the urinary system chapter 26. introduction n the kidneys are perfect examples of homeostatic organs...
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The Urinary System
Chapter 26
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Introduction The kidneys are perfect examples of
homeostatic organs Maintain constancy of fluids in our
internal environment Filter 200 liters of fluid a day Remove toxins, metabolic wastes, and
excess ions to leave the body in urine Return needed substances to the blood A primary organ of excretion
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Kidney Functions Kidneys regulate volume and chemical
makeup of the blood Maintain the proper balance between water
and salts as well as between acids and bases Gluconeogenesis - supply glucose during
fasting Produce enzyme renin which helps regulate
blood pressure and kidney function Produce hormone erythropoietin which
stimulates red blood cell production
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Urinary System Organs Structures of the
urinary system include;– Kidneys
– Urinary bladder
– Ureters
– Urethra
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Kidney Location The kidneys extend
approximately from the level of the 12th thoracic vertebra to the third lumbar vertebra
Receive some protection from ribs
Right lies somewhat lower than left as it is positioned under liver
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External Antomy The adult kidney
weights about 150 g (5 oz.)
Dimensions are 12 cm long, 6 cm wide, 3 cm thick
Lateral surface is convex while the medial surface is concave
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External Antomy Medial surface
has a vertical cleft called the renal hilus that leads into the space within the kidney called the renal sinus
Atop each kidney is an adrenal gland which is unrelated to kidney function
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External Anatomy Structures such
as the ureters, the renal blood vessels, lymphatics, and nerves enter the kidney at the hilus
These structures occupy the renal sinus
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Position of the Kidneys
The kidneys are retroperitoneal, or behind the peritoneum
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Position of the Kidneys
Kidneys supported by three layers of supportive tissue The renal capsule adheres directly to the kidney
surface and isolates it from surrounding region
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Position of the Kidneys
The adipose capsule attaches the kidney to the posterior body wall and cushions it against trauma
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Position of the Kidneys
The renal fascia is dense fibrous connective tissue which surrounds the kidney and anchors these organs to the surrounding structures
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Internal Anatomy The kidney has
three distinct regions– Cortex
– Medulla
– Pelvis
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Internal Anatomy The most
superficial region
The renal cortex is light in color and has a granular appearance
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Internal Anatomy Deep to the
cortex is the renal medulla
Darker tissue which exhibits cone shaped tissue masses called medullary or renal pyramids Medullary
pyramids
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Internal Anatomy Each renal
pyramid has a base which is convex, and a apex which tapers toward its apex or papilla
Medullarybase
Medullaryapex
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Internal Anatomy The apex, or
papilla, points internally
The pyramids appear striped because they are formed almost entirely of roughly parallel bundles of urine collecting tubules
Pyramidal stripes
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Internal Anatomy Inward
extensions of cortical tissue called renal columns separate the pyramids
Each medullary pyramid is surrounded by a capsule of cortical tissue to form a lobe
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Internal Anatomy Within the
renal sinus is the renal pelvis
This flat, funnel shaped tube is continuous with the ureter leaving the hilus
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Internal Anatomy Branching
extensions of the renal pelvis form 2-3 major calyces, each of which sub-divides to form several minor calyces
These cup shaped areas collect the urine which drain continuously from the papillae
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Internal Anatomy Urine flows
through the renal pelvis into the ureter, which transports it to the bladder
The walls of the calyces, pelvis, and ureter contain smooth muscle which contract to move urine
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Blood Supply The kidney continuously cleanse the blood
and adjust its composition Kidneys possess an extensive blood supply Under normal resting conditions, the renal
arteries deliver approximately one-fourth of the total systemic cardiac output (1200 ml) to the kidneys each minute
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Blood Supply The renal arteries
issue at right angles from the abdominal aorta
Each renal artery divides into five segmental arteries that enter the hilus
Each segmental artery divides into lobar and interlobar arteries
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Nephrons Each kidney contains over 1 million tiny
blood processing units called nephrons, which carry out the processes that form urine
In addition, there are thousands of collecting ducts, each of which collects urine from several nephrons and conveys it to the renal pelvis
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Nephron Each nephron
consists of a glomerulus, a tuft of capillaries associated with a renal tubule
The end of the renal tubule is a blind, enlarged, and cup-shaped and completely surround the glomerulus
Glomerulus
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Nephron
The renal corpuscle refers to the enclosed glomerulus and the capsule of the glomerulus referred to as Bowman’s capsule
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Nephron The glomerulus endothelium is fenestrated,
(penetrated by many pores), which make these capillaries exceptionally porous
The capillaries allow large amounts of solute-rich, virtually protein free fluid to pass from the blood into the glomerulus capsule
This plasma-derived fluid or filtrate is the raw material that is processed by the renal tubules to form urine
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Nephron
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Nephron The external parietal layer of the glomerular
capsule is simple squamous epithelium This layers contributes to the structure of the
capsule and plays no part in forming filtrate The visceral layer that clings to the
glomerulus consists of highly modified, branching epithelial cells called podocytes
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Nephrons
Podocytes terminate in foot processes, which intertwine and form filtration silts or slit pores
The silts allow filtrate to pass to the interior of capsule
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Nephrons The filtration
membrane is the actual filter that lies between the blood and the interior of the glomerular capsule
It is a porous membrane that allows free passage of water and solutes
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Nephrons It is a porous
membrane that allows free passage of water and solutes smaller that plasma proteins
The capillary pores prevent passage of blood cells, but plasma components are allowed to pass
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Nephron Once filtered out of
the plasma the urine enters the collecting duct
Urine passes into larger ducts until it reaches the ureters
It leaves the kidneys and moves toward the bladder in the ureters
Glomerulus
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Renal Physiology Skip to sections on Ureters located
on page 1029
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Ureters The ureters are
slender tubes that convey urine from the kidneys to the bladder
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Ureters Each leaves the
renal pelvis, decends behind the peritoneum to the base of the bladder, turns and then runs obliquely through the medial bladder wall
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Ureters The ureters are
protected from a backflow of urine because any increase within the bladder compresses and closes the ends of the ureters
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Ureters Histologically, the walls of the ureter is
trilayered– An inner layer of transitional epithelium
lines the inner mucosa– The middle muscularis layer is composed of
a an inner longitudinal layer and an outer circular layer
– The outer layer is composed of fibrous connective tissue
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Ureters The ureters play an active role in
transporting urine Distension of the ureters by incoming
urine stimulates the muscularis layer to contract, which propels the urine into the bladder
The strength and frequency of peristaltic waves are adjusted to the rate of urine formation
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Urinary Bladder
The urinary bladder is a smooth, collapsible, muscular sac that stores urine
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Urinary Bladder
In males, the bladder lies immediately anterior to the rectum
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Urinary Bladder
In females, the bladder is anterior to the vagina and uterus
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Urinary Bladder The interior of the
bladder has openings for both ureters and the urethra
The triangular region of the bladder base outlined by these openings is called the trigone which is a common site of infections
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Urinary Bladder The bladder wall has three layers
– A mucosa containing transitional epithelium– A thick muscular layers– A fibrous adventitia
The muscle layer consists of smooth muscle arranged inner and outer longitudinal layers
Collectively the muscle layer is called the detrusor muscle (literally to thrust out)
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Urinary Bladder The bladder is very distensible and
uniquely suited for its function of urine storage
It can expand for storage or collapse when empty
Empty its walls are thick and thrown into folds (rugae)
As it expands it becomes pear shaped and rises in the abdominal cavity
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Urinary Bladder The bladder can store more than 300 ml
or urine without a significant increase in internal pressure
A moderately full bladder holds approximately 500 ml and can about 1000 ml at capacity
Urine is held in the bladder until release is convenient
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Urethra The urethra is
a thin muscular tube that drains urine from the bladder and conveys it out of the body
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Urethra The epithelium of its mucosal lining is
mostly pseudostratified columnar epithelium
Near the bladder it is transitional epithelium and near its external opening it changes to a protective squamous epithelium
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Urethra At the bladder-urethra junction a
thickening of the detrusor muscle forms the internal sphincter
This voluntary sphincter keeps the urethra closed when urine is not being passed
A second sphincter, the external urethral sphincter, surrounds the urethra and is composed of skeletal muscle and thus is under voluntary control
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Urethra The levator ani muscle of the pelvic floor
also serves as a voluntary constrictor of the urethra
The length and functions of the urethra differ in the two sexes
In females the urethra is 3-4 cm long and is tightly bound to the anterior vaginal wall by fibrous connective tissue
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Urethra Its external
opening, the external urethral orifice, anterior to the vaginal opening and posterior to the clitoris
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Urethra
In males the urethra is 20 cm long with three regions– Prostatic
urethra– Membranous
urethra– Spongy or
penile urethra
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Urethra The male urethra has two basic functions
– It carries urine out of the body– It carries semen into the female reproductive
tract
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Micturition Micturition, also called voiding or
urination, is the act of emptying the bladder
Ordinarily, as urine accumulates, distension of the bladder walls activates stretch receptors
Impulses are transmitted via visceral afferent fibers to the sacral region of the spinal cord
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Micturition Spinal reflexes
– Initiate increased sympathetic outflow to the bladder that inhibits the detursor muscle and internal sphincter (temporarily)
– Stimulate contraction of the external urethral sphincter
When about 200 ml of urine has accumulated, afferent impulses are transmitted to the brain, at this point one feels the urge to void their bladder
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Micturition Contractions of the bladder become both
more frequent and urgent with time If the time is convenient to empty the
bladder voiding reflexes are initiated Visceral afferent impulses activate the
micturition center of the dorsolateral pons Acting as an on/off switch for urination, this
center signals the parasympathetic neurons to stimulate contraction of the detrusor muscle and relaxation of sphincters
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Micturition When one chooses not to void, reflex
bladder contractions subside within a minute or so and urine continue to accumulate
Because the external sphincter (and the levator ani) is voluntarily controlled, we can choose to keep it closed and postpone bladder emptying temporarily
The urge to void eventually becomes irresistible and micturition occurs
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Chapter 26 End of material from chapter 26