The delivery of oxygen to and the removal of carbon dioxide from the tissues. External respiration is the job of the respiratory system.

Cartilaginous rings that function to maintain an open airway, smooth muscle that functions to control the diameter of the airway, and ciliated epithelia that function to transport mucus and particles trapped in the mucus, such as bacteria, viruses, dust, and other irritants, out of the respiratory tract and up to the throat to be swallowed.

The breakdown of triglycerides to glycerol and fatty acids. Lypolysis, predominately of fats from adipose tissue, provides fatty acids as energy for most body cells during the post-absorptive state. This has the effect of sparing glucose for the brain.

The breakdown of glycogen. In the liver, glycogen is converted directly to glucose, which is then transported into the blood. In skeletal muscle glycogen is first converted to pyruvate or lactate via the glycolytic pathway. Pyruvate and lactate are then transported to the liver where they are reconverted back to glucose.

The synthesis of "new" glucose from other, smaller molecules. Gluconeogenesis only occurs in the liver. The liver can utilize lactate, pyruvate, glycerol (from fats) and keto acids (from proteins) as raw materials for the synthesis of glucose.

Glucose-dependent Insulinotropic Peptide