Summary: What Happens to the Blood During Exercise?

When you exercise, many of the processes that we have discussed in this and in previous tutorials work together to determine how the pH of the blood will change (Figure 5).

The following steps outline the processes that affect the buffers in the blood during exercise.

• Hemoglobin carries O₂ from the lungs to the muscles through the blood.
• The muscles need more O₂ than normal, because their metabolic activity is increased during exercise. The amount of oxygen in the muscle is therefore depleted in the muscles, setting up a concentration gradient between the muscle cells and the blood in the capillaries. Oxygen diffuses from the blood to the muscles, via this concentration gradient.
• The muscles produce CO₂ and H⁺ as a result of increased metabolism, setting up concentration gradients in the opposite direction from the O₂ gradient.
• The CO₂ and H⁺ flow from the muscles to the blood, via these concentration gradients.
• The buffering action of hemoglobin picks up the extra H⁺ and CO₂.
• If the amounts of H⁺ and CO₂ exceed the capacity of hemoglobin, they affect the carbonic acid equilibrium (Equation 10), as predicted by Le Ch â telier's Principle or the quantitative treatment in terms of equilibrium constants. As a result, the pH of the blood is lowered, causing acidosis.
• The lungs and kidneys respond to pH changes by removing CO₂, HCO₃^-, and H⁺ from the blood.

Hence, the body has developed finely-tuned chemical processes (based on buffering and acid-base equilibria) that work in combination to handle the changes that exercise produces.

References:

Brown, Lemay, and Bursten. Chemistry: the Central Science, 7th ed. p. 631.

Vander, A. et al. Human Physiology, 6th ed. WCB McGraw-Hill, Boston, 1994, p. 463-466, 492-3, 552-6.

Zumdahl. Chem. Principles, 4th ed.p. 192-5, 208-214.

Acknowledgements:

The authors thank Dewey Holten, Michelle Gilbertson, Jody Proctor and Carolyn Herman for many helpful suggestions in the writing of this tutorial.

The development of this tutorial was supported by a grant from the Howard Hughes Medical Institute, through the Undergraduate Biological Sciences Education program, Grant HHMI# 71195-502005 to Washington University.