MICTURITION
Micturition is the process of emptying the urinary bladder. When 300-400 ml of urine has been collected in the urinary bladder, the afferent autonomic nerve fibres in the bladder wall which are sensitive to stretch are stimulated.
MICTURITION REFLEX (SIMPLE REFLEX CONTROL)
In infants, fullness of the bladder initiates a spinal reflex action to cause micturition. The reflex pathway involves:
- Stretch-sensitive nerve endings in the bladder wall send nerve impulses to the spinal cord
- Autonomic efferent fibres carry impulses back to the bladder
- Detrusor muscles contract (squeeze the bladder)
- Internal urethral sphincter relaxes (opens the outlet)
- Urine is expelled through the external urethral orifice
Note: In infants, conscious effort cannot override this reflex action.
CONTROL OF MICTURITION IN ADULTS
In adults, the reflex contraction of the bladder wall and relaxation of the internal sphincter can be inhibited consciously for a limited time period. The cerebral cortex exerts voluntary control over micturition.
Process of Voluntary Micturition
- Contraction of detrusor muscles (reflex)
- Reflex relaxation of internal urethral sphincter (involuntary)
- Voluntary relaxation of external urethral sphincter (conscious control)
- Urine is expelled
Valsalva’s Manoeuvre
Micturition can be supported by lowering the diaphragm and contracting the abdominal muscles (Valsalva’s manoeuvre), which increases pelvic cavity pressure and helps expel urine.
Over-distended Bladder
When the bladder is over-distended, it becomes extremely painful. At this stage, there is an involuntary tendency to relax the external sphincter and release a small amount of urine (overflow incontinence).
COMPARISON: INFANTS VS ADULTS
| Feature | Infants | Adults |
|---|---|---|
| Control | Spinal reflex only (involuntary) | Cortical control (voluntary + reflex) |
| External sphincter | No conscious control | Voluntary relaxation possible |
Inhibition of reflexNot possible
| Can be inhibited consciously for limited time | |
| Can be inhibited consciously for limited time |
CLEARANCE TESTS
The volume of plasma that is cleared of a substance in unit time is known as the clearance of that substance. GFR is precisely measured by inulin clearance, as inulin is neither secreted nor absorbed by the renal tubules. Using serum creatinine value, estimated GFR (eGFR) is also calculated in clinical settings.
BLOOD TESTS
Serum Creatinine
Creatinine is a waste product formed by muscles during wear and tear. Creatinine levels vary with age, race, and body mass.
- Normal: Women <1.2 mg/dL; Men <1.4 mg/dL
- Abnormal: Higher levels indicate kidney dysfunction
- Creatinine level increases as renal disease worsens
Glomerular Filtration Rate (GFR)
GFR determines how successfully kidneys remove waste and extra fluid from blood. Calculated using blood creatinine level, age, gender, and race (MDRD or CKD-EPI equation).
- Normal GFR: ≥90 mL/min
- GFR <60: Indicates kidneys are not functioning properly
- GFR <15: High risk of needing dialysis or kidney transplant
- GFR decreases with age
Blood Urea Nitrogen (BUN)
The breakdown of protein in foods produces urea nitrogen.
- Normal BUN: 7-20 mg/dL
- BUN level rises as renal function declines
IMAGING TESTS
Ultrasound
Uses sound waves to image the kidney. Detects anomalies in kidney size or location, and blockages like stones or tumours.
CT Scan
Uses X-rays to image the kidneys. Checks for structural issues and blockages. May require intravenous contrast dye (problematic for patients with kidney disorders).
Kidney Biopsy
Involves slicing small sections of kidney tissue with a thin needle for microscopic examination.
Indications for Kidney Biopsy
- To identify a specific disease process and see if it will respond to treatment
- To assess the degree of renal damage
- To determine the reason behind kidney transplant failure
URINE TESTS
Urinalysis
Includes dipstick test and microscopic inspection. The dipstick is a chemically treated strip that changes colour when abnormalities are present.
Detects: Excess protein, blood, pus, germs, or sugar. Helps diagnose chronic kidney disease, diabetes, bladder infections, and kidney stones.
Urine Protein (Proteinuria)
Excess protein in urine. A positive dipstick test (1+ or above) should be validated with an albumin-specific dipstick or albumin-to-creatinine ratio.
Microalbuminuria
A sensitive dipstick test that detects trace amounts of albumin in urine. Recommended for individuals vulnerable to renal disease (diabetes, high blood pressure).
Creatinine Clearance
Compares creatinine level in blood to creatinine level in a 24-hour urine sample. Determines how much waste products the kidneys filter out per minute.
Cystatin C
A protein level in the blood that can also be used as a marker of renal function.
SUMMARY OF CLEARANCE TESTS
| Test | Normal Value | Clinical Significance | |
|---|---|---|---|
| Serum Creatinine | Women <1.2, Men <1.4 mg/dL | Increased in kidney dysfunction | |
| GFR | ≥90 mL/min | <60 = dysfunction; <15 = kidney failure risk | |
| BUN | 7-20 mg/dL | Rises as renal function declines | |
| Urinalysis | Negative for abnormalities | Detects protein, blood, pus, germs, sugar |
