1. R. A. Wallace, Biology: the World of Life, 6th ed., Harper Collins Publishers, 1992, p. 840.
2. P. O. Heller, Life. The Science of Biology, 3rd ed., p. 574.
3. L. S. Costanco, Physiology, 3rd ed., 2006, pp. 612.
4. I. Fox, Ph.D., Human Physiology, 2nd ed., p. 691.
5. M. Klavins, O. Nikodemus, V. Silins, V. Mellecis, M. Vircovs, K. Āboltiņa, “Environment science,” LU Academic Publisher, 2008, p. 599, ISBN 978-9984-825-09-7.