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Home , Peritubular capillaries

Urinary System Functions • Major excretory pathway for wastes transported by blood – Filtration, secretion, and by the ’s microscopic tubules – Includes wastes from metabolism, excess water and excess solutes, absorbed substances • Collection, transport, exit of urine – product of kidney’s function – Responsive to condition – Role in maintenance of blood volume and pressure • Urine release controlled by reflex actions • Regulation of hemopoesis Organs and Vessels of the Hepatic veins (cut) Esophagus (cut) Inferior vena cava Adrenal gland Aorta

Kidney Iliac crest - peristalsis

Rectum (cut) Uterus (part of female reproductive system)

Urethra Figure 25.2 Dissection of urinary system organs (male).

Kidney Renal artery Renal hilum Renal vein

Ureter

Urinary bladder Figure 25.3a Position of the kidneys against the posterior body wall. Three layers of supportive tissue surround kidney outer layer of dense fibrous connective tissue Perirenal fat capsule Fatty cushion Fibrous capsule Transparent capsule Anterior Inferior vena cava Aorta Peritoneum Peritoneal cavity (organs removed) Supportive Renal tissue layers vein • Renal fascia anterior Renal posterior artery • Perirenal fat capsule • Fibrous Body of capsule

vertebra L2 Body wall Posterior Kidney Function

Maintain the body’s internal environment by: – Regulating total water volume and total solute concentration in water – Regulating ion concentrations in ECF – Ensuring long-term acid-base balance – Excreting metabolic wastes, toxins, drugs – Producing erythropoietin (EPO) and renin – Activating vitamin D – Carrying out gluconeogenesis, if needed Figure 25.4 Internal anatomy of the kidney. Blood flow critical – 25% of each ejection enters kidney vessels. Kidneys monitor and facilitate • Favorable BP Renal hilum • Oxygen carrying ability

Renal cortex

Renal medulla

Major calyx

Papilla of pyramid

Renal pelvis

Minor calyx Ureter

Renal pyramid in

Fibrous capsule

Photograph of right kidney, frontal section Diagrammatic view Figure 25.5a Blood vessels of the kidney. Cortical radiate vein Cortical radiate Cortical radiate arteries supply artery the – location of blood filtration Arcuate artery Interlobar vein Glomeruli and Interlobar artery portions of Segmental arteries are Renal vein located in the

Renal artery About 8 lobes (pyramid & surrounding Ureter cortex) per kidney

11 cm x 6 cm x 3 Renal medulla cm 150 g / 5 oz

Renal cortex

Frontal section illustrating major blood vessels Physiology of Kidney

• Values for Average Human • 180 L of fluid processed daily, but only 1.5 L of urine is formed – Kidneys filter body’s entire plasma volume 60 times each day – “Filtrate” is basically blood plasma minus proteins • Urine is produced from filtrate – Urine • <1% of original filtrate • Contains metabolic wastes and unneeded substances • Kidneys consume 20–25% of oxygen used by body at rest Nephrons and Structure

• Structural and functional units that form urine • > 1 million per kidney • Two main parts – • Glomerulus – “tuft” of capillaries and visceral layer • Glomerular (or Bowman’s) capsule, parietal layer – Renal tubules • Including tubules, loop, and ducts • Collecting ducts in the thousands, drain nephrons Figure 25.6 Location and structure of nephrons.

Renal cortex

Renal medulla

Renal pelvis Glomerular capsule: parietal layer

Ureter Basement membrane Kidney Fenestrated Renal corpuscle endothelium of the glomerulus • Glomerular capsule • Glomerulus Glomerular capsule: visceral layer Distal convoluted Apical microvilli tubule Mitochondria

Highly Proximal infolded convoluted basolateral tubule membrane

Proximal convoluted tubule cells Cortex Apical side

Medulla Basolateral side Thick segment cells Thin segment Nephron loop • Descending limb • Ascending limb Collecting Nephron loop (thin-segment) cells duct Principal cell Intercalated cell

Terminates at renal papilla Collecting duct cells Histology of corpuscle and nearby tubules • • • Renal corpuscle Glomerulus space Glomerular capsular glomerular capsule of parietal layer of Squamous epithelium tubule Distal convoluted tubule convoluted Proximal microvilli) lumen due to long (clear (clear lumen) (fuzzy Figure 25.12a The filtration membrane. Glomerular capsular space Efferent Filtration Membrane: arteriole • Fenestrated capillaries • Visceral layer glomerular capsule with filtration slits • Produce solute rich, virtually protein free filtrate

Afferent arteriole Proximal convoluted Glomerular capillary tubule covered by Parietal layer that form the visceral layer of glomerular of glomerular capsule capsule Figure 25.12b The filtration membrane.

Cytoplasmic extensions of podocytes

Filtration slits

Podocyte cell body

Fenestrations (pores)

Glomerular capillary endothelium (podocyte covering and basement Foot processes membrane removed) of podocyte

Glomerular capillary surrounded by podocytes Figure 25.12d The filtration membrane. Capillary Filtration membrane • Capillary endothelium • Basement membrane • Foot processes of podocyte of glomerular capsule

Allows molecules smaller than 3 nm to pass: Water, , amino acids, nitrogenous wastes, most solutes *Normally no cells can pass*

Plasma proteins remain in Filtration slit plasma to maintain colloid osmotic pressure Slit diaphragm Prevents loss of all water to Plasma Filtrate in capsular space capsular *Proteins in filtrate indicate membrane problem* space Fenestration (pore) Foot processes of podocyte

Three layers of the filtration membrane Figure 25.10 Juxtaglomerular complex (JGC) of a nephron. Blood pressure in glomerulus high because: • are larger in Glomerular diameter than efferent arterioles capsule • Arterioles are high-resistance vessels Glomerulus

Parietal layer Foot processes of glomerular of podocytes Podocyte cell body capsule (visceral layer) Capsular Red blood cell space Afferent Efferent arteriole Proximal arteriole tubule cell

Juxtaglomerular complex • cells of the ascending limb of nephron loop • Extraglomerular Lumens of mesangial cells glomerular • Granular cells capillaries Endothelial cell Afferent arteriole of glomerular capillary

Glomerular mesangial cells

Juxtaglomerular complex Renal corpuscle Figure 25.9 Blood vessels of the renal cortex.

Peritubular capillary bed

Afferent arteriole

Glomerulus Efferent arteriole Renal Tubules and Collecting Duct

Renal tubule is about 3 cm (1.2 in.) long • Single layer of epithelial cells in 3 major parts 1. Proximal convoluted tubule (closest to renal corpuscle) • Cuboidal cells with dense “brush border” microvilli, confined to cortex – high surface area to interact with filtrate – large mitochondria to support functions • Functions in (significant) reabsorption and secretion 2. Nephron loop – formerly ‘’ –thick and thin limbs 3. Distal convoluted tubule • Farthest from renal corpuscle, confined to cortex • Cuboidal cells with very few microvilli • Function more in secretion than reabsorption • drains into collecting duct Cortical nephron Juxtamedullary nephron • 85% of all nephrons • Long nephron loop • Almost entirely in • Glomerulus closer to the cortex Glomerulus Efferent Cortical radiate vein cortex-medulla junction (capillaries) arteriole Cortical radiate artery Renal • Short nephron loop corpuscle Glomerular Afferent arteriole • Efferent arteriole supplies capsule Collecting duct • Glomerulus further Proximal Distal convoluted tubule from the cortex- • Long nephron loops deeply medulla junction convoluted Afferent arteriole tubule Efferent invade medulla • Efferent arteriole arteriole supplies peritubular • Important in production of capillaries Peritubular capillaries concentrated urine Ascending limb of nephron loop

Cortex-medulla junction Arcuate vein Kidney Vasa recta Arcuate artery Nephron loop Descending limb of nephron loop

‘Thin’ sections - Simple squamous Cuboidal or columnar Renal Tubule and Collecting Duct (cont.) • Collecting ducts – Cells lining ducts participate in fine-tuning of water, sodium ion, and acid-base balance between blood and urine – Receive filtrate from many nephrons – Run through medullary pyramids • Give pyramids their striped appearance – Ducts fuse together to deliver urine through papillae into minor calyces Nephron Capillary Beds

• Renal tubules are associated with two capillary beds – Glomerulus – Peritubular capillaries • Low-pressure, porous capillaries adapted for absorption of water and solutes • Arise from efferent arterioles • Cling to adjacent renal tubules in cortex • Empty into venules • Juxtamedullary nephrons are associated with Vasa recta – Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons – Arise from efferent arterioles serving juxtamedullary nephrons • Instead of peritubular capillaries – Function in formation of concentrated urine Juxtaglomerular Complex (JGC) • Each nephron has one • Modified portions of distal portion of distal tubule and Afferent arteriole • Regulates rate of filtrate formation and blood pressure • Three cell populations are seen in JGC: 1. Ascending limb macula densa contains chemoreceptors that sense NaCl content of filtrate 2. Granular cells (or JG cells) act as mechanoreceptors to sense blood pressure in afferent arteriole Enlarged, smooth muscle cells of arteriole Contain secretory granules that contain enzyme renin 3. Extraglomerular mesangial cells may be autoregulatory smooth muscle cells, may also contribute to renin secretion and perhaps EPO Steps in Urine Production

• Three processes are involved in urine formation and adjustment of blood composition: 1. Glomerular filtration: produces cell- and (mostly) protein-free filtrate 2. Tubular reabsorption: selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts 3. Tubular secretion: selectively moves substances from blood to filtrate in renal tubules and collecting ducts Figure 25.11 The three major renal processes.

Afferent arteriole Glomerular capillaries

Efferent arteriole Cortical radiate artery Glomerular capsule 1

Renal tubule and collecting duct containing filtrate

Peritubular 2 capillary

3 Three major renal processes: To cortical radiate vein 1 Glomerular filtration 2 Tubular reabsorption 3 Tubular secretion Urine Urine Production Step 1: Glomerular Filtration • Glomerular filtration is a passive process • No metabolic energy required • Hydrostatic pressure forces fluids and solutes through filtration membrane into glomerular capsule • Control over systemic blood pressure very important to this process - necessitates kidney’s role in BP control Pressures That Affect Filtration • Outward pressures promote filtrate formation • Hydrostatic pressure in glomerular capillaries (HPgc) • Essentially glomerular blood pressure • 55 mm Hg compared to ~ 26 mm Hg seen in most capillary beds • Efferent arteriole • high-resistance vessel • diameter smaller than afferent arteriole

• Inward Pressures inhibit filtrate formation: • Hydrostatic pressure in capsular space (HPcs) • 15 mm Hg capsule filtrate “back” pressure • Colloid osmotic pressure in capillaries (OPgc) • “pull” of proteins in blood; 30 mm Hg

• Filtrate formation the result of net filtration pressure (NFP): sum of forces • 55 mm Hg forcing out minus 45 mm Hg opposing = net outward force of 10 mm Hg • Main controllable factor determining glomerular filtration rate (GFR) Figure 25.13 Forces determining net filtration pressure (NFP). Glomerular capsule Efferent arteriole

HPgc = 55 mm Hg

OPgc = 30 mm Hg

Afferent HPcs = 15 mm Hg arteriole

NFP = Net filtration pressure = outward pressures – inward pressures Normal Glomerular Filtration Rate (GFR) = 120–125 ml/min = (HPgc) – (HPcs + OPgc) = (55) – (15 + 30) Dependent on glomerular hydrostatic = 10 mm Hg pressure and surface area for filtration Step 2: Tubular Reabsorption

• Quickly most of tubular contents and to blood • Whole volume of plasma enters proximal convoluted tubule every 22 minutes • Selective transepithelial process • Almost all organic nutrients like glucose and amino acids are reabsorbed • Water and ion reabsorption are hormonally regulated and adjusted • Peritubular capillaries nearby contain blood that just left the glomerulus • Includes active and passive tubular reabsorption • Primary and secondary active transport • Diffusion, facilitated diffusion, osmosis Transcellular and paracellular routes of tubular reabsorption.

The transcellular route Filtrate Tubule cell Interstitial involves: in tubule fluid Peri- 1 lumen Lateral Transport across the apical Tight tubular membrane. junction intercellular capillary space 2 Diffusion through the cytosol. 3 Transport across the 3 4 basolateral membrane.

1 2 3 4 H2O and solutes Transcellular route 4 Movement through the interstitial fluid and into the Apical capillary. membrane Capillary endothelial The paracellular route involves: cell • Movement through leaky Paracellular route tight junctions, particularly in H2O and solutes the proximal convoluted tubule. Basolateral • Movement through the membranes interstitial fluid and into the capillary. Reabsorption by PCT cells

Nucleus 1 Filtrate Interstitial At the basolateral membrane, Peri- Na+ is pumped into the interstitial in tubule fluid + + lumen Tubule cell tubular space by the Na -K ATPase. Active capillary Na+ transport creates concentration gradients that drive: 2 “Downhill” Na+ entry at the Na+ 2 apical membrane. 3Na+ 3Na+ 3 1 Reabsorption of organic + + nutrients and certain ions by 2K 2K cotransport at the apical 3 membrane. Glucose K+ 4 Reabsorption of water by Amino acids osmosis through aquaporins. Some ions Water reabsorption increases the Vitamins 4 concentration of the solutes that are left behind. These solutes can H2O then be reabsorbed as they move down their gradients: 5 Lipid-soluble 5 Lipid-soluble substances substances diffuse by the transcellular route. 6 Various ions 6 Various ions (e.g., Cl−, Ca2+, and urea Tight junction Paracellular K+) and urea diffuse by the route paracellular route.

Primary active transport Transport protein Secondary active transport Ion channel Passive transport (diffusion) Aquaporin Tubular Reabsorption of Nutrients, Water, and Ions • Passive tubular reabsorption of water • Movement of Na+ and other solutes creates osmotic gradient for water • Water is reabsorbed by osmosis, aided by membrane pores called aquaporins • Obligatory water reabsorption: • Aquaporins are always present in PCT • Facultative water reabsorption: • Aquaporins are inserted in collecting ducts only if ADH is present

• Passive tubular reabsorption of solutes • Solute concentration in filtrate increases as water is reabsorbed Transport Maximum

• Transcellular transport systems are specific and limited

• Transport maximum (Tm) exists for almost every reabsorbed substance • Reflects number of carriers in renal tubules that are available

• When carriers for a solute are saturated, excess is excreted in urine • Example: hyperglycemia leads to high blood glucose levels that exceed Tm, and glucose spills over into urine Hormonal Control Over Reabsorption • Antidiuretic hormone (ADH) • Released by posterior pituitary • Causes principal cells of collecting ducts to insert aquaporins in apical membranes, increasing water reabsorption

• Aldosterone • Mineralocorticoid from the adrenal cortex • Targets collecting ducts and distal DCT • Promotes synthesis of apical Na+ and K+ channels, and basolateral Na+-K+ pumps for Na+ reabsorption • As a result, most Na+ reabsorbed (and water follows)

• Atrial natriuretic peptide Step 3: Tubular Secretion • Occurs mostly in PCT with ‘fine-tuning’ in DCT and collecting duct

• Selected substances are moved from peritubular capillaries through tubule cells out into filtrate + + + • K , H , NH4 , creatinine, organic acids and bases

• Substances synthesized in tubule cells also are secreted – (example: HCO3 )

• Important for: • Disposing of substances, such as drugs or metabolites • Eliminating undesirable substances that were passively reabsorbed (example: urea and uric acid) • Ridding body of excess K+ • Controlling blood pH by altering amounts of H+ or HCO – in urine 3 Summary of tubular reabsorption and secretion. No water No can ascending ascending leave the No stimulus No automatic limb needed Secretion Reabsorption - Cortex medulla Inner medulla Outer • H reabsorbed 65%of filtrate volume and and other nutrients • all glucose, amino acids, other ions • Na canleave the descending descending No solutes solutes No • Some • Some drugs • H 2 O + , HCO + limb • H and and NH 2 O 3 − , and and many, 4 + and and NH involves H described 26; in Chapter maintain to blood pH • Reabsorption or secretion • Na Regulated reabsorption • Ca 4 Cl hormone) + − + + 2 • Na • K secretion Regulated • Urea • Urea (increased • Na • H reabsorption Regulated • K secretion Regulated , follows) ( + aldosterone) byADH) follows) aldosterone; byaldosterone; aldosterone) HCO (byparathyroid + 2 + O (by ADH)O (by + + (by (by , K (by 3 − + , , Cl − Cl − Regulation of Urine Concentration and Volume • Kidneys make adjustments needed to maintain body fluid osmotic concentration at around 300 mOsm • Same as plasma • Limits osmosis between cells and fluids = balance, no cells swell or shrink

• osmolality is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg) • osmolarity is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L) • Body fluids expressed in milliosmoles (mOsm) = 0.001 osmol • 1 mole NaCl/l = 2 osmoles NaCl/l

• Urine volume and concentration vary based on • Your hydration status: drinking, sweating • Small amount of concentrated urine if the body is dehydrated • Large amount of dilute urine if overhydrated Regulation of Urine Concentration and Volume • Accomplish this by using countercurrent mechanism • Fluid flows in opposite directions in two adjacent segments of same tube • Countercurrent mechanisms work together to: • Establish and maintain medullary osmotic gradient from renal cortex through medulla • Gradient runs from 300 mOsm in cortex to 1200 mOsm at bottom of medulla • Countercurrent multiplier creates gradient • Countercurrent exchanger preserves gradient • Collecting ducts can then use gradient to vary urine concentration Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to produce urine of varying concentration.

300

300

The osmotic 400 gradient: The long nephron loops of The osmolality (solute 600 juxtamedullary nephrons create concentration) of the the gradient. They act as 900 medullary interstitial countercurrent multipliers. fluid progressively 1200 increases from the 300 mOsm of normal body fluid to 1200 mOsm at The big picture: the deepest part of the Three key players medulla. 300 interact with the medullary osmotic Lower 300 gradient. concentration of solutes 400

600 The vasa recta preserve the gradient. They act as 900 countercurrent exchangers (see Figure 25.18). Higher 1200 concentration of solutes

300

300

400 The collecting ducts of all nephrons use the gradient 600 to adjust urine osmolality 900 (see Figure 25.19).

1200 Long nephron loops of juxtamedullary nephrons create the gradient.

The countercurrent multiplier depends on three properties These properties establish a positive feedback cycle of the nephron loop to establish the osmotic gradient. that uses the flow of fluid to multiply the power of the salt pumps.

Filtrate flows in the opposite direction (countercurrent) through two adjacent parallel Water leaves the Interstitial fluid sections of a descending limb osmolality nephron loop.

H2O NaCI H2O NaCI Start The ascending here The descending limb is limb is permeable impermeable to Osmolality of filtrate Salt is pumped out to water, but not water, and pumps in descending limb of the ascending limb to salt. out salt.

Active transport Passive transport Water impermeable Osmolality of filtrate entering the ascending limb

As water and solutes are reabsorbed, the loop first concentrates the filtrate, then dilutes it.

300 300 100 Cortex 100 300 300 1 Filtrate entering the 5 Filtrate is at its most dilute nephron loop is isosmotic H2O NaCI as it leaves the nephron loop. to both blood plasma and At 100 mOsm, it is hypo- ) cortical interstitial fluid. osmotic to the interstitial fluid. H2O NaCI

400 400 200 mOsm H O NaCI 2 4 Na+ and Cl– are pumped out Outer of the filtrate. This increases the NaCI interstitial fluid osmolality. medulla H2O 600 600 400 2 Water moves out of the H2O filtrate in the descending limb down its osmotic gradient. NaCI This concentrates the filtrate. H2O 900 900 700

H2O 3 Filtrate reaches its highest Osmolality of interstitial fluid ( fluid interstitial of Osmolality concentration at the bend of the Inner loop. medulla Nephron loop 1200 1200 Vasa recta preserve the gradient.

• The vasa recta are highly permeable to water and solutes. • Countercurrent exchanges occur between each section of the vasa recta and its surrounding fluid. As a result: • The blood within the vasa recta remains nearly isosmotic to the surrounding fluid. • The vasa recta are able to reabsorb water and solutes into the general circulation without undoing the osmotic gradient created by the countercurrent multiplier.

Blood from To vein efferent 300 arteriole 300 325 NaCI NaCI H2O H2O 300 400 400 The counter- current flow of fluid moves NaCI 400 NaCI through two H2O H2O adjacent parallel sections of vasa recta. 600 600

NaCI 600 NaCI H2O H2O

900 900

NaCI NaCI 900 H2O H2O

1200 1200 Vasa recta Formation of Dilute or Concentrated Urine • Established medullary osmotic gradient can now be used to form dilute or concentrated urine • Without gradient, would not be able to raise urine concentration > 300 mOsm to conserve water

• Overhydration produces large volume of dilute urine • ADH production decreases; urine ~100 mOsm • If aldosterone present, additional ions can be removed, causing water to dilute to ~50 mOsm • Dehydration produces small volume of concentrated urine • Maximal ADH is released; urine ~1200 mOsm If we were so overhydrated we had no ADH...

Osmolality of extracellular fluids

ADH release from posterior pituitary

Number of aquaporins (H2O channels) in collecting duct

H2O reabsorption from collecting duct

Large volume of dilute urine

Collecting duct Descending limb 300 of nephron loop 100

DCT 100

Cortex

300 100 300 )

NaCI mOsm

H2O NaCI 600 400 600 100 Outer medulla

NaCI

900 700 900 Osmolality of interstitial fluid ( fluid interstitial of Osmolality

H2O Urea

Inner medulla 1200 100 1200

Large volume Active transport of dilute urine Passive transport If we were so dehydrated we had maximal ADH...

Osmolality of extracellular fluids

ADH release from posterior pituitary

Number of aquaporins (H2O channels) in collecting duct

H2O reabsorption from collecting duct

Small volume of concentrated urine

Collecting duct

H2O Descending limb 300 of nephron loop 100 150

H2O DCT 300

Cortex ) 300 100 300 300 H2O

NaCI mOsm 400

H2O H2O NaCI 600 400 600 600

Outer medulla H2O NaCI Urea

900 700 900 900 Osmolality of interstitial fluid ( fluid interstitial of Osmolality

H2O

H2O Urea

Inner H O medulla 2 1200 1200 1200

Small volume of Urea contributes to the concentrated urine osmotic gradient. ADH increases its recycling. Urine • Chemical composition • 95% water and 5% solutes • Nitrogenous wastes • Urea (from amino acid breakdown): largest solute component • Uric acid (from nucleic acid metabolism) • Creatinine (metabolite of creatine phosphate) • Other normal solutes found in urine + + 3– 2– 2+ 2+ – • Na , K , PO4 , and SO4 , Ca , Mg and HCO3 • Abnormally high concentrations of any constituent, or abnormal components may indicate pathology • pH • Urine is slightly acidic (~pH 6, with range of 4.5 to 8.0) Substances Filtered and Reabsorbed by the Kidney per 24 Hours Substance Amount filtered (g) Amount reabsorbed (g) Amount in urine (g) % Reclaimed Water 180 L 178- 179 L 1-2 L 99 Small Proteins 2 1.9 0 95-100 Chlorine 630 625 5 99 Sodium 540 537 3 99 Potassium 28 24 4 86 Bicarbonate 300 299.7 0.3 99.9 Glucose 180 180 0 100 Urea 53 28 25 50 Uric acid 8.5 7.7 0.8 91 Creatinine 1.4 0 1.4 0

Source: Open Stax. Anatomy and Physiology (open source text book) https://opentextbc.ca/anatomyandphysiology/chapter/25-6- tubular-reabsorption/ . Accessed 10/8/18

Jon Barron of the Baseline of Health Foundation. 03/26/2012. What Is the Urinary System, Functions | Kidneys & Urinalysis Newsletter. https://jonbarron.org/article/physiology-urinary-system . Accessed 10/8/18 Peritoneum

Ureter

Rugae Structure Detrusor Ureteric orifices of the Trigone of bladder Bladder neck Internal urethral urinary sphincter Prostate Prostatic bladder Intermediate part of the urethra External urethral sphincter and Urogenital diaphragm urethra

Spongy urethra

Erectile tissue of penis

External urethral orifice Male. The long male urethra has three regions: prostatic, intermediate, and spongy. Peritoneum Structure of the urinary Ureter bladder and urethra. Rugae Detrusor

Ureteric orifices

Bladder neck Internal urethral sphincter

Trigone

External urethral sphincter Urogenital diaphragm Urethra External urethral orifice Female. Urinary Bladder (cont.)

• Urine storage capacity • Collapses when empty • Rugae appear • Expands and rises superiorly during filling without significant rise in internal pressure • Moderately full bladder is ~12 cm long (5 in.) and can hold ~ 500 ml (1 pint) Micturition, also called urination or voiding Three simultaneous events must occur 1. Contraction of detrusor by ANS 2. Opening of internal urethral sphincter by ANS 3. Opening of external urethral sphincter by somatic nervous system