Urine Composition and Analysis

Normal Urine Composition

• Normal urine volume is approximately 1200 mL/day, with an osmotic concentration of 1000 mOsm/L.
• Urine changes can indicate abnormalities in the urinary system or other parts of the body.
• Urine is composed primarily of water (95%).
• The remaining 5% consists of electrolytes and solutes from cellular metabolism:
  • Urea (from protein breakdown)
  • Creatinine (from creatine phosphate breakdown in muscle fibers)
  • Uric acid (from nucleic acid breakdown)
  • Urobilinogen (from hemoglobin breakdown)
• Also includes exogenous substances (like drugs) and small amounts of fatty acids, pigments, enzymes, and hormones.

Urinalysis

• Urinalysis involves analyzing a urine sample.
• Red blood cells may indicate infection, kidney stones, or other conditions.
• White blood cells often suggest a urinary tract infection.
• Nitrites might point to a bacterial infection converting nitrates to nitrites.
• Leukocyte esterase also indicates infection.
• Proteins in urine suggest kidney disease.
• Glucose in urine can indicate diabetes.

Blood Tests for Kidney Function

• Blood Urea Nitrogen (BUN): Measures blood nitrogen, a part of urea resulting from amino acid breakdown. A steep rise signifies a severe decrease in glomerular filtration rate (GFR) – a common sign of renal disease.
• Plasma Creatinine: Measures plasma levels of creatinine, arising from creatine phosphate breakdown in muscle cells. A sharp increase indicates low GFR and is a sign of renal disease. Used to estimate GFR (eGFR).
• Inulin Clearance: Measures the volume of blood cleared of inulin per unit of time. This is the best blood test for kidney function because inulin passes through the kidneys but is neither secreted nor reabsorbed. A higher clearance rate signifies efficient filtration and excretion, while a lower rate indicates inefficiency. Inulin clearance closely estimates GFR, and is considered the gold standard; however, it is not always clinically accessible.
• PAH Clearance: Measures renal plasma flow (typically 650 mL/min). PAH (para-aminohippuric acid), when administered intravenously, is completely filtered and secreted in a single pass through the kidneys.

Kidney (Renal) Failure

• Kidney failure occurs when the kidneys fail to filter wastes from the blood and maintain homeostasis.
• This is marked by a drop in GFR.
• Kidney failure disrupts bodily functions, leading to reduced urine production, elevated blood pressure, anemia, and central nervous system complications (e.g., sleeplessness, seizures, delirium).

Chronic Kidney Failure

• The most common causes are diabetes and hypertension.
• Kidney function deteriorates gradually.
• Associated problems accumulate over time.
• Progression can be slowed, but not reversed.
• Management involves restricting water, salt, and protein intake, to reduce strain on the urinary system.
• Acidosis associated with renal failure can be managed by ingesting bicarbonate ions.

Acute Kidney Injury (AKI)

• AKI is characterized by a rapid decline in kidney function over a few days.
• Causes include exposure to toxic drugs, renal ischemia, urinary obstructions (like UTIs), trauma, and allergic responses to medications.
• Partial or complete recovery is possible when the initial incident is survived.

Dialysis

• Dialysis is a process of passive diffusion across a selectively permeable membrane.
• Hemodialysis uses an artificial membrane as a substitute for the natural glomerular membrane in the kidney. A dialysis machine regulates blood composition. The membrane allows diffusion of ions, nutrients, and wastes, but not plasma proteins.

Kidney Failure Treatment

• Dialysis relieves symptoms of kidney failure but isn't a cure.
• Kidney transplant is the only true cure for severe renal failure, with a high success rate (over 90% at 2 years post-transplant). A close relative donor increases success. Immunosuppressive drugs prevent transplant rejection.

The Urinary Tract

• The urinary tract consists of ureters, urinary bladder, and urethra.
• Ureters: Receive urine from kidneys, conducting it to the bladder via gravity and peristalsis.
• Urinary Bladder: Stores urine; contraction of the bladder muscle (detrusor muscle) drives urination.
• Urethra: Transports urine from the bladder to the outside of the body (and, in males, semen).

Ureters

• Paired muscular tubes that extend from the kidneys to the bladder.
• Composed of three layers: Mucosa (transitional epithelium), Muscularis (two layers of smooth muscle for peristalsis), and Adventitia (outer connective tissue layer).

Urinary Bladder

• Lies posterior to the pubic symphysis.
• Shaped differently based on distension (filled/empty state)
• Filled by ureters, drained by urethra.
• Contains rugae (folds) that allow for bladder expansion
• Surrounded by supporting ligaments (lateral & medial umbilical) The bladder sits outside the peritoneal cavity.

Urethra

• Extends from the bladder neck to the body's exterior.
• Males: longer, transporting semen and urine; different sections (prostatic, membranous, spongy).
• Females: approximately 4 cm long, opening between the clitoris and vagina.

Urination

• Urine storage reflex: Stretch receptors signal sympathetic stimulation to detrusor muscle and the internal urethral sphincter as the bladder fills.
• Micturition reflex (urination): Coordinates urination using a local reflex pathway and a central pathway (through the cerebral cortex)
• Afferent information of bladder fullness is relayed to the thalamus, from there to cerebral cortex for volitional control of the external urethral sphincter. This leads to contraction of the bladder (detrusor muscle) and relaxation of the internal urethral sphincter to facilitate urination.

Urinary Disorders

• Primary signs: Changes in urine volume, appearance, and frequency.
• Polyuria: Excessive urine production (hormonal/metabolic problems e.g., diabetes, glomerulonephritis.)
• Oliguria: Reduced urine production (50–500 mL/day).
• Anuria: Severely reduced urine production (0–50 mL/day). Severe kidney problems possible.
• Incontinence: Inability to control urination (stress, urge, overflow).
• Urinary retention: Inability to urinate even with a full bladder (e.g., male enlarged prostate).
• Pain: Pain in the pelvic region (bladder/ureters), lumbar/flank region (kidney infections/stones).
• Dysuria: Painful or difficult urination (cystitis, urethritis or urinary obstructions - e.g., prostate).
• Fever: Common with urinary tract infections (UTIs).

This quiz covers the normal composition of urine, including essential components like urea and creatinine, and highlights the significance of urinalysis in identifying various health conditions. Learn about the normal urine volume, osmotic concentration, and what changes in urine can indicate regarding the urinary system. Test your knowledge on how different substances in urine can suggest specific infections or abnormalities.