Urination and Osmolarity Shifts

• Urination is also known as micturition

Urine Concentration

• In absence of ADH, product is dilute
• ADH regulates concentration to maintain homeostasis
• Osmosis controls movement of water, via osmolarity of tubule and capillary, and wall permeability

Osmolarity Shifts

• Glomerular fluid and plasma have the same effect, osmolarity equals 300 mOsm/L
• At PCT, fluid and plasma have the same osmolarity, water moves via aquaporin-1
• Descending limb moves into medulla, osmolarity increases, water is reabsorbed
• 900 mOsm/L osmolarity at the base of the nephron loop
• Ascending limb returns to the cortex, osmolarity decreases, solutes are reabsorbed
• Thin section involves passive ion reabsorption
• Thick section involves active ion reabsorption
• 100 mOsm/L occurs by the end of the DCT
• Collecting duct branches off between dilute and concentrated urine
• Dilute section: solutes leave, water can't, osmolarity drops to 65 mOsm/L
• Concentrated section: aquaporin-2 allows reabsorption of water, osmolarity rises to 1200 mOsm/L

Normal Urine

• Volume is 1-2L/day
• Color is yellow or amber based on concentration and diet
• Pigmentation made of urobilin and urochrome
• Blood indicates pathologies such as kidney stones
• Turbidity is transparent, but cloudy on standing
• Odor is mildly aromatic due to ammonia
• Ammonia-like on standing
• Methyl mercaptan is like asparagus
• Fruity indicates ketones
• pH is 4.6-6

Ureter Anatomy

• Length is 25-30 cm
• Width is 1-10 diameter
• Location is retroperitoneal
• Renal pelvis is where one ureter exits each kidney
• Ureteral openings are holes on low posterior wall of bladder
• Physiological valve opens when relaxed, compress/close when stretched
• Peristalsis, gravity, and hydrostatic pressure facilitate movement

Ureter Histology

• Mucosa is the innermost layer, of transitional epithelium and lamina propria
• Transitional epithelium allows stretch in walls of tissue
• Goblet cells secrete mucus, protecting from urine's pH and toxins
• Muscularis is smooth muscle, involved in peristalsis
• Inner layer has longitudinal fibers
• Next layer has circular fibers
• Outer layer in distal 1/3 of ureter has longitudinal fibers
• Adventitia contains blood vessels, nerves, and lymphatic vessels, anchors structures

Urinary Bladder Anatomy

• Description: hollow, distensible, and muscular
• Capacity: 700-800 mL
• Location: posterior and superior to pubic symphysis, anterior and inferior to urethra, subperitoneal
• Histology similar to ureters
• Rugae lining provides SA for expansion
• Detrusor muscle has 3 smooth muscle layers, contracts to push urine into urethra
• Adventitia same as ureters
• Internal urethral orifice connects bladder to urethra, surrounded by smooth muscle/internal urethral sphincter
• Trigone is triangular region bounded by ureteral openings and internal urethral orifice; facilitates smooth surface
• External urethral orifice connect to outside and is surrounded by skeletal external urethral sphincter
• Perineum has pelvic floor muscles controlling voluntary urination

Micturition Reflex

• Micturition is urination
• Micturition reflex controls urination
• Mechanism: stretch receptors on bladder walls detect when urine volume reaches 200-400 mL
• Information sent to micturition center in S2-3 of spinal cord
• PSNS motor reflex contracts detrusor and relaxes internal sphincter, signals cerebral cortex that the bladder is full
• Cerebral cortex initiates micturition by relaxing external sphincter, or delays by contracting external sphincter
• Incontinence occurs when PSNS intervention cuts off motor signals contracting external sphincter

Urethra

• Function: opening for pee expulsion
• Composition: mostly mucosa and muscularis
• Mucosa transitions from transitional, to pseudostratified columnar, to stratified squamous epithelium
• Muscularis is single layer of circular smooth muscle

Male Urethra

• Length: 20 cm
• Prostatic urethra is the proximal region, passes through prostate gland
• Prostate gland adds reproductive excretions
• Internal urethral sphincter prevents backflow of reproductive excretions into bladder
• Mucosa starts as transitional, moves into pseudostratified columnar epithelium
• Intermediate (membranous) urethra is the shortest region, passes through the perineum, contains the external urethra sphincter
• Dual pathway carries both urine and reproductive secretion
• Mucosa contains pseudo stratified columnar epithelium
• Spongy urethra is the longest segment, enters the penis
• Mucosa starts as pseudo stratified columnar, becomes stratified squamous epithelium

Female Urethra

• Location is anterior to vulva
• Uterus is on top of bladder
• Length is 4 cm
• External orifice sits between clitoris and vagina