TUBULAR SECRETION
Tubular secretion is the process by which substances move from blood into the urine (the opposite of reabsorption). Tubular secretion occurs by active or passive transport before filtrates leave the body as urine (see Fig. 27-7). Molecules secreted in peri-tubular capillaries (see Figs. 27-3 and 27-6) move into tubular cells, then into the tubular lumen. Secretions into urine include end products of metabolism and other body processes, such as ammonia, bile pigments, and urea, along with metabolites (end products) of drugs. Ions, such as hydrogen (H+) and potassium (K+), are also subject to tubular secretion. Acid-base regulation by the kidneys depends on tubular secretion of hydrogen ions.
Key Concept In tubular reabsorption,substances move: URINE -) BLOOD
In tubular secretion (opposite), substances move: BLOOD -) URINE
WATER REABSORPTION UNDER THE INFLUENCE OF ADH
Urine formation in the kidneys is mostly regulated by three processes. In addition to water reabsorption via osmosis, some water (about 10%) is reabsorbed under the influence of antidiuretic hormone (ADH, discussed in “Role of Hormones and Other Substances” section). When secreted, ADH allows water to be reabsorbed from the distal convoluted tubule and collecting duct (back into the blood) (see Fig. 27-7). This results in less water being excreted; therefore, urine is more concentrated. The secretion of ADH depends on the amount of water in the blood. The lower the concentration of water in the blood, the more concentrated the blood is (elevated osmotic pressure). When there is high osmotic pressure because of concentrated blood, more ADH is secreted, allowing additional water reabsorption back into the blood. The reverse effect also occurs. This regulatory mechanism helps maintain the body’s homeostasis.
Key Concept Urine production is influenced by:
• Kidney functioning (glomerular filtration rate)
• Renal blood flow
• Hormonal balances
• Changes in the body’s fluid/electrolyte status
• Salt intake
• General physical condition, illnesses, exercise
• Some medications.
NCLEX Alert Multiple factors affect urine production. These may be incorporated throughout the exam due to the impact on many diseases and conditions (e.g., hypertension or renal failure). Be able to identify normal I aboratory values.
CHARACTERISTICS AND COMPOSITION OF URINE
The kidneys secrete fluids and dissolved substances, which are stored in the urinary bladder and excreted through the urethra. The result is urine. The body excretes about 1,000-1,500 mL (2-3 pints) of urine daily. Many factors influence the quantity of urine, including the amount of fluid and salt a person takes in, perspiration, hemorrhage, blood pressure, vomiting, external temperature, drugs, fever, and various diseases.
Urine is initially a clear yellow liquid, with a characteristic odor. Urine color can vary, depending on the person and the circumstances. Amber or dark yellow urine may indicate a decrease in water and dehydration. Normally, urine has an acidic pH, but this can vary depending on the diet. Urine may become cloudy (turbid), or an ammonia-like smell can result if urine is left sitting for a time.
The specific gravity of urine denotes the relationship between that urine and pure water. The specific gravity of pure water is 1.000. Normal urine is only slightly concentrated, with a specific gravity of about 1.010-1.025. A higher specific gravity indicates more concentrated urine and could be secondary to dehydration or urinary retention. Low (lower than the normal range) specific gravity indicates dilute urine and may be secondary to overhydration or a physical disorder. When kidneys are diseased, they are often unable to concentrate urine; therefore, the specific gravity remains at a dilute level.
Certain wastes are always present in urine, but careful analysis will show whether substances not normally found in urine are present. The composition of normal urine is about 95% water (a solvent) and 5% solutes. Solutes may be composed of the following:
• Nitrogenous waste products from breakdown of proteins; common protein wastes are urea, uric acid, and creatinine.
• Excess minerals from the diet, such as sodium, potassium, chloride, calcium, sulfur, and phosphate.
• Toxins and certain drug metabolites.
• Hormones (especially those related to the sex of the person).
• Pigments caused by certain foods or drugs may also appear in the urine.
In case of disease or malfunction, abnormal products, such as blood, glucose, pus, large amounts of bacteria, casts, ketone bodies, bile, and albumin (protein), may be present in the urine. Urine is normally sterile (free from microorganisms). Non-sterile urine implies disease or infection.
Key Concept Kidney transplants are the most frequently performed type of organ transplant. Ihey were also the first organ transplant to be successful, with many recipients surviving 35 years or more.
MICTURITION
The release of urine (urination) from the body is also called voiding, or micturition. Involuntary micturition is called urinary incontinence. The verb “to void” means “to vacate” or “to make empty.” Urine flows from the collecting tubules into the renal pelvis, down the ureters, and slowly enters the bladder. As urine distends the bladder, it stimulates nerve endings in the bladder walls. The brain interprets the message of fullness and the nervous system stimulates the internal and external sphincter muscles controlling the urethral opening to relax. However, the external sphincter is controlled voluntarily. Therefore, when the person wills it,external muscles in the bladder wall contract, forcing out the accumulated urine. Many conditions can cause urinary incontinence.
EFFECTS OF AGING ON THE SYSTEM
The ability of kidneys to filter blood, reabsorb electrolytes, and secrete wastes decreases with age. Table 27-3 summarizes some major effects of aging on the urinary system. Although aging kidneys still function, they have less ability to return to normal after abrupt changes in blood volume, electrolytes, and acid-base balance, owing to decreasing renal reserve. Direct insults to the kidneys from trauma, obstructions, and infections also contribute to decreased renal reserve. The number and size of nephrons normally decrease after approximately age 40, with an approximate 30%-50% loss of functioning nephrons occurring by age 80. This functional loss decreases the GFR, resulting in a decreased clearance of protein waste products (urea, creatinine, and uric acid). Drugs may reach toxic levels in the kidneys because they are not adequately filtered and removed. Secretion and removal of substances, such as ammonia, are not as efficient. The threshold for glucose decreases, and higher blood sugar levels may be noted.
TABLE 27-3. Effects of Aging on the Urinary System
|
FACTOR |
RESULT |
NURSING IMPLICATIONS |
|
20% loss in kidney weight and 30%-50% decrease in number of nephrons by age 80. |
Lessened ability to concentrate urine and form urine |
Be aware of intake and output. Assess for dehydration and edema. Assess blood pressure. Weigh daily, as necessary. Measure intake and output as needed. |
|
Nephron membranes thicken. |
Decrease in rate of filtration, excretion, and reabsorption Rise in blood urea nitrogen (BUN), creatinine, and uric acid May be more susceptible to gout |
Be aware that medications will concentrate in the blood. Watch for toxic levels of medications or electrolytes (either high or low levels). Watch laboratory reports for abnormal levels of BUN, creatinine, and uric acid. |
|
Blood flow to kidney decreases. |
Lessened urine formation |
Watch intake and output. Check for edema or dehydration. Offer fluids throughout day (maintain minimum of 2,000 mL/d). Administer medications, such as Lasix, carefully. Administer supplemental medications, such as potassium, as required. |
|
Bladder lining may become fibrotic. Client may have decreased ability to control bladder contractions. Reduced bladder sensation |
Decreased capacity of bladder Loss of muscle tone may occur causing nocturia or incontinence. May not completely empty bladder (residual urine) |
Allow for frequent bathroom visits. Make available devices or pads to absorb leaks for ambulatory clients. Watch for bladder infection and urinary retention. Allow 3 hours between administration of last evening fluids and bedtime. Advise clients: in the evening, do not drink fluids that stimulate voiding (e.g., coffee, tea, colas, alcohol). Make bathroom and bedroom safe for nighttime visits—move obstacles, keep a night light on. |
|
Cancer or benign hypertrophy of the prostate is common in men. |
Frequent urge to void Retention of urine Sexual dysfunction |
Encourage frequent testicular self-examination. Encourage medical evaluation. Treat symptomatically. |
|
Pelvic muscles may weaken and relax or atrophy in women, owing to decreased levels of estrogen and perineal trauma from childbirth. Reduced creatinine clearance in some people |
Incontinence Uterine or bladder prolapse Bladder infections common Indicates impaired kidney function |
Instruct in pelvic exercises. Do not use paper incontinence pads for clients confined to bed (to prevent ulcers); use cloth pads. Offer bedpan or assist client to bathroom every 2 hours. Provide adult incontinence pads for ambulatory clients. Assess symptoms of bladder infections. Teach proper feminine hygiene. Monitor laboratory values. Treat as ordered. |
The bladder of the older adult often has a smaller capacity; therefore, urinary frequency (inability to wait) and nocturia (waking up to void at night) are common. The bladder muscles often become weaker as a result of aging or because of trauma or childbirth, leading to urinary retention (abnormal holding), dribbling, and stress incontinence (involuntary voiding on actions such as sneezing or coughing).
Enlargement or cancer of the prostate may also cause urinary difficulties in older men.
NCLEX Alert When reading an NCLEX scenario or situation, note possible changes relative to the aging process of the anatomical systems. The urinary system has normal as well as abnormal changes; either can impact effective nursing actions.
KEY POINTS
• The urinary system eliminates wastes, controls water volume, regulates electrolyte levels, maintains pH balance, activates vitamin D, secretes renin and erythropoietin, and helps to regulate blood pressure.
• The kidneys lie behind the peritoneum (retroperitoneal).
• Nephrons, the functional units of kidneys, form urine, which is eliminated by the rest of the urinary system.
• Nephrons consist of renal corpuscles (glomerulus, Bowman’s capsule) and renal tubules (proximal convoluted tubule, loop of Henle, distal convoluted tubule).
• Urine is 95% water and 5% solutes (salts, nitrogenous waste products, metabolites, hormones, toxins).
• Urine is formed by three processes: glomerular filtration, tubular reabsorption, and tubular secretion. In addition, ADH assists in regulation of water balance in the kidneys, to maintain the body’s homeostasis.
• Micturition (voiding) is the release of urine; involuntary voiding is called urinary incontinence.
• As the body ages, the number of functional nephrons decreases.
