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Graphics are used with permission of: Graphics are used with permission of: adam.com
adam.com ((
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)) Benjamin Cummings Publishing Co (Benjamin Cummings Publishing Co (
http://www.aw.com/bc
http://www.aw.com/bc
))O
Over
ver
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:
R
Resp
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S
Syst
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Or
r
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gan
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s
• Air enters the
• Air enters the
nose
nose
by passing through two openings called the
by passing through two openings called the
external nares
external nares
, or nostrils.
, or nostrils.
• Within the nose, the air passes through the
• Within the nose, the air passes through the
nasal cavity
nasal cavity
, and then travels through the
, and then travels through the
pharynx
pharynx
, a
, a
muscular tube which carries both food and air throughout most of its length.
muscular tube which carries both food and air throughout most of its length.
• Air then enters the
• Air then enters the
larynx
larynx
..
• After passing through the larynx, air enters the
• After passing through the larynx, air enters the
trachea
trachea
, which is held open by incomplete rings of
, which is held open by incomplete rings of
cartilage.
cartilage.
• The trachea divides into a right and a left
• The trachea divides into a right and a left
primary bronchus
primary bronchus
, which carry the air into the lungs.
, which carry the air into the lungs.
• Although not part of the respiratory system, the diaphragm and
• Although not part of the respiratory system, the diaphragm and the intercostal muscles play important
the intercostal muscles play important
roles in breathing.
roles in breathing.
D
Dem
emon
onst
st
r
r
at
at
i
i
o
on
n
o
of
f
P
Pl
l
eu
eur
r
ae
ae
an
and
d
t
t
h
he
e
L
Lu
un
ngs
gs
••
Each lung is surrounded by two layers of
Each lung is surrounded by two layers of serous membrane known as the pleurae.
serous membrane known as the pleurae.
• The relationship between the pleurae and the lungs can be demonstrated by
• The relationship between the pleurae and the lungs can be demonstrated by pushing a
pushing a fist
fist into a water-
into a
water-filled balloon. The balloon represents the pleurae, and the fist represents the lung.
filled balloon. The balloon represents the pleurae, and the fist represents the lung.
• As the fist pushes into the balloon, notice how the balloon wraps around it, and the opposite
• As the fist pushes into the balloon, notice how the balloon wraps around it, and the opposite
surfaces of the balloon almost touch.
surfaces of the balloon almost touch.
••
The inner
The inner part of the balloon which wraps around the fist represents the visceral pleura.
part of the balloon which wraps around the fist represents the visceral pleura. The
The
visceral pleura is the part of the pleura which covers the surface of the lungs.
visceral pleura is the part of the pleura which covers the surface of the lungs.
• The outer part of the balloon represents the parietal pleura, which lines the mediastinum, the
• The outer part of the balloon represents the parietal pleura, which lines the mediastinum, the
diaphragm, and the thoracic wall.
diaphragm, and the thoracic wall.
• Notice that the visceral and parietal pleurae are actually a continuation of the same membrane.
• Notice that the visceral and parietal pleurae are actually a continuation of the same membrane.
• The water-filled space between the two layers
• The water-filled space between the two layers represents the pleural cavity, which contains pleural
represents the pleural cavity, which contains pleural
fluid.
fluid.
V
Vi
i
s
sc
ce
er
r
a
al
l
a
an
nd
d
P
Pa
ar
r
i
i
e
e
t
t
a
al
l
P
Pl
l
e
eu
ur
r
a
a
••
The visceral pleura and parietal pleura enclose each lung in a separate sac. The frosty layer you see
The visceral pleura and parietal pleura enclose each lung in a separate sac. The frosty layer you see
here covering the lung is the portion of the parietal pleura that lines the anterior thoracic wall.
here covering the lung is the portion of the parietal pleura that lines the anterior thoracic wall.
• The visceral pleura covers the surface of the lungs and the cut edges of the parietal pleura.
• The visceral pleura covers the surface of the lungs and the cut edges of the parietal pleura.
• The pleural cavity is an extremely thin, slit-like space between the pleurae, separating them by a thin
• The pleural cavity is an extremely thin, slit-like space between the pleurae, separating them by a thin
layer of pleural fluid.
layer of pleural fluid. The pleural fluid assists in breathing movements by acting as a lubricant.
The pleural fluid assists in breathing movements by acting as a lubricant.
• The parietal pleura lines the mediastinum, the superior surface of the diaphragm, and the inner thoracic
• The parietal pleura lines the mediastinum, the superior surface of the diaphragm, and the inner thoracic
wall.
wall.
B
Br
r
o
on
nc
c
h
hi
i
a
a
l
l
T
T
r
r
e
e
e
e
••
The lungs contain many branching airways which collectively are known as the bronchial tree.
The lungs contain
many branching airways which collectively are known as the bronchial tree.
••
Air enters the lungs through the primary bronchi, which branch into secondary bronchi, which in turn
Air enters the lungs through the primary bronchi, which branch into secondary bronchi, which in turn
branch into tertiary bronchi.
branch into tertiary bronchi.
• The trachea and all the bronchi have supporting cartilage which keeps the airways open.
• The trachea and all the bronchi have supporting cartilage which keeps the airways open.
• Air flows deeper into the lungs as the tertiary bronchi branch repeatedly into smaller bronchi, which
• Air flows deeper into the lungs as the tertiary bronchi branch repeatedly into smaller bronchi, which
eventually branch into bronchioles.
eventually branch into bronchioles.
•
• Bron
Bronchi
chioles lack
oles lack car
cartila
tilage
ge and contai
and contain
n more smoot
more smooth
h musc
muscle
le in
in thei
their
r wall
walls
s than the
than the bron
bronchi. These
chi. These
features allow airflow regulation by altering the diameter of the bronchioles.
features allow airflow regulation by altering the diameter of the bronchioles.
• Bronchioles branch further into terminal bronchioles.
• The airways from the nasal cavity through the terminal bronchioles are called the conducting zone. The
air is moistened, warmed, and filtered as it flows through these passageways.
• Beyond the terminal bronchioles, the air enters the respiratory zone, the region of the lung where gas
exchange occurs.
Respi
r
at
or
y
Zone
•
Beyond the terminal bronchioles lie the structures of the respiratory zone, where we begin to find
alveoli, tiny thin-walled sacs where gas exchange occurs.
• Respiratory bronchioles have scattered alveoli in their walls. They lead into alveolar ducts, which are
completely lined by alveoli. These ducts end in clusters of alveoli called alveolar sacs.
Al
v
eol
i
a
nd
Pul
mona
ry
Ca
pi
l
l
a
r
i
e
s
• The pulmonary arteries carry blood which is low in oxygen from the heart to the lungs.
• These blood vessels branch repeatedly, eventually forming dense networks of capillaries that
completely surround each alveolus.
• This rich blood supply allows for the efficient exchange of oxygen and carbon dioxide between the air
in the alveoli and the blood in the pulmonary capillaries.
• Blood leaves the capillaries via the pulmonary veins, which transports the freshly oxygenated blood
out of the lungs and back to the heart.
St
r
uc
t
ur
e
of
a
n
Al
v
eol
us
•
Structure of the inside of an individual alveolus shows three types of cells:
1. simple squamous epithelium
2. alveolar macrophages
3. surfactant-secreting cells
• The wall of an alveolus is primarily composed of simple squamous epithelium, or Type I cells. Gas
exchange occurs easily across this very thin epithelium.
• The alveolar macrophages, or dust cells, creep along the inner surface of the alveoli, removing debris
and microbes.
• The alveolus also contains scattered surfactant-secreting, or Type II, cells.
Rol
e
of
Sur
f
act
ant
•
The inside surface of the alveolus is lined with alveolar fluid.
• The water in the fluid creates a surface tension. Surface tension is due to the strong attraction between
water molecules at the surface of a liquid, which draws the water molecules closer together.
• As seen here, this force pulls the alveolus inward and reduces its size. If an alveolus were lined with
pure water, it would collapse.
• Surfactant, which is a mixture of phospholipids and lipoproteins, lowers the surface tension of the fluid
by interfering with the attraction between the water molecules, preventing alveolar collapse.
• Without surfactant, alveoli would have to be completely reinflated between breaths, which would take
an enormous amount of energy.
St
r
uct
ur
e
of
t
he
Respi
r
at
or
y
Membr
ane
• The wall of an alveolus and the wall of a capillary form the respiratory membrane, where gas exchange
occurs.
• The respiratory membrane is made up of two layers of simple squamous epithelium and their basement
membranes. This membrane is extremely thin, averaging 0.5 micrometers in width.
• Notice also that in many regions of the membrane there is no interstitial fluid. This is because
pulmonary blood pressure is so low that little fluid filters out of the capillaries into the interstitial
space. Oxygen and carbon dioxide can diffuse easily across this thin respiratory membrane.
Gas
T
r
anspor
t
Graphics are used with permission of:adam.com (http://www.adam.com/)
Benjamin Cummings Publishing Co (http://www.awl.com/bc)
Oxygen
Tr
anspor
t
• Transport of oxygen during external respiration: •
With its low solubility, only approximately 1.5% of the oxygen is transported dissolved in plasma.
•
The remaining 98.5% diffuses into red blood cells and chemically combines with hemoglobin.
Hemogl
obi
n
• Within each red blood cell, there are approximately 250 million hemoglobin molecules. • Each hemoglobin molecule consists of:
1. A globin portion composed of 4 polypeptide chains. 2. Four iron-containing pigments called heme groups.
• Each hemoglobin molecule can transport up to 4 oxygen molecules becauseeach iron atom can bind one oxygen molecule.
• When 4 oxygen molecules are bound to hemoglobin, it is 100% saturated; when there are fewer, it is partiallysaturated.
• Oxygen binding occurs in response to the high partial pressure of oxygen in the lungs. • When hemoglobin binds with oxygen, it is called oxyhemoglobin.
• When one oxygen binds to hemoglobin, the other oxygen molecules bind more readily. This is called cooperative binding. Hemoglobin's affinity for oxygen increases as its saturation increases.
Oxyhemogl
obi
n
and
Deoxyhemogl
obi
n
• The formation of oxyhemoglobin occurs as a reversible reaction, and is written as in this chemical equation:
• In reversible reactions, the direction depends on the quantity of products and reactants present. •Inthelungs,wherethepartialpressureofoxygen ishigh,thereaction proceedstotheright,forming
oxyhemoglobin.
• In organs throughout the body where the partial pressure of oxygen is low, the reaction reverses,
proceeding to the left. Oxyhemoglobin releases oxygen, forming deoxyhemoglobin, which is also called reduced hemoglobin.
• Notice that the affinity of hemoglobin for oxygen decreases as its saturation decreases.
• Carbon dioxide transport:
• Carbon dioxide is produced by cells throughout the body.
• It diffuses out of the cells and into the systemic capillaries, where approximately 7% is transported dissolved in plasma.
•
The remaining carbon dioxide diffuses into the red blood cells. Within the red blood cells,
approximately 23% chemically combines with hemoglobin, and 70% is converted to bicarbonate ions, which are then transported in the plasma.
CO2
Tr
anspor
t
:
Car
bami
nohemogl
obi
n
(
Ti
ssues)
• Of the total carbon dioxide in the blood, 23% binds to the globin portion of the hemoglobin molecule to form carbaminohemoglobin,aswritten inthisequation:
• Carbaminohemoglobin forms in regions of high PCO2, as blood flows through the systemic capillaries inthetissues.
CO2
Tr
anspor
t
:
Car
bami
nohemogl
obi
n
(
Lungs)
• The formation of carbaminohemoglobin is reversible.
• In the lungs, which have a lower PCO2, carbon dioxidedissociates from carbaminohemoglobin, diffusesintothealveoli,and is exhaled.
CO2Transport
:
Bi
c
ar
bona
t
e
I
ons
(
Ti
s
sue
s)
• Of the total carbon dioxide in the blood, 70% is converted into bicarbonate ions within the red blood cells, in a sequence of reversible reactions. The bicarbonate ions then enter the plasma.
• In regions with high PCO2, carbon dioxide enters the red blood cell and combines with water to form carbonic acid. This reaction is catalyzed by the enzyme carbonic anhydrase. The same reaction occurs in the plasma, but without the enzyme it is very slow.
• Carbonic acid dissociates into hydrogen ions and bicarbonate ions. The hydrogen ions produced in this reaction are buffered by binding to hemoglobin. This is written as HHb.
• In order to maintain electrical neutrality, bicarbonate ions diffuse out of the red blood cell and chlorideionsdiffusein.Thisiscalledthechlorideshift.
• Within the plasma, bicarbonate ions act as a buffer and play an important role in blood pH control.
CO2Transport
:
Bi
car
bonat
e
I
ons
(
Lungs)
• In the lungs, carbon dioxidediffuses out of the plasma and into the alveoli. This lowers the PCO2in the blood, causing the chemical reactions to reverseandproceedtotheleft.
• In the lungs, the bicarbonate ions diffuse back into the red blood cell, and the chloride ions diffuse outoftheredbloodcell.Recallthatthisiscalled thechlorideshift.
• The hydrogen ions are released from hemoglobin, and combine with the bicarbonate ion to form carbonicacid.
• Carbonic acid breaks down into carbon dioxide and water. This reverse reaction is also catalyzed by the enzyme carbonic anhydrase.
Summar
y:
Lungs
• Summary of external respiration in the lungs.
• Although we will look at the processes step by step, they actually occur simultaneously. • Remember, gases always follow their partial pressure gradients.
• As you go through the steps, place the numbers that correspond with the steps in the blanks on this diagram:
• During external respiration, a small amount of oxygen (1) remains dissolved in the plasma. However, the majority of the oxygen (2) continues into the red blood cells, where it combines with deoxyhemoglobin (3) to form oxyhemoglobin (4), releasing a hydrogen ion (5).
• When hemoglobin is saturated with oxygen, its affinity for carbon dioxide decreases. Any carbon dioxide combined with hemoglobin (6) dissociates and diffuses out of the red blood cell (7), through the plasma and into the alveoli. In other words, oxygen loading facilitates carbon dioxide unloading from hemoglobin. This interaction is called the Haldane effect.
• The hydrogen ion released from hemoglobin combines with a bicarbonate ion (8), which diffuses into the red blood cell from the plasma in exchange for a chloride ion (9). Recall that this exchange is the chloride shift. The reaction between hydrogen and bicarbonate ions forms carbonicacid(10).
• Carbonic acid then breaks down into water and carbon dioxide (11), catalyzed by the enzyme carbonic anhydrase. The water produced by this reaction may leave the red blood cell, or remain as part of the cytoplasm. The carbon dioxide diffuses out of the red blood cell (12) into the plasma and then into the alveoli. The small amount of carbon dioxide transported in the plasma diffuses into the alveoli
Summar
y:
Ti
ssues
Summary of internal respiration in the tissues.
• As you go through the steps, place the numbers that correspond with the steps in the blanks on this diagram:
• During internal respiration,a small amount of carbon dioxide (1) remains dissolved in the plasma, but most of the carbon dioxide (2) continues into the red blood cells where much of it combines with water to form carbonic acid (3) or combines with hemoglobin to form carbaminohemoglobin (11). This reaction is catalyzed by carbonic anhydrase. The carbonic acid then dissociates into hydrogen and bicarbonate ions (4).
• During the chloride shift, bicarbonate ions (5) diffuse out of the red blood cell in exchange for chlorideions(6).Bicarbonate ions act as buffers within the plasma, controlling blood pH. • Within the red blood cell, hydrogen ions (7) are buffered by hemoglobin (8). When hemoglobin
binds hydrogen ions, it has a lower affinity for oxygen. As a result, oxygen (9) dissociates from hemoglobin, diffuses out of the red blood cell and into the tissues. The interaction between hemoglobin's affinity for oxygen and its affinity for hydrogen ions is called the Bohr effect. By forming hydrogen ions, carbon dioxide loading facilitates oxygen unloading.
• The small amount of oxygen transported in the dissolved state (10) also diffuses out of the plasma and into the tissue cells.
Pul
monar
y
Vent
i
l
at
i
on
Graphics are used with permission of: adam.com (
http://www.adam.com/
)Benjamin Cummings Publishing Co (
http://www.aw.com/bc
)Boyl
e'
s
Law:
Rel
at
i
onshi
p
Bet
ween
Pr
essur
e
and
Vol
ume
• In order to understand ventilation, we must first look at the relationship between pressure and volume.
• Pressure is caused by gas molecules striking the walls of a container.
• The pressure exerted by the gas molecules is related to the volume of the container.
• This large sphere contains the same number of gas molecules as the original sphere. Notice that in this
larger volume, the gas molecules strike the wall less frequently, thus exerting less pressure.
• In this small sphere, the gas molecules strike the wall more frequently, thus exerting more pressure.
Notice that the number of gas molecules has not changed.
• These demonstrations illustrate Boyle's Law, which states that the pressure of a gas is inversely
proportional to the volume of its container. Thus, if you increase the volume of a container, the
pressure will decrease, and if you decrease the volume of a container, the pressure will increase.
Qui
e
t
I
ns
pi
r
a
t
i
on:
Mus
cl
e
Cont
r
a
ct
i
on
• The volume of the thoracic cavity is changed by muscle contraction and relaxation.
• During quiet inspiration, the diaphragm and the external intercostal muscles contract, slightly enlarging
the thoracic cavity.
• As we learned from Boyle's Law, increasing the volume decreases the pressure within the thoracic
cavity and the lungs.
• Notice how the diaphragm flattens and moves inferiorly while the external intercostal muscles elevate
the rib cage and move the sternum anteriorly. These actions enlarge the thoracic cavity in all
dimensions.
• As we learned from Boyle's Law, increasing the volume decreases the pressure within the thoracic
cavity and the lungs.
Qui
e
t
Ex
pi
r
a
t
i
on:
Mus
cl
e
Re
l
a
xa
t
i
on
• Quiet expiration is a passive process, in which the diaphragm and the external intercostal muscles
relax, and the elastic lungs and thoracic wall recoil inward.
• This decreases the volume and therefore increases the pressure in the thoracic cavity.
• As the diaphragm relaxes, it moves superiorly. As the external intercostal muscles relax, the rib cage
and sternum return to their resting positions. These actions decrease the size of the thoracic cavity in
all dimensions, and therefore increase the pressure in the thoracic cavity.
Mus
cl
e
s
of
De
ep
I
ns
pi
r
a
t
i
on
a
nd
Ex
pi
r
a
t
i
on
• Deep breathing uses forceful contractions of the inspiratory muscles and additional accessory muscles
to produce larger changes in the volume of the thoracic cavity during both inspiration and expiration.
During deep inspiration, the diaphragm and the external intercostal muscles contract more forcefully
than during quiet breathing. Additionally, the sternocleidomastoid and scalenes contract, lifting the
rib cage higher. These actions further increase the volume. As we learned from Boyle's Law, this
decreases the pressure within the thoracic cavity.
• Deep or forceful expiration is an active process. The internal intercostal muscles depress the rib cage,
and the external oblique, internal oblique, transversus abdominis and rectus abdominis muscles
compress the abdominal organs, forcing them superiorly against the diaphragm. These actions can
dramatically decrease the volume, and further increase the pressure within the thoracic cavity,
producing forceful expiration.
I
nt
r
apul
monar
y
Pr
essur
e
Changes
• Now let's look at the specific pressure changes that occur in the lungs during breathing. For reasons
described later, the lungs closely follow the movements of the thoracic wall.
• The pressure within the lungs is called the intrapulmonary, or intra-alveolar, pressure.
• Between breaths, it equals atmospheric pressure, which has a value of 760 millimeters of mercury at
sea level. When discussing respiratory pressures, this is generally referred to as zero.
• During inspiration, the volume of the thoracic cavity increases, causing intrapulmonary pressure to fall
below atmospheric pressure. This is also known as a negative pressure. Since air moves from areas
of high to low air pressure, air flows into the lungs. Notice that at the end of inspiration, when the
intrapulmonary pressure again equals atmospheric pressure, airflow stops.
• During expiration, the volume of the thoracic
cavity decreases, causing the intrapulmonary
pressure to rise above atmospheric pressure.
Following its pressure gradient, air flows out
of the lungs, until, at the end of expiration,
the intrapulmonary pressure again equals
atmospheric pressure.
I
nt
r
apl
eur
al
Pr
essur
e
• Intrapleural pressure is the pressure within the pleural cavity. Intrapleural pressure is always negative,
which acts like a suction to keep the lungs inflated.
• The negative intrapleural pressure is due to three main factors:
1. The surface tension of the alveolar fluid.
• The surface tension of the alveolar fluid tends to pull each of the alveoli inward and therefore
pulls the entire lung inward. Surfactant reduces this force.
2. The elasticity of the lungs.
• The abundant elastic tissue in the lungs tends to recoil and pull the lung inward. As the lung
moves away from the thoracic wall, the cavity becomes slightly larger. The negative
pressure this creates acts like a suction to keep the lungs inflated.
3. The elasticity of the thoracic wall.
• The elastic thoracic wall tends to pull away from the lung, further enlarging the pleural cavity
and creating this negative pressure. The surface tension of pleural fluid resists the actual
separation of the lung and thoracic wall.
I
nt
r
apl
eur
al
Pr
essur
e
Changes
• Intrapleural pressure changes during breathing:
• As the thoracic wall moves outward during
inspiration, the volume of the pleural cavity
increases slightly, decreasing intrapleural
pressure.
• As the thoracic wall recoils during expiration, the
volume of the pleural cavity decreases, returning
the pressure to minus 4, or 756 millimeters of
mercury.
Effect
of
Pneumot
hor
ax
• If you cut through the thoracic wall into its pleural cavity, air enters the pleural cavity as it moves from
high pressure to low pressure. This is called a pneumothorax.
• Normally, there is a difference between the intrapleural and intrapulmonary pressures, which is called
transpulmonary pressure. The transpulmonary pressure creates the suction to keep the lungs inflated.
In this case, when there is no pressure difference there is no suction and the lung collapses.
• The lungs are completely separate from one another, each surrounded by its own pleural cavity and
pleural membranes. Therefore, changes in the intrapleural pressure of one lung do not affect the
other lung.
Anat
omy
Revi
ew:
Ur
i
nar
y
Syst
em
Graphics are used with permission of:adam.com (
http://www.adam.com/
)Benjamin Cummings Publishing Co (
http://www.aw.com/bc
)The
Ur
i
nar
y
Syst
em
•
The urinary system is composed of paired kidneys and ureters, the urinary bladder, and the urethra. • Urine is produced in the kidneys, and then drains through the ureters to the urinary bladder, where
the urine is stored. Urine is eliminated from the body through the urethra.
Ex
t
e
r
na
l
St
r
uc
t
ur
e
of
t
he
Ki
dne
ys
•Each bean-shaped kidney is embedded in a fatty adipose capsule.
• The kidneys are retroperitoneal, lying against the dorsal body wall in the upper abdomen. • An adrenal gland, which is part of the endocrine system, lies on top of each kidney.
•Severalstructuresenterorexittheconcavesurfaceofthekidneyattherenalhilus,includingthe ureter and the renal vein, which drains into the inferior vena cava.
Bl
ood
Suppl
y
of
t
he
Ki
dneys
•
When the renal vein is removed and the kidney is shown in frontal section, you can see the deeper renal artery and its connection to the abdominal aorta.
• Branching from the renal artery are the segmental and lobar arteries.
• Together, these vessels provide the kidneys with a rich blood supply under high pressure that allows them to continuously filter and cleanse the blood.
I
nt
er
nal
St
r
uct
ur
e
of
t
he
Ki
dne
y
•Internally, the human kidney is composed of three distinct regions:
1.Renal Cortex
• The outermost layer is called the renal cortex. It contains about one million nephrons, the filteringunitsthatform urine.
• Types of Nephrons:
•Corticalnephrons- liecompletelywithinthecortex
• Juxtamedullary nephrons - lie in both the cortex and medulla 2.Renal Medulla
• The middle layer is called the renal medulla, in which you can see the triangular renal pyramids. These pyramids look striated because of parallel bundles of ducts carrying urine from the nephrons.
• The areas between pyramids are the renal columns. They are extensions of the cortex that provide a route for the passage of blood vessels and nerves to and from the outer cortex. 3.Renal Pelvis
•Thefunnel-shaped renalpelvisiswithin therenalsinus.Therenalpelviscollectsurinefrom the pyramids and conveys it into the ureter for passage to the urinary bladder.
• The nephron is the structural and functional unit of the kidneys.Itconsistsofaspecialized tubular structureandcloselyassociatedbloodvessels.
Nephr
on
St
r
uct
ur
e:
Associ
at
ed
Bl
ood
Vessel
s
• Blood entering the kidney through the renal artery flows first into the segmental arteries and then intothelobararteries.
•From there,itenterstheinterlobararteries,thearcuatearteries,thesmallinterlobulararteries,and thestillsmallerafferentarterioles,whichemptyintoacapillarybedcalledtheglomerulus.
• Leading away from the glomerulus is the efferent arteriole. Notice that the afferent arteriole is larger indiameterthantheefferentarteriole.
•Bloodpassesfrom theefferentarterioleintotheperitubularcapillariesandvasarecta.
• From there, blood drains into the interlobular vein, flows into the arcuate vein and enters the interlobarvein,eventuallyreachingtherenalvein.
Nephr
on
St
r
uct
ur
e:
Tubul
ar
Segment
s
• The expanded ‘cup-shaped’ end of the tubule surrounding the glomerulus is called the glomerular, or Bowman’s, capsule.
• Water and solutes pass from the blood into the glomerular capsule, and then flow into the proximal convoluted tubule, or PCT.
• After many loops and convolutions, the tubule straightens out, and fluid flows down the descending, or thin, segment of the loop of Henle into the medullary region, and then up the ascending, or thick,segmentbackintothecorticalregion.
• From the loop of Henle, the fluid then enters the twists and turns of the early and late distal convolutedtubule,orDCT,eventuallyemptyingintoacorticalcollectingduct.
• This duct extends into the medulla, forming the medullary collecting duct, which carries the urine through the tubules of the renal pyramids to the renal pelvis.
Ce
l
l
ul
a
r
F
ea
t
ur
e
s
of
t
he
Re
na
l
Cor
pus
cl
e
•
The glomerulus, with its larger incoming afferent arteriole and smaller outgoing efferent arteriole, is nested within the glomerular capsule something like a fist thrust into a balloon. Together, these structuresarecalledtherenalcorpuscle.
• The visceral layer of the glomerular capsule is made up of specialized cells called podocytes, which surround the permeable capillaries.
• Between the visceral and parietal layers of the capsule lies the capsular space, which collects the fluid and solutes being filtered from the blood.
•In longitudinalsection,theendothelialliningshowssmallopeningscalledfenestrations,which allow for the passage of water and solutes such as ions and small molecules.
•Therearefenestrationsbetween endothelialcellsinthecapillary. • The porous basement membrane encloses the capillary endothelium.
• Surrounding the basement membrane is a layer of podocytes. These cells have large ‘leg-like’ extensions,whichinturnhavesmall‘fringe-like’extensionscalledpedicels.
•Pedicelsfrom adjacentareasinterdigitatelooselytoform spacescalledfiltrationslits.
• Substances being filtered must pass first through the fenestrations, then through the basement membrane, and finally through the filtration slits and into the capsular space.
• Together, the capillary endothelium, basement membrane, and podocytes make up the filtration membrane.
•Extendingfrom thepodocytecellbodyareleg-likeextensionscontainingthefringe-likepedicels.The extensionsandpedicelswraparoundthecapillaryandinterdigitatetoform thefiltration slits.
•
A cross section of the filtration membrane reveals a large podocyte with its nucleus and pedicels. The white areas are portions of the capsular space. Gaps between the pedicels are the filtration slits. • The basement membrane of the capillary endothelium separates the podocyte from the capillary with
itsfenestrations.
• Notice that the filtration membrane permits the escape of small molecules, while preventing large molecules from leaving the bloodstream and passing through into the capsular space.
Cel
l
s
of
t
he
Pr
oxi
mal
Convol
ut
ed
Tubul
e
(
PCT)
•
The simple cuboidal cells of the proximal convoluted tubule are called brush border cells because of their numerous microvilli, which project into the lumen of the tubule.
• These microvilli greatly expand the surface area of the luminal membrane, adapting it well for the processofreabsorption.
• Tight junctions between adjacent cells permit passage of water but limit the escape of large molecules from the tubular lumen into the interstitial space.
• The highly folded basolateral membrane of the cells contains numerous integral proteins involved in passive or active transport of substances between the intracellular and interstitial spaces. Numerous mitochondria provide the ATP necessary for these active transport processes.
• The key feature of these cells is that they are highly permeable to water and many solutes.
Cel
l
s
of
t
he
Thi
n
Loop
of
Henl
e
• The cells of the thin segment of the descending loop of Henle are simple squamous epithelial cells. • These cells lack brush borders, which reduces their surface area for reabsorption.
• These cells continue to be permeable to water, they possess relatively few integral proteins that function asactivetransportmoleculesforreabsorbingsolutesfrom thefiltrate.
•Thekeyfeatureofthesecellsisthattheyarehighlypermeabletowaterbutnottosolutes.
Cel
l
s
of
t
he
Thi
ck
Ascendi
ng
Loop
of
Henl
e
and
Ear
l
y
DCT
•Theepitheliaofthethickascending loop of Henle and the early distal convoluted tubule are similar. They are composed of cuboidal cells, but they have several structural differences compared to the
cells of the proximal convoluted tubule. For example, these cells have fewer and smaller microvilli projecting into the lumen.
•Inaddition,theluminalmembraneiscoveredbyaglycoprotein layer,which,alongwith‘tighter’tight junctions,greatlyrestrictsthediffusion ofwater.
• The basolateral membrane is similar to that of the PCT, containing many integral proteins and closely associated mitochondria for passive and active membrane transport processes.
• The key feature of these cells is that they are highly permeable to solutes, particularly sodium chloride,butnottowater.
The
Juxt
agl
omer
ul
ar
Appar
at
us
•
As the thick ascending loop of Henle transitions into the early distal convoluted tubule, the tubule runs adjacent to the afferent and efferent arterioles.
• Where the cells of the arterioles and of the thick ascending loop of Henle are in contact with each other, they form the monitoring structure called the juxtaglomerular apparatus.
• The modified smooth muscle cells of the arterioles (mainly the afferent arteriole) in this area are called juxtaglomerular or JG cells. These enlarged cells serve as baroreceptors sensitive to blood pressurewithinthearterioles.
• Cells of the thick ascending segment in contact with the arterioles form the macula densa. These cells monitor and respond to changes in the osmolarity of the filtrate in the tubule.
•
The cuboidal cells of the late distal convoluted tubule and the cortical collecting duct fall into two distinctstructuralandfunctionaltypes:principalcellsandintercalatedcells.
1.Principal Cells The more numerous principal cells have few microvilli and basolateral folds. These specialized cells respond to certain hormones that regulate the cell’s permeability to water and solutes, specifically sodium and potassium ions. The key feature of principal cells is
that their permeability to water and solutes is physiologically regulated by hormones.
2.Intercalated Cells When the acidity of the body increases, the intercalated cells secrete hydrogen ions into the urine to restore the acid/base balance of the body. The key feature of intercalatedcellsistheirsecretion ofhydrogenionsforacid/basebalancing.
Ce
l
l
s
of
t
he
Medul
l
ar
y
Col
l
e
ct
i
ng
Duc
t
•Principal cells of the medullary collecting duct are mostly cuboidal in shape. The luminal and
basolateral membranes are relatively smooth, and the cells possess few mitochondria. The permeability of these cells to water and urea is hormonally regulated as the fluid passes through thisregion.
• The key feature of these cells is their hormonally regulated permeability to water and urea.
Phot
omi
cr
ogr
aphs
of
Col
l
ect
i
ng
Duct
s
•In photomicrographs of a longitudinal section and a cross section of collecting ducts, one will notice
that the ducts are composed of cuboidal cells. The lumen of the collecting duct, shown in cross section, is much larger than the lumens of the adjacent thick ascending tubules. This reflects the volume of fluid the collecting ducts contain as they gather the fluid from many nephrons.
