NURS 370: Module 7: Renal: Assessment of Kidney and Urinary Function

Nephron

is the microscopic structural and functional unit of the kidney responsible for filtering blood, reabsorbing needed substances, and creating urine.

Ureters

Tubes that carry urine from the kidneys to the bladder.

Bladder

Stores urine until excretion.

Urethra

The canal through which urine exits the body from the bladder.

Glomerulus

filters blood.

Tubules

adjust water/electrolyte levels.

Collecting duct

gather final urine for output.

Fewer than 20% of Nephrons are Functioning

Indicates the consideration for kidney replacement therapies like dialysis or transplantation to maintain health.

This is due to the kidneys not being able to effectively filter waste and regulate fluid and electrolytes effectively.

Glomerular Membrane Function

Acts as a filter by allowing small molecules (like water and waste products) to pass through, while blocking larger molecules such as blood cells and proteins like albumin.

Pressure changes and the selective permeability of this membrane help move fluids and dissolved substances from the blood into the Bowman capsule, starting the process of urine formation.

Urine is formed through three main processes in the nephron

1. Glomerular Filtration

2. Tubular Reabsorption

3. Tubular Secretion

Glomerular filtration

Blood is filtered by the glomerulus to extract water and small molecules. Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule.

Tubular reabsorption

Useful substances (like water and electrolytes) are reabsorbed back into the blood from the tubules.

Tubular secretion

Additional waste products are secreted into the tubules for elimination in urine.

Key Renal Physiology Concepts

These values together describe how blood is directed to the kidneys, how much is filtered, the force behind that filtration, and the total volume of filtrate produced, which are essential concepts in understanding kidney function.

1. Renal Fraction

2. Filtration Fraction

3. Net Filtration Pressure

4. Glomerular Filtration Rate (GFR)

Renal Fraction

This refers to the percentage of total cardiac output (blood flow) that goes to the kidneys, typically about 20-25%.

Filtration Fraction

This is the percentage of plasma entering the kidneys that actually gets filtered through the glomerulus to become filtrate, usually around 20%.

Net Filtration Pressure

This is the effective pressure that drives fluid across the glomerular membrane into Bowman's capsule, leading to filtration; an average value is about 10 mmHg.

Glomerular Filtration Rate (GFR)

GFR measures how much filtrate is produced by all nephrons in both kidneys per minute, with a normal rate of 120-125 mL/min.

Regulation of Glomerular Filtration

Glomerular Filtration is regulated by two path ways divided into Intrinsic (Local Kidney) and Extrinsic (Systemic) Controls

Intrinsic Controls

(Kidney Self-Regulation) Kidneys regulate their own blood flow (auto-regulation) to maintain steady GFR.

1. The kidneys use autoregulation to keep the glomerular filtration rate (GFR) nearly constant when mean arterial pressure (MAP) is between 80-180 mmHg.

2. Macula densa (MD) cells in the nephron monitor sodium (Na+) and chloride (Cl-) levels in the filtrate.

3. If filtration flow (and hence NaCl in the filtrate) is low, the MD signals juxtaglomerular cells in the afferent arteriole to relax (vasodilate) and to secrete renin—helping restore GFR.

Extrinsic Controls

(Systemic Body Responses) The body's emergency systems (SNS, RAAS) can override to preserve BP and circulation during major stress or shock. During stress or blood pressure (BP) emergencies, the body overrides local kidney controls:

1. The sympathetic nervous system releases norepinephrine; the adrenal medulla releases epinephrine.

2. Both hormones constrict the afferent arterioles, decreasing filtration and triggering renin release.

3. The renin-angiotensin-aldosterone system (RAAS) increases blood pressure and volume.

4. Additional modifiers include:

A) Prostaglandin E2 (vasodilator) prevents kidney damage.

B) Intrarenal angiotensin II enhances hormonal angiotensin II effects.

C) Adenosine acts as a vasoconstrictor of renal blood vessels.

Subjective Assessment: Renal and Urinary History

Consists of

1. Voiding Symptoms

2. Pain

3. Lifestyle and Medication Review

4. Pas Medical History

5. Family History

Subjective Assessment: Renal and Urinary History: Voiding Symptoms

1. Frequency (how often)

2. Urgency (sudden need)

3. Dysuria (painful urination)

4. Hematuria (blood in urine)

5. Nocturia (nighttime urination)​

Subjective Assessment: Renal and Urinary History: Pain

1. Flank (side of the body, below ribs)

2. Suprapubic (above the pubic bone)

3. Dysuria (pain during urination)​

Subjective Assessment: Renal and Urinary History: Lifestyle and Medication Review

1. Fluid intake and diet

2. Medications (with specific attention to any nephrotoxic drugs)​

Subjective Assessment: Renal and Urinary History: Past Medical History

1. Hypertension (HTN)

2. Diabetes Mellitus (DM)

3. Urinary tract infections (UTIs)

4. Kidney stones

5. Chronic renal disease​

Subjective Assessment: Renal and Urinary History: Family History

1. Polycystic kidney disease (PKD)

2. Renal cancer

3. Autoimmune diseases (that may affect the kidneys)​

Objective Assessment: Physical Exam

Examine for physical manifestations of urinary and renal disease. Consists of:

1. Inspection

2. Palpation

3. Percussion

4. Auscultation

Objective Assessment: Physical Exam: Inspection

1. Edema (swelling)

2. Pallor (pale skin)

3. Bruising

4. Skin changes, notably uremic frost (crystals on skin surface due to advanced kidney disease)​

Objective Assessment: Physical Exam: Palpation

1. Costovertebral angle (CVA) tenderness (suggests kidney infection or inflammation)

2. Bladder distension (possible urinary retention or obstruction)​

Objective Assessment: Physical Exam: Percussion

Costovertebral Angle Pain (CVA) pain with percussion indicates possible kidney inflammation​

Objective Assessment: Physical Exam: Auscultation

Renal bruits (vascular sounds) may indicate renal artery stenosis​

Average Daily Intake (Adult)

1. Total daily intake: ~2,000-3,000 mL/day (varies by age, activity, climate, health).​

2. 1,200-1,500 mL/day comes from beverages (water, coffee, tea, juice, milk, etc.).​ Other Sources include IV and Blood Infusions.

3. Average diet contributes 700-1,000 mL/day of water (from food).​

Fluid Output

1. 800 mL lost through skin (perspiration and evaporation).​

2. 200 mL lost through feces (insensible loss).​ 3. Other Sources Include: Bleeding, Suctioning, Wound Vac/Drainage.

Urinary Output

1. Typical urine output: 800-2,000 mL per 24 hours (reflects kidney function and hydration).​

Weight Change and Fluid Balance

1. Daily weight monitoring helps assess fluid status.

2. A change of 1 pound (lb) is roughly equivalent to 500 mL of fluid.

Urinalysis (UA)

color, clarity, pH, specific gravity

Special Markers

Proteinuria, hematuria, ketones, glucose, WBCs, casts may point to kidney pathology or systemic disease.

Urine C&S

Essential to confirm infection and guide effective treatment.

Urine: Color Clarity

1. Abnormal Findings: Dark, cloudy, bloody

2. Nursing Significance / Interpretation: Dehydration, infection, hematuria

Urine: Odor

1. Abnormal Findings: Strong, foul, fruity

2. Nursing Significance/Interpretation: UTI, DKA, dehydration

Urine: pH

1. Normal Findings: 4.5 to 8.0

2. Abnormal Findings: Acidic (low) or alkaline (high)

3. Nursing Significance/Interpretation: Acidic: dehydration/protein diet; Alkaline: infection

Urine: Protein( Proteinuria)

1. Abnormal Findings: Positive

2. Nursing Significance/Interpretation: Kidney disease, glomerulonephritis, diabetes

Urine: Blood (Hematuria)

1. Abnormal Findings: Positive

2. Nursing Significance/Interpretation: Infection, stones, trauma, tumors

Urine: Glucose

1. Abnormal Findings: Positive

2. Nursing Significance/Interpretation: Hyperglycemia, diabetes mellitus

Urine: Ketones

1. Abnormal Findings: Positive

2. Nursing Significance/Interpretation: DKA, starvation, severe illness

Urine: WBCs/Leukocyte Esterase

1. Abnormal Findings: Positive

2. Nursing Significance/Interpretation: UTI, inflammation

Urine: Casts

1. Normal Findings: None/few Hyaline Casts

2. Abnormal Findings: RBC/WBC/granular casts

3. Nursing Significance/Interpretation: Glomerulonephritis, tubular damage

Urine: Specific Gravity

1. Abnormal Findings: ↑ SG = concentrated; ↓ SG = dilute

2. Nursing Significance/Interpretation: High: dehydration; Low: overhydration/renal damage

Osmolarity

1. Definition: The ratio of solute particles to water, measured as the number of solute particles per liter of solution (solute particles/1 liter solvent).​

2. Use: Tracks overall concentration of urine and other body fluids in clinical settings.​

Osmolality

1. Definition: The degree of dilution or concentration in urine, measured as the number of solute particles per kilogram of solvent (solute particles/1 kg solvent).​

2. Use: More precise for physiological measurements because it's not affected by temperature/pressure changes.​

Clinical Relevance of Osmolarity and Osmolality

1. Both terms indicate the solution's "osmotic activity," i.e., its ability to cause water movement (osmosis).​

2. Normal body fluid osmolality is around 285-300 mOsm/L, commonly referenced as about 300 mOsm/L.​

3. Understanding these parameters is important for assessing hydration status, kidney concentrating function, and risk for electrolyte imbalance.​

Urine Osmolality and Concentrating Ability Why It Matters

The ability to concentrate and dilute urine is an early indicator of kidney health and fluid balance issues.​

Osmolality Test Definition

Measures how concentrated the urine is, based on the number of dissolved particles (solutes) per kilogram of water.​

Osmolality Test: Normal Value

About 300 mOsm/L (ranges from 50 mOsm/L for dilute urine to 1200 mOsm/L for very concentrated urine).​

Osmolality Test What It Means

1. High osmolality: Concentrated urine—dehydration, SIADH.​

2. Low osmolality: Dilute urine—overhydration, diabetes insipidus, renal tubular damage.​

Osmolality Test Clinical Use

Evaluates the kidney's ability to concentrate urine, helping diagnose the cause of fluid imbalances.​

Osmolality Test Nursing Tip

"Osmolality = how salty or concentrated the urine soup is".​

24-Hour Urine Collection Definition

Collects all urine voided in 24 hours to assess total excretion of substances like creatinine, protein, urea, sodium; estimates overall kidney function (creatinine clearance/GFR).​

24-Hour Urine Collection Nursing Instructions

1. Discard first morning void, then collect every drop for the next 24 hours.​

2. Keep specimens refrigerated or on ice; label start/end times clearly.​ 3. Missing a single void invalidates the test.​

24-Hour Urine Collection Purpose

Provides a comprehensive view of renal function and fluid balance.​

24-Hour Urine Collection Teamwork Tip

"It's a team effort—every void counts".​

Specific Gravity (SG) Definition

Compares the density of urine to water; a quick bedside test of urine concentration.​

Specific Gravity (SG) Normal Range

1.010-1.030.​

Specific Gravity (SG) Interpretation

1. Increased SG: Concentrated urine—dehydration, fluid loss, SIADH.​

2. Decreased SG: Dilute urine—excess fluids, renal disease, diabetes insipidus.​

Specific Gravity (SG) Preparation

1. Restrict fluid intake 12-14 hours before the test to avoid false low results.​

2. Use a morning sample if possible for accuracy.​

Specific Gravity (SG) Convenience

Easier and faster than osmolality testing; can be done at the bedside.​

"Specific gravity = concentration snapshot; osmolality = lab deep dive".​

Renal Clearance

Measures the kidney's ability to clear (remove) a particular solute from the plasma, often reflecting kidney "cleaning" function.​

Renal Clearance Clinical Use

1. Used to determine glomerular filtration rate (GFR).​

2. Detects glomerular damage.​

3. Follows the progress of renal disease over time.​

4. Clearance decreases with aging.​

Renal Clearance Primary Test

24-hour urine collection is the gold standard for assessing renal clearance and estimating GFR.​

Renal Clearance Substance Clearance Interpretation

1. RC = GFR = 125 mL/min: Substance is freely filtered by the glomerulus, not reabsorbed or secreted (e.g., inulin).​

2. RC < 125 mL/min: Substance is partly reabsorbed (e.g., urea).​

3. RC = 0: Substance is completely reabsorbed (e.g., glucose under normal conditions).​

4. RC > 125 mL/min: Substance is secreted into the filtrate (e.g., drug metabolites).​

Renal Clearance Key Biomarker

Creatinine (an endogenous waste product of skeletal muscle) is commonly used to estimate GFR via creatinine clearance.​

Renal Clearance Summary Points

Lower clearance indicates poor kidney function.​

Nursing's vital role: ensure precise, complete urine collection and patient/family education for accurate results.​

Renal Clearance Teaching tip

Renal clearance = kidney's cleaning power; creatinine clearance is the best practical estimate of GFR; errors in collection can mislead interpretation.

Renal clearance is a measure of the kidney's capacity to remove solutes from the plasma and is a key indicator of kidney health and filtration efficiency.​

Renal Clearance Key Concepts and Uses

It changes with aging and is used to:

1. Determine GFR (glomerular filtration rate)

2. Detect glomerular damage

3. Track the progression of renal disease​

4. The primary clinical test is a 24-hour urine collection, with creatinine (a muscle-derived waste product) used for estimating clearance.

Renal Clearance: Substance is Freely Filtered

RC~=125 mL/min Ex. Inulin

Renal Clearance: Substance is Partially Reabsorbed

RC~=<125 ml/min Ex. Urea

Renal Clearance: Substance is Completely Reabsorbed

RC=0 Ex. GLucose

Renal Clearance: substance is Secreted

RC>125 mL/min Ex. Drug Metabolites, PAH

Summary for Nursing Practice: Renal Clearance

1. Reflects the kidney's "cleaning power," and creatinine clearance is the best bedside estimate of GFR.

2. Low clearance signals poor kidney function.

3. Nursing priorities are accurate urine collection (esp. 24-hour) and thorough patient teaching about the procedure to ensure valid results.

Creatine Clearance Formular

Volume of Urine (mL/min) x Urine Creatinine (mL/dL) / Serum Creatinine (mg/dL)

Low GFR is Associated With

Increased levels of BUN, Creatinine, and Potassium

Antidiuretic Hormone (ADH) Source and Name

1. Secreted by the posterior portion of the pituitary gland.

2. Also called vasopressin.

Antidiuretic Hormone (ADH) Trigger for Release

Decreased water intake or increased blood osmolality (more concentrated blood) stimulates the release of ADH.

Antidiuretic Hormone (ADH) Actions in the Kidney

1. Acts on the kidney to increase water reabsorption, especially in the distal tubules and collecting ducts.

2. This reduces urinary water loss, helping restore normal blood osmolality.

Antidiuretic Hormone (ADH) Purpose

Maintains water balance and keeps blood osmolality within a normal range.

Antidiuretic Hormone (ADH) In summary

1. ADH is a hormone secreted when the body senses an increase in blood solute concentration (osmolality) or low water intake; it acts in the kidney to conserve water and restore normal fluid balance.Antidiuretic hormone (ADH), also known as vasopressin, is secreted by the posterior pituitary gland in response to increased blood osmolality or decreased water intake.

2. ADH acts on the kidneys to increase water reabsorption, which concentrates the urine, reduces urine output, and returns blood osmolality to normal levels by retaining water in the body.

3. This mechanism is central for maintaining proper hydration and electrolyte balance.

Increased ADH

Causes fluid retention

Aldosterone

Causes Sodium and Fluid Retention

Angiotension

Causes Vasoconstriction

Diabetes Insipidus (DI)

Is a disorder of water balance due to: Lack of antidiuretic hormone (ADH, i.e., central DI), or Kidneys not responding to ADH (nephrogenic DI).​

Diabetes Insipidus (DI) ADH Role

Normally, ADH (vasopressin) helps the kidneys reabsorb water, concentrating the urine.​ What Happens in DI:

1. Without enough ADH or if kidneys do not respond, the renal tubules cannot reabsorb water.​

2. Result: Kidneys lose water continuously, producing very large amounts of dilute (watery) urine.​

Urine Output in Diabetes Insipidus (DI)

Normal output is 1-2L/Day

In DI: Extremely high volume: 4-18 liters/day.​

Very dilute (SG <1.005, osmolality <300 mOsm/kg), pale/clear like water.

Normal Specific Gravity

1.01 to 1.03

Specific Gravity During DI

<1.005

Nursing Implications Diabetes Insipidus (DI)

1. Monitor Input & Output: Track urine output and fluid intake vigilantly.​

2. Watch for Dehydration: Dry mucous membranes, hypotension, tachycardia can signal water depletion.​

3. Monitor Blood Sodium: Serum sodium commonly increases (hypernatremia) due to excess water loss.​

4. Therapeutic Strategies:

Central DI: Administer desmopressin (DDAVP).​

Nephrogenic DI: Provide fluids and close monitoring; correct underlying causes.​

Erythropoietin (EPO) Hormone

1. Healthy kidneys make EPO, a hormone that stimulates the bone marrow to produce red blood cells (RBCs).​

2. EPO helps maintain optimal oxygen-carrying capacity in the blood.​

Erythropoietin (EPO) Hormone Physiological Feedback Loop ("Teaching Pearl")

1. Healthy kidneys → make EPO → EPO makes RBCs → RBCs carry oxygen → oxygen level signals kidneys to regulate EPO production.​

2. When blood oxygen is adequate, EPO secretion decreases; when oxygen is low, EPO secretion increases.​

Erythropoietin (EPO) Hormone Release Triggers

1. EPO is released in response to decreasing oxygen tension in renal blood flow.​

A) Anemia (low RBCs)

B) Arterial hypoxia (low oxygen in blood)

C) Inadequate blood flow to kidneys​

2. These states stimulate the kidney to secrete EPO, increasing red blood cell production to compensate.​

Erythropoietin (EPO) Hormone Visual Summary

Anemia or hypoxia → kidney increases EPO → EPO boosts bone marrow RBC production → normalizes blood oxygen.​