Understanding Respiration, Exercise Physiology, and Hydration
Respiration
Ventilation, Diffusion, and Transport
Ventilation refers to the airflow in and out of the lungs. Diffusion involves the exchange of oxygen (O2) and carbon dioxide (CO2) between the blood and lungs/tissue. Transport is the movement of O2 and CO2 between the lungs and tissues. Finally, internal respiration is the cellular exchange of O2 and CO2.
Oxygen Affinity Determinants
- Temperature: As temperature increases, oxygen affinity decreases.
- pH: A decrease in pH leads to a decrease in oxygen affinity. For instance, blood with a pH of 7.20 will have a decreased affinity compared to blood with a pH of 7.60.
- PCO2: An increase in PCO2 reduces oxygen affinity due to CO2 competition with O2 (carbamino effect).
- 2,3-Diphosphoglycerate (2,3-DPG): Produced in erythrocytes in response to low oxyhemoglobin, 2,3-DPG decreases oxygen affinity, promoting the release of bound O2 from hemoglobin.
Erythrocyte Synthesis
Erythrocyte synthesis takes place in the bone marrow and is regulated by erythropoietin (EPO). Produced in the kidneys, EPO secretion is stimulated by low arterial O2 pressure.
Exercise Physiology
Adaptations to Training
Training for strength, power, or speed impacts aerobic capacity, leading to changes in ATP, PCr, and glycolytic pathways. Aerobic endurance training increases capillary density, mitochondrial size and number, TCA cycle activity, and the oxidation of lipids and carbohydrates. It also elevates intramuscular stores of myoglobin, glycogen, and triacylglycerol.
Endurance training benefits the cardiovascular system by enhancing blood volume, stroke volume, cardiac output, and arteriovenous oxygen difference. Intramuscular triacylglycerol appears to be the primary source of increased lipid oxidation post-training. Additionally, endurance training reduces the hormonal response to acute exercise.
Overtraining
Overtraining hinders beneficial adaptations and increases the risk of immune system suppression. High levels of glucocorticoids, such as cortisol, can cause immunosuppression and inhibit IFN-gamma.
Nutritional Considerations
Low muscle glycogen levels and high-fat diets may enhance the mRNA content of genes involved in exercise metabolism. While exercise increases free radical generation, the benefits of nutritional antioxidant supplementation remain inconclusive.
Skeletal Muscle Structure and Function
The sarcomere contains the contractile proteins actin and myosin. Tropomyosin inhibits actin-myosin interaction, while troponin, in the presence of calcium, activates actin, allowing myosin cross-bridges to bind to active sites.
Type 1 (slow-twitch) fibers generate energy aerobically, while Type 2 (fast-twitch) fibers produce quick, powerful contractions anaerobically. Factors like MAPK, calcium, and PGC-1alpha regulate fiber type.
Dynamics of Pulmonary Ventilation
Buffer System and pH Regulation
The body’s buffer system, regulated by acid-base balance, utilizes bicarbonate, phosphate, and protein to defend against pH changes. Buffers consist of a weak acid and its corresponding salt. The lungs and kidneys also play a crucial role in pH regulation. Changes in alveolar ventilation rapidly alter free hydrogen ion concentration in extracellular fluids, while renal tubules act as the final line of defense by secreting hydrogen ions in urine and reabsorbing bicarbonate. Anaerobic exercise increases the demand for buffering, making pH regulation more challenging.
Hydration in Exercise
Importance and Issues
Hydration is crucial for performance and requires careful assessment and rehydration strategies tailored to individual sweating rates and sex. Dehydration risk increases when starting exercise dehydrated, engaging in multiple training sessions, or using diuretics.
Hyponatremia
Hyponatremia (sodium levels < ~130 mmol/L) can lead to Exercise-Associated Hyponatremic Encephalopathy (EAH), characterized by headache, nausea, dizziness, muscle weakness, and in severe cases, pulmonary edema. Overdrinking during exercise can contribute to hyponatremia and negatively impact endurance performance, cardiovascular function, thermoregulation, and muscle strength and power.
Thermoregulation
Hyperthermia (increased body temperature and low body water content) exacerbates dehydration. It increases sweat rate and cutaneous vasodilation, reduces blood volume and circulatory efficiency, and impairs acclimatization. Cool drinks can help mitigate these effects. Osmolarity and isotonic hypovolemia are important considerations.
Consequences of Dehydration
Dehydration can lead to:
- Increased 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 euhydrated (well-hydrated with sodium-containing fluids).
- Pay attention to thirst cues.
- Drink 5-10 ml of fluid per kg body weight in the two hours before exercise, aiming for pale yellow urine.
- Avoid fluid overloading, which can lead to increased urine production, gastrointestinal issues, and increased body weight.
- Glycerol hyperhydration, while increasing osmotic pressure and expanding extracellular/intracellular space, should be approached with caution.
Sodium Intake
- Oral rehydration solutions (ORS): 90 mmol/L sodium
- Post-exercise: ~50 mmol/L (2-3 g/L) sodium
- Commercial sports drinks: 10-25 mmol/L sodium for palatability
Hydration Guidelines
Before Exercise
- Start well-hydrated.
- Listen to your thirst.
- Drink 5-10 ml/kg body weight in the last two hours before exercise.
- Aim for pale yellow urine.
During Exercise
- Strength/power athletes: drink to replace sweat loss.
- Endurance athletes: drink to thirst or prevent >2-3% body weight loss.
- Consume cold drinks for thermoregulation.
After Exercise
- Replace 150% of sweat loss.
- Include sodium and glucose in rehydration fluids.
- Drink slowly.
- Prioritize palatability; 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