25.2 Gross Anatomy of Urine Transport

what is urine

is a fluid of variable composition that requires specialized structures to remove it from the body safely and efficiently

How is urine formed throughout the day?

Blood is filtered continuously, and the resulting filtrate is turned into urine at a steady rate throughout the day

What happens to urine after it is produced?

Urine is stored in the body until it can be conveniently excreted

What are the functions of the urinary transport and storage system?

This system stores urine, protects body tissues from damage due to the urine's changing pH and osmolarity, helps prevent infections, and in males, it also supports reproductive functions

How does the urinary system help prevent infections?

By properly storing and transporting urine, the system reduces the chance for harmful microbes to enter or remain in the body

what is the urethra

transport urine from the bladder to the outside of the body for disposal

what is notable of the urethra

urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical

where does the urethra in both female and male begin

begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone

what is the trigone

area at the base of the bladder marked by the two ureters in the posterior–lateral aspect and the urethral orifice in the anterior aspect oriented like points on a triangle

what is the urethra’s anatomical position as it moves through the lower pelvis

the urethra tracks posterior and inferior to the pubic symphysis (The urethra travels behind and below the pubic symphysis.)

what is the proximal urethra lined by in both males and females

transitional epithelium

what is the terminal portion of the urethra lined by in both males and females

nonkeratinized, stratified squamous epithelium

which tissues lines the urethra between these two cell types (transitional epithelium and stratified squamous epithelium) in males

pseudostratified columnar epithelium

what is voiding (release of urine) regulated by

an involuntary autonomic nervous system-controlled internal urinary sphincter

what is the internal urinary sphincter

smooth muscle at the juncture of the bladder and urethra; relaxes as the bladder fills to allow urine into the urethra

what is the external urinary sphincter

skeletal muscle; must be relaxed consciously so void urine

Where is the external urethral orifice located in females

It is in the anterior vaginal wall below the clitoris above the vaginal opening and between the labia minora

How long is the female urethra

It is about 4 centimeters long

Why are urinary tract infections more common in females

Because the female urethra is shorter and provides less of a barrier to fecal bacteria than the longer male urethra

What nerve controls the voluntary function of the external urethral sphincter

The pudendal nerve controls it

Where does the pudendal nerve originate

It starts in the sacral region of the spinal cord and travels through the S2 to S4 nerves of the sacral plexus

Where does the male urethra begin its path

It begins by passing through the prostate gland just below the bladder

How long is the average male urethra

It averages about 20 centimeters in length though it varies between individuals

What are the four regions of the male urethra

the preprostatic urethra, the prostatic urethra, the membranous urethra, and the spongy or penile urethra

what is the characteristic of the preprostatic urethra

very short and incorporated into the bladder wall

where does the prostatic urethra pass through

the prostate gland

What is the role of the prostatic urethra during sexual activity

It receives sperm from the ejaculatory ducts and secretions from the seminal vesicles

What do the Cowper's (bulbourethral glands) glands do

produce and secrete mucus into the urethra to buffer urethral pH during sexual stimulation

what is the function of the mucus in the urethra produced by cowpers gland

The mucus neutralizes the usually acidic environment and lubricates the urethra, decreasing the resistance to ejaculation

Where is the membranous urethra located

It passes through the deep muscles of the perineum and is surrounded by urethral sphincters

What is the final section of the urethra before urine or semen exits the body

The spongy urethra which exits through the external urethral orifice at the tip of the penis after passing through the corpus spongiosum

Why are mucous glands present along the male urethra

They protect the urethra from the extreme pH levels of urine

Is the nerve supply to the urethra the same in males and females

Yes the innervation is the same in both

what is the function of the urinary bladder

collects urine from both ureters

where is the bladder located in females

The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum

what happens to the bladder during late pregnancy

its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination.

where is the bladder located in males

the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder

what is the characteristic of the bladder

The bladder is a retroperitoneal organ whose "dome" distends superiorly when the bladder is filling with urine

what is retroperitoneal

behind the peritoneum; in the case of the kidney and ureters, between the parietal peritoneum and the abdominal wall

what are detrusor muscles

smooth muscle in the bladder wall; fibers run in all directions to reduce the size of the organ when emptying it of urine

why is the bladder a distensible (ability to stretch) organ

comprised of irregular crisscrossing bands of smooth muscled called detrusor muscle

what is the interior surface of the bladder made of

transitional cellular epithelium

why is the interior surface of the bladder made of transitional cellular epithelium

for the ability of the large volume fluctuations

when the bladder is empty, what tissue does it resemble

columnar epithelia

when the bladder is stretched, what tissue does it resemble

squamous

how much volume of urine can adults hold

500-600 mL

what happens to the bladder’s strength with age

diminishes but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying

what is micturition

urination or voiding

the result of micturition (urination or voiding) is from

an interplay of involuntary and voluntary actions by the internal and external urethral sphincters

what happens when the bladder volume reaches 150 mL

an urge to void is sensed but is easily overridden

what does voluntary control of urination rely on

consciously preventing relaxation of the external urethral sphincter to maintain urinary continence

what is incontinence

loss of ability to control mucturition

what happens as voluntary constraint of the bladder fails

incontinence

when does incontinence occur

when bladder reaches volumes 300-400 mL

What is the result of parasympathetic neural outflow during micturition

It causes the detrusor muscle to contract and the internal urethral sphincter to relax

What triggers the micturition reflex

Stretch receptors in the bladder wall send signals to the sacral region of the spinal cord

How does the external urethral sphincter relax during urination

The spinal cord inhibits somatic motor neurons which leads to relaxation of the external sphincter

How do children gain control over urination

They learn to override the reflex by controlling the external urethral sphincter

Can the micturition reflex still occur after spinal cord injury

Yes the reflex may still function even in paraplegia or quadriplegia

Why might catheterization be necessary in some cases of spinal cord injury

Because the external sphincter may not relax properly making it hard to empty the bladder naturally

what are the nerves involved in the control of urination

the hypogastric, pelvic, and pudendal

What is required for voluntary urination to occur

An intact spinal cord and a functional pudendal nerve from the sacral micturition center

What type of muscle is the external urinary sphincter and how is continence maintained during bladder filling

It is voluntary skeletal muscle and is kept contracted by cholinergic neurons

How does the sympathetic nervous system affect the detrusor muscle

It suppresses detrusor muscle contraction via the hypogastric nerves

What happens when the bladder stretches further

Afferent signals from the pelvic nerves activate parasympathetic neurons

What is the result of parasympathetic activation in the bladder

Efferent neurons release acetylcholine which causes detrusor muscle contraction and bladder emptying

Where are the kidneys and ureters located in relation to the peritoneum

They are completely retroperitoneal

Where does the urine go after being formed in the kidneys

It drains into the calyces which merge to form the renal pelvis and then into the ureter

How is urine moved through the ureter to the bladder

It is propelled by waves of peristalsis not by passive flow

What path do the ureters take as they enter the pelvis and bladder

They sweep laterally along the pelvic walls then turn medially and pierce the bladder wall obliquely

Why is the oblique angle of ureter entry into the bladder important

It forms a one way valve that allows urine in but prevents it from flowing back into the ureters

What condition can occur if a child is born without the oblique ureter path

Vesicoureteral reflux which increases the risk of urinary tract infections

Why might pregnancy increase the risk of reflux and UTI

Because it can alter the angle and pressure at which the ureters connect to the bladder

How long are the ureters on average

About 30 centimeters long

What type of epithelium lines the inside of the ureter

Transitional epithelium lines the inside of the ureter

What cells in the ureter secrete protective mucus

Scattered goblet cells

What muscle layers are found in the ureter wall

Longitudinal and circular smooth muscles

What is the purpose of the muscle layers in the ureter

To create peristaltic contractions that move urine without gravity

What makes up the outer layer of the ureter and what is its function

A loose adventitial layer of collagen and fat that anchors the ureter in place

what does peristaltic contractions help with

to move urine through the lumen with contributions from fluid pressure and gravity

what is the physiological sphincter

sphincter consisting of circular smooth muscle indistinguishable from adjacent muscle but possessing differential innervations, permitting its function as a sphincter; structurally weak

what is the anatomical sphincter

smooth or skeletal muscle surrounding the lumen of a vessel or hollow organ that can restrict flow when contracted

what is the sacral micturition center

group of neurons in the sacral region of the spinal cord that controls urination; acts reflexively unless its action is modified by higher brain centers to allow voluntary urination