HUMAN BIOCHEMISTRY - L. V. Kapilevich - 2016

PART 1. SPORTS BIOCHEMISTRY

BIOCHEMICAL SHIFTS IN THE BODY DURING MUSCULAR ACTIVITY

Biochemical changes occurring in skeletal Muscles

The concentration of creatine phosphate decreases while creatine accumulates.

Muscle Glycogen levels decline, whereas lactic acid concentration increases. As a result, acidity and osmotic pressure rise, causing Water to flow into myocytes and making them swell (in athletic training, this phenomenon is often referred to as muscle "tightness" or "congestion").

The Rate of protein degradation increases, particularly during strength exercises, primarily affecting the contractile Proteins that make up myofibrils. Protein breakdown leads to elevated levels of free Amino Acids and their subsequent degradation products, such as keto acids and ammonia.

Finally, the most detrimental outcome that can occur is damage to intracellular structures—myofibrils, Cell/35.html">Mitochondria, and various Biomembranes.

Biochemical changes occurring in the Brain

Physical exertion increases brain activity and Energy Expenditure. The brain actively consumes oxygen and glucose from the bloodstream. Consequently, any disruption in the supply of oxygen or glucose inevitably leads to a decline in functional activity, which in athletes may manifest as dizziness or fainting.

Biochemical changes occurring in the myocardium

During muscular activity, the workload of The Heart increases. The Energy supply of the myocardium is maintained primarily through the aerobic resynthesis of ATP.

During intense exercise, the myocardium can extract lactate from the Blood and oxidize it to generate ATP. The heart's ability to oxidize lactate is of great biological significance. Utilizing lactate as an energy source helps maintain the required blood glucose concentration for a longer period, which is crucial for the Bioenergetics of Nerve Cells, for which Glucose serves as almost the sole oxidative substrate. The oxidation of lactate in The cardiac muscle also helps normalize the acid-base balance by reducing the concentration of this acid in the blood.

Biochemical changes occurring in the Liver

First, glycogen breaks down into glucose. Second, Ketone Bodies are formed from Lipids. Ketone bodies serve as vital Energy Sources and are transported via the bloodstream from the liver to working Organs—the myocardium and skeletal muscles. In these Tissues, ketone bodies are converted back into acetyl-CoA, which is immediately oxidized aerobically in the Krebs cycle (TCA cycle) into carbon dioxide and water, releasing a large amount of energy.

When glucose is scarce, it is synthesized from glycerol, amino acids, and lactate.

Another crucial process taking place in the liver is the detoxification of ammonia. Physical exertion accelerates The breakdown of Muscle Proteins, leading to The formation of free amino acids, which are subsequently deaminated to release NH₃. Ammonia is a cellular toxin; its detoxification occurs in the liver, where it is converted into urea. Urea synthesis requires a significant amount of energy. Under exhausting workloads that exceed the body's functional capacity, the liver may fail to neutralize ammonia effectively, resulting in systemic ammonia intoxication, which leads to a decrease in physical performance.

Biochemical changes occurring in the blood

Changes in The chemical composition of the blood reflect the biochemical shifts that occur during muscular activity in various Internal Organs, skeletal muscles, and the myocardium. Therefore, analyzing blood chemistry makes it possible to assess the biochemical processes taking place during exercise. This is of great practical importance, as blood is the most accessible bodily tissue for research.

An increase in protein concentration is observed in Blood Plasma. This happens for two reasons. First, heavy sweating leads to a reduction in plasma water content, causing hemoconcentration, which raises the concentration of all plasma components, including proteins. Second, due to the damage to cell membranes, intracellular proteins leak into the blood plasma. However, during very prolonged exercise, a decrease in plasma protein concentration is also possible. In this case, some proteins pass from the bloodstream into the urine, while others are utilized as energy sources.

At the onset of exercise, blood glucose levels rise. This is because the liver holds large glycogen reserves at the start, and Gluconeogenesis proceeds at a high rate. On the other hand, muscles also possess significant glycogen stores at the beginning of activity, which they use for their own energy supply, and therefore they do not extract glucose from the bloodstream. As exercise continues, glycogen content decreases in both The Liver and muscles. Consequently, the liver releases less and less glucose into the blood, whereas muscles, conversely, begin to rely more heavily on blood glucose for energy. During prolonged exercise, a drop in blood glucose concentration (hypoglycemia) is frequently observed due to the depletion of glycogen stores in the liver and muscles.

There is also an increase in lactate concentration, the magnitude of which largely depends on The Nature of the exercise performed and the athlete's level of conditioning. The sharpest rise in blood lactate is noted during physical exertion in the submaximal power zone, since anaerobic Glycolysis becomes the primary energy source for working muscles, leading to the production and accumulation of lactic acid.

Blood pH changes. During submaximal Physical Exercise, blood pH drops to 7.1–7.2 in moderately trained athletes, while in world-Class athletes, this hydrogen ion index may fall as low as 6.8.

Elevated concentrations of free Fatty acids and ketone bodies are observed during prolonged muscular work as a result of fat mobilization from adipose depots followed by hepatic ketogenesis. The increase in ketone body concentrations (acetoacetic and beta-hydroxybutyric acids) also causes an increase in acidity and a drop in blood pH.

Blood urea levels rise. During short-term exercise, the blood urea concentration increases only slightly, whereas during prolonged physical exertion, blood urea levels can increase 4-to-5-fold. The cause of this elevated blood urea content is the intensification of Protein Catabolism induced by physical loads, particularly strength training. Protein degradation, in turn, leads to the accumulation of free amino acids, the breakdown of which produces large amounts of ammonia. In the liver, the majority of this generated ammonia is converted into urea.

Biochemical changes occurring in urine

Physical exertion also leads to significant shifts in the Chemical composition of urine and substantially affects its physicochemical properties.

Following the completion of muscular work, the most characteristic feature is the appearance in the urine of chemical substances that are practically absent at rest. These compounds are often referred to as pathological components, as they emerge in the urine not only after physical exertion but also in A number of pathological conditions.

Detection of protein in urine. This phenomenon is known as proteinuria. Particularly pronounced proteinuria is observed after excessive workloads that do not match the athlete's functional state. The likely cause of proteinuria is the damage to renal membranes induced by muscular exertion, as well as the appearance in the blood during physical work of tissue protein degradation products—various Polypeptides—that easily pass through the renal filter from the bloodstream into the urine.

Glucose content in urine (glucosuria). This can be caused by two main reasons. First, as already noted, physical exercise elevates blood glucose levels (hyperglycemia), which may exceed the renal threshold, causing a portion of the glucose to escape reabsorption in the convoluted tubules of the nephron and remain in the urine. Second, damage to the renal membranes impairs glucose reabsorption in the Kidneys, which also leads to The Development of glucosuria.

Ketone bodies in urine. Following competitive or training loads, large quantities of ketone bodies—acetoacetic and beta-hydroxybutyric acids, along with their breakdown product, acetone—may be excreted in the urine. This phenomenon is called ketonuria or acetonuria. The causes of ketonuria are similar to those that induce glucosuria: an elevated concentration of ketone bodies in the blood (hyperketonemia) and a decline in the renal reabsorptive function during muscular work.

Appearance of lactate in urine. The presence of lactic acid in the urine is typically observed after workouts involving submaximal power exercises. Each such exercise triggers a sharp surge in blood lactate concentration, followed by its transfer from the bloodstream into the urine. Consequently, lactic acid accumulates in the urine. Therefore, urinary lactate excretion can serve as an indicator of the overall contribution of the glycolytic ATP resynthesis pathway to the energy supply of the entire workout performed by the athlete.

Along with its impact on chemical composition, physical exertion alters the PHYSICOCHEMICAL PROPERTIES OF urine. Urine specific gravity increases due to the enhanced role of extrarenal water excretion pathways and the appearance of substances in the urine that are absent during rest. On average, pre-exercise urine specific gravity ranges from 1.010 to 1.025 g/mL. Following training, this figure may reach 1.030–1.035 g/mL or even higher.

Urine acidity also shifts due to the post-workout excretion of lactic acid and ketone bodies. Under normal dietary conditions prior to exercise, urine pH is 5–6. After exertion, especially intense workouts, urine pH can drop to 4–5, which corresponds roughly to a tenfold increase in urinary hydrogen ion concentration.

Review Questions

1. Which human body systems regulate muscular activity?

2. What biochemical changes occur in skeletal muscles during physical work?

3. What biochemical shifts are observed in the brain during muscular work?

4. Name the biochemical changes taking place in the liver during physical work.

5. How does Blood Glucose Level change during physical activity?

6. As a result of which processes do ketone bodies appear in the blood during muscular work?

7. Why does lactate appear in the urine after physical work?

8. How does urine pH change during physical work?