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Related Concept Videos

Physiological Control of Respiration01:23

Physiological Control of Respiration

Introduction
Breathing, a seemingly passive process, is regulated by the respiratory center in the brainstem. This center coordinates the involuntary control of respirations, which means it occurs without conscious effort, ensuring a smooth and uninterrupted pattern.
Regulation of Ventilation
The body maintains ventilation by monitoring levels of carbon dioxide (CO2), oxygen (O2), and hydrogen ion concentration (pH) in the arterial blood. Among these factors, the level of CO2 plays a crucial...
Chemical Factors Affecting Respiration Centers01:31

Chemical Factors Affecting Respiration Centers

Chemical factors such as changing CO2, O2, and H+ levels in arterial blood play a critical role in influencing respiration depth and rates. These variations are detected by chemoreceptors—specialized sensors located in two primary body areas. Central chemoreceptors are found throughout the brain stem, including the ventrolateral medulla, while peripheral chemoreceptors are located in the aortic arch and carotid arteries.
CO2 has a potent influence on respiration and is strictly regulated. Under...
Hypoxia01:23

Hypoxia

Hypoxia is a medical condition characterized by an inadequate oxygen supply to body tissues. It typically manifests as a bluish discoloration of the skin and mucosae, especially in fair-skinned individuals, when hemoglobin (Hb) saturation drops below 75%.
Types of Hypoxia
There are four primary types of hypoxia, each resulting from a different cause:
1. Anemic hypoxia: This type occurs due to insufficient oxygen delivery caused by a lack of red blood cells (RBCs) or RBCs with abnormal or...
Other Factors Affecting Respiration Centers01:17

Other Factors Affecting Respiration Centers

Breathing is primarily an involuntary activity regulated by the brainstem respiratory centers. However, it can also be consciously controlled, allowing us to hold our breath or take deeper breaths when needed. This voluntary control is facilitated by the cerebral motor cortex, which bypasses the medullary centers to stimulate the respiratory muscles directly.
However, the ability to hold one's breath voluntarily is not limitless. When the CO2 concentration in the blood reaches a critical level,...
The Parasympathetic Nervous System01:14

The Parasympathetic Nervous System

Overview
Physiology of Respiration II: Neurogenic Control of Respiration01:22

Physiology of Respiration II: Neurogenic Control of Respiration

The neurogenic control of respiration coordinates various neural networks and pathways to regulate breathing rate and depth, meeting the body's oxygen and carbon dioxide exchange requirements. This system adapts to physiological and environmental conditions, ensuring optimal breathing patterns.
Central Control
The brainstem is the primary site of central control, hosting respiratory centers:

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Related Experiment Video

Updated: Jun 20, 2026

Delivery of In Vivo Acute Intermittent Hypoxia in Neonatal Rodents to Prime Subventricular Zone-derived Neural Progenitor Cell Cultures
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Mechanisms reducing parasympathetic activity in chronic hypoxia.

Lauren E Maier1,2, Andrew Douglas1, Katharine Foster3

  • 1Cardiff School of Sport and Health Sciences, Cardiff Metropolitan University, Cardiff, UK.

The Journal of Physiology
|June 19, 2026
PubMed
Summary

Chronic hypoxia decreases parasympathetic activity, mainly due to increased pulmonary ventilation, not arterial chemoreflex, hypocapnia, or hypovolemia. This study used beta-adrenergic blockade to assess heart rate as an index of parasympathetic control.

Keywords:
altitudeautonomicheart ratevagalβ‐adrenergic antagonists

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Area of Science:

  • Cardiovascular Physiology
  • Altitude Physiology
  • Autonomic Nervous System Research

Background:

  • Reduced parasympathetic activity is observed in chronic hypoxia, but the specific mechanisms remain incompletely understood.
  • Investigating these mechanisms is crucial for understanding cardiovascular adaptations to high altitude environments.

Purpose of the Study:

  • To elucidate the mechanisms responsible for reduced parasympathetic activity during chronic hypoxia.
  • To determine the roles of arterial chemoreflex, pulmonary ventilation, pulmonary stretch, hypocapnia, and hypovolemia in this phenomenon.

Main Methods:

  • Utilized beta-adrenergic blockade (propranolol) to isolate parasympathetic control of heart rate (HR) in 13 lowlanders at sea level (SL) and high altitude (HA, 3800m) after 9-12 days.
  • Manipulated variables including arterial chemoreflex (oxygen breathing), pulmonary ventilation (paced breathing), pulmonary stretch (apnea), hypocapnia (CO2 rebreathing), and hypovolemia (saline infusion).

Main Results:

  • Heart rate under beta-adrenergic blockade was significantly higher at HA than SL, confirming parasympathetic withdrawal.
  • Increased pulmonary ventilation contributed to parasympathetic withdrawal, as matching ventilation between SL and HA reduced the HR acceleration.
  • Arterial chemoreflex inhibition, hypocapnia correction, and hypovolemia restoration did not significantly alter the HA-induced HR increase.

Conclusions:

  • Increased pulmonary ventilation is a significant contributor to parasympathetic withdrawal in chronic hypoxia.
  • Arterial chemoreflex activation, hypocapnia, and hypovolemia are unlikely to be primary drivers of reduced parasympathetic activity at high altitude.
  • The mechanism by which increased ventilation reduces parasympathetic activity appears independent of pulmonary stretch.