# Passion and Health: How Winter Swimming Influences Blood Morphology and Rheology
## 1 Introduction  
The beneficial effects of cold on the human body have been known since long ago [1]. Cold water swimming is applied to induce organ- and system-related physiological defense responses that are advantageous and efficient in maintaining or restoring human body homeostasis [2, 3, 4, 5, 6, 7, 8]. Winter swimming is a short-term plunge in the cold water of a lake, sea, or other body of water practiced in a group during the autumn and winter seasons, resulting in increased immunity, an improved thermoregulatory system, as well as enhanced cardiovascular function, skin blood flow, and mood. Winter swimming can help reduce body fatigue and soreness [9]. Cold temperature diminishes swelling and tissue breakdown. Ice baths cause a constriction of blood vessels. This has been suggested to be a mechanism that helps with the flushing of waste products, such as lactic acid, out of the affected tissue. Moreover, [PERSON] et al. [10] demonstrated that cold baths had a positive effect on improving post-workout recovery. Individuals who regularly practicewinter swimming exhibit greater tolerance to low temperatures [11]. Regular exposure to cold increases tolerance to cold owing to a number of adaptive mechanisms. Low temperatures exert analgesic and anti-swelling effects, improves circulation, has antioxidant properties, reduces inflammation, and positively influences the function of the neuromuscular system [12; 13]. Swimming in icy water has been proved to have a positive effect on the human psyche [14; 15; 16] and to bring about antidepressant effects [17]. Regular winter swimming leads to a rise in general well-being in individuals who suffer from rheumatism, fibromyalgia, or asthma [14]. Owing to an increase in catecholamine concentration, swimming in cold water can remedy depression, as it activates the sympathetic nervous system and raises norepinephrine and \(\beta\)-endorphin concentrations [18]. The stress hormones of adrenalaine, norepinephrine, and cortisol, released by cold water exposure, stimulate the immune system to mobilize and positively affect the psyche. People who perceive themselves as tolerant of cold experience stable emotions and are characterized by greater activity and vigor [19]. The mechanism of adaptation is considered to be habituation or acclimatization [20]. Cold water immersions (e.g., adapted cold showers, partial or total immersion, cold swimming) are now increasingly used as a complementary procedure to improve the effects of the primary treatment of various clinical conditions, including depressive and anxiety disorders [21]. Sex-specific differences in the response to cold exposure and swimming have also been observed in the literature, and they may play a significant role in how males and females adapt to such conditions. For instance, males and females differ in their thermoregulatory mechanisms, such as skin blood flow and heat conservation, partly due to variations in body composition, including fat distribution and muscle mass. Hematological differences, such as baseline hemoglobin levels and red blood cell counts, are also notable and may influence oxygen delivery and blood viscosity during cold exposure. The rationale for combining cold water and sports pool swimming was based on the hypothesis that alternating exposure to extreme cold and moderate temperatures could enhance thermoregulatory adaptability and cardiovascular benefits. Cold water immersion activates the sympathetic nervous system, induces vasoconstriction, and stimulates non-shivering thermogenesis, while subsequent swimming in a warmer pool allows for muscle relaxation and recovery. This combination is thought to optimize the balance between stress-induced adaptation and recovery, fostering systemic resilience. The aim of this study was to investigate the effects of winter swimming and swimming in a sports pool on the morphological and rheological characteristics of blood in female and male winter swimmers.