Human Anatomy and Physiology - N. I. Fedyukovich 2003

Internal Organs
Urogenital System
Renal Physiology

Urine Formation consists of three main processes: filtration, reabsorption, and tubular secretion.

The formation of urine in the Kidney begins with the ultrafiltration of Blood Plasma at the interface between the glomerular capillaries and the nephron capsule (Bowman's capsule, or the Shumлянский-Bowman capsule) driven by a blood pressure gradient. Water, salts, glucose, and other blood components pass from the glomerular capillaries into the capsular space, forming the glomerular filtrate, which is devoid of Blood Cells and Proteins. Approximately 1,200 mL of blood passes through the Kidneys every minute, accounting for about 25% of the total Cardiac Output. The volume of fluid filtered from the glomeruli into the capsule per minute is known as the Glomerular Filtration rate (GFR). Under normal conditions, the GFR in both kidneys is 125 mL/min in men and 110 mL/min in women, amounting to 150–180 liters per day. This fluid is referred to as primary urine.

From the capsule, primary urine enters the convoluted tubules, where the reabsorption of fluid and dissolved components—such as glucose and salts—takes place. For instance, out of every 125 liters of filtrate, the human kidneys reabsorb 124 liters. As a result, only 1.5–1.8 liters of definitive (final) urine are produced from 180 liters of primary urine. Certain End products of METABOLISM, such as creatinine, uric acid, and sulfates, are reabsorbed poorly and pass from the tubular lumen into the surrounding capillaries via diffusion. In addition, renal tubular cells actively transport a significant amount of waste products from the blood into the filtrate. This process, known as tubular secretion, is the primary mechanism for urine concentration. A drop in blood pressure can halt filtration and urine production entirely.

The Regulation of urine formation is carried out through neurohumoral mechanisms. The Nervous system and Hormones control the lumen of renal Blood Vessels, maintain blood pressure at a specific level, and promote normal urine production.

Pituitary Hormones exert a direct effect on urine formation. Growth Hormone (somatotropin) and thyroid-stimulating hormone increase diuresis, whereas antidiuretic hormone (ADH) decreases urine output by stimulating tubular reabsorption. A deficiency of antidiuretic hormone leads to diabetes insipidus.

The act of urination (Micturition) is a complex reflex process that occurs periodically. As the Urinary Bladder fills, urine exerts pressure on its walls and stimulates mechanoreceptors in the mucous membrane. The resulting impulses travel via afferent nerves to the Brain, from which efferent signals are sent back to the muscular layer of the bladder and its sphincter; the contraction of the bladder Muscles then expels urine through the Urethra.

The micturition reflex center is located at the level of the second through fourth sacral segments of the Spinal Cord and is modulated by higher brain centers—inhibitory influences originate in the Cerebral Cortex AND Midbrain, while excitatory influences come from the Pons and posterior Hypothalamus. Cortical signals, which initiate voluntary micturition, cause the bladder muscles to contract, increasing intravesical pressure. This is followed by the opening of the bladder neck, the widening and shortening of the posterior urethra, and the relaxation of the sphincter. Consequently, the contraction of the bladder muscles raises intravesical pressure while lowering urethral pressure, causing the bladder to enter its emptying phase and expel urine through the urethra.

The normal daily urine volume (diuresis) in an adult is 1.2–1.8 liters, depending on fluid intake, ambient Temperature, and other factors. Normal urine is straw-yellow in color, a characteristic largely determined by its specific gravity. The reaction of urine is mildly acidic, with a specific gravity ranging from 1.010 to 1.025. Urine consists of 95% water and 5% solid solutes, the major components being urea (2%), uric acid (0.05%), and creatinine (0.075%). Daily urine contains approximately 25–30 g of urea, 15–25 g of inorganic salts, as well as sodium and potassium salts. Only trace amounts of glucose are normally detectable in urine.