Learning Outcomes
By the end of this section, you should be able to:
- 15.1.1 Recognize the normal function of the renal system.
- 15.1.2 Recognize cues of nutritional impact on the renal system.
- 15.1.3 Analyze cues of nutritional impact on the renal system.
Normal Function of the Renal System
The renal system contains the kidneys, urinary bladder, ureters, and urethra (Figure 15.2) The kidneys (the primary organs in this system) filter blood and remove serum wastes, excrete wastes from the body, maintain blood pressure (National Institutes of Health, National Cancer Institute, n.d.), maintain acid-base balance, and balance fluids. The kidneys are also responsible for reabsorption of glucose, amino acids, bicarbonate, water, and phosphate, as well as chloride, sodium, magnesium, calcium, and potassium ions (Newman, 2023).
There are two kidneys, each located between the twelfth thoracic and third lumbar vertebrae on either side of the vertebral column. The right kidney is slightly lower than the left because of the liver. A fibrous connective tissue known as the renal capsule covers the outermost layer of the kidneys (Figure 15.3).
Renal Cortex and Medulla
The next layer of the kidney is the renal cortex, followed by the renal medulla. The renal medulla consists of a series of renal pyramids that contain straight tubular structures and blood vessels. The top point of these pyramid-like structures, known as the renal papilla, points toward the innermost layer of the kidney, called the renal pelvis, which is found within the renal sinus. The renal sinus (cavity) collects urine that is produced by the functioning layer of the kidney, the parenchyma (the renal cortex and medulla together). Inside the parenchyma are the nephrons, the basic functioning units of the kidney (Figure 15.4).
Nephrons
Each kidney has roughly 1 million nephrons (Cleveland Clinic, 2023b, 2023c). A nephron is the functional unit of the kidney and contains two parts: the glomerulus, which is a cluster of capillaries and nerve endings that cluster around its second part, the renal tubule. Urine created in the nephrons will pass into collecting ducts and flow to the minor calyxes, to the major calyxes, into the renal pelvis, and out of the kidney into the ureter.
The kidneys remove three primary wastes: urea, uric acid, and drugs and their metabolites. Urea are wastes from protein breakdown, while uric acid is waste from the breakdown of nucleic acids (Newman, 2023).
Hormone Production
The kidneys also play a role in the production of certain hormones that support other body processes. For example, they help balance blood pressure by retaining water and sodium in the body; they also produce renin that helps to maintain appropriate blood pressure by managing water reabsorption (Merck Manuals, 2023). The kidneys assist with red blood cell production: they create and release erythropoietin, a hormone that signals the bone marrow to produce red blood cells. Another hormone, calcitriol, which is a form of vitamin D that helps the body absorb calcium and regulates the parathyroid hormone, is produced by the kidneys. The kidneys also help to raise glucose levels through employing gluconeogenesis—the formation of glucose from precursors—in the renal cortex.
Renal Artery, Renal Vein, and Ureters
Two other important kidney components are the renal artery and the renal vein. The renal artery controls blood flow to the kidneys for both supply to the organ and for filtering purposes. Blood flows through this artery at a rate of 1.2 L/min (Dalal, Bruss, & Sehdev, 2022). The renal vein carries filtered blood back to the heart, away from the kidneys.
The ureters are small tubes that run from the renal pelvis to the urinary bladder, allowing for urine passage (National Institutes of Health, National Cancer Institute, n.d.). The urine is moved by way of peristalsis, or the involuntary constriction and relaxation of musculature that promotes movement. The layers of the ureters include the fibrous coat (outer layer), the muscular coat (middle layer), and the mucosa (inner layer).
Bladder
The urine that passes down the ureters travels to the urinary bladder, a temporary storage area for urine located in the pelvic cavity, posterior to the symphysis pubis. The layers of the bladder include the outer superior surface layer, the parietal peritoneum, and the outer fibrous connective tissue. The detrusor muscle is made up of interwoven smooth muscle fibers that control the contraction of the bladder during urination; the submucosa, which supports the mucous membrane and is composed of connective tissue; and the innermost layer, the mucous membrane, which is composed of transitional epithelium that runs continuous with the ureters.
To prevent backflow of urine into the ureters, small flaps of mucosa cover the opening of the ureters into the bladder and act as valves; the internal urethral sphincter, located at the bottom of the bladder, is controlled by the detrusor muscle to control exit of urine through the urethra.
Urethra
The urethra is a thin-walled, hollow tube-like duct that allows urine to pass from the bladder to the outside of the body. The urine enters the urethra through the internal urethral sphincter; to exit the urethra, it passes through the final controlled muscle, the external urethral sphincter, which is composed of striated muscle fibers. The urethra’s layers include the fibroelastic connective tissue layer (outer layer), smooth muscle layer (middle layer), and epithelial spongy submucosa (inner layer).
Adrenal Gland
Although adrenal glands are not part of the renal system, they are an extremely important accessory gland. One adrenal gland, or suprarenal gland, sits atop each kidney. These glands release a hormone called aldosterone, which helps to balance the blood pH and fluid balance by signaling the excretion of potassium and the reabsorption of sodium by the kidneys; aldosterone is part of the renin-angiotensin-aldosterone system to help manage blood pressure (Merck Manuals, 2023). The other major role adrenal glands play is in the release of cortisol. Cortisol has several uses in the body such as helping to maintain blood pressure, increase glucose levels, reduce inflammation, and control metabolism. The relationship to the kidneys comes into play when cortisol signals the kidneys to reabsorb specific electrolytes and excrete others based on the function it is trying to accomplish in the body at a given time.
Assessment of Nutrition and the Function of the Renal System
Assessment of the renal system starts with collecting subjective data about urinary symptoms the client is experiencing. Assessing for fluid retention, over- or underproduction of urine, and fluid deficit is useful when assessing renal function. Swelling, unexplained weight gain, difficulty breathing, and very clear urine are some symptoms a client might report that are important to the assessment of the renal system. Additional symptoms might include dysuria (painful or difficult urination) (Urology Care Foundation, 2023), urinary urgency, or urinary frequency. Other information concerning urinary incontinence, passage of urinary stones, color and clarity of urine, retention of urine, and nocturia (nighttime urinary frequency). Objective data collected to validate these subjective findings can include assessment of limb edema, crackles in the lungs, intake and output measurements, and testing of the urine directly.
After collecting subjective data, the nurse should complete a physical examination of the renal system. This can be done through organ evaluation and measurement of function results. The physical assessment of the organs should begin with the abdominal assessment procedure as described in The Digestive Process with some special considerations to focus on the renal system during this assessment.
The first step is an inspection of the skin, during which the nurse should examine the color and look for masses, bulging areas, or distention. Skin discoloration may indicate anemia, if pale or cyanotic; a bulging area above the symphysis pubis may indicate urinary retention or a full bladder; and distention may indicate fluid build-up. Next, during auscultation of the abdomen, hyperactive bowel sounds can indicate diarrhea, which can result from hyperkalemia (Cleveland Clinic, 2023a), possibly related to renal issues. During palpation of the abdomen, hyperkalemia may manifest as abdominal tenderness; severe skin dryness may mean dehydration; and an overdistended bladder seen through palpation may indicate urinary retention issues. To palpate the right kidney, the nurse should place one hand under the right flank of the client and press downward against the outer edge of the right abdomen to “sandwich” the kidney, which should be smooth and firm (Roscoe, 2022). The left kidney cannot normally be palpated—unless enlarged—due to its location behind the bowel (Roscoe, 2022). Abdominal palpation can also identify urinary retention and a full bladder by identifying fluid in an enlarged bladder. During the final step—percussion of the abdomen—a dullness above the symphysis pubis may indicate bladder distension (Maddukuri, 2022b).
Safety Alert
Contraindications for Deep Palpation
Deep palpation is contraindicated in clients with suspected abdominal aortic aneurysm, appendicitis, polycystic kidney disease, kidney transplantation history, or a tender spleen, due to a risk for injury to the client and rupture of organs (Roscoe, 2022).
Bladder Scanning
Bladder scanning is another way to evaluate the bladder and its ability to effectively empty. This scan can assist in determining whether a client is retaining urine, indicated by bladder distention and abdominal fullness and discomfort. Post void residual (PVR) volume can be measured with a bladder scan machine, which is a portable, hand-held, non-invasive, ultrasound device. A wand is placed at the suprapubic area for examination immediately after the client voids and attempts to fully empty their bladder (Ballstaedt & Woodbury, 2022). Results help to determine if the bladder is being emptied adequately during micturition, or urination; determine if urine is being produced in those with renal illness; and assess for any bladder tumors, diverticula, or stones that may be present. For young and middle-aged adults (ages 18–65 years), a PVR of less than 50 mL is considered normal, over 200 mL is considered inadequate; for older adults (over age 65), 50 to 100 mL is considered normal; and for children, more than 20 mL is considered abnormal. A PVR over 500 mL, combined with identified neurologic findings, is highly predictive of cauda equina syndrome, a dysfunction affecting the multiple lumbar and sacral nerve roots of the cauda equina, which can cause a need to strain or an altered flow or altered awareness of the need to urinate.
Safety Alert
Bladder Scanning During Pregnancy
Bladder scanning is not suitable on clients who are pregnant, those with uterine prolapse, or those with severe abdominal scarring and abdominal ascites because these conditions may prompt false high results (Ballstaedt & Woodbury, 2022).
Other Body Assessments
In assessing the renal system, the nurse should examine the chest, skin, costovertebral angle (the area on the back flank at the bottom of the 12th rib forming a 90-degree angle to the spine, as shown in (Figure 15.5), rectum, groin and genitals, and neurologic system. In examining these areas, the nurse should:
- Auscultate the chest for heart and lung sounds (Maddukuri, 2022a).
- Examine the skin for color, turgor, and temperature.
- Evaluate neurologic indicators such as level of consciousness, orientation, ability to speak and write clearly, and movements of the hands.
- Review the costovertebral angle for pain (Maddukuri, 2022b).
- Determine if there is any prostate enlargement by completing a rectal examination. (Although the prostate has no relation to the renal system itself as far as function, if it enlarges, it can press against the urethra, preventing urine from exiting the body).
- Examine the groin and genitals for herniation, deviation from normal anatomy, especially of the urethra, possible signs of infection, and any masses.
Glomerular Filtration Rate, Serum Creatinine, and Blood Urea Nitrogen Tests
In addition to physical assessment, several pertinent tests are used to determine the proper functioning of the renal system and are the true indicators of health. The three major serum tests are the glomerular filtration rate (GFR), serum creatinine, and blood urea nitrogen (BUN) (Centers for Disease Control and Prevention, 2022; National Kidney Foundation, n.d.). The GFR measures how well the kidneys remove excess fluids and waste products from the blood (factoring in age and sex). This test is related to the 24-hour creatinine clearance test, because it can also identify how well the kidneys filter out wastes. The GFR can be used to evaluate the client for acute kidney injury (AKI) as well as to determine the stage of CKD and when the kidneys have failed completely, requiring transplantation or dialysis. Normal GFR for adults (18–69 years) is above 90 mL/min/1.73 m2; adults (70 years and older) should be greater than 60 mL/min/1.73 m2; children and adolescents (3–17 years) should be greater than 80 mL/min/1.73 m2; and newborn to 2 years should be 40–60 mL/min/1.73 m2 (Mitchell, Strafford, & Tavares, 2022; National Kidney Foundation, 2023a).
Serum creatinine is a measurement of the waste product from muscle energy consumption and muscle damage from the kidneys, whereas BUN is a waste product that comes from ingested proteins excreted by the kidneys. Serum creatinine levels should be 0.74–1.35 mg/dL for men 19 years and older, 0.59–1.04 mg/dL for women 19 years and older, and 0.5–1.0 mg/dL for children ages 2–18 years. Normal BUN levels for all ages are 6–24 mg/dL.
Urine
Testing urine can indicate how well the renal system maintains fluid balance by measuring hydration status and volume produced. The 24-hour creatinine clearance test measures the amount of creatinine excreted directly into the urine and is conducted through a urine sample collection process completed over 24 hours (Centers for Disease Control and Prevention, 2022; National Kidney Foundation, n.d.). If the creatinine clearance amount is low, then the kidneys are not filtering properly. A full urinalysis can test for pH, sodium, potassium, urea, creatinine, uric acid, phosphorus, calcium, chloride, ammonia, water, ketones, nitrites, blood, glucose, protein, and leukocytes to evaluate for infection and renal damage (Mitchell, Strafford, & Tavares, 2022). Most often, a urine dipstick test is used, which can test for blood, urine specific gravity, urine pH, glucose, protein, nitrates, and leukocytes (Mitchell, Strafford, & Tavares, 2022). A more specific dipstick urinalysis test, the albumin dipstick test, checks for microalbuminuria that is caused by smaller protein amounts than the standard urine dipstick test can detect. This test can help identify kidney disease in clients who have a negative test for excess protein in the urine in the regular urinalysis testing but who are at high risk for kidney disease (National Kidney Foundation, n.d.). Another specific urine test is the albumin-to-creatinine-ratio that uses the amount of albumin and creatinine levels to form a ratio, which indicates the level of kidney function.
Image and Biopsy Tests
Imaging tests—such as ultrasound and computed tomography (CT) scans—and biopsies can also be conducted to monitor the function of the kidneys. An ultrasound image of the kidneys can identify obstructions in the urinary tract and evaluate the size and position of the kidneys. A CT scan of the kidneys can help identify obstructions, structural abnormalities (National Kidney Foundation, n.d.), and kidney disease, which would present as abscesses, masses, cystic masses, blockages, or lesions. A kidney biopsy is conducted by taking a sample of the kidney tissue, usually through CT-guided or ultrasound-guided sharp needle aspiration. The tissue sample is then evaluated under the microscope for irregularities like inflammation, protein deposits, or damage, such as scarring or cancerous anomalies (National Kidney Foundation, 2023b).
Serum Electrolyte Testing
Serum electrolyte testing monitors fluid balance and electrolyte levels, both of which are regulated by the kidneys. Irregular values can indicate issues with function. The major electrolytes—sodium, potassium, calcium, and phosphorus— are monitored; however, bicarbonate, chloride, and magnesium are also important indicators of kidney function. This list describes the importance of monitoring these nutrients:
- Sodium plays a significant role in maintaining fluid and acid-base balance and is primarily regulated by the kidneys. If the kidneys are not functioning properly, sodium values can be impacted (Newman, 2023).
- Potassium, primarily regulated by the kidneys, is an important electrolyte to monitor because minor deviations can cause a client to experience life-threatening problems such as cardiac dysrhythmias, also known as an irregular heartbeat.
- Calcium and phosphorus have an inverse relationship—when one is high, the other is low. Calcium is extremely important to body functions and the musculoskeletal system. If calcium levels get too low, bone structure can break down and be life threatening for a client who has renal disease. Although calcium is not excreted through the kidneys, increased phosphorus levels is a common issue in real failure. Phosphorus levels must be restricted to prevent low calcium levels.
- Although the kidneys regulate bicarbonate levels, they are not as easily attributed to kidney function—other pathologies in the body and compensatory mechanisms can impact these levels.
- Chloride is excreted and reabsorbed, along with sodium, from the blood. It is, however, found in higher amounts in interstitial compartments and lymph fluid than in the blood, so monitoring sodium is more important than monitoring chloride for fluid balance because most of these storage areas cannot be measured. It is still important to ensure excessive intake in chloride is addressed, as this can lead to an increase in sodium levels, fluid retention, and edema.
- Magnesium plays an important role when renal failure is present because excessive intake can lead to hypermagnesemia and can exacerbate or cause hypocalcemia. Regular functioning kidneys remove excess magnesium, but in the case of failure and an inability to filter, hypocalcemia can become a serious issue leading to renal bone dysplasia, a life-threatening condition in which the body breaks down the bones to increase blood calcium levels by releasing the calcium stored in bones.
Special Considerations
COVID-19 and Renal Function
Clients who have a history of COVID-19 infection, particularly severe COVID-19 infection, should be evaluated for renal damage and failure, despite prior history because “30% of clients who are hospitalized with COVID 19 develop AKI” and “more than 50% who are admitted to the intensive care unit with kidney injury require dialysis.” This relationship is still being researched, but several hypotheses are currently being considered:
- COVID-19 directly targets renal cells.
- The decrease in oxygen from respiratory failure starves and damages the kidneys.
- COVID-19 causes a cytokine storm that destroys renal cells.
- COVID-19 causes clots that damage the small vessels in the kidneys (Sperati, 2022).
Analysis of Nutrition and the Renal System
Subjective and objective data should be collected, as previously discussed, to evaluate abnormal findings and the possibility of malfunction in the renal system. The nurse should complete an analysis of the client’s nutritional status through the client’s lab values and all assessment findings. A baseline of the renal system should then be established and used to develop a nutritional plan.
When assessing the client, the nurse should ask about dysuria (Urology Care Foundation, 2023), urinary urgency, urinary frequency, urinary incontinence, passage of renal stones, color and clarity of urine, retention of urine, and any issues with nocturia. These findings can indicate several pathologies, including cystitis (inflammation of the bladder caused by infection from bacteria). Cystitis is commonly referred to as a urinary tract infection (UTI). Dysuria can also be related to pyelonephritis, an infection of the upper urinary tract that involves the parenchyma and kidney pelvis. Pyelonephritis is also a UTI, but it is more serious and is usually accompanied by fever, flank pain, nausea and vomiting, hematuria, and suprapubic tenderness, all of which can be subjective data that is also objectively verifiable.
A report of nocturia, oliguria (low urine output), or anuria (absent urine output) with foamy quality urine can be related to CKD. CKD will very often include many other subjective complaints that will be verifiable by objective data as well. Other assessment data that could indicate CKD include lethargy, fatigue, headache, breathlessness, edema, symptoms relating to anemia, anorexia, nausea, vomiting, weight loss, cramping muscles, hematuria (blood in the urine), and pruritis (uncomfortable itching). Findings of colicky pain, hematuria, nausea, and vomiting can be an indication of renal lithiasis, or kidney stones. Discharge from the penis or vagina, dysuria, frequency, and purulent drainage from the penis all can be indicative of sexually transmitted infection.
When physical assessment findings indicate potential pathologies in the renal system, testing can assist in diagnosing the issue. GFR will indicate if there is a decrease in the function of the kidneys when the results are less than normal. Serum creatinine should be tested with the BUN results to evaluate kidney function. If the creatinine is high and the BUN is high, kidney functioning is insufficient. Table 15.1 lists laboratory tests typically performed to evaluate possible renal system issues and their relationship to the renal system.
| Lab Test | Results | Relationship |
|---|---|---|
| White blood cell count | Elevated | UTI |
| Potassium | Elevated | CKD |
| Sodium | Decreased | Fluid overload and edema from CKD |
| Calcium | Decreased or elevated | CKD/hyperparathyroidism related to CKD/renal osteodystrophy |
| Bicarbonate | Decreased | CKD |
| Phosphorus | Elevated | CKD |
| Magnesium | Elevated | CKD or AKI |
| Chloride | Decreased | Fluid overload and edema from CKD |