Urinary System: Ureters to Urethra, Physiology, and Clinical Considerations

Ureters

Structure and Function

The ureters are muscular tubes that transport urine from the kidneys to the urinary bladder. Each ureter is approximately 25 cm long and originates at the renal hilus.

• Mucosal epithelium with tight junctions lines the lumen, providing a barrier to urine leakage.

• Muscle layers: Two layers of smooth muscle (inner longitudinal and outer circular) facilitate peristaltic movement of urine.

Example: Peristalsis in the ureter propels urine toward the bladder, even against gravity.

Urinary Bladder

Anatomy and Storage Capacity

The urinary bladder is a hollow, muscular, and elastic organ located on the pelvic floor. Its primary function is to store urine until micturition (urination).

• Composed of detrusor muscle (smooth muscle) and lined by transitional epithelium.

• Can hold 600–800 ml of urine; the urge to urinate is typically felt when bladder pressure exceeds 300–400 ml.

Example: The bladder expands as it fills, and stretch receptors signal the need to void.

Urethra

Male vs. Female Urethra

The urethra is the tube that carries urine from the bladder to the external environment. Its length and structure differ between sexes.

Feature	Male Urethra	Female Urethra
Length	~20 cm	~3–4 cm
Pathway	Passes through prostate and penis	Short, straight tube to external orifice
Control	Voluntary and involuntary	Voluntary and involuntary

Additional info: The male urethra also serves as a passage for semen during ejaculation.

External Urethral Sphincter

Control of Micturition

The external urethral sphincter is a circular band of skeletal muscle located where the urethra passes through the urogenital diaphragm. It acts as a valve and is under voluntary control.

• Maintains resting muscle tone to prevent urine leakage.

• Voluntary relaxation allows urination (micturition).

Example: Adults can consciously delay urination by contracting the external sphincter.

Neural Control of Micturition

Reflex Pathway

Micturition is regulated by a complex neural circuit involving the brain and spinal cord.

• As the bladder fills, stretch receptors send signals via interneurons to the thalamus and cerebral cortex.

• Motor nerves from the sacral region of the spinal cord mediate voluntary relaxation of the external sphincter, followed by the internal sphincter.

Equation: $\text{Micturition Reflex} = \text{Bladder Stretch} \rightarrow \text{Afferent Signal} \rightarrow \text{CNS Processing} \rightarrow \text{Efferent Signal} \rightarrow \text{Sphincter Relaxation}$

Developmental and Age-Related Changes

Infant Urinary System

• Infants lack voluntary control over urination due to immature CNS connections.

• Incontinence is the inability to control urination voluntarily.

Elderly Urinary System

• Decline in number of functional nephrons reduces kidney efficiency.

• Reduced sensitivity to antidiuretic hormone (ADH) impairs water reabsorption.

• Problems with reflexes and motor nerves can lead to incontinence.

Micturition Problems

Urinary Retention in Males

• Enlarged prostate gland may compress the urethra, restricting urine flow and causing retention.

Example: Benign prostatic hyperplasia (BPH) is a common cause of urinary retention in older men.

Urine Analysis and Renal Thresholds

Renal Threshold for Glucose

• Renal threshold is the plasma concentration at which a substance begins to appear in urine.

• For glucose, threshold is approximately 180 mg/dl.

• If plasma glucose exceeds this, tubular maximum (Tm) is surpassed and glucose appears in urine (glycosuria).

Renal Threshold for Amino Acids

• Lower than glucose: ~65 mg/dl.

• Amino acids may appear in urine after a protein-rich meal (aminoaciduria).

Renal Threshold for Acids

• Ketoacidosis lowers blood pH due to presence of ketones from amino acid digestion.

• Occurs in starvation or diabetes when glucose/glycogen is insufficient.

Equation: $\text{Renal Threshold} = \frac{\text{Tm}}{\text{GFR}}$

Additional info: Tm = tubular maximum; GFR = glomerular filtration rate.

Kidney Stones

Formation and Passage

• Kidney stones are solid crystals of dissolved minerals formed inside the kidneys or ureters.

• Size varies from grains of sand to as large as a grapefruit.

• Stones typically leave the body by passage in the urine, which can be painful.

Example: Calcium oxalate is a common type of kidney stone.

Urinary Tract Infections (UTIs)

Etiology and Symptoms

• UTIs are bacterial infections affecting any part of the urinary tract (kidneys, bladder, urethra).

• Bacteria multiply in urine, causing symptoms such as pain, urgency, and fever.

Example: Escherichia coli is the most common cause of UTIs.

Diuretics and Dialysis

Diuretics

• Drugs that promote water loss in urine (diuresis).

• Used to reduce blood volume, blood pressure, and extracellular fluid volume.

Dialysis

• Medical procedure that removes waste products from the blood when kidneys fail.

• Blood is pumped through a dialysis membrane where diffusion removes urea and other wastes.

Parameter	Effect of Diuretics
Blood Volume	Decreases
Blood Pressure	Decreases
Extracellular Fluid	Decreases

Equation: $\text{Diffusion Rate} = k \times (C_{blood} - C_{dialysate})$

Additional info: Dialysis is life-saving for patients with end-stage renal disease.