The Urinary System
Chapter 26
Functions of the Urinary System
1. Removal of metabolic wastes (especially
nitrogenous wastes e.g. urea & uric acid).
2. Water balance (and therefore blood pressure).
3. Control of electrolyte balance.
4. Control of pH.
5. Removal of toxins.
Anatomy of the Urinary System
• The Kidneys: the functional heart of the urinary
system.
• The Ureters: pipeline from the kidneys to the
bladder.
• The Urinary Bladder: holding tank of urine.
• The Urethra: avenue of relief
(word of the day “micturition” = voiding the bladder)
Urinary
System
Components
The Kidneys
• Location: retroperitoneal against the dorsal wall
of the abdominal cavity. The right kidney is
slightly lower than the left.
• Size & weight: approximately 150 grams (about
5 ounces) each and 12 cm x 10 cm x 4 cm.
• Shaped like a bean (or are beans shaped like
kidneys?)
The
Position of
the Kidneys
Figure 26–2
Gross
Anatomy
of
the
Urinary
System
Figure 26–3
The Structure of the Kidney
Figure 26–4
Blood Supply to Kidneys
• Kidneys receive 20–25% of total cardiac
output
• 1200 - 1500 ml of blood flows through
kidneys each minute
• Kidney receives blood through renal artery
Blood
Supply
to the
Kidneys
Figure 26–5
A slice of kidney
Cortex
Glomeruli
Capsule
Medulla
The Nephron: functional unit of the kidney
Interlobular artery
Afferent Arteriole
Glomeruli
Functional
Anatomy
of
Nephron
and
Collecting
System
Figure 26–6
Renal Corpuscle
The
Nephron
and
Collecting
System
Table 26–1
Filtrate
& Urine
flow
Cortical and Juxtamedullary Nephrons
Figure 26–7
The renal corpuscle and the Juxtaglomerular apparatus
The renal filtration membrane:
Podocytes and fenestrated capillaries
Filtration
slits
Pedicles
The filtration membrane
An
Overview
of Urine
Formation
Figure 26–9 (Navigator)
Blood pressure drops due to peripheral resistance
Filtration pressures: NFP must be positive for U2P
GFR
GFR is “Glomerular Filtration Rate”.
• It is directly proportional to NFP.
• It is a measurement of FLOW in milliliters
per minute (ml/min).
• If NFP drops more than 15% below 10
mmHg, GFR goes to 0.
• If NFP goes up less than 30% above
normal, the kidneys can handle it without
major compensatory mechanisms kicking
in.
Creatinine Clearance Test
• Is used to estimate GFR
• A more accurate GFR test uses inulin:
– which is not metabolized
Response to
Reduction in
GFR
Figure 26–11
Tubular reabsorption
•Reabsorption of filtered solutes occurs in
the Proximal Convoluted Tubules.
•Most solutes are reabsorbed by
secondary active transport with Na+. Does
this look familiar?
4 Types of
Carrier-Mediated Transport
1.
2.
3.
4.
Facilitated diffusion
Active transport
Cotransport
Countertransport
The Transport Maximum (Tm)
• If nutrient concentrations rise in tubular
fluid:
– reabsorption rates increase until carrier
proteins are saturated
• Concentration higher than transport
maximum:
– exceeds reabsorptive abilities of nephron
– some material will remain in the tubular fluid
and appear in the urine
• Determines the renal threshold
Transport
Activities
at the PCT
Figure 26–12 (Navigator)
Reabsorption
of sodium
Reabsorption
of glucose
Reabsorption
of electrolytes
and water
Reabsorption of
ions in the Loop
of Henle
Reabsorption
• 60–70% of filtrate volume produced at
glomerulus:
– is reabsorbed before tubular fluid reaches
loop of Henle
The Loop of Henle
• Reabsorbs about 1/2 of water, and 2/3 of
sodium and chloride ions remaining in
tubular fluid by the process of
countercurrent exchange
Countercurrent Multiplication
• Is exchange that occurs between 2 parallel
segments of loop of Henle:
– the thin, descending limb
– the thick, ascending limb
Countercurrent
• Refers to exchange between tubular fluids
moving in opposite directions:
– fluid in descending limb flows toward renal
pelvis
– fluid in ascending limb flows toward cortex
Multiplication
• Refers to effect of exchange:
– increases as movement of fluid continues
Parallel Segments
of Loop of Henle
• Are very close together, separated only by
peritubular fluid
• Have very different permeability
characteristics
Formation
of dilute
urine
Countercurrent Multiplication
and Concentration of Urine
Figure 26–13a (Navigator)
Countercurrent Multiplication and
Concentration of Urine
Figure 26–13b, c
—
Na —K /2 Cl Transporter
+
+
• Each cycle of pump carries ions into
tubular cell:
– 1 sodium ion
– 1 potassium ion
– 2 chloride ions
Tubular Fluid at DCT
• Arrives with osmotic concentration of 100
mOsm/L:
– 1/3 concentration of peritubular fluid of renal
cortex
• Rate of ion transport across thick
ascending limb:
– is proportional to ion’s concentration in tubular
fluid
Regional Differences
—
• More Na and Cl are pumped into
medulla:
+
– at start of thick ascending limb
– than near cortex
• Regional difference in ion transport rate:
– causes concentration gradient within medulla
Normal Maximum
Solute Concentration
• Of peritubular fluid near turn of loop of
Henle:
– 1200 mOsm/L
The Concentration
Gradient of the Medulla
—
• 2/3 (750 mOsm/L) from Na and Cl :
+
– pumped out of ascending limb
• Remainder from urea
Urea and the Concentration
Gradient (1 of 2)
• Thick ascending limb of loop of Henle,
DCT, and collecting ducts:
– are impermeable to urea
• As water is reabsorbed:
– concentration of urea rises
Urea and the Concentration
Gradient (2 of 2)
• Tubular fluid reaching papillary duct:
– contains 450 mOsm/L urea
• Papillary ducts are permeable to urea:
– concentration in medulla averages 450
mOsm/L
2 Benefits of
Countercurrent Multiplication
1. Efficiently reabsorbs solutes and water:
– before tubular fluid reaches DCT and
collecting system
2. Establishes concentration gradient:
– that permits passive reabsorption of water
from tubular fluid in collecting system
In the
collecting
duct
Reabsorption
in the Distal
Convoluted
Tubule &
Collecting
Duct
Differences
between
Solute
Composition
in Urine and
Plasma
Table 26–2
Summary of
tubular
reabsorption/
excretion
Renal
autoregulation
Compensatory
Mechanisms
to maintain
GFR
A
Pyelogram
Figure 26–17
After the
Kidneys:
the
bladder
&
urethra
Male
Figure 26–18a
Female
Figure 26–18c
The
Human
Bladder:
It can hold
a maximum
of 800 –
1000 ml!
Histology
Physical characteristics of Urine
• Color - Clear to deep yellow, almost rusty, depending on
concentration.
• Odor - Fresh urine is slightly aromatic, stale urine smells
like downtown Tacoma on a Sunday morning.
• pH – range of 4.5 – 8 depending on diet. High protein
leads to low pH (acid ash diet), vegetarian (alkaline ash)
diet leads to high pH. Heavy vomiting and bacterial
infection can also lead to alkaline urine.
• Specific gravity – Normal range is 1.001 – 1.030.
Distilled water has a s.g. of 1.000. Anything solutes
cause the specific gravity of a liquid to go up.
Concentrated urine has a higher s.g. than dilute urine.
General Characteristics of Normal Urine
Table 26–5
Typical
Values
Obtained
from
Standard
Urinalysis
Table 26–6
Composition of Urine
Normal
Abnormal
Urea
Glucose
“glycosuria”
Uric acid
Proteins
“proteinuria” or
Creatinine
“albuminuria”
Na
Ketones
“ketonuria”
K
Hemoglobin “hemoglobinuria”
Phosphates
Erythrocytes “hematuria”
Sulfates
Bile pigments “bilirubinura”
Bicarbonate
Leukocytes “pyruia”
Ca
Abnormally low output = oliguria
Mg
No output = anuria
Abnormally high output = polyuria
Diuresis = increased urine output
Diuretic = substance that leads to diuresis
A Summary
of Renal
Function
Figure 26–16a
A
Summary
of Renal
Function
Figure 26–16b
The Micturition Reflex
Figure 26–20 (Navigator)
Incontinence
• Is the inability to control urination
voluntarily
• May be caused by trauma to internal or
external urethral sphincter
Age-Related Changes
in Urinary System
•
•
•
•
Decline in number of functional nephrons
Reduction in GFR
Reduced sensitivity to ADH
Problems with micturition reflex
Stages of Renal Disease
• Stage 1: signs of kidney damage w/ GFR ≥ 90.
• Stage 2: signs of kidney damage w/ GFR 60 –
89.
• Stage 3: GFR 30 – 59.
• Stage 4: GFR 15 – 29.
• Stage 5: < 15
Renal Clearance
The volume of plasma that is cleared of a particular
substance in a given time (usually one minute).
RC = UV/P
U = concentration of the substance in urine (mg/ml)
V = flow rate of formation (ml/min)
P = concentration of the substance in the plasma
(mg/ml)
High renal clearance values means that the substance is being
effectively cleared, low values means that more is being
reabsorbed. For some solutes low is good (glucose should be 0).
For others, high RC would be expected (creatinine should be
complete, urea should be about 80%).
The Excretory System
• Includes all systems with excretory
functions that affect body fluids
composition:
– urinary system
– integumentary system
– respiratory system
– digestive system
Kidney stones
Renal Calculi
“Well Mr.
Osborne, I
don’t think
that it’s kidney
stone after all”