The physiological effects of slow breathing in the healthy human
Authors: Marc A. Russo, Danielle M. Santarelli, Dean O’Rourke
DOI / Source: https://doi.org/10.1183/20734735.009817
Date: Devember 2017
Reading level: Beginner
Why This Matters for Freedivers
Slow breathing practice is one of the simplest ways to train a calmer autonomic state: it can increase HRV and improve the “brake” (vagal tone) that helps you relax and save oxygen, which is directly useful for pre-dive preparation and recovery. It also supports the idea that how you breathe on the surface can change reflex sensitivity and body rhythm — so a consistent slow-breathing routine can be a real performance and safety tool, not just a vibe.
Synopsis
Slow breathing has gone mainstream (yoga, pranayama, “box breathing,” Buteyko), but many divers still think of it as just a way to “feel calmer.” This review explains that slow breathing can do more than change mood: it reliably shifts how the respiratory system, heart, blood vessels, and autonomic nervous system work together. The authors define “slow breathing” as roughly 4–10 breaths per minute, and they point out that around 6 breaths per minute often seems to be a sweet spot where the body’s rhythms start to “lock in.” 
The main effects described are surprisingly relevant to freedivers. Slowing the breathing rate usually makes each breath deeper, which can improve ventilation efficiency (less wasted dead-space breathing) and can slightly improve oxygenation in some settings. It also tends to increase heart rate variability (HRV) and strengthen reflexes that help regulate blood pressure (the baroreflex). In plain terms: slow breathing can shift you toward a more “rest-and-recover” state (more parasympathetic/vagal influence) and improve the coordination between breathing and heartbeats (respiratory sinus arrhythmia and cardiorespiratory coupling). The review also highlights findings that slow breathing can reduce chemoreflex sensitivity to CO₂ and low oxygen, meaning the body may react a bit less aggressively to changes in blood gases during controlled breathing practice (this is not “magic,” but a measurable shift in reflex responsiveness in lab studies). 
The authors are careful: they’re not claiming slow breathing cures everything. They emphasize that the field still needs better long-term studies. But as a physiology summary, the message is clear: controlled slow breathing is a simple tool that can measurably tune the systems freedivers care about most — calm, heart rate control, and efficient breathing mechanics — without needing complicated equipment.
Abstract
Slow breathing practices have been adopted in the modern world across the globe due to their claimed health benefits. This has piqued the interest of researchers and clinicians who have initiated investigations into the physiological (and psychological) effects of slow breathing techniques and attempted to uncover the underlying mechanisms. The aim of this article is to provide a comprehensive overview of normal respiratory physiology and the documented physiological effects of slow breathing techniques according to research in healthy humans. The review focuses on the physiological implications to the respiratory, cardiovascular, cardiorespiratory and autonomic nervous systems, with particular focus on diaphragm activity, ventilation efficiency, haemodynamics, heart rate variability, cardiorespiratory coupling, respiratory sinus arrhythmia and sympathovagal balance. The review ends with a brief discussion of the potential clinical implications of slow breathing techniques. This is a topic that warrants further research, understanding and discussion.