In animal physiology,<unk> is the transport of oxygen from the ambient air to the tissue cells and the transport of carbon dioxide in the opposite direction. This is in contrast to the biochemical definition of<unk>  which refers to cellular<unk>  the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and ATP (energy). Although physiologic<unk> is necessary to sustain cellular<unk> and thus life in animals, the processes are distinct: cellular<unk> takes place in individual cells of the animal, while physiologic<unk> concerns the bulk flow and transport of metabolites between the organism and external environment.
In unicellular organisms, simple diffusion is sufficient for gas exchange: every cell is constantly bathed in the external environment, with only a short distance for gases to flow across. In contrast, complex multicellular organisms such as humans have a much greater distance between the environment and their innermost cells, thus, a respiratory system is needed for effective gas exchange. The respiratory system works in concert with a circulatory system to carry gases to and from the tissues.
In air-breathing vertebrates such as humans,<unk> of oxygen includes four stages:
Ventilation from the ambient air into the alveoli of the lung.
Pulmonary gas exchange from the alveoli into the pulmonary capillaries.
Gas transport from the pulmonary capillaries through the circulation to the peripheral capillaries in the organs.
Peripheral gas exchange from the tissue capillaries into the cells and mitochondria.
Note that ventilation and gas transport require energy to power mechanical pumps (the diaphragm and heart respectively), in contrast to the passive diffusion taking place in the gas exchange steps.
Respiratory physiology is the branch of human physiology concerned with<unk>
Respiration 
Oxygenation
Thermoregulation
Hemostasis
Breathing
Coagulation
Erection
Control of respiration