints due to expansion of tissues). Drops in atmospheric pressure also have an impact on headaches, particularly sinus headaches. When atmospheric pressure drops (such as in an ascending airplane or before a storm), gases in the sinuses and ears are at a higher pressure than those of the surrounding air. The air pressure tries to equalize, causing pain in the face and ears. Those that suffer from chronic sinusitis or have a cold have the most issues, as the air becomes trapped in the sinuses and is unable to equalize.