Urinary system anatomy and physiology review

The urinary system is responsible for the production and elimination of urine when a type of waste called urea is removed from the blood. Urea is produced when protein-containing foods are broken down in the body. Urea is carried in the bloodstream to the two kidneys, where urine is formed. The two ureters, the urinary bladder, and the urethra, are responsible for the elimination of urine. Figure illustrates the urinary system in relationship to the body, and Figure illustrates the interior and exterior features of the urinary system organs.

The urinary system

FIGURE. The urinary system

Frontal section of the right kidney showing internal structures and blood vessels

FIGURE. (A) Frontal section of the right kidney showing internal structures and blood vessels. (B) Magnified section of the kidney shows several nephrons. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 422, with permission.)

The kidneys, each about the size of a fist, help to regulate the water, electrolyte (ionized salt), and acid-base content of the blood, and they selectively filter the waste products of metabolism.

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They also play an important role in regulating blood pressure. Each kidney contains more than 1 million nephrons, which are the principal filtering units of the kidney. Each nephron houses a ball of tiny blood capillaries called the glomerulus and a renal tubule. It is here that the three-part process of selective filtration of wastes, reabsorption of vital minerals and fluid, and secretion of waste products and other substances takes place. As urine is formed, it passes through the nephrons into the renal tubules of the kidneys.

Nephron with associated blood vessels

FIGURE. Nephron with associated blood vessels

From the kidneys, urine travels into the ureters, thin tubes about 8 to 10 inches in length, on its way to the urinary bladder. Small amounts of urine drip into the urinary bladder about every 12 seconds. The urinary bladder is a balloon-shaped muscular organ that stores urine until it is emptied. It can hold about 16 ounces of urine for 2 to 5 hours. Nerves in the urinary bladder indicate when it should be emptied. Sphincter muscles keep urine from leaking into the urethra too soon and relax when it is time to urinate.

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It is worth emphasizing the reabsorption process of the kidneys’ nephrons. Were it not for this process, the body would rapidly be depleted of its fluid. Typically, only 1% of the fluid passing through a nephron is excreted as urine.

A routine diagnostic test for suspected urinary disease is a urinalysis, which includes testing the specific gravity; pH; and presence of protein, blood, sugar, and ketones. It includes a microscopic examination for the presence of white blood cells (WBCs) and red blood cells (RBCs), casts, bacteria, and crystals. Normal urine is amber in color with a slightly acid reaction, has a peculiar odor, and frequently deposits a precipitate of phosphates when fresh. The specific gravity varies from 1.005 to 1.030. The greater the rate of urine excretion, the lower is the specific gravity. Refer to Table, Significance of Changes in Urine, throughout the chapter, noting possible abnormalities and their significance to the disease in question.

Table. Significance of Changes in Urine

Significance of Changes in Urine

Acute Tubular Necrosis ICD-9: 584.5 Description Acute tubular necrosis is the rapid destruction or degeneration of the tubular segments of nephrons in the kidneys. The disease is characterized by a sudden deterioration in renal function, with resulting accumulation of nitrogenous wastes in the body. Impaired or interrupted renal function from acute tubular necrosis is considered reversible. Etiology The majority of cases of acute tubular necrosis are due to renal ischemia, or the interruption or impairment of blood flow in and out of the kidneys. This disease is the most common cause of acute renal failure in critically ill persons. Although there can be numerous causes for such impairment, renal ischemia leading to acute tubular necrosis is most frequently produced by severe bodily trauma or as a complication following surgery. The renal tubules also can be damaged in other ways. Acute tubular necrosis may be toxin induced (as a result of exposure to solvents, heavy metals, or certain medication...
End-stage renal disease ICD-9: 585.6 Description End-stage renal disease (ESRD), usually the result of chronic renal failure, is the gradual, progressive deterioration of kidney function to the point that the kidneys cannot sustain their necessary dayto-day life activity. As the kidney tissue is progressively destroyed, the kidney loses its ability to excrete the nitrogenous end products of metabolism, such as urea and creatinine, which accumulate in the blood and eventually reach toxic levels. As kidney function diminishes, every organ in the body is affected, and dialysis or kidney transplantation is eventually needed for survival. Etiology Causes of ESRD include diabetes mellitus (leading cause), hypertension, chronic glomerulonephritis, pyelonephritis, obstruction of the urinary tract, congenital anomalies such as polycystic kidneys, vascular disorders, infections, medications, and toxic agents. Signs and Symptoms The early signs and symptoms are oliguria and azotemia, or the presence of n...
Glomerulonephritis (Acute) ICD-9: 580.9 Description Glomerulonephritis, which is inflammation of the glomeruli in the kidney’s nephrons, causes the rate of blood filtration to be reduced. Retention of water and salts follows, resulting in injury to the glomeruli, which allow RBCs and serum protein to pass into the urine. Both kidneys are affected. Etiology The cause is often unknown. However, it is also known as acute poststreptococcal glomerulonephritis (APSGN), following a streptococcal infection of the respiratory tract. This inflammation is a consequence of an infection elsewhere in the body, most frequently following an infection of the upper respiratory tract or the middle ear by streptococcal bacteria. APSGN is less common today owing to the antibiotic therapy used for streptococcal infections. Other bacteria, however, and certain viruses and parasites, such as impetigo, mumps, Epstein-Barr virus, and hepatitis B and C as well as HIV (AIDS), also may induce glomerulonephritis. The disease also...
Neurogenic or overactive bladder ICD-9: 596.54 Description Neurogenic bladder refers to any loss or impairment of bladder function caused by central nervous system injury or by damage to nerves supplying the bladder. Overactive bladder function may be manifested as either incontinence (loss of voluntary control of micturition) or loss of the autonomic reflex, producing the sensation that the bladder is full. This is also referred to as urinary incontinence. Etiology Neurogenic bladder may present in one of the following two ways: (1) specific bladder dysfunction in which the neurological lesions are above sacral nerves S2 through S4 or (2) flaccid bladder dysfunction in which the lesions are below sacral nerves S2 through S4. Physical trauma to the spinal cord is a frequent cause of neurogenic bladder. Neurogenic bladder may arise as a consequence of multiple sclerosis, dementia, and Parkinson disease. Other causes can include nerve damage as a consequence of chronic alcoholism or heavy-metal poisoning. Me...
Nephrotic syndrome ICD-9: 581.9 Description Nephrotic syndrome is a condition or a complex of signs and symptoms (syndrome) of the basement membrane of the glomerulus. (The basement membrane surrounds each of the many tiny capillaries comprising a glomerulus.) The disease is characterized by severe proteinuria, often to the extent that the body cannot keep up with the protein loss, which is known as hypoalbuminemia. The disease is further characterized by hyperlipemia (excessive levels of fatlike substances called lipids in the blood), lipiduria (lipids in the urine), and generalized edema. Etiology Nephrotic syndrome may result from a variety of disease processes having the capacity to damage the basement membrane of the glomerulus. Between 70% and 75% of the cases of nephrotic syndrome result from some form of glomerulonephritis. The syndrome also may arise as a consequence of diabetes mellitus, systemic lupus erythematosus, neoplasms, or reactions to drugs or toxins. The disease is occasio...