The Urinary System

Chapter 26

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

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

Urinary System Organs Structures of the

urinary system include;– Kidneys

– Urinary bladder

– Ureters

– Urethra

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

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

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

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

Position of the Kidneys

The kidneys are retroperitoneal, or behind the peritoneum

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

Position of the Kidneys

The adipose capsule attaches the kidney to the posterior body wall and cushions it against trauma

Position of the Kidneys

The renal fascia is dense fibrous connective tissue which surrounds the kidney and anchors these organs to the surrounding structures

Internal Anatomy The kidney has

three distinct regions– Cortex

– Medulla

– Pelvis

Internal Anatomy The most

superficial region

The renal cortex is light in color and has a granular appearance

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

Internal Anatomy Each renal

pyramid has a base which is convex, and a apex which tapers toward its apex or papilla

Medullarybase

Medullaryapex

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

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

Internal Anatomy Within the

renal sinus is the renal pelvis

This flat, funnel shaped tube is continuous with the ureter leaving the hilus

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

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

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

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

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

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

Nephron

The renal corpuscle refers to the enclosed glomerulus and the capsule of the glomerulus referred to as Bowman’s capsule

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

Nephron

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

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

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

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

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

Renal Physiology Skip to sections on Ureters located

on page 1029

Ureters The ureters are

slender tubes that convey urine from the kidneys to the bladder

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

Ureters The ureters are

protected from a backflow of urine because any increase within the bladder compresses and closes the ends of the ureters

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

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

Urinary Bladder

The urinary bladder is a smooth, collapsible, muscular sac that stores urine

Urinary Bladder

In males, the bladder lies immediately anterior to the rectum

Urinary Bladder

In females, the bladder is anterior to the vagina and uterus

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

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)

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

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

Urethra The urethra is

a thin muscular tube that drains urine from the bladder and conveys it out of the body

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

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

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

Urethra Its external

opening, the external urethral orifice, anterior to the vaginal opening and posterior to the clitoris

Urethra

In males the urethra is 20 cm long with three regions– Prostatic

urethra– Membranous

urethra– Spongy or

penile urethra

Urethra The male urethra has two basic functions

– It carries urine out of the body– It carries semen into the female reproductive

tract

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

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

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

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

Chapter 26 End of material from chapter 26