Vasodilation and Vasoconstriction: The Real Story

What is vasodilation? Definition of vasodilation

Vasodilation (definition) = the increase in the internal diameter of blood vessels that is caused by relaxation of smooth muscle within the wall of the vessels, thus causing an increase in blood flow. The opposite effect is vasoconstriction.

During vasodilation, when blood vessels dilate, the blood flow is increased due to a decrease in vascular resistance. However, for practical purposes, dilation of arteries and arterioles has the most significant therapeutic value since these blood vessels are the main contributors to systemic-vascular resistance and, therefore, dilation of arteries and arterioles leads to an immediate decrease in arterial blood pressure and heart rate. Hence, chemical-arterial dilators are used to treat heart failure, systemic and pulmonary hypertension, and angina. Dilation of venous-blood vessels decreases venous-blood pressure. Such agents can be used to reduce cardiac output, venous-and-arterial pressure, tissue edema (due to better capillary-fluid filtration), and myocardial oxygen demands. Let us consider, unlike useless official medical sources, practical or real-life aspects of vasodilation and vasoconstriction.

Content of this page:
Vasodilation and CO2: most potent vasodilator
Who is going to suffer from vasoconstriction?
Studies related to CO2-induced vasodilation and vasoconstriction
Vasodilation and vasoconstriction in simple terms
YouTube Video about CO2 - Vasodilation effect
References

Vasodilation, vasoconstriction and CO2: most potent vasodilator

Among arterial dilators, the natural vasodilation agent CO2 is probably the most powerful chemical. The vasodilation effect is present in healthy people due to normal arterial CO2 concentration. According to Dr. M. Kashiba, MD and his medical colleagues from the Department of Biochemistry and Integrative Medical Biology, School of Medicine, Keio University in Tokyo, CO2 is a "potent vasodilator" (Kashiba et al, 2002), while Dr. H. G. Djurberg and his team from the Department of Anesthesia, Armed Forces Hospital, in Riyadh, Saudi Arabia wrote that "Carbon dioxide, a most potent cerebral vasodilator..." (Djurberg et al, 1998).

Nitric oxide is another very potent vasodilator, which is generated within the human body from foods. More about Most Potent Natural Vasodilators: CO2 and NO.

Who is going to suffer from vasoconstriction?

Since CO2 is the most potent vasodilator, vasoconstriction should be a problem for those people who suffer from arterial hypocapnia. This relates to people with hyperventilation (or breathing more than the medical norms) and a normal or nearly normal ventilation-perfusion ratio (e.g., no problems with lungs). Indeed, people with, for example, COPD, may hyperventilate, but their blood CO2 is generally higher than normal. Here are some studies that explain blood flow and vasodilation/vasoconstriction in the healthy and sick people.

Minute ventilation rates (chronic diseases)

Condition	Minute ventilation	Number of people	All references or click below for abstracts
Normal breathing	6 L/min	-	Medical textbooks
Healthy Subjects	6-7 L/min	>400	Results of 14 studies
Heart disease	15 (±4) L/min	22	Dimopoulou et al, 2001
Heart disease	16 (±2) L/min	11	Johnson et al, 2000
Heart disease	12 (±3) L/min	132	Fanfulla et al, 1998
Heart disease	15 (±4) L/min	55	Clark et al, 1997
Heart disease	13 (±4) L/min	15	Banning et al, 1995
Heart disease	15 (±4) L/min	88	Clark et al, 1995
Heart disease	14 (±2) L/min	30	Buller et al, 1990
Heart disease	16 (±6) L/min	20	Elborn et al, 1990
Pulm hypertension	12 (±2) L/min	11	D'Alonzo et al, 1987
Cancer	12 (±2) L/min	40	Travers et al, 2008
Diabetes	12-17 L/min	26	Bottini et al, 2003
Diabetes	15 (±2) L/min	45	Tantucci et al, 2001
Diabetes	12 (±2) L/min	8	Mancini et al, 1999
Diabetes	10-20 L/min	28	Tantucci et al, 1997
Diabetes	13 (±2) L/min	20	Tantucci et al, 1996
Sleep apnea	15 (±3) L/min	20	Radwan et al, 2001
Liver cirrhosis	11-18 L/min	24	Epstein et al, 1998
Hyperthyroidism	15 (±1) L/min	42	Kahaly, 1998
Epilepsy	13 L/min	12	Esquivel et al, 1991
CHV	13 (±2) L/min	134	Han et al, 1997
Panic disorder	12 (±5) L/min	12	Pain et al, 1991
Bipolar disorder	11 (±2) L/min	16	MacKinnon et al, 2007
Dystrophia myotonica	16 (±4) L/min	12	Clague et al, 1994

Studies related to CO2-induced vasodilation and vasoconstriction

Dr. K. P. Buteyko and his colleagues found that there were vasoconstrictive effects of hypocapnia (CO2 deficiency) on arteries and peripheral blood vessels (Buteyko et al, 1964a; Buteyko et al, 1964b; Buteyko et al, 1964c; Buteyko et al, 1965; Buteyko et al, 1967), while additional CO2 causes vasodilation, which is a normal state of arteries and arterioles.

As western physiological studies found, vasodilation requires normal arterial CO2 concentration, while hypocapnia (low CO2 concentration in the arterial blood) decreased perfusion of the following organs due to vasoconstriction:
- brain (Fortune et al, 1995; Karlsson et al, 1994; Liem et al, 1995; Macey et al, 2007; Santiago & Edelman, 1986; Starling & Evans, 1968; Tsuda et al, 1987),
- heart (Coetzee et al, 1984; Foëx et al, 1979; Karlsson et al, 1994; Okazaki et al, 1991; Okazaki et al, 1992; Wexels et al, 1985),
- liver (Dutton et al, 1976; Fujita et al, 1989; Hughes et al, 1979; Okazaki, 1989),
- kidneys (Karlsson et al, 1994; Okazaki, 1989),
- spleen (Karlsson et al, 1994),
- colon (Gilmour et al, 1980).
Some abstracts from these studies are provided at the bottom of this page.

Vasodilation and vasoconstriction in simple terms

What is the physiological mechanism of the reduced blood flow to vital organs? It is like a car service NYC. Arteries and arterioles have their own tiny smooth muscles that can constrict or dilate (causing vasodilation) depending on CO2 concentrations. When we breathe more, our arterial CO2 level becomes smaller, blood vessels constrict and vital organs (like the brain, heart, kidneys, liver, stomach, spleen, colon, etc.) get less blood supply. Similarly, hypocapnia causes spasm of all other smooth muscles of the human body: airways or bronchi and bronchioles, diaphragm, colon, bile ducts, etc.

This effect explains why sick people have less blood going to their brains, heart, liver, and other vital organs. A normal breathing pattern provides people with normal perfusion and oxygen supply for all vital organs due to CO2 vasodilation. However, since modern people breathe more than the medical norm (hyperventilate), they have to suffer from CO2-deficiency effects.

Are there any related systemic effects? The state of these blood vessels (arteries and arterioles) define total resistance to the systemic blood flow in the human body. Hence, hypocapnia increases strain on the heart. Normal CO2 parameters make resistance to blood flow in the cardiovascular system small. Hence, breathing directly participates in regulation of the heart rate. The father of cardiorespiratory physiology, Yale University Professor Yandell Henderson (1873-1944), investigated this effect about a century ago.

Among his numerous physiological studies, he performed experiments with anaesthetized dogs on mechanical ventilation. The results were described in his publication "Acapnia and shock. - I. Carbon dioxide as a factor in the regulation of the heart rate". In this article, published in 1908 in the American Journal of Physiology, he wrote, "... we were enabled to regulate the heart to any desired rate from 40 or fewer up to 200 or more beats per minute. The method was very simple. It depended on the manipulation of the hand bellows with which artificial respiration was administered... As the pulmonary ventilation increased or diminished the heart rate was correspondingly accelerated or retarded" (p.127, Henderson, 1908).

Be observant. When you get a small bleeding cut or a wound, deliberately hyperventilate and see if that can help stop the bleeding. It should due to vasoconsctriction. As an alternative, perform comfortable breath holding and breathe less and accumulate CO2. What would happen with your bleeding? (It should increase due to vasodilation.) Now you know what to do after dental surgeries, brain traumas, and other accidents involving bleeding. It is natural for humans and other animals to breathe heavily in such conditions. Hence, hyperventilation can be life-saving in cases of severe bleeding.

As many health professionals found, blood flow to vital organs is directly proportional to blood CO2 concentrations. Consider this example of vasodilation - vasoconstriction. According to the Handbook of Physiology (Santiago & Edelman, 1986), cerebral blood flow decreases 2% for every mm Hg decrease in CO2 pressure. When people have 20 mmHg CO2 in their blood (half of the official norm), they have about 40% less blood supply to the brain in comparison with normal conditions. Only skeletal muscles can get more blood in conditions of hyperventilation.

"…cerebral blood flow decreases 2% for every mm Hg decrease in CO2" Professor Newton, University of Southern California Medical Center, Hyperventilation Syndrome, 2004 June 17, Topic 270, p. 1-7 (www.emedicine.com).

Personal experiment. Take 100 deep and fast breaths through the mouth and you can pass out due to ... lack of oxygen and poor blood supply for the brain. Why? Because CO2 is a vasodilator (dilator of blood vessels).

Note that there is another powerful chemical NO (nitric oxide) that is also able to produce vasodilation, while its lack causes vasoconstriction. Humans generate nitric oxide in sinuses and, hence, mouth breathing prevents us from inhaling our own nitric oxide (see web page: Nasal Nitric Oxide Effects). Meanwhile, as some medical studies claim, CO2 is a most powerful known vasodilator.

YouTube Video about CO2 - Vasodilation-Vasoconstriction effect

A first part of this video clip explains how and why voluntary forceful hyperventilation leads to fainting: when we start to breathe heavily, CO2 content in the arterial blood sharply falls within seconds and blood vessels (arteries and arterioles) constrict since CO2 is the key factor in vasodilation.

References about effects of CO2 on vasodilation.

Reference pages: Breathing norms and medical facts:
- Breathing norms: Parameters, graph, and description of the normal breathing pattern
- 6 breathing myths: Myths and superstitions about breathing and body oxygenation (prevalence: over 90%)
- Hyperventilation: Definitions of hyperventilation: their advantages and weak points
- Hyperventilation syndrome: Western scientific evidence about prevalence of chronic hyperventilation in patients with chronic conditions (37 medical studies)
- Normal minute ventilation: Small and slow breathing at rest is enjoyed by healthy subjects (14 studies)
- Hyperventilation prevalence: Present in over 90% of normal people (24 medical studies)
- HV and hypoxia: How and why deep breathing reduces oxygenation of cells and tissues of all vital organs
- Body-oxygen test (CP test) : How to measure your own breathing and body oxygenation (two in one) using a simple DIY test
- Body oxygen in healthy: Results for the body-oxygen test for healthy people (27 medical studies)
- Body oxygen in sick : Results for the body-oxygen test for sick people (14 medical studies)
- Buteyko Table of Health Zones: Clinical description and ranges for breathing zones: from the critically ill (severely sick) up to super healthy people with maximum possible body oxygenation
- Morning hyperventilation: Why people feel worse and critically ill people are most likely to die during early morning hours

References: pages about CO2 effect:
- Vasodilation: CO2 expands arteries and arterioles facilitating perfusion (or blood supply) to all vital organs
- The Bohr effect: How and why oxygen is released by red blood cells in tissues
- Cell oxygen levels: How alveolar CO2 influences oxygen transport
- Oxygen transport: O2 transport is controlled by vasoconstriction-vasodilation and the Bohr effects, both of which rely on CO2
- Free radical generation: Reactive oxygen species are produced within cells due to anaerobic cell respiration caused by cell hypoxia
- Inflammatory response: Chronic inflammation in fueled by the hypoxia-inducible factor 1, while normal breathing reduces and eliminates inflammation
- Nerve stabilization: People remain calm due to calmative or sedative effects of carbon dioxide in neurons or nerve cells
- Muscle relaxation: Relaxation of muscle cells is normal at high CO2, while hypocapnia causes muscular tension, poor posture and, sometimes, aggression and violence
- Bronchodilation: Dilation of airways (bronchi and bronchioles) is caused by carbon dioxide, and their constriction by hypocapnia (low CO2)
- Blood pH: Regulation of blood pH due to breathing and regulation of other bodily fluids
- CO2: lung damage: Elevated carbon dioxide prevents lung injury and promotes healing of lung tissues
- CO2: Topical carbon dioxide can heal skin and tissues
- Synthesis of glutamine in the brain, CO2 fixation, and other chemical reactions
- Deep breathing myth: Ignorant and naive people promote the idea that deep breathing and breathing more air at rest is beneficial for health
- Breathing control: How is our breathing regulated? Why hypocapnia makes breathing uneven, irregular and erratic.

* Illustrations by Victor Lunn-Rockliffe

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