Apnea diving challenges the human body's ability to perform under conditions of oxygen deprivation. One key physiological adaptation observed during apnea is the acute hematological response, specifically changes in hemoglobin (Hb) concentration and hematocrit (Hct). These changes are thought to enhance oxygen transport and utilization, which are critical for sustaining performance and safety during prolonged breath-holds.
While previous studies have documented these hematological responses, there remains a lack of comprehensive analysis regarding the magnitude of these changes and the underlying mechanisms. Additionally, the implications of these physiological adaptations for training and safety protocols in apnea diving are not fully understood. A systematic review and meta-analysis aims to bridge these gaps.
Magnitude of the increase in Hb and Hct
Hemoglobin concentration (Hb) and hematocrit (Hct) levels increase significantly during apnea bouts. On average
Hb levels showed an acute increase ranging from 3% to 10%, depending on factors such as the duration and intensity of the apnea bout, as well as the individual's baseline physiological characteristics.
Hct levels increased by a similar magnitude, with reported changes ranging from 2% to 8%. These changes were more pronounced in trained apnea divers compared to untrained individuals, highlighting the role of physiological adaptation through regular practice.
What is hemoglobin (Hb) and Hematocrit (Hct)?
While hemoglobin (Hb) is the protein in red blood cells that binds and carries oxygen, hematocrit is the percentage of your blood that is made up of red blood cells. It reflects the proportion of red blood cells compared to the total blood volume (which includes plasma and other components). A higher Hct means a greater concentration of red blood cells, which improves the blood's ability to transport oxygen.
Mechanism behind the increase
The spleen plays a central role in the acute hematological response to apnea and is part of the mammalian dive reflex. It functions as a reservoir for red blood cells, storing approximately 10-20% of the total red blood cell volume under normal conditions. During apnea, the body triggers splenic contraction as part of the dive reflex, a suite of physiological adaptations that conserve oxygen during submersion.
Activation of splenic contraction
The onset of apnea and the associated hypoxia (low oxygen levels) stimulate the sympathetic nervous system. This activation causes the spleen’s smooth muscle tissue to contract, ejecting stored red blood cells into the systemic circulation.
What is smooth muscle tissue?
Smooth muscle tissue is a type of involuntary muscle found in the walls of internal organs and structures such as blood vessels, the digestive tract, and the spleen. Unlike skeletal muscle, smooth muscle contracts automatically without conscious control. Its contraction is regulated by the autonomic nervous system and various chemical signals. In the spleen, smooth muscle fibers are distributed within the organ's capsule and trabeculae (supporting framework). When these fibers contract, they reduce the spleen's volume, forcing red blood cells stored within it into the bloodstream.
The hematological response to breath-hold diving is a fascinating adaptation that ultimately reflects our evolutionary origins in the sea.
Increase in blood cell count
The additional red blood cells increase the hemoglobin concentration (Hb) and hematocrit (Hct), thereby boosting the blood’s oxygen-carrying capacity. This adaptation enhances oxygen delivery to critical organs such as the brain and heart, helping to sustain life during prolonged breath-hold periods.
Oxygen Content of Arterial Blood (CaO2)
It represents the total amount of oxygen carried in the blood and is calculated using the following formula:
CaO2=(Hb×SaO2×1.34)+(0.003×PaO2
In simple terms, CaO2 measures how much oxygen is available in your blood to be delivered to your body. Most of this oxygen is carried by hemoglobin. A smaller portion is dissolved directly in the blood’s plasma. When hemoglobin levels increase due to splenic contraction during apnea, the oxygen-carrying capacity of the blood also increases, allowing more oxygen to reach vital organs during a dive. This formula shows that an increase in Hb concentration during splenic contraction directly raises the oxygen-carrying capacity of the blood (first term), while dissolved oxygen in plasma contributes a smaller, secondary effect (second term).
Hb: Hemoglobin concentration (in g/dL), which binds oxygen and is the primary carrier of oxygen in the blood.
SaO2: Arterial oxygen saturation (in %), indicating the percentage of hemoglobin saturated with oxygen.
1.34: A constant representing the oxygen-carrying capacity of hemoglobin (mL of O2 per gram of Hb).
PaO2: Partial pressure of oxygen in the arterial blood (in mmHg), representing the amount of oxygen dissolved in the plasma.
0.003: A constant for the solubility of oxygen in plasma
Duration of the effect
The splenic contraction-induced rise in Hb and Hct typically persists for several minutes to hours after apnea, depending on the individual’s physiology and level of training. This ensures prolonged benefits even after the dive has ended.
Dependence on duration and style of apnea diving
Duration of Apnea: Longer breath-hold durations tend to induce more pronounced increases in Hb and Hct levels. This is likely because extended apnea stimulates a stronger splenic contraction as the body compensates for prolonged oxygen deprivation.
Intensity and Type of Dive: Static apnea, where the divers remain stationary while holding their breath, often produces smaller increases in Hb and Hct compared to dynamic apnea, which involves swimming or other movements. Dynamic apnea typically demands higher oxygen consumption, thus triggering a more robust hematological response.
Training Level: Regular apnea training enhances the efficiency and magnitude of splenic contraction, resulting in faster and greater increases in Hb and Hct during dives. Experienced divers exhibit more pronounced hematological responses compared to novices.
Conclusion
The hematological response to breath-hold diving is a fascinating adaptation that ultimately reflects our evolutionary origins in the sea.
A single session of apnea bouts in physically active, healthy individuals significantly increases hemoglobin concentration (Hb) and hematocrit (Hct), improving breath-holding duration through hematological and cardio-respiratory adaptations. Future research should investigate these effects in sedentary individuals and broader contexts to evaluate whether patients with cardiovascular or respiratory diseases could potentially benefit from this response.
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