Download CHAPTER 20: URINARY SYSTEM

UNIT 5 - CHAPTER 20: URINARY SYSTEM
LEARNING OUTCOMES:
20.1
Introduction
1.
20.2
20.3
20.4
20.5
Name the organs of the urinary system and list their general functions.
Kidneys
2.
Describe the location of the kidneys and the structure of a kidney.
3.
List the functions of the kidneys.
4.
Trace the pathway of blood flow through the major vessels within a kidney.
5.
Describe a nephron and explain the function of its major parts.
Urine Formation
6.
Explain how glomerular filtrate is produced and describe its composition.
7.
Explain how various factors affect the rate of glomerular filtration and identify
ways that this rate is regulated.
8.
Explain tubular reabsorption, and its role in urine formation.
9.
Identify the changes in the osmotic concentration of the glomerular filtrate as it
passes through the renal tubule.
10.
Explain tubular secretion, and its role in urine formation.
11.
Identify the characteristics of a countercurrent mechanism, and explain its role in
concentrating the urine.
12.
Explain how the final composition of urine contributes to homeostasis.
Elimination of Urine
13.
Describe the structures of the ureters, urinary bladder, and urethra.
14.
Explain how micturition occurs, and how it is controlled.
Life-Span Changes
15.
Describe how the components of the urinary system change with age.
20-1
UNIT 5 - CHAPTER 20: URINARY SYSTEM
20.1
INTRODUCTION
The major function of the urinary system is to remove metabolic wastes from blood,
and direct them out of the body. In doing so, blood homeostasis is maintained, as well.
The organs of the urinary system include the kidneys, ureters, urinary bladder, and
urethra.
See Fig. 20.1 and Fig 20.2, page 773.
20.2
KIDNEYS
A.
B.
The term renal refers to the kidney.
Location of Kidneys:
Fig 20.1, page 773 - Fig 20.3, page 774.
1.
2.
3.
C.
high on posterior abdominal wall
retroperitoneal
right kidney lies just below left. Why?
Kidney Structure:
Fig 20.4a, page 775.
D.
1.
2.
3.
4.
5.
6.
Renal capsule
Renal cortex
Renal medulla
Renal pyramids
Ureter
Renal pelvis
7.
8.
Major calyces
Minor calyces
=
=
=
=
=
=
tough fibrous shell around kidney.
outer portion of kidney.
inner portion of kidney.
cone shaped masses of tissue in renal medullae.
tube leading from away from kidney.
superior end of ureter which is expanded to
form a funnel shape.
= divisions of renal pelvis (2-3 tubes).
= divisions of major calyces.
Functions of the Kidneys:
1.
to remove metabolic wastes from blood and excrete them to outside in
urine.
2.
maintenance of blood homeostasis:
a.
regulation of RBC formation (hormone erythropoietin)
b.
blood pressure (enzyme renin)
c.
blood volume (hormone ADH)
d.
blood composition
e.
blood pH
20-2
UNIT 5 - CHAPTER 20: URINARY SYSTEM
20.2
KIDNEYS
E.
Renal Blood Vessels:
1.
Macroscopic = Fig 20.5, page 775 and Fig 20.6, page 777.
2.
Microscopic = Fig 20.10, page 780, and Fig 20.14, page 783.
3.
Blood Flow Summary = Fig 20.15, page 20.15.
Aorta
↓
Renal Artery (to each kidney)
↓
Interlobar artery (between each pyramid)
↓
Arcuate artery (between medulla & cortex)
↓
Cortical Radiate artery (within cortex)
↓
Afferent arteriole (leading to glomerulus)
↓
Glomerular capillaries (site of filtration)
↓
Efferent arteriole (leading away from glomerulus)
↓
Peritubular capillaries/vasa recta (around renal tubule)
↓
Cortical Radiate vein (within cortex)
↓
Arcuate vein (between cortex and medulla)
↓
Interlobar vein (between pyramids)
↓
Renal vein (from each kidney)
↓
Inferior vena cava
20-3
UNIT 5 - CHAPTER 20: URINARY SYSTEM
20.2
KIDNEYS
F.
Nephrons are the functional unit of the kidney. Fig 20.4c, page 775.
1.
Structure of a Nephron:
a.
A nephron is composed of a renal corpuscle and a renal
tubule.
o
Renal Corpuscle = glomerulus (specialized capillaries
which serve as filtration unit) within glomerular
(Bowman's) capsule.
o
Renal Tubule =
1.
proximal convoluted tubule
2.
descending limb of nephron loop (loop of Henle)
3.
ascending limb of nephron loop (loop of Henle)
4.
distal convoluted tubule
(5.
collecting duct)
Each collecting duct empties into a minor calyx,
which leads to a major calyx and into the renal
pelvis.
Refer to Fig 20.7 page 778, an SEM of glomeruli within the renal tubules,
and Fig 20.11, page 780, illustrating light micrographs of the kidney.
G.
Juxtaglomerular Apparatus (JGA) = point of contact between the afferent
arteriole and ascending nephron loop (just before distal convoluted
tubule). See Fig. 20.12, page 781.
1.
2.
H.
G.
Macula Densa = cells in ascending nephron loop in contact with afferent
arteriole.
Juxtaglomerular cells = specialized smooth muscle cells in afferent
arteriole.
a.
The JGA is very important in regulating renin secretion (and in
turn glomerular filtration rate; see below)
Cortical and Juxtamedullary Nephrons
1.
80% of nephrons are found mostly in the cortex and are therefore termed
“cortical nephrons”.
2.
20% of nephrons have nephron loops that extend deep into the renal
medulla and are therefore termed “juxtamedullary nephrons”.
a.
Very important in the production of concentrated urine at times
of dehydration, as will be discussed later.
b.
See Figure 20.13 page 782.
Blood Supply of a Nephron:
See blood flow chart on page 3 of outline.
a.
Blood passing through 2 capillaries (glomerulus and peritubular capillary
system/vasa recta) as it moves through kidney is an example of a portal
system.
20-4
UNIT 5 - CHAPTER 20: URINARY SYSTEM
9.3
URINE FORMATION
A.
Introduction
1.
The nephrons function to remove wastes from blood (excretion) and
regulate water and electrolyte concentrations.
2.
Urine is the end-product of these functions.
3.
Urine formation involves three major steps including
a.
glomerular filtration
b.
tubular reabsorption
c.
tubular secretion
4.
Urine excreted = Glomerular Filtration + Tubular Secretion – Tubular
Reabsorption
See Figure 20.16b, page 784.
B.
Glomerular Filtration
Fig Fig 20.17, page 784.
1.
The fenestrated glomerular capillaries filter water and dissolved
materials (remember plasma components) from blood.
a.
This "filtrate" is collected in the glomerular (Bowman's) Capsule.
b.
Proteins are not filtered out of blood!
2.
Filtration Pressure
See Figure 20.18 page 786.
a.
3.
Filtration is due to a force, net filtration pressure.
1.
outward + 60 mmHg called glomerular hydrostatic
pressure (GHSP) inside glomerular capillaries.
2.
inward - 18 mmHg in Bowman’s capsule.
3.
inward – 32 mmHg colloid osmotic pressure in glomerular
capillaries.
4.
Net Filtration Pressure = 10 mmHg outward
(Glomerular) Filtration Rate (GFR)
a.
Kidneys produce =125 ml fluid per minute (glomerular filtration
rate/GFR)
1.
Most of this is reabsorbed back into the blood through the
proximal convoluted tubule.
2.
See Fig 20.19, page 787.
20-5
UNIT 5 - CHAPTER 20: URINARY SYSTEM
9.3
URINE FORMATION
B.
Glomerular Filtration
4.
Control of (Glomerular) Filtration Rate
a.
Normal GFR is approximately 125ml/minute (70 kg adult male,
both kidneys) primarily due to glomerular hydrostatic pressure
(GHSP).
GFR remains relatively constant through two
mechanisms, which include:
1. Autoregulation by vasomotor center in medulla
2. Renin-angiotensin system
b.
Autoregulation (AR) by Vasomotor Center
1.
The vasomotor center in the medulla regulates arteriole
smooth muscle allowing for AR.
2.
Under normal conditions, the parasympathetic autonomic
nervous system (ANS) maintains AR through
vasoconstriction of the afferent arteriole to decrease GFR
when elevated or vasoconstriction of the efferent arteriole
to increase GFR when low.
3.
AR can be overridden by the sympathetic ANS during
significant volume loss or gain.
○
A large blood volume loss, which markedly
decreases blood pressure causes vasoconstriction of
afferent arterioles, decreasing GFR, which decreases
urine output to conserve water.
○
A large blood volume gain, which markedly
increases blood pressure causes vasodilation of
afferent arterioles, increasing GFR, which increases
urine output to eliminate the excess water.
20-6
UNIT 5 - CHAPTER 20: URINARY SYSTEM
9.3
URINE FORMATION
B.
Glomerular Filtration
4.
Control of Glomerular Filtration Rate (GFR)
c.
Renin-Angiotensin System
1.
2.




See Fig 20.20, page 787.
A decrease in blood volume (BV) causes a decrease in
blood pressure (BP), which in turn decreases the net
filtration pressure (NFP) and GFR.
Receptors in the Juxtaglomerular Apparatus (JGA) detect
this decrease in two ways:
○
Baroreceptors in the JG cells of the afferent arteriole
(AA) detect a decrease in stretch and secrete the
enzyme renin.
○
Chemoreceptors in the macula densa cells in the
ascending nephron loop detect a decrease in the
levels of sodium (Na+), potassium (K+), and
chloride (Cl-), and further stimulate the JG cells of
the AA to secrete the enzyme renin.
a.
In blood, renin converts the plasma protein,
angiotensinogen to angiotensin I.
b.
Primarily in the lungs where endothelial
capillaries produce angiotensin converting
enzyme (ACE), ACE converts angiotensin I
to Angiotensin II.
e.
Angiotensin II targets four sites that work
together to maintain sodium balance,
water balance and blood pressure (which
directly affects GFR).
The efferent arterioles vasoconstrict, increasing NFP,
which directly increases GFR back to normal.
The adrenal cortex secretes the hormone aldosterone,
which targets the DCT, causing reabsorption of Na+ (and
H20). This increases BV, which increases BP, which
increases NFP, and restores GFR back to normal.
The posterior pituitary gland secretes the antidiuretic
hormone (ADH), which targets the DCT, causing
reabsorption H20. This increases BV, which increases BP,
which increases NFP, and restores GFR back to normal.
The hypothalamus triggers thirst, which increases fluid
intake. This increases BV, which increases BP, which
increases NFP, and restores GFR back to normal.
20-7
UNIT 5 - CHAPTER 20: URINARY SYSTEM
9.3
URINE FORMATION
C.
Tubular Reabsorption:
See Fig 20.21a, page 788.
D.
1.
Tubular reabsorption is the process by which substances are transported
out of the tubular fluid in the renal tubule (into interstitial fluid which then
diffuse) into blood in the peritubular capillaries.
2.
Most reabsorption occurs in the PCT which has microvilli (increasing
surface area) through the process of active transport.
a.
Water is reabsorbed through osmosis (see below)
3.
The peritubular capillary walls are especially permeable and they carry
blood that is under low pressure (blood that has passed through two
arterioles).
4.
Reabsorbed substances include:
a.
glucose (via active transport)
b.
amino acids (via active transport)
c.
water (osmosis, see below)
d.
ions (sodium [see below], chloride, phosphate, sulfate, potassium
via active transport)
(e.
small proteins by endocytosis)
f.
creatine, lactic acid. citric acid, uric acid, ascorbic acid, urea (via
active transport)
5.
Substances that remain in filtrate become concentrated as water is
reabsorbed.
Sodium and Water Reabsorption
1.
2.
See Fig 20.22, page 790.
Sodium ions are reabsorbed by active transport.
a.
Negatively charged ions accompany positively charged sodium
ions out of the filtrate.
Water is passively reabsorbed by osmosis, in response to active transport
reabsorbing sodium and other solutes.
20-8
UNIT 5 - CHAPTER 20: URINARY SYSTEM
9.3
URINE FORMATION
E.
Tubular Secretion: See Fig 20.21b, page 788 and Fig 20.23, page 791.
1.
2.
3.
4.
5.
F.
Regulation of Urine Concentration and Volume: See Fig 20.24- Fig 20.26,
pages 792-793.
1.
2.
3.
4.
5.
G.
Tubular secretion is the process by which substances are transported from
the blood plasma in the peritubular capillaries into the tubule (primarily
DCT).
Some substances are actively secreted.
a.
organic compounds (urea, trace amino acids)
b.
hydrogen ions
Potassium ions are secreted actively and passively in the DCT and
collecting duct.
Tubular secretion maintains ion concentrations in blood (i.e. if the blood is
high in K+, K+ will be secreted into urine).
Tubular secretion allows for secretion of metabolic wastes (see below)
The hormone anti-diuretic hormone (ADH) promotes the reabsorption of
water through the DCT and collecting ducts, preventing excessive amounts
of water from being lost in the urine. This negative feedback mechanism
prevents dehydration.
a.
See table 20.3, page 793.
Countercurrent Mechanism – juxtamedullary nephrons actively resorb
NaCl in the ascending loop causing hypertonic medullary interstitial fluid
Countercurrent Multiplier – descending loop loses water to hypertonic
medulla, further increasing tonicity of medulla
Vasa recta ensures NaCl stays in the medulla
ADH opens water channels (aquaporin) in DCT and CD that allows water
to be resorbed by osmosis, due to the osmotic pressure set up by the
juxtamedullary nephrons
Urea and Uric Acid Excretion
1.
Wastes are by-products of metabolism:
a.
urea from amino acid metabolism
○
plasma concentration reflects protein in diet
○
enters tubules through glomerular filtration
○
50% is passively reabsorbed in PCT.
○
50% is excreted in urine.
○
A countercurrent mechanism with urea helps reabsorb
water.
b.
uric acid from nucleotide metabolism
○
enters tubules through glomerular filtration
○
Most is reabsorbed by AT.
○
Some is secreted into urine.
20-9
UNIT 5 - CHAPTER 20: URINARY SYSTEM
9.3
URINE FORMATION
H.
Urine Composition
1.
95% water
2.
other 5% includes:
a.
urea
b.
uric acid
c.
trace amino acids
d.
electrolytes
e.
drugs
I.
Renal Clearance
1.
Rate at which a particular chemical is removed from the plasma
a.
Inulin Clearance Test
b.
Creatinine Clearance Test
c.
Paraminohippuric Acid Test
2.
Used to calculate GFR and efficiency by comparing blood and urine values
Use Table 20.1, page 785 to compare the levels of various substances present in
plasma, glomerular filtrate, and urine.
J.
Urine Formation Summary (Keyed at the end of this outline)
Major Step in Urine
Formation
Location in Nephron
Substances
Transported and
Mode of Transport
From where to
where?
(i.e. from blood to
glomerular filtrate)
20-10
UNIT 5 - CHAPTER 20: URINARY SYSTEM
20.4
ELIMINATION OF URINE
A.
Ureters are small tubes that carry urine from each kidney to the urinary bladder
through peristaltic movements.
1.
2.
3.
25 - 39 cm in length See Fig 20.1 and 20.2, page 773.
retroperitoneal
Three layers:
a.
Inner mucosa = transitional epithelium.
b.
Middle muscularis = inner circular layer of smooth muscle and
outer longitudinal layer (peristalsis).
c.
Outer serosa = fibrous CT.
See Fig 20.27, page 795.
B.
Urinary Bladder
See Fig 20.28, page 796 and Fig 20.29, page 797.
1.
Location:
within pelvic cavity
behind symphysis pubis
2.
Structure:
a.
b.
c.
d.
3.
C.
hollow, distensible, muscular organ
lined by transitional epithelium
detrusor muscle = 3 layers of smooth muscle
covered by fibrous CT
Function: storage of urine
The Urethra is a tube that carries urine from the urinary bladder to the outside.
See Fig 20.31, page 797 and Fig 20.32, page 798.
1.
2.
Length depends on sex:
a.
female = 4 cm
b.
male = 20 cm
Histology depends on sex:
a.
female = 3 layers
b.
males = 2 layers
20-11
UNIT 5 - CHAPTER 20: URINARY SYSTEM
20.4
ELIMINATION OF URINE
D.
Micturition = the process by which urine is expelled from urinary bladder to
outside.
1.
2.
3.
*
E.
Micturition reflex center is in sacral spinal cord.
Parasympathetics cause detrusor muscle to contract in response to stretch
of the urinary bladder.
External urethra sphincter (skeletal muscle) is last “doorway” to pass, and
therefore micturition can be (and usually is) inhibited until released by
conscious control.
See Table 20.5, page 799.
Starts at glomerulus where glomerular filtrate is collected in
Bowman's capsule
PCT
nephron loop
DCT
collecting duct (urine)
minor calyx
major calyx
renal pelvis
ureter
(peristalsis)
urinary bladder
(micturition)
urethra
outside
Use this flow chart as a review by adding key points where they belong (i.e. reabsorption,
reabsorption of water under influence of ADH, secretion, etc.)
20-12
UNIT 5 - CHAPTER 20: URINARY SYSTEM
20.5
LIFE-SPAN CHANGES
A.
As one ages, kidneys, ureters, and urethra changes occur, however nephrons are so
numerous that the following changes are essentially masked.
1.
2.
3.
4.
5.
B.
The kidneys become grainy and scarred.
GFR decreases significantly as glomeruli atrophy, fill with connective
tissue, or unwind.
Fat accumulates on the exterior of the renal tubules, making them
asymmetric.
a.
Reabsorption and secretion become slow or impaired.
b.
The rate of drug clearance decreases.
Cardiovascular changes lead to decreased rates through urinary system.
The kidney:
a.
slows in its response to changes.
b.
is less efficient at activating Vitamin D.
Finally, elasticity of urinary organs declines.
a.
Changes in urination patterns result.
OTHER INTERESTING TOPICS:
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
N.
A Medical Mystery. See Introduction on page 772, Aristolocholic Acid Nephropathy.
Clinical Application 20.1, page 776, Chronic Kidney Failure.
Continuous ambulatory peritoneal dialysis. See box on 776.
Development of specialized glomerular cells. See box on page 778.
Clinical Application 20.2, page 779, Glomerulonephritis.
Hemolytic Uremic Syndrome. See box on page 787.
Clinical Application 20.3, page 789, The Nephrotic Syndrome.
Glucosuria. See box on page 789.
Diuretics. See box on 794.
Gout. See box on page 794.
Infants and dehydration. See box on page 795.
Kidney Stones. See box on page 796.
Urinry Tract Infections (UTIs). See box on page 797.
Urinalysis: Clues to Health. See Clinical Application 20.4, page 799.
INNERCONNECTIONS OF THE URINARY SYSTEM. See page 801.
CHAPTER SUMMARY – see pages 802-804.
CHAPTER ASSESSMENTS – see pages 804-805.
INTEGRATIVE ASSESSMENTS/ CRITICAL THINKING – see page 805.
20-13
UNIT 5 - CHAPTER 20: URINARY SYSTEM
Urine Formation SUMMARY TABLE
Major Step in Urine
Formation
glomerular filtration
tubular reabsorption
tubular secretion
Location in Nephron
Glomerulus
primarily through
proximal convoluted
tubule (PCT)
primarily through
distal convoluted
tubule (DCT)
Substances
Transported and
Mode of Transport
All plasma
constituents except
proteins (i.e.
glucose, amino
acids, water, ions,
creatine, lactic acid,
urea, uric acid,
ascorbic acid, etc);
filtration due to Net
Filtration Pressure
(NFP)
glucose, amino
acids, water, ions,
creatine, lactic acid,
urea, uric acid,
ascorbic acid, etc.
excess ions, trace
amino acids, urea,
uric acid, drugs
from blood in
glomerulus to
“filtrate” in
Bowman’s capsule
from “filtrate” in
PCT to blood in
peritubular
capillaries
From where to
where?
(i.e. from blood to
glomerular filtrate)
Most by active
transport; water
passively by osmosis
from blood in
peritubular
capillaries to “urine”
in DCT
20-14