Chapter 14
The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Kidneys
• Overview of kidney functions
– Maintain H2O balance in the body
– Maintain proper osmolarity of body fluids, primarily through
regulating H2O balance
– Regulate the quantity and concentration of most ECF ions
– Maintain proper plasma volume
– Help maintain proper acid-base balance in the body
– Excreting (eliminating) the end products (wastes) of bodily
metabolism
– Excreting many foreign compounds
– Producing erythropoietin
– Producing renin
– Converting vitamin D into its active form
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Urinary System
Know organs and functions
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Nephron
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Functional unit of the kidney
Smallest unit that can perform all the
functions of the kidney
Approximately 1 million
nephrons/kidney
Each nephron has two components
– Vascular component
– Tubular component
Arrangement of nephrons within kidney
gives rise to two distinct regions
– Outer region
• Renal cortex (granular in
appearance)
– Inner region
• Renal medulla
• Made up of striated triangles
called renal pyramids
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Nephron
• Two types of nephrons
• Distinguished by location and length of their
structures
– Juxtamedullary
nephrons
– Cortical
nephrons
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Nephron
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Basic Renal Processes
• Glomerular filtration
– Tubular reabsorption
– Tubular secretion
Urine results from these three
processes.
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Glomerular Filtration
• Fluid filtered from the glomerulus into Bowman’s
capsule pass through three layers of the glomerular
membrane
– Glomerular capillary wall
• Single layer of endothelial cells
• More permeable to water and solutes than capillaries
elsewhere in the body
– Basement membrane
• Acellular gelatinous layer
• Composed of collagen and glycoproteins
– Inner layer of Bowman’s capsule
• Consists of podocytes that encircle the glomerulus tuft
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Glomerular Filtration and Layers of Glomerular Membrane
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Forces Involved in Glomerular Filtration
• Three physical forces
involved
– Glomerular capillary
blood pressure
– Plasma-colloid
osmotic pressure
– Bowman’s capsule
hydrostatic pressure
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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Glomerular Filtration Rate
Unregulated influences on the GFR
– Pathologically plasma-colloid osmotic pressure and Bowman’s capsule hydrostatic
pressure can change
• Plasma-colloid osmotic pressure
– Severely burned patient ↑ GFR
– Dehydrating diarrhea ↓ GFR
• Bowman’s capsule hydrostatic pressure
– Obstructions such as kidney stone of enlarged prostate can decrease filtration
and elevate capsular hydrostatic pressure
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Controlled adjustments in GFR
– Glomerular capillary blood pressure can be controlled to adjust GFR to suit the body’s
needs
– Two major control mechanisms
• Autoregulation (aimed at preventing spontaneous changes in GFR)
– Myogenic mechanism
– Tubuloglomerular feedback (TGF)
• Extrinsic sympathetic control (aimed at long-term regulation of arterial blood
pressure)
– Mediated by sympathetic nervous system input to afferent arterioles
– Baroreceptor reflex
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Adjustments of Afferent
Arteriole Caliber to Alter
The GFR
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Glomerular Filtration Rate
• Influenced by changes in filtration coefficient
– This coefficient is not constant but is subject to
physiological control.
– The coefficient depends on surface area and the
permeability of the glomerular membranes. Both can be
modified by contractile activity within the membrane.
• The kidneys receive 20 to 25 percent of the cardiac output
– The total blood flow through the kidneys average 1,140 ml
per minute. If the cardiac output is 5 liters, this figure is 22
% of the cardiac output.
– This kidneys need to receive this large blood flow to
monitor and control the ECF
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Basic Renal Processes
– Glomerular filtration
• Tubular reabsorption
– Tubular secretion
Urine results from these three
processes.
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Reabsorption
Transepithelial
Transport
•Passive reabsorption
No energy is required for the substance’s net movement
Occurs down electrochemical or osmotic gradients
Active reabsorption
Occurs if any one of the steps in transepithelial transport of a
substance requires energy
Movement occurs against electrochemical gradient
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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An active Na+ - K+ ATPase pump in
basolateral membrane is essential for
Na+ reabsorption
Of total energy spent by kidneys, 80%
is used for Na+ transport
Na+ is not reabsorbed in the
descending limb of the loop of Henle
Water follows reabsorbed sodium by
osmosis which has a main effect on
Na+ Reabsorption
Tubule area
% of Na+
Role of Na+
reabsorbed reabsorption
Proximal
tubule
67%
Plays role in
reabsorbing
glucose, amino
acids, H2O, Cl-,
and urea
Ascending
limb of the
loop of Henle
25%
Plays critical role
in kidneys’ ability
to produce urine
of varying
concentrations
Distal and
collecting
tubules
8%
Variable and
subject to
hormonal
control; plays
role in regulating
ECF volume
blood volume and blood pressure
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Sodium Reabsorption
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
RAAS
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Renin-angiotensin-aldosterone system
Most import and best known hormonal system involved in regulating Na+
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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Glucose and amino acids are
reabsorbed by sodiumdependent, secondary active
transport.
Electrolytes other than Na+ that
are reabsorbed by the tubules
have their own independently
functioning carrier systems
within the proximal tubule
The reabsorption of water in the
proximal tubule increases the
concentration of urea in the
tubule, as water is lost from the
tubule. This produces a
concentration gradient for urea
from the tubule into the
interstitial fluid.
Generally, unwanted waste
products are not reabsorbed
Passive Reabsorption of Urea
at the End of the Proximal
Tubule
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Basic Renal Processes
– Glomerular filtration
– Tubular reabsorption
• Tubular secretion
Transfer of substances from
peritubular capillaries into the
tubular lumen
Involves transepithelial
transport (steps are reversed)
Kidney tubules can selectively
add some substances to the
substances already filtered
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Tubular Secretion
• Most important secretory systems are for
– H+
• Important in regulating acid-base balance
• Secreted in proximal, distal, and collecting tubules
– K+
• Keeps plasma K+ concentration at appropriate level to
maintain normal membrane excitability in muscles and
nerves
• Secreted only in the distal and collecting tubules under
control of aldosterone
– Organic ions
• Accomplish more efficient elimination of foreign organic
compounds from the body
• Secreted only in the proximal tubule
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Plasma Clearance
• Volume of plasma cleared of a particular substance per
minute (not the amount of the substance removed)
• Varies for different substances, depending on how the
kidneys handle each substance
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Urine Excretion
• Depending on the body’s state of hydration, the kidneys secrete
urine of varying concentrations.
• Too much water in the ECF
establishes a
hypotonic ECF (result = dilute urine)
• A water deficit establishes a
hypertonic ECF (result = conc. urine)
• A large, vertical osmotic
gradient is
established in the
interstitial fluid of the
medulla
(from 100 to 1200 mosm/liter
to 1200 mosm/liter). This increase
follows
the hairpin loop of Henle
deeper and deeper into the medulla.
• This osmotic gradient exists between the tubular lumen and the
surrounding interstitial fluid.
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Countercurrent Multiplication
• The medullary vertical osmotic gradient is established by
countercurrent multiplication
• Comparing the descending and ascending limbs of the loop of
Henle:
• The descending ling is highly permeable to water but does not
extrude sodium for reabsorption.
• The ascending limb actively transports NaCl out of the tubular lumen
into the surrounding interstitial fluid. It is impermeable to water.
Therefore, water does not follow the salt by osmosis.
• There is a countercurrent flow produced by the close proximity of the
two limbs.
• The ascending limb produces an interstitial fluid that becomes
hypertonic to the ascending limb. It does this by pumping out
sodium ions. Water does not follow. This interstitial fluid faces
against the flow of fluid (countercurrent) in the descending limb,
attracting the water by osmosis for reabsorption
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Role of Vasopressin
• Vasopressin-controlled, variable water reabsorption occurs in the
final tubular segments.
• 65 percent of water reabsorption is obligatory in the proximal tubule.
In the distal tubule and collecting duct it is variable, based on the
secretion of ADH.
• The secretion of vasopressin increases the permeability of the
tubule cells to water. An osmotic gradient exists outside the tubules
for the transport of water by osmosis.
• Vasopressin is produced in the hypothalamus and stored in the
posterior pituitary. The release of this substance signals the distal
tubule and collecting duct, facilitating the reabsorption of water.
• Vasopressin works on tubule cells through a cyclic AMP
mechanism.
• During a water deficit, the secretion of vasopressin increases. This
increases water reabsorption.
• During an excess of water, the secretion of vasopressin decreases.
Less water is reabsorbed. More is eliminated.
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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Nephron
Vascular component
– Dominant part is the glomerulus
• Ball-like tuft of capillaries
• Water and solutes are filtered through glomerulus as blood passes through it
– Filtered fluid then passes through nephron’s tubular component
– From renal artery, inflowing blood passes through afferent arterioles which deliver
blood to glomerulus
– Efferent arteriole transports blood from glomerulus
– Efferent arteriole breaks down into peritubular capillaries which surround tubular
part of nephron
– Peritubular capillaries join into venules which transport blood into the renal vein
Tubular component
– Hollow, fluid-filled tube formed by a single layer of epithelial cells
– Components
• Bowman’s capsule
• Proximal tubule
• Loop of Henle
– Descending limb
– Ascending limb
• Juxtaglomerular apparatus
• Distal tubule
• Collecting duct or tubule
Chapter 14 The Urinary System
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Countercurrent
Renal Anatomy
Renal Processes 1
Renal Processes 2
Urine Concentration
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Remaining class periods 4/13, 4/16, 4/20, 4/23 and 5/2 for students not graduating.
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Exam 4
– Exam IV (9, 12 14) will be available 4/17 at 12:00 PM until 4/18 at
5:00 PM. I will not reset this exam after 4/17 at 5:00 PM
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Lab exam 3
ECG and Endocrinology 4/20
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Endocrinology lectures 4/20 and 4/23
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Last Exam
(18,19) cumulative material will be available 4/26 at 12:00 PM until 4/27 at 5:00
PM. I will not reset this exam after 4/26 at 12:00 PM
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5/2 review final exam and determine final grades.