Note: Thousands of scientific studies have been conducted on respiration.  The following represent a small sample.

Breathing-control lowers blood pressure. Grossman E., Grossman A., Schein M.H., Zimlichman R., Gavish B. Journal of Human Hypertension 2001; 15:263-269.

Slow breathing reduces chemoreflex response to hypoxia and hypercapnia, and increases baroreflex sensitivity. Bernardi L, Gabutti A, Porta C, Spicuzza L. J Hypertens. 2001 Dec;19(12):2221-9. PMID: 11725167 [PubMed - indexed for MEDLINE] 

Controlled breathing protocols probe human autonomic cardiovascular rhythms. Cooke WH, Cox JF, Diedrich AM, Taylor JA, Beightol LA, Ames JE 4th, Hoag JB, Seidel H, Eckberg DL. Am J Physiol. 1998 Feb;274(2 Pt 2):H709-18. PMID: 9486278 [PubMed - indexed for MEDLINE]

Slow breathing increases arterial baroreflex sensitivity in patients with chronic heart failure. Bernardi L, Porta C, Spicuzza L, Bellwon J, Spadacini G, Frey AW, Yeung LY, Sanderson JE, Pedretti R, Tramarin R. Circulation. 2002 Jan 15;105(2):143-5. PMID: 11790690 [PubMed - indexed for MEDLINE] 

Respiration and Blood Pressure. Parati G, Izzo JL Jr, Gavish B., in Hypertension Primer, Third Edition. JL Izzo and HR Black, Eds. Baltimore, Lippincott, Williams, and Wilkins, 2003; Ch. A40, p117-120.