Kidneys... the weight of the kidneys constitutes only about .5 percent of total body weight, yet they normally receive 20% to 25% of cardiac output. The kidney functions to eliminate waste products while conserving water and important electrolytes (ions) and metabolites within the normal range for maintaining homeostasis, as well as preventing pH deviations in alkalinity of blood and cerebral spinal fluid.

The kidney has one of the highest metabolic rates in the body. It only needs to use 10 percent of the oxygen available to it, and it also has access to large quantities of electrolytes (which determine electrical activity of cells). Electrolyte balance is maintained by reabsorption of ions from blood plasma by the kidney. The kidneys will adjust the sodium ion (Na+) absorption up or down, placing more or less bicarbonate into the blood. If the amount of anions are insufficient to balance cations, body pH becomes acid. As long as the relative number of anions and cations is maintained, the total number of ions can go up or down and body pH will be sustained at normal alkalinity.

Cellular function is optimal when the pH of intracellular and extracellular fluid is held at a specific value. For example, protein requires acid base equilibrium to maintain its shape and function, and the balance of anions and cations inside and outside the cell determines the speed of polarization which affects cell growth and renewal. As long as all the tissues in the body are provided with sufficient electrolytes to maintain a slightly alkaline pH balance, the result will be an acceleration of cellular rejuvenation (if not cells wear out from acid stress).The body maintains a normal extracellular pH in two ways, through buffers like bicarbonate, and by excretion of CO2 by the lungs.

The kidneys are also endocrine organs producing kinins related to transcellular electrolyte transport, blood clotting, and a relaxation of vascular smooth muscle in blood vessels that lowers blood pressure  Although the theory that its mechanisms maintain constant blood flow regardless of perfusion pressure remains controversial, the large quantities of electrolytes in the kidneys are a clue to what is responsible for maintaining speed of metabolic activity, a chief determinant of blood flow. They also secrete renin which participates in glycogen metabolism, and they're the site of one of the principle humoral mechanisms involved in regulation of blood volume, and release adrenaline and cortico steroids.

Catecholamines constrict renal vessels, with the greatest effect from norepinphrine (which by the way is released during almost any kind of physical training). Metabolism can be stimulated by increasing the demand for electrolytes with specialized exercise that can supply the necessary electrolytes for pH balance through breathing that matches the needs of each movement so precisely there's never any hyperventilation observed at any intensity. Otherwise body pH drops due to stress and strain, as it usually does with an increase in physical activity. And it can take the kidneys hours if not days to restore homeostatic body pH (whereas it only takes the respiratory system minutes with engineering of appropriate physiologic mechanisms)

Body'Fit pH Fitness™ exercise (short form... pHx™) presents a new paradigm for promotion of metabolic function during exercise. By stimulating nerve and endocrine centers in the sequence that balances body pH almost instantaneously (within 7 to 14 minutes), the Body'Fit warm-up cool down workout routine serves to increase rather than decrease circulation to the kidneys as usually happens during exercise. Body'Fit pH Fitness exercises are uniquely designed to trigger mechanisms that transform the action of norepinephrine from vaso-constriction to vaso-dilation by stimulating release of adenosine in the abdomen immediately after increasing heart rate with venous return instead of adrenaline. The Body'Fit routine is designed to optimize the oxygen/ carbon dioxide ratio, bring the ventilation/ perfusion ratio in the lungs closer to a value of one... and there are many other examples covered in this website.