Overview
The urinary system (renal system) filters blood, removes waste products and excess water, regulates fluid and electrolyte balance, and produces urine.
It consists of the kidneys, ureters, urinary bladder, and urethra.
The kidneys also help:
- Regulate Blood Pressure: Adjust blood volume and release renin, which participates in blood pressure regulation.
- Stimulate Red Blood Cell Production: The kidneys release erythropoietin (EPO), which signals the bone marrow to produce red blood cells.
- Support Calcium Absorption: Activate vitamin D, helping the intestines absorb calcium.
- Maintain Acid-Base Balance: Remove excess acid and regulate bicarbonate levels.
The bladder stores urine until signals from the nervous system coordinate urination.
Major Structures
Upper Urinary Tract
- Kidneys: Two bean-shaped organs located toward the back of the abdomen, on either side of the spine, below the rib cage.
- Renal Cortex: Outer region containing parts of the structures that filter blood.
- Renal Medulla: Inner region containing structures involved in concentrating urine.
- Nephrons: Functional units of the kidneys that filter blood and form urine.
- Glomerulus: Network of capillaries that filters fluid and small substances from blood.
- Renal Tubules: Reabsorb needed substances and secrete additional wastes into the forming urine.
- Renal Pelvis: Collects urine and channels it into the ureter.
- Ureters: Two muscular tubes that transport urine from the kidneys to the bladder through peristalsis.
Lower Urinary Tract
- Urinary Bladder: Hollow, muscular organ in the pelvis that stores urine.
- Detrusor Muscle: Smooth muscle in the bladder wall that contracts during urination.
- Stretch Receptors: Detect bladder filling and send signals to the nervous system.
- Urethra: Tube that carries urine from the bladder to the outside of the body.
- Internal Urethral Sphincter: Involuntary smooth muscle at the bladder neck that helps prevent urine leakage.
- External Urethral Sphincter: Skeletal muscle under voluntary control that helps maintain continence.
Supporting Structures
- Pelvic Floor Muscles: Help support the bladder and maintain urinary continence.
- Renal Arteries: Supply blood to the kidneys for filtration.
- Renal Veins: Return blood from the kidneys to circulation.
Urine Pathway: Kidneys → Ureters → Bladder → Urethra → Outside the body.
Key Processes
Urine Formation
Urine is produced through three primary processes:
- Glomerular Filtration: Water and small dissolved substances move from blood into the nephron.
- Tubular Reabsorption: Needed water, glucose, electrolytes, and other substances return to the bloodstream.
- Tubular Secretion: Additional wastes, medications, and excess ions move from the blood into the renal tubules.
Remaining fluid and dissolved waste products become urine.
Urination (Micturition)
Urination requires coordination between the bladder, sphincters, spinal cord, and brain.
- The kidneys continuously produce urine.
- Urine travels through the ureters and collects in the bladder.
- As the bladder fills, stretch receptors send signals to the nervous system.
- The person becomes aware of the need to urinate.
- During normal voluntary urination, the detrusor muscle contracts and urethral sphincters relax.
- Urine flows through the urethra and exits the body.
Continence: Ability to control when and where urination occurs.
Incontinence: Involuntary urine leakage caused by impaired bladder storage, sphincter function, neurological control, or other factors.
Fluid & Electrolyte Regulation
- Water Balance: Kidneys adjust the amount of water excreted in urine.
- Antidiuretic Hormone (ADH): Increases water reabsorption, helping conserve body fluids.
- Aldosterone: Promotes sodium reabsorption and potassium excretion.
- Electrolyte Balance: Kidneys regulate sodium, potassium, calcium, and other electrolytes.
- Acid-Base Balance: Kidneys help maintain blood pH by excreting acid and regulating bicarbonate.
Blood Pressure & Hormone Regulation
- Renin: Released by the kidneys and activates the renin-angiotensin-aldosterone system (RAAS), which helps regulate blood pressure.
- Erythropoietin (EPO): Stimulates bone marrow to produce red blood cells.
- Vitamin D Activation: Kidneys convert vitamin D into its active hormonal form, calcitriol, which supports calcium absorption.
Effects of Aging
- Reduced Kidney Reserve: Kidney blood flow and filtration capacity may decline, reducing the ability to handle physiological stress.
- Altered Medication Clearance: Reduced kidney function can cause certain medications to accumulate.
- Reduced Urine-Concentrating Ability: Kidneys may become less effective at conserving water during dehydration.
- Reduced Bladder Capacity: Some older adults experience decreased functional bladder capacity and more frequent urination.
- Bladder Emptying Changes: Incomplete emptying may become more common, increasing the risk of urinary retention and infection.
- Pelvic Floor Changes: Reduced muscle support may contribute to urinary leakage.
- Changes in Prostate Size: Prostate enlargement in some older men can obstruct urine flow.
Clinical Implications
- Increased vulnerability to dehydration and electrolyte disturbances.
- Changes in urinary frequency, urgency, or nighttime urination.
- Increased risk of urinary retention and urinary tract infections in some residents.
- Greater sensitivity to medications cleared through the kidneys.
- Increased need for appropriate toileting assistance and observation of urinary patterns.
Important: Urinary incontinence, kidney failure, and urinary tract infections are not inevitable consequences of aging.
See Effects of Aging - Systems & Clinical Implications.
Common Diseases & Disorders
| Condition | Description | Common Causes / Risk Factors | CNA Considerations |
|---|---|---|---|
| Acute Kidney Injury (AKI) | Sudden decline in kidney function, occurring over hours to days | Dehydration, severe infection, reduced blood flow, medications, urinary obstruction | Report decreased urine output, new swelling, weakness, or acute changes in condition |
| Chronic Kidney Disease (CKD) | Long-term damage or reduced kidney function, potentially progressing to kidney failure | Diabetes, hypertension, cardiovascular disease, age, inherited kidney disorders | Observe edema, fatigue, changes in urine output, and fluid balance; follow prescribed restrictions |
| Urinary Tract Infection (UTI) | Infection affecting any part of the urinary tract, commonly the bladder | Bacterial entry into urinary tract, urinary retention, catheter use, urinary obstruction | Report painful urination, urgency, frequency, fever, or new urinary symptoms; observe changes from baseline |
| Urinary Incontinence | Involuntary leakage of urine | Pelvic floor weakness, overactive bladder, neurological disorders, medications, mobility limitations | Provide prompt incontinence care, support scheduled toileting, protect skin, and report changes |
| Urinary Retention | Inability to empty the bladder completely or at all | Prostate enlargement, medications, nerve disorders, urinary obstruction, postoperative changes | Observe difficulty voiding, bladder discomfort, distention, or frequent small amounts of urine; report promptly |
| Kidney Stones (Nephrolithiasis) | Hard mineral deposits forming within the kidneys or urinary tract | Dehydration, certain metabolic conditions, family history, dietary factors | Report severe flank pain, blood in urine, nausea, or difficulty urinating |
| Prolapsed Bladder (Cystocele) | Bladder descends into the vaginal wall due to weakened pelvic support | Childbirth, aging, menopause, pelvic floor weakness, obesity, chronic straining | Observe urinary difficulty, leakage, discomfort, or reported pelvic pressure; provide respectful assistance |
| Benign Prostatic Hyperplasia (BPH) | Noncancerous enlargement of the prostate that may restrict urine flow | Increasing age, hormonal changes, family history | Report weak urine stream, difficulty starting urination, frequent nighttime urination, or retention |
| Kidney Infection (Pyelonephritis) | Bacterial infection of kidney tissue, often arising from an ascending UTI | Untreated or complicated UTI, urinary obstruction, urinary reflux | Report fever, flank pain, chills, nausea, or sudden illness promptly |
Types of Urinary Incontinence
| Type | Description | Common Causes |
|---|---|---|
| Stress Incontinence | Urine leaks during coughing, laughing, sneezing, or lifting | Weak pelvic floor or urethral support |
| Urge Incontinence | Sudden intense urge to urinate followed by involuntary leakage | Overactive bladder, detrusor overactivity, neurological conditions |
| Overflow Incontinence | Leakage associated with a bladder that does not empty adequately | Urinary retention, obstruction, weak bladder contraction |
| Functional Incontinence | Person cannot reach or use the toilet in time despite potentially intact urinary function | Mobility limitations, cognitive impairment, environmental barriers |
| Mixed Incontinence | Combination of two or more incontinence types, commonly stress and urge | Multiple contributing mechanisms |
Important Distinctions
- AKI vs. CKD: Acute kidney injury develops suddenly; chronic kidney disease develops or persists over months or years.
- Urinary Retention vs. Incontinence: Retention is inadequate bladder emptying; incontinence is involuntary leakage. Both can occur together.
- Stress vs. Urge Incontinence: Stress leakage occurs with increased abdominal pressure; urge leakage follows a sudden compelling need to urinate.
- UTI vs. Kidney Infection: A bladder infection affects the lower urinary tract; pyelonephritis affects the kidney and may cause more serious illness.
- Oliguria vs. Anuria: Oliguria means abnormally low urine output; anuria means nearly absent urine output.
Important: Cloudy or foul-smelling urine alone does not establish a UTI, and a positive urine culture does not always mean a symptomatic infection. CNAs report objective findings rather than diagnosing infection.
CNAs do not independently diagnose urinary conditions, prescribe fluid restrictions, or perform urinary catheter procedures unless specifically authorized within their role and training.
When to Report
- New burning, pain, urgency, or difficulty with urination.
- New or worsening urinary incontinence.
- Inability to urinate or suspected urinary retention.
- Markedly decreased urine output or significant change from baseline.
- Blood in the urine.
- Fever, chills, flank pain, or suspected infection.
- New lower abdominal pain or distention.
- Unexpected changes in intake and output.
- New swelling, dizziness, or signs of dehydration.
- Sudden confusion or functional decline, particularly when accompanied by other signs of illness.
Emergency signs: Inability to urinate with severe pain or distention, severe flank pain with fever, sudden unresponsiveness, or signs of severe infection require urgent or emergency evaluation according to facility protocol.
Study Notes
Clinical Connections
- Elimination Care: Proper toileting support and observation help maintain comfort and dignity.
- Incontinence Care: Prompt hygiene reduces moisture exposure and skin breakdown.
- Fluid Balance: Kidneys regulate water and electrolyte levels.
- Dehydration: Reduced fluid intake or excessive fluid loss can affect kidney function.
- Skin Integrity & Breakdown Prevention: Urinary incontinence can contribute to moisture-associated skin damage.
- Vital Signs: Fever, blood pressure changes, and pulse changes may accompany urinary illness.
- Standard Precautions: Urinary infections and catheter care require appropriate infection-prevention practices.
- Recognizing Patient Distress: Acute urinary retention or severe infection may require urgent attention.
Questions
- What are the four major structures of the urinary system?
- What is the correct pathway of urine from the kidneys to outside the body?
- What are the primary functions of the kidneys?
- What is a nephron?
- How do filtration, reabsorption, and secretion contribute to urine formation?
- How do the detrusor muscle and urethral sphincters coordinate urination?
- What is the difference between voluntary and involuntary urinary control?
- How do ADH and aldosterone affect fluid balance?
- What are the functions of erythropoietin and activated vitamin D?
- How does aging affect kidney function and bladder control?
- What distinguishes acute kidney injury from chronic kidney disease?
- What is the difference between urinary retention and urinary incontinence?
- How do stress, urge, overflow, and functional incontinence differ?
- What symptoms can suggest a UTI or kidney infection?
- What urinary changes should a CNA report urgently?
Sources
- CNA Module 13 — Anatomy, Physiology & Aging
- NIDDK — Your Kidneys & How They Work
- NIDDK — The Urinary Tract & How It Works
- NIDDK — Urinary Incontinence
- MedlinePlus — Kidney Diseases
- MedlinePlus — Urinary Tract Infections
Flashcards
What are the four major structures of the urinary system?01-foundations / anatomy-physiology / urinary
Answer
Kidneys, ureters, bladder, and urethra.
What is the primary function of the kidneys?01-foundations / anatomy-physiology / urinary
Answer
Filter blood, remove wastes, and regulate fluid and electrolyte balance.
What is the correct pathway of urine through the urinary system?01-foundations / anatomy-physiology / urinary
Answer
Kidneys → ureters → bladder → urethra.
What is a nephron?01-foundations / anatomy-physiology / urinary
Answer
The functional unit of the kidney responsible for forming urine.
What are the three main processes of urine formation?01-foundations / anatomy-physiology / urinary
Answer
Glomerular filtration, tubular reabsorption, and tubular secretion.
What is the function of the ureters?01-foundations / anatomy-physiology / urinary
Answer
Transport urine from the kidneys to the bladder.
What is the function of the bladder?01-foundations / anatomy-physiology / urinary
Answer
Store urine until urination.
What is the detrusor muscle?01-foundations / anatomy-physiology / urinary
Answer
Smooth muscle in the bladder wall that contracts to expel urine.
What is the difference between internal and external urethral sphincters?02-mechanisms / anatomy-physiology / urinary
Answer
Internal is involuntary; external is under voluntary control.
What causes the sensation of needing to urinate?02-mechanisms / anatomy-physiology / urinary
Answer
Bladder stretch receptors signal the nervous system as the bladder fills.
What is the function of antidiuretic hormone (ADH)?01-foundations / anatomy-physiology / urinary
Answer
Increase water reabsorption by the kidneys.
What is the function of erythropoietin (EPO)?01-foundations / anatomy-physiology / urinary
Answer
Stimulate red blood cell production in bone marrow.
How do the kidneys support calcium absorption?02-mechanisms / anatomy-physiology / urinary
Answer
Activate vitamin D into calcitriol, helping intestinal calcium absorption.
How does aging commonly affect kidney function?02-mechanisms / anatomy-physiology / urinary
Answer
Reduced filtration reserve, concentrating ability, and medication clearance.
What is the difference between acute kidney injury and chronic kidney disease?02-mechanisms / anatomy-physiology / urinary
Answer
AKI develops suddenly; CKD is persistent kidney impairment.
What is the difference between urinary retention and urinary incontinence?02-mechanisms / anatomy-physiology / urinary
Answer
Retention is incomplete emptying; incontinence is involuntary leakage.
What is stress urinary incontinence?01-foundations / anatomy-physiology / urinary
Answer
Leakage during coughing, laughing, sneezing, or exertion.
What is urge urinary incontinence?01-foundations / anatomy-physiology / urinary
Answer
Leakage associated with a sudden, compelling urge to urinate.
What is overflow urinary incontinence?01-foundations / anatomy-physiology / urinary
Answer
Leakage associated with an inadequately emptied bladder.
What is functional urinary incontinence?01-foundations / anatomy-physiology / urinary
Answer
Inability to reach or use the toilet in time due to functional barriers.
What is a urinary tract infection (UTI)?01-foundations / anatomy-physiology / urinary
Answer
An infection involving part of the urinary tract.
What is pyelonephritis?01-foundations / anatomy-physiology / urinary
Answer
An infection of kidney tissue, often caused by an ascending UTI.
What is a cystocele?01-foundations / anatomy-physiology / urinary
Answer
Prolapse of the bladder into the vaginal wall due to weakened pelvic support.
What should a CNA do if a resident suddenly cannot urinate and has severe bladder pain?03-application / anatomy-physiology / urinary
Answer
Report immediately and summon urgent nursing assistance.