Breathing takes oxygen in and carbon dioxide out of the body. Aerobic organisms require oxygen to create energy via respiration, in the form of energy-rich molecules such as glucose. The medical term for normal relaxed breathing is eupnea.

Mechanics

In mammals, breathing in, or inhaling, is usually an active movement, with the contraction of the diaphragm muscles needed. This is known as negative pressure breathing. The diaphragm’s normal relaxed position is that of a recoiled one (decreasing the thoracic volume) whereas in the contracted position it is pulled downwards (increasing the thoracic volume). Relaxation of the diaphragm compresses the lungs, effectively decreasing their volume while increasing the pressure inside them. With a pathway to the mouth or nose clear, this increased pressure forces air out of the lungs by diffusion. Conversely, contraction of the diagraphm increases the volume of the (partially empty) lungs, decreasing the pressure inside, which creates a partial vacuum. Environmental air then follows its concentration gradient down to fill the lungs.

In amphibians, the process used is positive pressure breathing. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils (which uses the same mechanics - pressure, volume, and diffusion - as a mammalian lung). With the nostrils and mouth closed, the floor of the oral cavity is forced up, which forces air down the trachea into the lungs.

At rest, breathing out, or exhaling, is a combination of passive and active processes powered by the elastic recoil of the alveoli, similar to a deflating balloon, and the contraction of the muscular body wall. The following organs are used in respiration: the mouth; the nose and nostrils; the pharynx; the larynx; the trachea; the bronchi and bronchioles; the lungs; the diaphragm; and the terminal branches of the respiratory tree, such as the alveoli.

Gas exchange

Breathing is only part of the processes of delivering oxygen to where it is needed in the body and removing carbon dioxide waste. The process of gas exchange occurs in the alveoli by passive diffusion of gases between the alveolar gas and the blood passing by in the lung capillaries. Once in the blood the heart powers the flow of dissolved gases around the body in the circulation.

As well as carbon dioxide, breathing also results in loss of water from the body. Exhaled air has a relative humidity of 100% because of water diffusing across the moist surface of breathing passages and alveoli