Urinary System Part 3: Kidney Function

Urinary System Part 3: Evaluation of Kidney Function

• Normal urine volume is approximately 1200 mL/day with an osmotic concentration of 1000 mOsm/L.
• Urine characteristics include pH ranging from 4.5-8 (average 6.0), specific gravity between 1.003–1.030, osmotic concentration (osmolarity) of 855-1335 mOsm/L, water content of 93-97%, volume of 700-2000 mL/day, pale yellow color, variable odor, and sterile (no bacteria).
• Changes in urine composition can be tested and measured to detect urinary system abnormalities.

Normal Urine Composition

• Water comprises 95% of total urine volume.
• Remaining 5% consists of electrolytes and solutes derived from cellular metabolism (e.g., urea from protein breakdown, creatinine from creatine phosphate breakdown, uric acid from nucleic acid breakdown, urobilinogen from hemoglobin breakdown).
• Small quantities of fatty acids, pigments, enzymes, and hormones are also present.
• Exogenous substances like drugs are also present in small quantities.

Urinalysis

• Urine samples are analyzed to determine the presence of various substances.
• Red blood cells may indicate infection or kidney stones, while white blood cells may indicate urinary tract infection.
• Nitrites indicate bacterial infections, and leukocyte esterase confirms infection.
• Elevated protein levels may suggest kidney disease, and glucose suggests potential diabetes mellitus.

Blood Tests

• Blood Urea Nitrogen (BUN): Measures blood nitrogen, a component of urea resulting from amino acid catabolism and deamination. Elevated BUN often accompanies reduced glomerular filtration rate (GFR).
• Plasma Creatinine: Measures plasma creatinine levels, a byproduct of creatine phosphate metabolism in skeletal muscle. Elevated creatinine levels are strongly correlated with reduced GFR.
• Inulin Clearance: Measures blood clearance of inulin, a plant polysaccharide, to assess kidney filtration efficiency. High clearance indicates efficient filtration, and low clearance points to inefficiency. Inulin is not absorbed or secreted. Clearance rate equals GFR. Inulin is a gold standard for GFR but not always accessible in a clinical setting.
• PAH Clearance: Measures the amount of plasma cleared by the kidneys, using para-aminohippuric acid (PAH). PAH is filtered and completely secreted; thus, this test estimates renal plasma flow. Also used to assess GFR.

Kidney (Renal) Failure

• Occurs when kidneys cannot filter wastes effectively, leading to homeostatic imbalances. GFR drops below a critical threshold.
• Kidney failure impairs various bodily systems, including a decrease in urine production, increased blood pressure, anemia (due to reduced erythropoietin production), and central nervous system problems (sleepiness, seizures, delirium, coma).
• Chronic Kidney Failure: Diabetes and hypertension are common causes. Kidney function progressively declines, making it non-reversible. Management involves restrictions on water, salt, and protein to minimize the kidney's workload. Acidosis from waste products can be countered by bicarbonate ingestion.
• Acute Kidney Injury (AKI): Kidney function deteriorates quickly, potentially in just a few days. Causes include exposure to toxic drugs, renal ischemia, urinary obstruction, trauma, or allergic responses to medications.
• Dialysis: A process of passive diffusion across a selectively permeable membrane that resembles kidney functions by removing waste from the blood.

Dialysis

• Hemodialysis is the use of an artificial membrane as an alternative for the filtering function of the glomerulus.
• The pores in the membrane allow diffusion of ions, nutrients, and organic waste (but not plasma proteins), by running an appropriate solution on the opposite side.

Kidney Failure Treatment

• Dialysis temporarily alleviates symptoms but isn't a cure.
• Kidney transplantation is the only definitive cure for acute or chronic renal failure.
• Patient survival rates are high (over 90%) in the 2 years following the transplant, especially with close relative donors.
• Immunosuppressive medications are essential to prevent transplant rejection.

The Urinary Tract

• The urinary tract is composed of ureters, urinary bladder, and urethra.
• Ureters transport urine from the kidneys to the bladder.
• The urinary bladder stores urine.
• The urethra carries urine from the bladder to the outside of the body; in males it also conduits semen.

Ureters

• Paired muscular tubes extending 30 cm from kidneys to bladder.
• Attached to the posterior abdominal wall.
• Three layers: mucosa, muscularis, and adventitia.

Urinary Bladder

• Located posterior to the pubic symphysis.
• Urine fills the bladder and is drained by the urethra.
• The bladder is outside the peritoneum.
• Supported by ligaments and surrounding bones.
• Displays rugae (folds) that aid expansion.
• Has ureteral orifices, a slit-like shape to prevent backflow, and a trigone (triangular area).
• Has an internal urethral sphincter in the neck that is involuntary.

External Urethral Sphincter

• Located distal to the prostate in males and at the pelvic floor in females.
• Under voluntary control.

Urinary Bladder Details

• The urinary bladder has mucosa, submucosa, muscularis, and connective tissue layers.
• The mucosa is lined with transitional epithelium, allows expansion for urine storage.

Urethra

• Extends from the bladder neck to the exterior of the body; differing lengths and functions depending on male versus female anatomy.
• Male: ~20 cm, passes through prostate, with three sections (prostatic, membranous, spongy) carrying urine and semen.
• Female: ~4 cm, opens into the orifice between the clitoris and vagina.
• The urethra is lined with stratified epithelium, which varies by location.

Urination (Micturition Reflex)

• Reflect involves both a local and central pathway.

• The stretch receptors in the bladder wall trigger a reflex arc.

• Afferent impulses are sent to the pontine storage center of the brain.

• Decreased parasympathetic activity and increased somatic motor nerve activity of the external urethral sphincter.

• Urination involves relaxation of the internal and external urethral sphincters.

Urinary Disorders

• Primary signs of urinary disorders: Changes in urine volume (polyuria, oliguria, anuria), frequency, and urgency.
• Polyuria: Excessive urine production due to hormonal or metabolic issues (like diabetes).
• Oliguria: Reduced urine production (50-500 mL/day).
• Anuria: Severely reduced urine production (less than 50 mL/day). This, and oliguria, indicates potential kidney problems.
• Incontinence: Inability to control urination voluntarily (stress/urge/overflow).
• Urinary retention: Initially normal renal function is present, but urination does not occur. Typically in males, due to enlarged prostate.
• Pain: Pain in the superior pubic region can associate with bladder disorders. Pain in the lumbar region or flank can associate with kidney infections or kidney stones (renal calculi).
• Fever: Often associates with urinary system infections (cystitis, pyelonephritis).

This quiz evaluates knowledge of kidney function and urine composition. Learn about normal urine volume, characteristics, and how changes in urine can indicate health issues. Test your understanding of normal urine composition, including its major components and the influence of various substances.