1. Anatomy and Physiology
Marieb’s Human
Anatomy and Physiology
Ninth Edition
Marieb w Hoehn
Chapter 25
Urinary system
Lecture 16
Part 3: Urine Formation and the Micturition Reflex
Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1]
Slides 16 – 38; 50 min [Lecture 2]
118 min (38 slides plus review of course Web sites and syllabus)

2. Urine Urine composition varies depending upon
Diet
Level of activity
Major constituents of urine
H2O (95%)
Creatinine (remember, NONE of this is reabsorbed)
Urea (most abundant solute), uric acid
Trace amounts of amino acids
Electrolytes
Urochrome (yellow color), urobilin, trace of bilirubin
Normal urine output is L/day ( ml/hr)
Output below about 25 ml/hour = kidney failure (oliguria - anuria)

3. Terms to know… Anuria – absence of urine
Diuresis – increased production of urine
Dysuria – difficult or painful urination
Enuresis – uncontrolled (involuntary) urination
Glycosuria (glucosuria) – glucose in the urine
Hematuria – blood in the urine
Oliguria – scanty output of urine
Polyuria – excessive urine output

4. Elimination of Urine Flow of Urine Know this… nephrons
collecting ducts
renal papillae
minor and major calyces
renal pelvis
ureters
urinary bladder
urethra
outside world
Know this…

5. Ureters
25 cm long
extend downward posterior to the parietal peritoneum
parallel to vertebral column
in pelvic cavity, joins urinary bladder
peristaltic contractions
Figure from: Hole’s Human A&P, 12th edition, 2010
Walls of the ureters:
Mucous coat – transitional epithelium continuous with linings of the renal tubules and urinary bladder Muscular coat – smooth muscle in longitudinal and circular bundles; carries out peristalsis to move urine toward bladder Fibrous coat (adventitia)– CT layer continuous with the renal capsule and peritoneum

6. Urinary Bladder [Cyst(o)]
Temporary reservoir for storage of urine
Four layers:
Mucous layer (transitional epi.)
Submucous coat
Muscular coat
Serous layer
Figure from: Hole’s Human A&P, 12th edition, 2010
Muscular layer runs in all directions (detrusor muscle) under parasympathetic control. Contraction compresses the bladder and causes urine to flow into urethra
Internal urethral sphincter (IUS) is a thickening of the detrusor muscle at the neck of the bladder. When the detrusor muscle is relaxed, the IUS is CLOSED. When the detrusor muscle contracts during the micturition reflex, the IUS OPENS.
Frontal section, anterior
posterior
Note the internal sphincter at neck of bladder

7. Urinary Bladder and Urethra - Female
Urinary bladder is inferior to the uterus and is separated from the rectum by the vagina.
Note the short urethra (about 4 cm)
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Pelvic region, midsagittal section
Note the passage of the urethra through the urogenital diaphragm that forms the external urethral sphincter

8. Urinary Bladder and Urethra - Male
Base of the urinary bladder lies between the rectum and pubis symphysis.
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Pelvic region, midsagittal section

9. Urinary Bladder and Urethra - Male
Note the long urethra (about cm). There are three sections to the male urethra:
- Prostatic urethra - Membranous urethra - Penile urethra
Note the passage of the urethra through the urogenital diaphagm that forms the external urethral sphincter
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

10. The Pelvic Diaphragm (Female)
Figure from: Hole’s Human A&P, 12th edition, 2010
These are DEEP muscles that form the pelvic floor and extend between the pubic bones

11. Cross Section of Urethra
Extends from neck of bladder to exterior
Epithelium varies from
- Transitional at neck of bladder
- Stratified columnar at midpoint
- Stratified squamous near external urethral meatus
Figure from: Hole’s Human A&P, 12th edition, 2010

12. Micturition (Urination) Reflex
trigger = bladder distention & stimulation of stretch receptors
micturition center activated in sacral portion of spinal cord
parasympathetic nerve impulses cause detrusor muscle to contract (short reflex) and internal urethral sphincter to open
need to urinate is sensed (spinal cord -> thalamus - > cortex) - urge to urinate at about ml of urine - discomfort at about 300 ml of urine - maximum capacity of bladder is about ml
voluntary (tonal) contraction of external urethral sphincter prevents urination and also closes the internal sphincter
when decision is made to urinate, external and internal urethral sphincters relax, detrusor muscle contracts, and urine is expelled

13. Micturition Reflex
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

14. Review Elimination of Urine Know pathway of urine flow
Ureters transport urine from kidney to urinary bladder
Urinary bladder
is a temporary storage site for urine
has muscular coat (detrusor) under parasympathetic control
Urethra is the conduit for urine from the bladder to the exterior
Varies in length between males (18-20 cm) and females (4 cm)
Has different types of epithelium along its length

15. Review Micturition (urination) reflex
Begins with distension of urinary bladder
Activates mild contraction of the detrusor muscle
Sends impulses to higher brain centers
Requires voluntary relaxation of the external urethral sphincter in order for urination to occur