Download Section 6

‫فیزیولوژی تکمیلی‬
Advanced Physiology
(part 6, Urinary system)
By: A. Riasi
(PhD in Animal Nutrition & Physiology)
Animal Sci. Dep. Isfahan University of Technology
http://riasi.iut.ac.ir
Respiratory system

Reference:
Urinary Systems

Vertebrate kidneys come in a variety of morphologies and
functional capacities.
Urinary Systems

Mammalian kidneys have four major functions:

Regulating major inorganic solutes

Regulating plasma volume

Regulating of harmful or unneeded organic molecules

Control the osmotic balance
Urinary Systems

In mammals, kidneys have the following additional
functions

Secretion of erythropoietin

Secretion of renin, a hormone important for salt conservation

Conversion of vitamin D into its active form

Excretion of pheromones for sexual signaling, marking territories, and
so forth
Urinary Systems

Nephrons are functional
unit of each kidney:

Tubular component

Vascular component
Urinary Systems
Urinary Systems
Urinary Systems

The three layers for glomerular filtration:

The glomerular capillary wall consists of a single layer of flattened
endothelial cells.

The basement membrane, a non-cellular gelatinous layer composed of
collagen and glycoproteins


The collagen provides structural strength

Thee glycoproteins discourage the filtration of small plasma proteins
The inner layer of Bowman’s capsule. It consists of podocytes (podo,
“foot”), octopus-like cells that encircle the glomerular tuft.
Urinary Systems
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Bowman’s capsule podocytes
Urinary Systems
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Three physical forces for
glomerular filtration
Urinary Systems
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Autoregulation of GFR
Urinary Systems
Urinary Systems

Two intrarenal mechanisms involve in autoregulation

The myogenic mechanism
A
common property of arteriolar vascular smooth muscle, which contracts inherently
in response to the stretch accompanying increased pressure within the vessel.

The tubuloglomerular feedback (TGF) mechanism

Involves the juxtaglomerular apparatus (JGA).
Urinary Systems

Within the wall of the afferent arteriole, the smooth muscle
cells in the JGA are specialized.

The distal tubule at JGA are specialized and known as
macula densa.

The macula densa detect changes in the salt level of the fluid flowing
past them through the tubule.
Urinary Systems

In response to the resultant rise in salt delivery to the distal
tubule, the macula densa cells release ATP and adenosine.

These are paracrines, which act on the adjacent afferent arteriole,
causing it to constrict and thus reducing glomerular blood flow and
returning GFR to normal.
Urinary Systems

In the opposite situation, when less salt is delivered to the
distal tubule because of a spontaneous decline in GFR
accompanying a fall in arterial pressure, the macula densa
releases less paracrines.

The resultant afferent arteriolar vasodilation increases the
glomerular flow rate, restoring the GFR to normal.
Urinary Systems

To exert even more exquisite control over TGF, the macula
densa cells also secrete the vasodilator nitric oxide.

By means of the TGF mechanism, the tubule of a nephron is
able to monitor the salt level in the fluid flowing through it
and adjust its own GFR accordingly to maintain fluid and
salt delivery in the distal tubule.
Urinary Systems

The myogenic and TGF mechanisms work in unison to
autoregulate the GFR during the transient changes in blood
pressure that accompany daily activities unrelated to the need
for the kidneys to regulate H2O and salt excretion.
Urinary Systems

If autoregulation did not occur, the GFR could increase and
H2O and solutes would be lost needlessly whenever arterial
pressure rises during intense activity.

If the GFR were too low, the kidneys could not adequately
eliminate unneeded materials.
Urinary Systems
Urinary Systems

The GFR can be influenced by changes in the filtration
coefficient (Kf).

Research indicates that Kf is subject to physiological
control.

Two factors affect the Kf:

The surface area

The permeability of the glomerular membrane
Urinary Systems
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The surface area available within the glomerulus is
represented by the extent of the capillary surface.

Each tuft of glomerular capillaries is held together by
mesangial cells.

Contraction of the mesangial cells reduces the radius of the filtering
capillaries, which reduces the surface area available.

Sympathetic stimulation causes the mesangial cells to contract, thus providing a
second mechanism (besides afferent arteriolar vasoconstriction) by which
sympathetic activity can decrease the GFR.
Urinary Systems

Podocytes also possess actin
like contractile filaments.

The contraction or relaxation can,
respectively, decrease or increase the
number of filtration slits open in the
inner
capsule.
membrane
of
Bowman’s
Urinary Systems

Tubular reabsorption
Urinary Systems

Reabsorption of most substances occurs in the proximal
tubule.

Tubular reabsorption involves two transepithelial transport
methods, and act highly selective:

Passive transepithelial transport
 Active
transepithelial transport
Urinary Systems

Mammalian tubules typically reabsorb 99% of the filtered
H2O and salt and 100% of the filtered glucose and amino
acids.

The tight junctions largely prevent substances except H2O
from moving between the cells.

So materials must move transcellularly to leave the tubular lumen and
gain entry to the blood.
Urinary Systems

Substances that are actively reabsorbed against a gradient
and are of particular importance to the body include:

Organic nutrients such as glucose and amino acids

Electrolytes such as Na+and PO43−
Urinary Systems

The waste products remaining in the tubular fluid become
highly concentrated.

Urea molecules passively reabsorbed in peritubular
capillaries as a result of this concentrating effect.
 About
40% of the filtered urea is passively reabsorbed and only 60%
of the fitered urea is eliminated.
Urinary Systems
Urinary Systems

Effect of the renin–angiotensin–aldosterone system (RAAS)

Angiotensinogen is a plasma protein synthesized by the
liver and always present in the plasma in high
concentration.

Once renin secreted into the blood convert angiotensinogen
into angiotensin I.

In the lungs, angiotensin I is converted into angiotensin II
by angiotensin-converting enzyme (ACE).
Urinary Systems

Angiotensin II is the primary stimulus for the secretion of
the hormone aldosterone from the adrenal cortex. These do
the following effects:

Na+ reabsorption

Water retention

Vasoconstriction

Thirst and salt hunger
Urinary Systems
Urinary Systems

The second route of entry selected substances into the
tubules is tubular secretion.

The most important substances secreted by the tubules are:

Hydrogen ion (H+)

Potassium (K+)

Organic anions and cations
Urinary Systems

Hydrogen ion can be added to the filtered fluid by being
secreted by the proximal, distal, and collecting segments,
using proton-pumping.

The extent of H+ secretion depends on the acidity of the body fluids.
Urinary Systems
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Potassium secretion is controlled by aldosterone.

Changes in the plasma K+ concentration can be dangerous
and causes heart arrhythmia.

Potassium ion is selectively moved in opposite directions in
different parts of the tubule.

It is actively reabsorbed in the proximal tubule

It can also actively secreted by principal cells in the distal and
collecting tubules
Urinary Systems
Urinary Systems

During periods of K+ depletion, K+ secretion in the distal
portions of the nephron is reduced to a minimum.

The K+ secretion, not the filtration or reabsorption of K+, is varied in
a controlled fashion to regulate the rate of K+ excretion and maintain
the desired plasma K+ concentration.
Urinary Systems

Several factors can alter the rate of K+ secretion, the most
important being aldosterone.

An elevation in plasma K+ concentration directly stimulates
the adrenal cortex to increase output of aldosterone.
Urinary Systems

The manifestations of ECF K+ depletion are:

Skeletal muscle weakness

Diarrhea
 Abdominal
distention
 Abnormalities
in cardiac rhythm and impulse conduction