Understanding Respiration, Exercise Physiology, and Hydration

Respiration

Ventilation

Ventilation is the process of air flowing in and out of the lungs.

Diffusion

Diffusion is the exchange of oxygen (O2) and carbon dioxide (CO2) between the blood and the lungs/tissues.

Transport

Transport refers to the movement of O2 and CO2 between the lungs and tissues.

Internal Respiration

Internal respiration is the cellular exchange of O2 and CO2.

Factors Affecting Oxygen Affinity

  • Temperature: As temperature increases, oxygen affinity decreases.
  • pH: A decrease in pH (more acidic) leads to a decrease in oxygen affinity. For example, if blood pH drops to 7.20, oxygen affinity decreases, while a pH increase to 7.60 enhances affinity.
  • Partial Pressure of CO2 (PCO2): As PCO2 rises, oxygen affinity decreases due to competition between CO2 and O2 for binding to hemoglobin (carbamino effect).
  • 2,3-Diphosphoglycerate (2,3-DPG): Erythrocytes produce 2,3-DPG in response to low oxyhemoglobin levels. Increased 2,3-DPG reduces oxygen affinity, promoting the release of bound oxygen from hemoglobin.

Erythrocyte Synthesis

Erythrocyte (red blood cell) synthesis takes place in the bone marrow and is regulated by erythropoietin (EPO). The kidneys produce EPO, and its secretion is stimulated by low arterial O2 pressure.

Adaptations to Training

Aerobic Endurance Training

  • Increased capillary density
  • Increased mitochondrial size and number
  • Enhanced TCA cycle activity
  • Increased oxidation of lipids and carbohydrates
  • Elevated intramuscular stores of myoglobin, glycogen, and triacylglycerol
  • Improved cardiovascular function: increased blood volume, stroke volume, cardiac output, and arteriovenous oxygen difference
  • Increased lipid oxidation primarily from intramuscular triacylglycerol
  • Attenuated hormonal responses to acute exercise

Overtraining

  • Hinders beneficial adaptations
  • Increases susceptibility to immune system suppression

Other Factors

  • High levels of glucocorticoids (e.g., cortisol) can suppress the immune system and inhibit interferon-gamma (IFN-γ).
  • Low muscle glycogen levels and high-fat diets may enhance the expression of genes involved in exercise metabolism.
  • Exercise increases free radical generation, but the benefits of antioxidant supplementation remain inconclusive.

Skeletal Muscle Structure and Function

Sarcomere

The sarcomere, the basic unit of muscle contraction, contains the contractile proteins actin and myosin. Tropomyosin inhibits actin-myosin interaction. Calcium binding to troponin allows myosin cross-bridges to interact with actin’s active sites.

Muscle Fiber Types

  • Type 1 (Slow-Twitch): Primarily use aerobic metabolism for endurance activities.
  • Type 2 (Fast-Twitch): Generate rapid, powerful contractions through anaerobic metabolism.

Factors like MAPK, calcium, and PGCα regulate muscle fiber type.

Dynamics of Pulmonary Ventilation

Acid-Base Balance

The body maintains acid-base balance through buffer systems. Bicarbonate, phosphate, and proteins act as buffers, consisting of a weak acid and its salt. The lungs and kidneys also play crucial roles in pH regulation. Changes in alveolar ventilation rapidly alter hydrogen ion (H+) concentration in extracellular fluids. The renal tubules serve as the final line of defense by excreting H+ in urine and reabsorbing bicarbonate.

Anaerobic Exercise

Anaerobic exercise increases the demand for buffering, making pH regulation more challenging.

Hydration in Exercise

Importance

  • Impacts performance
  • Requires proper assessment
  • Necessitates effective rehydration strategies
  • Varies based on sex and sweat rate

Risks of Dehydration

  • Increased risk when starting exercise dehydrated
  • Higher risk with multiple training sessions
  • Diuretic use can exacerbate dehydration

Hyponatremia

Hyponatremia (low blood sodium levels, typically below ~130 mmol/L) can cause:

  • Exercise-associated hyponatremic encephalopathy (EAH): symptoms include headache, nausea, dizziness, muscle weakness, encephalopathy, pulmonary edema
  • Overdrinking effect: can impair endurance performance, cardiovascular function, thermoregulation, muscle strength, and power

Thermoregulation

  • Hyperthermia (elevated body temperature) increases dehydration risk and vice versa.
  • Causes of heat stress: increased sweat rate, cutaneous vasodilation, reduced blood volume, acclimatization
  • Strategies: consume cool fluids, consider osmolarity and isotonic hypovolemia

Consequences of Dehydration on Exercise Performance

  • Increased body temperature and heat storage (cognitive impairment)
  • Reduced exercise-induced heat strain
  • Impaired blood flow and sweat rate
  • Increased cardiovascular strain and reduced cardiac output
  • Increased glycogen utilization
  • Altered metabolic function
  • Reduced muscle blood flow

Pre-Exercise Hydration

  • Begin exercise well-hydrated
  • Pay attention to thirst cues
  • Drink 5-10 ml of fluid per kg body weight in the 2 hours before exercise
  • Aim for pale yellow urine
  • Hyperhydration can be beneficial
  • Avoid fluid overloading: can lead to increased urine production, gastrointestinal issues, and increased body weight
  • Glycerol hyperhydration can increase osmotic pressure and expand intracellular and extracellular fluid spaces
  • Goal: dilute plasma osmolality and sodium content to increase diuresis and reduce thirst

Sodium Intake

  • Oral rehydration solutions (ORS): typically contain 90 mmol/L sodium
  • Post-exercise: aim for ~50 mmol/L (2-3 g/L) sodium intake
  • Commercial sports drinks: sodium content varies from 10-25 mmol/L for palatability

Challenges of Hydration

  • Difficult to replace 100% of sweat losses, especially after significant dehydration (2-5% body mass loss) or with short recovery periods (6-8 hours) between exercise sessions
  • Gastric emptying and intestinal absorption rates can limit fluid intake
  • Excessive fluid consumption can lead to hyponatremia or gastrointestinal problems

Hydration Guidelines

Before Exercise

  • Start well-hydrated
  • Listen to your thirst
  • Drink 5-10 ml/kg body weight in the 2 hours before exercise
  • Aim for pale yellow urine
  • Consider hyperhydration

During Exercise

  • Strength/power athletes: drink to replace sweat losses
  • Endurance athletes: drink to thirst or prevent more than 2-3% body weight loss
  • Consume cold drinks to aid thermoregulation

After Exercise

  • Replace 150% of fluid losses
  • Include sodium and glucose in rehydration beverages
  • Drink fluids gradually
  • Choose palatable options
  • Consider solid foods for sodium replacement

Fluid Balance Calculation

Fluid balance (sweat rate) = pre-exercise body weight – post-exercise body weight + fluid intake – urine output