Human A&P II – THE URINARY SYSTEM

I. Introduction

A. The urinary system consists of two kidneys, two ureters, one urinary

bladder, and one urethra.

B. Urine is produced in the ____________________________

1. Urine is excreted from each kidney through its

____________________________

2. Urine is then stored in the ____________________________

3. Urine is expelled from the body through the ________________

in a process known as ____________________________.

II. OVERVIEW OF KIDNEY FUNCTIONS

A. The major work of the urinary system is done by the kidneys.

B. Kidneys contribute to homeostasis of body fluids by…

1. regulation of blood ionic composition

2. maintenance of blood osmolarity

3. regulation of blood volume

4. regulation of blood pressure

5. regulation of pH

6. endocrine secretions

a. Produce ____________________________ to help

regulate blood pressure

b. ____________________________ to stimulate RBC

production

7. regulating blood glucose level

a. ____________________________ during prolonged

fasting

8. excreting wastes and foreign substances

9. activation of vitamin D

C. The kidneys perform an amazing function

1. About 1000 – 1200 ml of blood pass through the glomerulus/min

a. ~ 650 ml of this blood is plasma

b. ~ 1/5 of this plasma is forced into the renal tubules

1) That’s equivalent to filtering your entire plasma

volume ____________________________ times/day!

2. In 24 hours, the kidneys filter ~ ____________ liters of fluid/day

a. Only ~ 1% of this leaves the body as urine (or almost

____________________________ of urine/day)

3. In 24 hours, the kidneys reclaim…

a. ~1,300 g of NaCl

b. ~400 g NaHCO₃

c. ~180 g glucose

III. ANATOMY OF THE KIDNEYS

A. Location & External Anatomy of Kidneys

1. The kidneys are bean-shaped organs that lie retroperitoneal in

the superior lumbar region.

a. They extend from the 12^th thoracic to the 3^rd lumbar

vertebrae

b. The right kidney is lower than the left because it is

crowded by the liver.

2. The lateral surface is convex

3. The medial surface is concave with a vertical fissure called the

____________________________

a. The ureter leaves and blood vessels, lymphatic vessels,

and nerves enter and exit through the hilus.

4. Layers of tissue supporting the kidney (from innermost to

outermost)

a. ____________________________ – fibrous, transparent

capsule that prevents kidney infection

b. Adipose capsule – fatty mass that cushions the kidney

and helps attach it to the body wall

c. Renal fascia – outer layer of dense fibrous connective

tissue that anchors the kidney and adrenal gland to

surrounding structures

B. Internal Anatomy of Kidneys

1. A frontal section shows three distinct regions

a. ____________________________ – the light colored,

granular superficial region

b. ____________________________ – exhibits cone-

shaped medullary (renal) pyramids

c. ____________________________ are made up of

parallel bundles of urine-collecting tubules

2. The renal cortex and renal pyramids constitute the functional

portion or parenchyma of the kidney.

3. The ____________________________ is the functional unit of

the kidney.

a. Nephrons perform 3 basic functions:

1) Glomerular filtration

2) Tubular reabsorption

3) Tubular secretion

b. The number of nephrons is constant from birth. They

may increase in size, but not in number.

4. Other internal anatomical features of the kidney…

a. Renal columns are inward extensions of cortical tissue

that separate the pyramids

b. The medullary pyramid and its surrounding capsule

constitute a lobe

c. ____________________________ – flat, funnel-shaped

tube lateral to the hilus within the renal sinus

d. ________________________________________ – large

branches of the renal pelvis

1) Major & Minor calyces collect urine draining from

____________________________

e. Urine empties into the renal pelvis

f. Urine then flows through the pelvis and ureters to the

bladder

C. Blood and Nerve Supply of the Kidneys

1. ____________________________ of systemic cardiac output

(1200 ml) flows through the kidneys each minute.

2. Blood enters the kidney through the renal artery and exits via

the renal vein, following similar paths.

a. Blood enters the kidneys from the aorta through…

1) renal arteries to

2) segmental arteries to

3) lobar arteries to

4) interlobar arteries to

5) arcuate arteries to

6) interlobular arteries to

7) afferent arteriole to

8) glomerulus (capillaries)

b. Blood returns to renal veins from

1) efferent arteriole to

2) peritublar capillaries & vasa recta to

3) interlobular veins to

4) arcuate veins to

5) interlobar veins

3. The nerve supply to the kidney is derived from the ___________

________________ (sympathetic division of ANS).

a. The renal plexus regulates renal blood flow by adjusting

the diameter of renal arterioles and influencing the urine-

forming role of the nephrons.

D. Nephrons – The structural & functional units that form urine,

consisting of a ____________________________ (where fluid is filtered)

& a ____________________________ (into which the filtered fluid

passes)

1. A renal corpuscle consists of a glomerulus & the glomerular

capsule

a. ____________________________ – a tuft of capillaries

associated with a renal tubule

b. ________________________________ – the blind, cup-

shaped end of a renal tubule that completely surrounds the

glomerulus

2. Anatomy of the Renal Corpuscle

a. The glomerular capsule consists of visceral and parietal

layers.

1) The parietal layer is a structural layer that consists

of simple squamous epithelium and forms the outer

wall of the capsule.

2) The visceral layer consists of modified simple

squamous epithelial cells called ________________.

a) Extensions of the octopus-like podocytes

terminate in foot processes

b) Filtration slits – openings between the foot

processes that allow filtrate to pass into the

capsular space

b. The filtration membrane lies between the blood and the

interior of the glomerular capsule, and allows free passage

of water and solutes.

1) The ___________________________________ –

fenestrated epithelium that allows solute-rich, virtually

protein-free filtrate to pass from the blood into the

glomerular capsule

c. Fluid filtered from the glomerular capillaries enters the

____________________________, the space between the

two layers of the glomerular capsule.

3. The renal tubule begins at the glomerular capsule & consists of

a ….

a. ________________________________________ (PCT)

1) The PCT is composed of cuboidal cells with

numerous microvilli & mitochondria.

2) Reabsorbs water & solutes from filtrate & secretes

substances into it

b. _________________________________ (nephron loop).

The loop of Henle consists of

1) descending limb similar to PCT

2) a thin ascending limb (simple squamous cells),

3) a thick ascending limb (cuboidal to columnar cells)

c. The ____________________________________ (DCT).

Cells here function more in secretion than reabsorption.

1) Two important cell types are found here…

a) Most of the cells are _________________

___________________ – cuboidal cells

without microvilli that help maintain the body’s

water and salt balance.

i) These cells have receptors for ADH

and aldosterone.

b) A smaller number are _________________

_______ – cuboidal cells with microvilli that

function in maintaining the acid-base balance

of the body.

4. The _______________________________________ arise from

efferent arterioles draining the glomerulus, and absorb solutes and

water from the tubules. These capillaries cling closely to adjacent

renal tubules and empty into nearby venules.

a. The ____________________________ are capillary

branches that supply loops of Henle in the medulla region of

the kidney.

5. The distal convoluted tubule empties into a collecting duct.

6. The distal convoluted tubules of several nephrons drain into to a

single ____________________________.

7. Many collecting ducts drain into a small number of

__________________________________ which extend through

the renal pyramid to the renal papilla, where they empty into a minor

calyx.

E. There are two types of nephrons that have differing structure and

function.

1. A ____________________ nephron usually has its glomerulus

in the outer portion of the cortex and a short loop of Henle that

penetrates only into the outer region of the medulla.

a. _________________ of nephrons are cortical nephrons

2. A ____________________________ nephron usually has its

glomerulus deep in the cortex close to the medulla; its long loop of

Henle stretches through the medulla and almost reaches the renal

papilla.

a. 15% of nephrons are juxtamedullary nephrons

b. These loops of Henle have extensive thin segments & are

involved in the production of concentrated urine.

F. Blood flow in the renal circulation is subject to high resistance in the

afferent and efferent arterioles.

1. Each glomerulus is fed by an _______________________

arteriole & drained by an ________________________ arteriole

2. Blood pressure in the glomerulus is high because:

a. Arterioles are high-resistance vessels

1) Afferent arterioles have larger diameters than

efferent arterioles

2) Fluids and solutes are forced out of the blood

throughout the entire length of the glomerulus due to

this high blood pressure

b. Peritubular beds are low-pressure, porous capillaries

adapted for absorption that:

1) Arise from efferent arterioles,

2) Cling to adjacent renal tubules, and

3) Empty into the renal venous system

3. Vascular Resistance in Microcirculation

a. Afferent and efferent arterioles offer high resistance to

blood flow

b. Blood pressure declines from 95mm Hg in renal arteries

to 8 mm Hg in renal veins

c. Resistance in afferent arterioles:

1) Protects glomeruli from fluctuations in systemic

blood pressure

d. Resistance in efferent arterioles:

1) Reinforces high glomerular pressure

2) Reduces hydrostatic pressure in peritubular

capillaries

4. The _______________________________________________

(JGA) is a structural arrangement between the afferent arteriole

and the distal convoluted tubule that forms juxtaglomerular cells

and macula densa cells.

a. Arteriole walls have ____________________________

(JG) cells

1) JG cells are enlarged, smooth muscle cells

2) JG cells have secretory granules containing rennin

3) JG cells act as mechanoreceptors

b. ____________________________

1) Tall, closely packed distal tubule cells that lie

adjacent to JG cells

2) Function as chemoreceptors or osmoreceptors

c. ____________________________: have phagocytic and

contractile properties

1) Influence capillary filtration

d. _____________________________________________

1) Filter that lies between the blood and the interior of

the glomerular capsule

2) It is composed of three layers

a) Fenestrated endothelium of the glomerular

capillaries

b) Visceral membrane of the glomerular

capsule (podocytes)

c) Basement membrane composed of fused

basal laminae of the other layers

e. The JGA helps regulate blood pressure and the rate of

blood filtration by the kidneys.

G. The filtrate contains all plasma components except protein

1. The filtrate loses water, nutrients, and essential ions to become

urine

2. The urine contains metabolic wastes and unneeded substances

IV. RENAL PHYSIOLOGY: Mechanisms of Urine Formation

A. Nephrons and collecting ducts perform three basic processes while

producing urine: glomerular filtration, tubular secretion, and tubular

reabsorption.

B. Step 1: ________________________________________________

1. Glomerular filtration is a passive, nonselective process in which

hydrostatic pressure forces fluids through the glomerular

membrane.

a. The fluid that enters the capsular space is termed

glomerular filtrate.

2. The Filtration Membrane

a. The filtering unit of a nephron is the endothelial-capsular

membrane.

1) It consists of the glomerular endothelium,

glomerular basement membrane, and slit membranes

between pedicels of podocytes.

b. The principle of filtration - to force fluids and solutes

through a membrane by pressure - is the same in glomerular

capillaries as in capillaries elsewhere in the body.

c. The three features of the renal corpuscle that enhance its

filtering capacity include…

1) the large surface area across which filtration can

occur

2) the thin and porus nature of the filtration

membrane

3) the high level of glomerular capillary blood

pressure.

3. Net Filtration Pressure = ____________________________

a. Glomerular filtration depends on three main pressures,

one that promotes and two that oppose filtration.

1) Filtration of blood is promoted by glomerular blood

hydrostatic pressure (BGHP) = 55 mmHg

2) It is opposed by capsular hydrostatic pressure

(CHP) – pressure exerted by fluids in the glomerular

capsule = 15 mmHg

3) It is opposed by blood colloid osmotic pressure of

glomerular blood (BCOP) = 30 mmHg

4. Glomerular Filtration Rate (GFR)

a. The amount of filtrate formed by both kidneys per minute;

1) in a normal adult, it is about 125 ml/minute. This

amounts to ____________________________.

b. GFR is directly related to the pressures that determine

net filtration pressure.

c. Surprisingly, when system blood pressure rises above the

normal resting level, net filtration pressure and GFR increase

very little.

5. Regulation of GFR

a. Maintenance of a relatively constant glomerular filtration

rate is important because reabsorption of water and solutes

depends on how quickly filtrate flows through the tubules.

1) If GFR is too high, needed substances cannot be

reabsorbed quickly enough & are lost in the urine.

2) If GFR is too low, everything is reabsorbed,

including wastes that are normally disposed of.

b. The mechanisms that regulate GFR adjust blood flow into

and out of the glomerulus and alter the glomerular capillary

surface area available for filtration.

1) The three principal mechanisms that control GFR

are intrinsic renal autoregulation, extrinsic neural

regulation, and extrinsic hormonal regulation.

c. ____________________________ of GFR – an intrinsic

control of GFR

1) Under normal conditions, renal autoregulation

maintains a nearly constant GFR.

a) Consists of the myogenic mechanism and

tubuloglomerular feedback.

2) The ____________________________ occurs

because stretching causes contraction of smooth

muscle cells in the wall of the afferent arteriole.

a) Responds to changes in pressure in the

renal blood vessels

3) _______________________________________

occurs as the macula densa provides feedback to the

glomerulus.

a) Senses changes in the juxtaglomerular

apparatus

b) Responds to the rate of filtrate flow in the

tubules

d. ____________________________ mechanisms –

regulation through the ANS.

1) When the sympathetic NS is at rest, renal blood

vessels are maximally dilated

a) Autoregulation mechanisms prevail

2) Under stress, Norepinephrine is released from the

sympathetic NS & epinephrine is released by the

adrenal medulla

a) Afferent arterioles constrict & filtration is

inhibited

b) The sympathetic NS also stimulates the

renin-angiotensin mechanism

e. ____________________________ regulation

1) The JG cells release ______________________.

This is triggered by…

a) reduced stretch of the granular JG cells

b) stimulation of the JG cells by activated

macula densa cells

c) direct stimulation of JG cells by renal nerves

2) Renin acts on angiotensinogen to release

angiotensin I

3) Angiotensin I is converted into ________________

a) Causes an increase in systemic blood

pressure & an increase in blood volume by

increasing Na⁺ reabsorption

4) The ____________________________ system

stabilizes systemic blood pressure & extracellular fluid

volume. Beyond increasing Na⁺ reabsorption, it…

a) stimulates the hypothalamus to release

ADH

b) activates the hypothalamic thirst center.

C. Step 2: ________________________________________________

1. Tubular reabsorption begins as soon as the filtrate enters the

proximal convoluted tubule, and involves near total reabsorption of

organic nutrients, and the hormonally regulated reabsorption of

water and ions.

a. The normal rate of glomerular filtration is so high that the

volume of fluid entering the proximal convoluted tubule in

half an hour is greater than the total plasma volume.

b. Reabsorption returns most of the filtered water and many

of the filtered solutes to the bloodstream using both active

and passive transport processes.

2. Reabsorption Routes

a. A substance being reabsorbed can move between

adjacent tubule cells or through an individual tubule cell

before entering a ____________________________.

b. Fluid leakage between cells is known as

_______________________________________________ .

c. In ___________________________________________,

a substance passes from the fluid in the tubule lumen

through the apical membrane of a tubule cell, across the

cytosol, and out into interstitial fluid through the basolateral

membrane.

3. Transport Mechanisms

a. Solute reabsorption drives water reabsorption.

1) The mechanisms that accomplish Na⁺

reabsorption in each portion of the renal tubule and

collecting duct recover not only filtered Na⁺ but also

other electrolytes, nutrients, and water.

b. Transport across membranes can be either active or

passive.

1) In primary active transport the energy derived from

ATP is used to “pump” a substance across a

membrane.

2) Passive tubular reabsorption is the passive

reabsorption of negatively charged ions that travel

along an electrical gradient created by the active

reabsorption of Na⁺.

c. Reabsorption of Water

1) The mechanism for water reabsorption by the

renal tubule and collecting duct is _______________.

2) About 90% of the filtered water reabsorbed by the

kidneys occurs together with the reabsorption of

solutes such as Na⁺, Cl^-, and glucose.

a) The most abundant cation of the filtrate is

Na⁺, and reabsorption of Na⁺ is always active.

b) ____________________________ water

reabsorption occurs in water-permeable

regions of the tubules in response to the

osmotic gradients created by active transport

of Na⁺.

3) Reabsorption of the final water, _______________

reabsorption, is based on need and occurs in the

collecting ducts and is regulated by ADH.

d. Transport Maximum (T_m)

1) Each type of symporter has an upper limit on how

fast it can work, called the transport maximum (T_m).

a) When the carriers (symporters) are

saturated, excess of that substance is

excreted.

2) When the blood concentration of glucose is above

200 mg/mL, the renal symporters cannot work fast

enough to reabsorb all the glucose that enters the

glomerular filtrate. As a result, some glucose remains

in the urine, a condition called glucosuria.

e. Substances that are not reabsorbed or incompletely

reabsorbed remain in the filtrate. This occurs due to…

1) a lack of carrier molecules

2) lipid insolubility

3) large size (urea, creatinine, and uric acid).

f. Different areas of the tubules have different absorptive

capabilities.

1) The proximal convoluted tubule is most active in

reabsorption, with most selective reabsorption

occurring there.

2) The descending limb of the loop of Henle is

permeable to water, while the ascending limb is

impermeable to water but permeable to electrolytes.

3) The distal convoluted tubule and collecting duct

have Na⁺ and water permeability regulated by the

hormones aldosterone, antidiuretic hormone, and

atrial natriuretic peptide.

D. Step 3: ________________________________________________

1. Tubular secretion is the transfer of materials from the blood and

tubule cells into tubular fluid.

a. Tubular secretion helps control blood pH

b. Disposes of unwanted solutes

c. Eliminates solutes that were reabsorbed

d. Rids the body of excess K⁺

2. Secretion in the Proximal Convoluted Tubule (PCT)

a. Tubular secretion is most active in the proximal

convoluted tubule

b. Secretion of NH₃ and NH₄⁺

1) The deamination of the amino acid glutamine by

PCT cells generates both NH₃ and new HCO₃^-.

2) At the pH inside tubule cells, most NH₃ quickly

binds to H⁺ and becomes NH₄⁺.

3) NH₄⁺ can substitute for H⁺ aboard Na⁺/H⁺

antiporters and be secreted into tubular fluid.

4) Na⁺-HCO₃^- symporters provide a route for

reabsorbed Na⁺ and newly formed HCO₃^- to enter the

bloodstream.

3. Secretion in the Distal Convoluted Tubule (DCT)

a. K⁺ secretion by principal cells occurs by increasing the

activity of existing sodium pumps and leakage channels and

stimulating the synthesis of new pumps and channels.

b. The amount of K⁺ secreted by principal cells is increased

by…

1) high K⁺ level in plasma

2) increased aldosterone

3) increased delivery of Na⁺

4. Secretion in the Collecting Duct

a. The secretion of K⁺ through K⁺ leakage channels in the

principal cells is the main source of K⁺ that is excreted in

urine.

V. RENAL PHYSIOLOGY: Regulation of pH, Urine Concentration, and

Urine Volume

A. Regulation of Blood pH

1. Occurs through Secretion of H⁺ and Absorption of HCO₃^- by

____________________________ of the ___________________

2. The apical surfaces of some intercalated cells include …

a. proton pumps (H⁺ ATPases) that secrete H⁺ into the

tubular fluid

b. Cl^-/HCO₃^- antiporters in their basolateral membranes to

reabsorb HCO₃^-.

3. Other intercalated cells have proton pumps in their basolateral

membranes and Cl^-/HCO₃^- antiporters in their apical membranes.

4. These two types of cells help maintain body fluid pH by …

a. excreting excess H⁺ when the pH is too low

b. excreting excess HCO₃^- when the pH is too high

B. Hormonal Regulation of Tubular Reabsorption and Secretion: Four

hormones affect the extent of Na⁺, Cl^-, and H₂O reabsorption and K⁺

secretion by the renal tubules.

1. The ____________________________ system

a. Angiotensin II increases blood volume and blood

pressure and is a major regulator of electrolyte reabsorption

and secretion

2. ____________________________

a. increases Na⁺ and water reabsorption by principal cells of

DCT

b. increases K⁺ secretion by principal cells of DCT

c. Aldosterone does this by increasing the activity of existing

sodium pumps and leakage channels and stimulating the

synthesis of new pumps and channels.

3. ____________________________ (ADH) regulates facultative

water reabsorption by increasing the water permeability of principal

cells.

4. _________________________________________________

can inhibit both water and electrolyte reabsorption.

C. Regulation of Urine Concentration

1. One of the critical functions of the kidney is to keep the solute

load of body fluids constant by regulating urine concentration and

volume.

2. The rate at which water is lost from the body depends mainly on

____________________________

a. ADH controls water permeability of principal cells in the

collecting duct (and in the last portion of the distal

convoluted tubule).

b. Low ADH levels cause the kidneys to produce ________

urine and excrete excess water;

1) in other words, renal tubules absorb more solutes

than water.

c. High ADH levels cause the kidneys to secrete

__________________________ urine and conserve water;

1) ADH makes the collecting ducts permeable to

water and increases water uptake from the urine.

2) a large volume of water is reabsorbed from the

tubular fluid into interstitial fluid, and the solute

concentration of urine is high.

3) In the presence of ADH, ______ of the water in the

filtrate is reabsorbed

3. Production of Concentrated Urine involves…

a. Ascending limb cells of the loop of Henle establishing the

osmotic gradient in the renal medulla

b. Collecting ducts reabsorbing more water and urea

c. Urea recycling causing a build up of urea in the renal

medulla.

d. The countercurrent mechanism also contributes to the

excretion of concentrated urine.

4. The ____________________________

a. The countercurrent mechanism involves interaction

between…

1) filtrate flow through the loops of Henle (the

countercurrent multiplier) of juxtamedullary

nephrons.

2) the flow of blood through the vasa recta (the

countercurrent exchanger).

b. Since water is freely absorbed from the descending limb

of the loop of Henle, filtrate concentration increases and

water is reabsorbed.

c. The ascending limb is permeable to solutes, but not to

water.

d. In the collecting duct, urea diffuses into the deep

medullary tissue, contributing to the increasing osmotic

gradient encountered by filtrate as it moves through the loop.

e. The vasa recta aids in maintaining the steep

concentration gradient of the medulla by cycling salt into the

blood as it descends into the medulla, and then out again as

it ascends toward the cortex.

5. Since tubular filtrate is diluted as it travels through the ascending

limb of the loop of Henle, production of a dilute urine is

accomplished by simply allowing filtrate to pass on to the renal

pelvis.

6. ____________________________ are drugs that increase urine

flow rate. They work by a variety of mechanisms.

VI. URINE: Physical & Chemical Characteristics

A. Physical Characteristics

1. Color – fresh urine is usually clear & pale to deep yellow in

color.

a. The color is due to ____________________________ –

a pigment resulting from the body’s destruction of

hemoglobin.

b. The more concentrated the urine, the deeper the yellow

color.

2. Odor – fresh urine is slightly aromatic

a. If allowed to stand, urine develops an ammonia odor due

to bacterial metabolism of its urea solutes.

3. pH – Urine is slightly acidic (____________________________)

a. The pH can vary from about 4.5–8.0 in response to

changes in metabolism or diet.

4. Specific gravity – Urine, due to solutes, is more dense than

water (1.0). Urine’s specific gravity is 1.001 to 1.035

5. Chemical composition – Urine is 95% water.

a. Solutes in urine, in order of decreasing concentration,

include…

1) urea;

2) sodium,

3) potassium,

4) phosphate,

5) sulfate ions;

6) creatinine; &

7) uric acid.

b. The largest solute fraction is devoted to removal of

nitrogenous wastes & includes urea, creatinine, and uric

acid.

VII. URINE STORAGE AND ELIMINATION

A. How does urine leave the kidneys?

1. Urine drains through papillary ducts into minor calyces

2. Minor calyces join to become major calyces

3. Major calyces unite to form the renal pelvis.

4. From the renal pelvis, urine drains into the ureters

5. The ureters empty into the urinary bladder

6. Urine finally leaves the body by way of the urethra.

B. Ureters

1. Each of the two ureters connects the renal pelvis of one kidney

to the urinary bladder.

2. The ureters transport urine from the renal pelvis to the urinary

bladder, primarily by ____________________________, but

hydrostatic pressure and

gravity also contribute.

3. The ureters are retroperitoneal

4. They consist of …

a. An inner mucosa continuous with the kidney pelvis & the

bladder.

b. A double-layered muscularis

c. A connective tissue adventitia covering the external

surface.

C. Urinary Bladder

1. The urinary bladder is a hollow muscular organ situated in the

pelvic cavity posterior to the pubic symphysis.

a. It expands as urine is produced by the kidneys to allow

storage until voiding is convenient.

2. Anatomy and Histology of the Urinary Bladder

a. In the floor of the urinary bladder is a small, smooth

triangular area, the ____________________________.

1) The ureters enter the urinary bladder near two

posterior points in the triangle;

2) The urethra drains the urinary bladder from the

anterior point of the triangle.

b. Histologically, the urinary bladder wall consists of three

layers. From deep to superficial, they are…

1) a mucosa (with rugae) – highly folded to allow

distension of the bladder without a large increase in

internal pressure

2) a lamina propria

3) a muscularis (detrusor muscle)

c. The bladder is surrounded by an outer adventitia (a

serous coat).

d. In the area around the opening to the urethra, the circular

fibers of the muscularis form the internal urethral

sphincter.

e. Below the internal sphincter is the external urethral

sphincter, which is composed of skeletal (voluntary)

muscle.

D. Micturition Reflex

1. Micturition, (a.k.a. urination or voiding) is the act of emptying the

bladder.

2. When the volume of urine in the bladder reaches a certain

amount (usually ____________________________), stretch

receptors in the urinary bladder wall transmit impulses that initiate a

spinal micturition reflex. This regulates urination in incontinent

adults and young children.

a. Newborns void most frequently, because the bladder is

small and the kidneys cannot concentrate urine until two

months of age.

b. From two months of age until adolescence, urine output

increases until the adult output volume is achieved.

c. Voluntary control of the urinary sphincters depends on

nervous system development, and complete control of the

bladder even during the night does not usually occur before

4 years of age.

3. Older children and adults may also initiate or inhibit micturition

voluntarily.

a. As urine accumulates, distention of the bladder activates

stretch receptors, which trigger spinal reflexes, resulting in

storage of urine.

b. Voluntary initiation of voiding reflexes results in activation

of the micturition center of the pons.

1) This signals parasympathetic motor neurons that

stimulate …

a) contraction of the detrusor muscle

b) relaxation of the urinary sphincters.

4. In old age, kidney function declines due to shrinking of the

kidney as nephrons decrease in size and number; the bladder also

shrinks and loses tone, resulting in frequent urination.

5. A lack of voluntary control over micturition is referred to as

____________________________.

6. Failure to void urine completely or normally is termed

____________________________.

E. Urethra

1. The urethra is a tube leading from the floor of the urinary

bladder to the exterior.

2. Histologically, the wall of the urethra consists of either three

coats in females or two coats in males.

3. The function of the urethra is to discharge urine from the body.

a. The male urethra also serves as the duct for ejaculation

of semen (reproductive fluid).

VIII. DISORDERS: HOMEOSTATIC IMBALANCES

A. Crystals of salts present in urine can precipitate and solidify into renal

calculi or ____________________________.

1. They may block the ureter and can sometimes be removed by

shock wave lithotripsy.

B. The term urinary tract infection (UTI) is used to describe either an

infection of a part of the urinary system or the presence of large numbers

of microbes in urine.

C. GLOMERULAR DISEASES

1. Glomerulonephritis (Bright’s disease) is an inflammation of

the glomeruli of the kidney.

a. One of the most common causes is an allergic reaction to

the toxins given off by steptococcal bacteria that have

recently infected another part of the body, especially the

throat.

b. The glomeruli may be permanently damaged, leading to

acute or chronic renal failure.

2. Chronic renal failure refers to a progressive and generally

irreversible decline in glomerular filtration rate that may result from

chronic glomerulonephritis, pyelonephritis, polycystic disease, or

traumatic loss of kidney tissue.

3. Polycystic kidney disease is one of the most common

inherited disorders.

a. In infants it results in death at birth or shortly thereafter.

b. In adults, it accounts for 6-12% of kidney

transplantations.

c. In this disorder, the kidney tubules become riddled with

hundreds or thousands of cysts

1) inappropriate apoptosis of cells in noncystic tubules

leads to progressive impairment of renal function and

eventually to renal failure.