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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

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  • 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
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
Urinary System Organs
  • Structures of the urinary system include;
    • Kidneys
    • Urinary bladder
    • Ureters
    • Urethra
kidney location
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
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 antomy1
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
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
Position of the Kidneys
  • The kidneys are retroperitoneal, or behind the peritoneum
position of the kidneys1
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 kidneys2
Position of the Kidneys
  • The adipose capsule attaches the kidney to the posterior body wall and cushions it against trauma
position of the kidneys3
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
Internal Anatomy
  • The kidney has three distinct regions
    • Cortex
    • Medulla
    • Pelvis
internal anatomy1
Internal Anatomy
  • The most superficial region
  • The renal cortex is light in color and has a granular appearance
internal anatomy2
Internal Anatomy
  • Deep to the cortex is the renal medulla
  • Darker tissue which exhibits cone shaped tissue masses called medullary or renal pyramids



internal anatomy3
Internal Anatomy
  • Each renal pyramid has a base which is convex, and a apex which tapers toward its apex or papilla





internal anatomy4
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



internal anatomy5
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 anatomy6
Internal Anatomy
  • Within the renal sinus is the renal pelvis
  • This flat, funnel shaped tube is continuous with the ureter leaving the hilus
internal anatomy7
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 anatomy8
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
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 supply1
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
  • 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
  • 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


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


renal physiology
Renal Physiology
  • Skip to sections on Ureters located on page 1029
  • The ureters are slender tubes that convey urine from the kidneys to the bladder
  • 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
  • The ureters are protected from a backflow of urine because any increase within the bladder compresses and closes the ends of the 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
  • 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
Urinary Bladder
  • The urinary bladder is a smooth, collapsible, muscular sac that stores urine
urinary bladder1
Urinary Bladder
  • In males, the bladder lies immediately anterior to the rectum
urinary bladder2
Urinary Bladder
  • In females, the bladder is anterior to the vagina and uterus
urinary bladder3
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 bladder4
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 bladder5
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 bladder6
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
  • The urethra is a thin muscular tube that drains urine from the bladder and conveys it out of the body
  • 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
  • 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
  • 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
  • Its external opening, the external urethral orifice, anterior to the vaginal opening and posterior to the clitoris
  • In males the urethra is 20 cm long with three regions
    • Prostatic urethra
    • Membranous urethra
    • Spongy or penile urethra
  • The male urethra has two basic functions
    • It carries urine out of the body
    • It carries semen into the female reproductive tract
  • 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
  • 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
  • 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
  • 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
Chapter 26
  • End of material from chapter 26