When a person reaches a mountain, he feels difficulty in breathing especially at about 8000 feet from the sea level. This is due to a considerable fall in the partial pressure of oxygen in the air. Consequently, the alveolar PO2 reduces and oxygen cannot diffuse into the blood. Thus, the process of oxygenation of the blood is progressively decreased to such an extent that the person feels mountain sickness which is characterised by a number of symptoms like breathlessness, headache, dizziness, nausea, vomiting and cyanosis i.e bluish discolouration of skin, nails and lips.
It is also known as Dysbarism or diver’s palsy or caisson’s disease. When a diver descends to a great depth underwater, his body is subjected to the high pressure of the surrounding water. This leads to a collapse of the lungs unless he breathes the compressed air under high pressure. But breathing of air at high pressure increases the partial pressure of gases in the alveoli. The nitrogen is the main constituent of air and it diffuses into the blood and body fats. This makes the diver lose his strength and work capacity and he fells down. If he is lifted rapidly to the sea surface, he develops several severe symptoms of decompression sickness. With rapid fall of pressure, nitrogen is evolved from the body fluids and forms gas bubbles in the blood and tissues. This may block the pulmonary vessels producing serious shortness of breath. Subsequently, itching and local pain in peripheral nerves, dizziness and paralysis may be caused by the involvement of the brain and spinal cord. So in order to avoid the disease, the diver should be lifted very slowly to the sea surface.