Dive reflex mammals and humans

Diving into mysteries: exploring the dive reflex in mammals and humans

The mammalian dive response (MDR) also known as the mammalian diving reflex is a unique and complex physiological phenomenon found in many mammals, including humans, which allows them to stay underwater for some time by holding their breath. This reflex includes a whole series of reactions that ensure survival and organ functioning under conditions of oxygen starvation.

When a mammal submerges, its body begins undergoing completely unique changes. Heart rate decreases, peripheral blood vessels constrict, blood pressure increases, the spleen activates to increase red blood cell count in the blood, and many other reactions occur. All these changes are aimed at one goal - to maximize the use of a limited oxygen supply, preserve the integrity and functioning of vital organs and tissues, and ensure survival underwater.

This reflex is not solely the purview of professional divers or hobbyists in underwater activities. Every person, regardless of training or physical fitness, possesses the ability to activate the diving reflex. However, the degree of its manifestation and effectiveness can vary significantly depending on many factors, including individual physiological characteristics, fitness level, underwater experience, depth and duration of dives, and many others.

In this article we will examine the main components of the mammalian diving reflex in more detail, the mechanisms of their operation, as well as the physiological and medical aspects of this amazing phenomenon.

Carotid Body Chemoreceptors and Their Role in the Diving Reflex

Carotid body chemoreceptors, also known as carotid bodies, are located at the bifurcation of the common carotid arteries in the neck. They are important components of the cardiorespiratory system and play a key role in modulating respiratory and cardiovascular responses to changes in blood oxygen (O²) and carbon dioxide (CO²) concentrations.

When diving and holding breath, blood CO² concentration begins to rise as it continues to be produced in the body but cannot be efficiently removed due to lack of lung ventilation. This change in blood gas composition normally triggers carotid body chemoreceptors to send signals to the brain about increasing breathing activity to restore normal CO² levels.

However, during diving this response to hypercapnia (elevated blood CO² levels) is suppressed. The diving reflex triggers a specific series of physiological reactions that allow the body to maximize use of the available oxygen supply and minimize its expenditure. One such reaction is suppression of breathing activity, allowing the body to hold its breath for a longer time.

The complex interaction between chemoreceptors, brain, and diving reflex continues to be studied by scientists to better understand how our body adapts to extreme diving conditions and how these processes can be applied in medicine and sports.

Cardiovascular System Responses During Immersion

The diving reflex triggers a whole cascade of changes in the cardiovascular system that allow the body to efficiently use the available oxygen supply and maintain vital functions when breathing is held. One of the most noticeable changes is vasoconstriction or narrowing of peripheral blood vessels. This process reduces blood flow to the skin, skeletal muscles, and other non-vital organs. Through this vasoconstriction, the body concentrates oxygen-rich blood on maintaining functions of vital organs such as the brain and heart.

At the same time, most mammals, including humans, exhibit bradycardia or decreased heart rate in response to immersion. This lowers overall metabolism and oxygen demand, helping preserve oxygen reserves in the body. Some research also points to spleen activation during diving. The spleen contains red blood cell reserves that can be rapidly released into the bloodstream under conditions of oxygen deficit, increasing blood's overall oxygen carrying capacity.

Bradycardia and Cardiac Output During Immersion: Optimizing Oxygen Utilization

Bradycardia, the slowing of heart rate, plays a key role in the diving reflex. It is commonly observed in mammals, including humans, as a reaction to submersion and breath-holding. The mechanism of bradycardia helps conserve oxygen reserves in the body by reducing overall metabolic activity and oxygen needs.

Cardiac output, the volume of blood ejected from the heart with each beat, may also change in response to immersion. Despite slowing heart rate, cardiac output can remain relatively stable due to increased blood volume ejected per heart contraction.

Spleen Activation During Diving: Increasing Blood Oxygen Capacity

The spleen is a key organ of blood cell production and the immune system, serving as a reservoir of red blood cells. It plays a vital role in acclimatizing the body to immersion by releasing red blood cells from its stores into circulation.

This process, known as spleen contraction or "spleenic reaction," usually begins immediately upon submersion, even preceding bradycardia. Contraction releases spleen stores of red blood cells into circulation, increasing total red blood cell count in the blood.

Red blood cells carry hemoglobin, the protein capable of binding and transporting oxygen. Thus, increasing red blood cells in the blood leads to heightened blood oxygen capacity. This means more oxygen can be delivered to vital organs and tissues, assisting organismal function under oxygen deficit conditions.

Blood Shifting During Immersion: Redistributing Blood to Vital Organs

Peripheral vasoconstriction, represents a key component of the diving reflex optimizing available oxygen usage. It involves redistributing blood from peripheral tissues like skin and skeletal muscles toward vital organs including the brain and heart.

In response to submersion and breath-holding, vessels in peripheral tissues constrict while those servicing vital organs remain open. This directs most oxygen-rich blood toward locations where it’s needed most. Blood shifting also helps reduce oxygen consumption by skeletal muscles and lesser organs.

The blood shift is a process that occurs in the body when diving to significant depths. As water pressure increases, the air in the lungs compresses, and to prevent their collapse, they fill with blood.

This phenomenon provides physiological adaptation to increased pressure and allows for diving to depths that would be inaccessible without this mechanism. Blood shift allows the lungs to adapt to changes in air volume during a dive, providing protection from compression and collapse.

It's important to note that this process is actively being researched, and some aspects of the blood shift mechanism are still the subject of scientific discussion. However, overall, it's believed to play a crucial role in enabling mammals, including humans, to stably withstand diving to great depths.

Kidney and Water Balance Responses During Immersion: Regulating Diuresis and Electrolyte Balance

Immersion and breath-holding can significantly impact kidney function and whole-body water-electrolyte balance. Kidneys regulate volume and composition of bodily fluids, eliminating excess water and electrolytes like sodium and potassium.

During immersion with normal hydration, kidneys typically augment diuretic activity, increasing urine volume or diuresis. This process also correlates with elevated sodium and potassium excretion. This may constitute part of a broader diving reflex optimizing oxygen usage and maintaining water-electrolyte balance.

If the body is dehydrated pre-submersion, the diuretic response may diminish due to renin-angiotensin-aldosterone system activation regulating blood pressure and water-electrolyte balance. Dehydration stimulates retaining water and salts, decreasing diuresis and electrolyte clearing.

These kidney adaptations enable organisms to cope with oxygen deficiency and blood pressure shifts during diving. Scientists explore them to enhance diving physiology comprehension and leverage related insights in healthcare and performance contexts.


Overall, all these adaptations, from blood shift to spleen activation and kidney function changes, are key elements of the diving reflex and highlight the amazing ability of mammals to adapt to extreme conditions. This is a complex and multifaceted process that evolved over millions of years and allows mammals to successfully cope with survival tasks in an aquatic environment.

However, despite our knowledge, much remains unknown. These adaptive mechanisms continue to intrigue scientists and are the subject of active research. Understanding these processes could have significant implications not only for medicine, where they could help develop new treatment and rehabilitation methods, but also for freediving, where they could provide insight into how to improve performance and safety of athletes.

In addition to this, the diving reflex and its mechanisms offer a unique look at human physiology, revealing to us the potential of our bodies for adaptation and survival in extremely adverse conditions. This reminds us of the deeply rooted connection between humans and nature and the need to continue studying and understanding our world for a better understanding of ourselves.
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