HUMAN BIOCHEMISTRY - L. V. Kapilevich - 2016

PART 1. SPORTS BIOCHEMISTRY

BIOCHEMISTRY OF THE KIDNEYS AND URINE

Urine, much like Blood, is frequently used as an object of biochemical research in athletes. Urinalysis provides coaches with essential information regarding an athlete's functional state and the biochemical shifts occurring in the body during various physical loads. Because drawing blood carries the risk of infection (such as hepatitis or HIV), urine testing has become increasingly preferable in recent times. Therefore, coaches and physical education instructors must understand The Mechanism of urine Formation, its physicochemical properties and chemical composition, and how urine parameters change during training and competitive workloads.

Mechanism of Urine Formation

Several hypotheses and theories have been proposed regarding The Mechanism of urine formation.

The pioneering work of I. P. Pavlov, conducted back in 1883, was of great importance in studying the Functions of the Kidneys.

It was established that urine formation occurs in two phases.

The first phase is filtration. It takes place in the renal corpuscle (capsule) and involves the Formation of primary urine (ultrafiltrate). It is hypothesized that primary urine is filtered from the capillaries of the Malpighian glomerulus into the capsular space. For filtration to occur, a significant pressure gradient between the vessels and the capsule is required. This relatively high pressure in the Malpighian glomerulus is maintained because the renal Arteries branch directly off the Abdominal Aorta, supplying blood to these vessels under high pressure. Measurements have shown that blood pressure in the Malpighian glomerulus is 60–70 mm Hg.

Such high vascular pressure and the specialized Structure OF THE capsule confirm that primary urine is filtered directly from the blood.

Since formed blood elements and Plasma Proteins cannot pass through the vascular walls, primary urine represents Blood Plasma free of proteins.

Unlike primary urine formed in the capsules, the urine excreted from the body is called final (or definitive) urine. Final urine differs sharply in composition from primary urine: it lacks sugar, Amino Acids, and other salts, but features a drastically increased concentration of Metabolic waste products, such as urea.

Urine undergoes these modifications during the second phase of formation, which involves the reabsorption of Water and certain constituents of primary urine from the convoluted tubules back into the bloodstream.

As urine flows through the proximal and distal convoluted tubules, the Cells lining these tubule walls actively reabsorb water, glucose, amino acids, and certain salts. From there, the substances recovered from the primary urine pass into the venous capillary network surrounding the convoluted tubules. Urea, creatinine, and sulfates are not reabsorbed.

In addition to reabsorption, secretion takes place in the tubules, which involves The Active Transport of specific substances into the tubular lumen.

As noted previously, the composition of final urine differs dramatically from that of primary urine. Final urine contains no glucose or amino acids, and exhibits a reduced concentration of sodium chloride, whereas the concentration of urea increases almost 70-fold. While the urea concentration in blood plasma is 0.03%, in final urine it reaches 2%.

From the renal pelvis, final urine travels through the Ureters into the Urinary Bladder and is subsequently eliminated from the body. An average person excretes approximately 1.5 liters of urine per day.

Regulation of urine Formation

The urine-forming function of the kidneys is governed by neuroendocrine regulation.

The kidneys are densely innervated by the Autonomic Nervous system, receiving nerve signals via sympathetic fibers and the Vagus nerve.

The Effect of the sympathetic nervous system can be observed during stimulation of the celiac plexus. Such stimulation results in decreased urine output. Urine production drops because celiac nerve stimulation causes vasoconstriction and, consequently, a reduced Blood supply to the kidneys. As renal blood flow declines, glomerular pressure drops, leading to decreased filtration of primary urine.

A drastic reduction in urine output, up to complete cessation (anuria), is observed in response to painful stimuli. Pain-induced or reflex anuria can occur As a result of reflex vasoconstriction within the renal Vascular System, causing a sharp drop in renal Blood supply and, consequently, in urine formation. Painful stimulation is also accompanied by the release of large amounts of adrenaline and vasopressin, which further provoke anuria. METABOLISM/18.html">The Influence of The Nervous System is not limited solely to regulating vascular tone.

Kidney function is also influenced by the Central Nervous System, particularly the Cerebral Cortex.

Enhanced urine formation has been observed following stimulation of the optic thalamus, tuber cinereum, Cerebellum, and the floor of the Fourth ventricle of the Brain.

Kidney activity is modulated by Hormones secreted into the bloodstream by the Endocrine glands; these hormones circulate in the blood and, upon reaching the kidneys, alter their function. Vasopressin is one such hormone. It is secreted by the posterior Pituitary Gland. Under the influence of vasopressin, urine excretion decreases sharply.

The effect of vasopressin is sometimes so powerful that it causes a complete cessation of urine production, leading to total anuria.

Cessation of urine formation occurs during trauma, surgery, and other intense painful stimuli. The anuria that develops during severe pain results from a massive release of vasopressin into the bloodstream by the posterior pituitary gland. Conversely, Hormones of the anterior pituitary promote a sharp increase in urine production, known as polyuria.

An increase in urine output is also triggered by the thyroid hormone thyroxine, whereas adrenaline, an adrenal hormone, causes a decrease in urine production.

For therapeutic purposes, patients are often prescribed Diuretics such as urea, caffeine, sodium nitrate, and certain other substances. Under the influence of these compounds, urine formation is enhanced.

PHYSICOCHEMICAL PROPERTIES OF Urine

Volume of Urine (Diuresis). On average, an adult excretes about 1.5 liters of urine per day. However, this figure is not constant and varies within a fairly wide range. For example, the volume of excreted urine increases after drinking large amounts of fluid or consuming significant quantities of protein, the breakdown products of which stimulate renal activity. Conversely, urine production decreases when a person consumes little fluid, eats a low-protein diet, or experiences excessive sweating, resulting in significant water loss through perspiration.

The intensity of urine formation fluctuates throughout the day. Urine is produced more intensively during the day than at night, even if the nighttime fluid intake is identical to the daytime volume.

The lowest volume of urine is produced between 2:00 and 4:00 AM. The reduction in nocturnal urine output is associated with decreased physiological activity during Sleep and a slight drop in blood pressure, which in turn lowers renal perfusion pressure and decreases filtration.

Physical exertion also influences urine formation. During prolonged physical activity, The amount of excreted urine decreases, firstly because the capillary network in the Muscles dilates and blood is diverted to the muscles, thereby reducing blood supply to the kidneys, and secondly because physical work is usually accompanied by sweating, which also leads to a reduction in urine output.

Diuresis also increases following the intake of large amounts of fluid and diuretic foods, and decreases due to sweating, diarrhea, and vomiting.

Polyuria—increased urine output (exceeding 2000 ml per day)—is observed in such renal diseases as chronic nephritis and Pyelonephritis, as well as in Diabetes Mellitus, nutritional dystrophy, and other conditions.

Oliguria—reduced urine output (not lower than 800 ml per day)—is observed in renal pathologies such as acute diffuse nephritis, circulatory insufficiency, increased tissue hydrophilicity, sodium retention in Tissues, etc.

Anuria—a daily urine output of 200 ml or less—is typically the result of severe kidney damage (parenchymal injury). Prolonged anuria leads to uremia, a condition of systemic poisoning by urinary toxins.

Color of Urine. Urine is a transparent, light-yellow liquid. Upon standing, a precipitate forms. The resulting turbidity consists of salts and mucus.

The color of urine can range from pale yellow to deep yellow. Normally, this depends on the presence of pigments such as urochrome, uroerythrin, urorosein, urobilin, and others. The shade varies depending on the specific gravity and the volume of excreted urine. An intense yellow color corresponds to a high specific gravity, whereas pale urine typically has a low specific gravity. In pathological states, the color of urine may change.

Reaction of Urine (Urinary pH). On a standard mixed diet, the urine of a healthy person has a slightly acidic reaction (pH ranging from 5.0 to 7.0). The urinary reaction varies depending on diet.

When consuming a predominantly meat-based diet and other protein-rich foods, the urine reaction becomes acidic; conversely, a plant-based diet causes some alkalinization, making the urine reaction neutral or even alkaline.

A markedly acidic reaction is observed in febrile states, diabetes, fasting, renal insufficiency, etc. An alkaline reaction of urine is noted in cystitis, pyelitis, Hematuria, after vomiting and diarrhea, during the resorption of exudates, and following the intake of soda or mineral water. Urinary reaction should be determined exclusively in native, i.e., fresh urine.

Specific Gravity of Urine. The specific gravity of urine fluctuates depending on fluid intake. A high volume of water consumed causes a drop in specific gravity; conversely, restricting water intake increases the specific gravity of urine. On average, the specific gravity is 1015-1020 g/cm³.

Normal kidneys exhibit wide fluctuations in specific gravity throughout the day, determined by food and water intake, perspiration, and Respiration. Low specific gravity values (1.005-1.012), known as hyposthenuria, indicate impaired renal concentration capacity, such as in chronic nephritis or a contracted kidney. As a temporary phenomenon, low specific gravity is observed in nutritional dystrophy, after heavy drinking, or during the reduction of edema. A high specific gravity of urine (greater than 1020) is observed in acute nephritis and The formation of intracavity exudates. Interestingly, in cases of diabetes mellitus, a high specific gravity of urine is noted even in the presence of polyuria.

Chemical composition of Urine

Urine contains water, protein breakdown products (nitrogenous substances), salts, and certain Other Compounds. On average, about 60 g of salts are excreted in the urine per day.

Nitrogen is excreted predominantly as urea, which accounts for approximately 90% of the nitrogen produced by protein degradation.

Normal urine does not contain protein because, being a colloid, it cannot pass through capillary walls. The appearance of protein in the urine indicates kidney disease. Protein may appear in the urine either as a result of pathological changes in Capillary Wall permeability, allowing them to leak protein into the urine, or due to inflammatory processes in the kidneys.

However, under heavy physical exertion, protein may temporarily appear in the urine even in healthy individuals. This is particularly characteristic of runners. The appearance of protein in the urine in such cases is the result of altered permeability of the renal vascular system due to increased workload. Shortly after cessation of heavy Physical Exercise, protein disappears from these individuals' urine, and normal renal function is restored.

The appearance of protein in the urine is referred to as albuminuria.

Sugar can appear in the urine of both healthy individuals and patients with underlying conditions.

In patients, The excretion of sugar in the urine is typically associated with diabetes mellitus. In healthy individuals, however, glucosuria may occur after consuming large amounts of sugar or sugar-rich foods (such as preserves or chocolate), as well as in athletes following strenuous training. The excretion of sugar in the urine is known as glucosuria.

Normal components of urine include the pigments urobilin and urochrome, which give urine its characteristic color. These urinary pigments are formed in the intestines and kidneys from Bile pigments, which in turn are derived from Hemoglobin breakdown products.

The presence of blood in the urine, or hematuria, is observed in cases of Hemorrhage within the kidneys or Urinary Tract.

Erythrocytes detected in urine may be intact (containing hemoglobin) or altered, hemoglobin-free, appearing as single- or double-contoured rings. When urine is visibly red, the condition is termed gross (macroscopic) hematuria. In microhematuria, red Blood Cells are detected only microscopically. Renal hematuria is associated with organic kidney damage, such as acute and chronic nephritis, hemorrhagic diathesis, or malignant neoplasms, and may also occur following heavy physical exertion. Extrerenal hematuria develops due to disorders or trauma affecting the bladder, renal pelves, or ureters.

An elevated WHITE BLOOD Cell count in the urine indicates inflammatory processes in the kidneys or urinary tract (such as Renal tuberculosis, pyelitis, cystitis, or pyelonephritis).

Renal epithelial cells are not found in normal urine; their presence indicates nephritis, nephrosis, intoxication, febrile states, or infectious diseases.

Cells detected in the urine are proteinaceous cellular structures of tubular origin that take the form of casts. These include hyaline, granular, waxy, epithelial, erythrocyte, pigmented, and leukocyte casts. The presence of numerous various casts (cylindruria) is observed in organic kidney diseases (nephritis, nephrosis), infectious conditions, renal congestion, and acidosis.

Self-Control Questions

1. What are the reasons why urinalysis is often preferred over blood tests?

2. What are the MAIN STAGES OF urine formation?

3. How is The process of urine formation regulated?

4. What does the term "oliguria" mean?

5. What can cause anuria?

6. What determines the specific gravity (density) of urine?

7. The presence of which compound gives urine its yellow color?

8. Urea is the end product of The breakdown of which compounds?

9. Which components of urine are classified as pathological?