Pranayama is a conscious breathing movement in which inhales and exhales are slowed and controlled. Pranayama accompanies asanas, or poses, in Vinyasa yoga. Pranayama usually occurs in 3 stages: inhale, retention (holding the breath), and exhale. In a vinyasa yoga session, each movement is purposefully accompanied with one or more of these stages.
Pranayama and slow breathing exercises have been found to decrease heart rate (Pal & Velkumary (2004), Bhargava, Gogate, & Mascarenhas (1988)). It has also been shown to decrease blood pressure (Bhargava, Gogate, & Mascarenhas (1988)).
It is believed that this breathing decreases sympathetic activity by increasing blood oxygenation. This was found during a study investigating pranayama during hypoxia (Bernardi et al. (2001)). This increase in oxygenation improves autonomic functioning (Telles, Desiraju (1991)).