Urinary Anatomy and Micturition

The urinary system consists of 

• Two kidneys

• Two ureters

• One bladder

• One urethra

• All these structures are retroperitoneal

8 Functions of the Kidneys

• Excreting Wastes (urea, creatinine, uric acid, urobilin, toxins, drugs)

• Regulating Blood Ionic Composition (electrolyte levels)

• Regulating Blood pH

• Regulating Blood Volume (water content)

• Regulating BP (RAA-pathway)

• Maintaining Blood Osmolarity

• Producing Hormones

• Regulating Blood Glucose (account for 50% of the gluconeogenesis in the body)

External Anatomy of the Kidney (3 Layers)

• The kidney is bean shaped and about the size of a bar of soap or deck of cards

• Three layers of tissue surround the kidney

1) Renal capsule (protective layer)

2) Perirenal fat capsule (also trauma protection)

3) Renal Fascia (anchoring)

• The perirenal fat capsule is the absolute last bit of fat that your body will break down in need. This is because without the fat, the kidney can start to droop (ptosis) and that can kink the ureter. The fat basically holds the kidneys up

Internal Anatomy of the Kidney

• Renal Cortex (outer layer. Contains renal corpuscles which is the glomerulus and it's capsule as well as the PCT and DCT)

• Renal medulla

  • Contains renal pyramids which contain the nephron loop and collecting duct. Between the renal pyramids are the renal columns. Urine is first collected in the collecting duct of the renal pyramids. Renal pyramids end at the renal papilla which dumps the urine from the collecting duct into the papillary duct into the minor calyx where it then goes to the major calyx and renal pelvis before going into the ureters. The renal pelvis is very large so stones can easily develop there.

Nephrolithiasis

• Form when urine is super saturated with salts, or there's changes in pH

• Form from calcium salts (80%), uric acid, struvite or cysteine

• Large kidney stones are extremely painful and can cause flank or lower back pain

• Treatment includes nephrolithotomy (cutting them out) or lithotripsy (breaking them with sound waves)

• Risk factors include family or personal history, UTI's, low fluid intake (most common), diabetes, obesity, gout, HTN, and male (twice as likely)

3 Places in the Ureter Where Kidney Stones can Get Stuck

1) Ureteropelvic junction (this is because the funnel shape of the renal pelvis empties into the thin ureter)

2) When the ureter goes over the iliopsoas muscle and internal iliac artery

3) Ureterovesical Junction (right before entering the bladder is a bit narrow.)

Hydronephrosis vs Hydroureter

• Blockage of the ureters for any reason (pregnancy, trauma, kidney stones, renal ptosis, etc) can lead to expansion of the renal pelvis (hydronephrosis) and/or ureters (hydroureter.)

3 Layers of the Ureters + Structure and Function

1) Outer adventitia (adheres them to the peritoneum)

2) Muscularis (inner longitudinal, outer circularis)

3) Mucosa

• Peristalsis moves the urine to the bladder.

• The ureter crosses the muscular bladder wall at an oblique angle. This helps create a physiological valve that prevents urinary reflux.

Bladder Relational Anatomy in Males

In males, the bladder lies anteriorly to the rectum and posteriorly to the pelvic bone.

3 Segments of the Urethra in Males

• 1) Prostatic urethra near the prostate

• 2) Membranous urethra that goes through the pelvic floor muscles which also forms the external urethral sphincter

• 3) Penile/spongy urethra which goes through the spongy portion of the penis.

Relational Anatomy of the Bladder in Females

• In females the bladder lies anterior to the vagina and inferior to the uterus (Water under the bridge)

Cystitis and Pylonephritis

• Cystitis: Inflammation of the bladder, usually due to E. Coli infection, but also chemo, radiation and catheters.

• Pyelonephritis: Inflammation of the kidneys caused by an ascending UTI. Can lead to sepsis, shock and renal failure.

• Risk factors for these include short urethra (female), urethral proximity to anus, urinary catheterization, urinary stasis (obstruction, dehydration), vesicoureteral reflux (not oblique ureter.)

Bladder Linings (3)

• The bladder is covered by peritoneum, which aids in maintaining its position in the pelvis

• The bladder is lined by transitional epithelium (epithelium that goes from cuboidal to squamous when stretched) which also forms rugae when empty.

• The wall of the bladder contains involuntary smooth muscle (detrusor muscle)

Bladder Trigone and Sphincters

• The openings of the paired ureters, and single urethra form the apices of the bladder trigone.

• There is an internal urethral sphincter (smooth muscle) and an external urethral sphincter (skeletal muscle) that is derived from deep pelvic floor muscles. In males the internal urethral sphincter blocks the bladder during ejaculation and contracts to prevent incontinence. A female's internal sphincter just contracts to prevent incontinence. The external one is responsible for concious relaxation so you can pee.

Micturition Reflex

• When the bladder fills with about 200-400mL of urine, the stretch receptors in the bladder are stimulated, and send signals to the sacral portion of the spinal cord. This creates an autonomic parasympathetic reflex which causes the detrusor muscle to contract and the internal sphincter (in males) to relax.

• The external urethral sphincter is under conscious control.