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MOLECULAR PHYSIOLOGY
Excretory system
Punam Verma*, Vineet Mehrotra and Sunita Mittal
*Associate Professor
Department of Physiology
SGRRIM&HS
Patel Nagar
Dehradun
3-Jul-2006 (23-May-2007)
CONTENTS
Introduction
Functions of kidney
Kidney structure
Structure of nephron
Types of nephron
Renal blood flow
Mechanism of urine formation
Glomerular filtration
Tubular transport process
Hormonal regulation of tubular reabsorption and tubular secretion
Renal handling of some common solutes and water
Keywords
Excretory system; Urine formation; Filtration; Reabsorption; Secretion
Introduction
Excretion literally, the word excretion means elimination of any matter from the body of an
organism. Different organs and systems like digestive system, respiratory system, excretory
system and skin are involved in the process of excretion. However, here the term excretion
refers to elimination of principal products of metabolism except carbon dioxide i.e. removal
of ammonia, urea, uric acid, creatinine, various pigments and inorganic salts. Thus in strict
sense kidneys are excretory organs. Together with a pair of ureter and a urinary bladder
kidneys constitute excretory system (Fig 1).
Fig. 1: Urinary system
Functions of kidney
The kidneys do the major function of the urinary system. The other parts of the system are
mainly passageways and storage areas. Functions of kidney are:
1. Homeostatic function: It maintains constancy of the interval environment of body
by
(a)
Regulating blood ionic composition Kidneys regulate the blood levels of
several ions like Na+, K+, Ca++, Cl- etc.
(b)
Regulating blood pH - It maintain blood pH by excreting variables amount of
H+ into urine and conserving H CO3- in the blood.
(c)
Regulating blood volume - It adjust blood volume by conserving or
eliminating water in urine.
(d)
Regulating blood pressure - It regulate blood pressure by secreting renin .
(e)
Maintain blood osmolarity - By separately regulating loss of water and loss
of solutes in the urine. Kidneys maintain relatively constant blood osmolarity
300 milli osmoles/liters.
2. Endocrine function: It secretes:
(a)
Erythropoietin hormonewhich stimulates production of RBC from stem cell.
(b)
25-Dihydroxycholecalciferol (active form of vitamin D),which helps to
regulate calcium homeostasis.
(c)
Renin from Juxta-glomerular cells in response to low BP. It converts
angiotensinogen into angiotensin I (Ag I) which in presence of angiotensin
converting enzyme is converted into angiotensin II (AgII) which is a powerful
vasoconstrictor and helps to maintain BP.
3. Regulating blood glucose level: Kidneys can also synthesize glucose from amino
acid, glutamine etc.
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4. Excreting waste and foreign substances: By forming urine kidneys help to excrete
wastes.
Kidney structure
The kidneys are paired organs, each weighting 150 gm in adults. They are located
retroperitonealy in the upper dorsal region of the abdominal cavity on either side of vertebral
column. It is bean shaped organ approximately 10 cm long, 5 cm wide and 2.5 cm thick.
Right kidney occupies slightly lower position due to pressure of liver on that side. The lateral
border of each kidney is convex and its medial side is deeply concave. In the middle of the
medial side there is depression leading to a hollow chamber called renal sinus through which
blood vessels, nerves, lymphatic renal pelvis and ureter passes. Vertical section of kidney
(Fig. 2) shows:
1. Outer cortex reddish in colour looks granular due to arrangements of nephrons. It
forms shell-surrounding medulla.
2. Inner medulla is pale in colour. It contains 10 – 15 conical mass of tissues called
pyramids whose bases are directed towards convex surface of the kidney. They
terminate medially in the renal papillae. Papillae projects into calyces, such 10 – 15
minor calyces join to form two major calyces, which come out through pelvis of
kidney to the widened end of the ureter.
Fig. 2: Vertical section of kidney
Structure of nephron
The basic functional unit of the kidney is the nephron. There are about 1.2 million nephrons
in each kidney, which drain into pelvis. Total length of nephrons is 45-65mm. Each nephron
is formed by two parts (Fig. 3):
1. Renal corpuscle or malpighian corpuscle
2. Renal tubule
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Fig. 3: Structure and histology of nephron
Renal Corpuscle – It is a rounded structure comprising glomerulus, surrounded by
glomerular capsule also called as Bowman’s capsule (Fig. 4).
Fig. 4: Renal Corpuscle
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Glomerulus – It is a tuft of anastmozing capillaries. Blood enters the glomerulus through
afferent arterioles and leaves it through efferent arterioles. Capillaries are made up of single
layer of endothelial cells.
Bowman’s capsule – It is a cup shaped structure having two layers: i) Visceral layer,
closely applied to the loops of capillaries from all sides and ii) Parietal layer form the outer
layer having glomerulus and is continuous with epithelial lining of proximal tubule. A space
between visceral layer and parietal layer is called as Bowman’s space (BS).
Structure of glomerular membrane
It is the membrane through which blood filters from capillaries into Bowman’s space and it is
also called as filtration barrier. It is made up of three layers (Fig. 5):
1. Endothelial layer of capillaries
2. Basement membrane
3. Visceral layer of Bowman’s capsule/ layer of podocytes
Fig. 5: Glomerular capillary membrane
Endothelial layer of capillaries: The endothelium is fenestrated i.e. contains holes of about
70-90 nm and is freely permeable to water, small solutes such as sodium, urea and glucose
and even very small proteins but not all since this layer express negatively charged
glycoproteins (sialoproteins) on their surface, which retard the filtration of large anionic
proteins.
Basement membrane: This is a porous matrix (pore size <8nm) of extra cellular proteins
including collagen, laminin, fibronectin and other negatively charged proteins. It is an
important filtration barrier to plasma protein. Unlike other basement membrane it is very
thick.
Visceral layer: It has special type of cells called podocytes, which have finger like
projections. These projections interdigitate to cover the basement membrane and are
separated by gaps called filtration slits. Each filtration slit is bridged by a thin diaphragam,
which contains pores with dimensions of 25 nm. Therefore, filtration slit retard the filtration
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of some proteins and macromolecules that pass through endothelium and basement
membrane.
Renal tubule
The tubular portion of nephrons is the continuation of Bowman’s capsule. It is made up of
four parts:
1. Proximal convoluted tubule (15 mm)
2. Loop of Henle (14 - 26 mm)
3. Distal convoluted tubule (5 mm)
4. Collecting duct (20 mm)
Proximal convoluted tubule (PCT)
The initial coiled, portion is called as pars convoluta while distal portion is straight and is
called as pars recta. The epithelial lihing is continuous with the parietal layer of Bowman’s
capsule. The epithelial cells are metabolically active as they have large number of
mitochondria in it. The apical surface has brush border due to presence of microvill in order
to increase the absorptive surface area. These cells are united with each other, with tight
junction, at their apex.
Function: These cells take part in active reabsorption and secretion of some organic
substances and ions. Glucose, amino acid, lactic acid, uric acid, ascorbic acid, phosphate,
sulphate, potassium, calcium, sodium ions and water are reabsorbed while penicillin,
histamine, creatinine and H+ are actively secreted.
Loop of Henle (LH)
It consists of i) Descending thin segment (DTS), ii) Ascending thin segment (ATS) and iii)
Ascending thick limb (TAL). The fluid in the descending limb runs towards renal pelvis and
in ascending limb run towards cortex. The DTS and ATS are made up of single layer of
squamous epithelial cells which do not have brush border and have very few or no
mitochondrias so these are metobolically inactive and only passive transport process can take
place in these segments. The TAL has cuboidal epithelial cell layer with prominent brush
border and has large number of mitochondrias. Therefore, TAL has functions similar to that
of PCT.
Function
From descending limb water is reabsorped
From ascending limb Na, K and Cl ions are reabsorbed
Distal convoluted tubule (DCT)
TAL is continuous with DCT. This tubule comes very close to its own glomerulus and
establishes a close proximity to the afferent and efferent arteriole of the glomerulus. At this
point the cells of the tubule get modified and closely crowned together and called as Macula
densa cells.
Function: DCT is an important site for active secretion of ions, acids etc. Na ions are
actively reabsorbed and water is reabsorbed in presence of ADH and H ions are secreted
actively
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Collecting duct (CD)
The collecting duct, strictly speaking is not a part of nephron because it is derived from the
ureteric buds. Depending upon the region where it is present, is divisible into three parts:
i) Cortical collecting duct (CCD)
ii) Outer medullary collecting duct (OMCD).
iii) Inner medullary collecting duct (IMCD)
The last part of DCT and continuing collecting duct has two different types of cells:
i) Principal cell (P-cell):- They are abundant in number, have moderately invaginated
basolateral membrane and contains very few mitochondrias . They have receptor for
ADH & aldosterone hormone so increase the permeability for water & Na+.
ii) Intercalated cell (I- cell):- Few in number and have large number of mitochondrias.
They play role in maintaining blood pH by secreting H+ in the urine.
Several IMCDS join together before finally opening at the tip of Renal papilla.
Types of nephron
There are two types of nephrons cortical nephron and Juxta medullary nephron. The
differences between these are listed in Table 1 and Fig. 6.
Table 1: Difference between cortical and juxtamedullary nephron
Cortical nephron
Juxta medullary nephron
Number of nephron
Size and location of
glomeruli
80 – 85%
Small size glomerulus in renal
cortex
Loop of henle
Short loop of henle may reach
upto medulla
In ascending limb ATS is
absent
Vascular supply is in the form
of peritubular capillaries
Mainly involved in the
formation of urine
15%.
Large size, located at the
junction of cortex and
medulla.
Long loop of henle goes deep
into the medulla
Both ATS and TAL are
present in ascending limb
It is in the form of vasa recta.
Vascular supply
Function
Involved in the concentration
of urine
Juxta glomerular apparatus (JGA)
It is a combination of specialized tubular and vascular cells located at the vascular pole
where the afferent and efferent arterioles enter and leave the glomerulus (Fig. 7). It is
composed of three types of cells:
a) Juxta glomerular cells – These are specialized myoepithelial cells located in the
media of afferent arterioles. They have well developed golgi apparatus, endoplasmic
reticulum, mitochondria and ribosomes. They synthesize store and release an enzyme
called renin. These cells act as baroreceptor as they respond to change in blood
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pressure and also monitor vascular volume. They are richly innervated with
sympathetic fibers.
Fig. 6: Cortical and juxtamedullary nephron
Fig. 7: Juxtaglomerular apparatus
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b) Macula densa cells –These cells refer to the specialized renal tubular epithelial cells
near the vascular pole. These cells have prominent nuclei and golgi complex. They
act as chemo receptors and are stimulated by decreased NaCl load in DCT, thereby
cause increase renin release.
c) Mesangial cells or Lacis cells - These are supporting cells of JGA and found
between capillary loops. They are contractile in nature and play role in glomerular
filtration by increasing or decreasing the surface area of filtration membrane (Table
2).
Table-2 Agents causing contraction and relaxation of mesangial cell
Contraction
Angitensin II
ADH
Endothelins
Histamine
Norepinephrin
PGF2
Platelet activating factor
GFR Relaxation
GFR
Atrial natriuretric peptide (ANP)
c-AMP
Doapmine
PGE2
Renal blood flow
Renal blood vessels
To perform its function, kidneys are abundantly supplied with blood vessels. Although they
constitute less than 0.5% of the total body mass but they receive 20-25% of the resting
cardiac output via the renal artery. Renal artery enter the kidney at the pelvis and then branch
into several segmental arteries, which enter the parenchyma and pass through the renal
columns between the renal pyramids as the interlobar arteries which at the base of renal
pyramid arches between the cortex and medulla as arcuate artery. Division of arcuate arteries
give rise to series of interlobular arteries, which enter the renal cortex and branches called as
afferent arteriole. It is short, thick walled and each divded into multiple capillary branches to
form, glomerulus. The glomerular capillaries, exhibit higher pressure of about 45 mm Hg
than that in other capillary bed, join to form efferent arteriole whose diameter is smaller to
afferent arteriole but possess a thinner wall. They have relatively high resistance. These
breakup to from peritubular capillary plexus in cortical nephrons and surrounds all the
convoluted tubule in the cortex. The juxtamedullary nephrons form peritubular capillary
plexus and also sub divides into bands of straight vessels called Vasa Recta (VR) which run
parallel to the convoluted tubules into the medulla. Both these join to form stellate vein
which drains into inter lobular vein, actuate vein, inter lobar vein and finally into renal vein
which leave the kidney at renal pelvis (Fig. 8).
Characteristics of RBF
1)
Amount and rate of blood flow
i.
The rate of blood flow in this region of the body is quite high in comparison to
blood flow in other region. Renal blood flow (RBF) is 1.2 – 1.3 lit / min (400 ml
/100gm/ min) or 1700–1800 L / day.
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Renal plasma flow (RPF) is 650 ml / min or 900 L / day
Coronary blood blow - 60 – 80 ml / 100 gm / min
Brain blood flow
- 55 ml / 100 gm / min
Skin blood flow
- 3 – 4 ml / 100 gm / min
ii. Higher blood flow in kidney is related to its excretory function rather than its
metabolic requirement.
iii. Afferent arteriole, efferent arterioles and intralobular artery are the major resistance
vessels in kidney and they determine renal vascular resistance.
iv. Renal oxygen consumption is very high (6 ml / 100 gm / min) as compared to that in
heart (8 ml / 100 gm / min).
Fig. 8: Blood supply of kidney: A - Blood vessels in cortex; B - V/S showing major
arteries and vein
2)
Intra renal distribution of RBF - Of the total RBF, 90 % blood perfuses cortex and
only 10 % perfuses renal medulla
Blood flow in cortex
5 ml / gm/ min
in outer medulla
2.5 ml / gm/ min
in inner medulla
0.6 ml / gm/ min
10
Low blood flow to medulla plays an important role in concentration of urine and is because
of high vascular resistance in vasa recta which may be due to:
i) long length of vasa recta (40 mm);
ii) increase viscosity of blood near the hair pin band of loop of Henle;
iii) small number of these vessels;
iv) low hydrostatic pressure head because the diameter of efferent arterioles arising from
JM nephron is small.
Regulation of Renal Blood Flow
Same regulatory mechanisms affect both RBF and GFR therefore this section will be
discussed with regulation of GFR.
Mechanism of urine formation
Three process are involved in the formation of urine (Fig. 9):
1) Glomerular filtration - It is the function of renal corpuscle of nephrons
2) Tubular Reabsorption
3) Tubular Secretion
Reabsorption and secretion are the function of tubular part of nephron. Both these process are
together are called tubular transport process.
Fig. 9: Mechanism of urine formation
Glomerular filtration
Glomerular filtration is defined as the ultra filtration of plasma from the glomerular
capillaries into the Bowman’s capsule. It is the first step in the urine formation. To
understand this process this section is divided into following sub headings:
i)
Characteristics of filtration membrane.
ii)
Composition of glomerular filtrate.
iii)
Dynamics of glomerular filtration.
iv)
Glomerular filtration rate (GFR)
v)
Filtrations fraction.
vi)
Factors affecting glomerular filtration.
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vii)
viii)
Regulation of glomerular filtration.
Measurement of glomerular filtration.
i) Characteristics of filtration membrane: As discussed previously, filtration membrane
consists of three layers: capillary endothelium, glomerular basement membrane and visceral
layer. The characteristic features of the membrane are:
1) It is highly permeable to water and to water soluble substances due to its porous
nature
2) This membrane also exhibits high degree of selective permeability which depends
upona) Pore size – Pore size of endothelial layer is 70 –90nm , of basement
membrane is 8nm and of visceral layer is 25 nm. Therefore,
1. Molecules less than 4 nm in size can freely pass.
2. Molecules with diameter more than 8nm are not filtered at all.
3. Filtration of molecules having diameter between 4nm –8nm is
inversely proportional to their diameter.
b) Electrical charge – This membrane is negatively charged due to presence of
glycoproteins rich in silica acid. Therefore the negatively charged molecules
are less permeable than neutral and positive charged molecules. This is the
reason that albumin, although has 7nm molecular diameter, is not filtered.
ii) Composition of glomerular filtrate: It is similar to plasma composition except that it is
devoid of cells and proteins. The composition may alter in some diseases due to alteration in
permeability of membrane. For example in glomerular disease the negative charge of
membrane is lost and protein filters across membrane and appears in the urine (Proteinuria /
albuminuria ) in significant amount.
iii) Glomerular filtration rate (GFR): The total quantity of filtrate formed in all the
nephrons of both kidney in the given unit time is called as glomerular filtration rate (GFR).
The normal value of GFR in an average man is 125ml/mn or 180 L/day. It is directly
proportional to the surface area but in women it is 10% lower than in man. After age of 30
year, GER declines with age.
iv) Dynamics of glomerular filtration: The forces that determine the filtration of plasma are
similar to the forces, which determine the absorption of fluid in interstitial space through
capillaries. GFR is the product of filtration coefficient and net filtration pressure.
GFR = Kf x Net filtration pressure.
Where Kf is filtration coefficient, which depends upon the glomerular capillary membrane
permeability and the surface area of filtration membrane and is equal to 12.5. Net filtration
pressure is the difference of the forces that favours the filtration and the forces that opposes
the filtration.
Basically four forces (Starling forces) act at the filtration membrane (Fig. 10)
1. Hydrostatic pressure in glomerulus (PG) = 45 mm Hg.
2. Hydrostatic pressure in Bowman’s space (PB) = 10 mm Hg.
3. Oncotic pressure in glomerular capillaries (πG) = 25 mmHg
4. Oncotic pressure in Bowman’s space (πB), which is taken as 0 as it does not contain
proteins.
PG and πB favors filtration while Pb and πG opposes the filtration
Net filtration pressure = (PG + πB) – (PB + πG)
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So, GFR
=
=
=
=
Kf x [(PG + πB) – (PB + πG)]
12.5 x (45 + 0 – 25 – 10)
12.5 x 10
125 ml / min
Fig. 10: Dynamics of glomerular filtration
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v) Filtration fraction (FF): It is the fraction of plasma that flows through kidneys get filters
at glomerulus into Bowman’s space.
GFR
125
FF =
=
=
0.16 – 0.22
RPF
620 – 700
=
16 % – 22 %
Therfore, filtration fraction is 16 % – 22 % of RPF i.e., only 16 % – 22 % of plasma, that
flows through kidneys, filters at glomerulus.
vi) Factors affecting Glomerular Filtration Rate
1. GFR decreases with advancing age due to decrease in renal plasma flow, cardiac
output and renal tissue mass.
2. GFR changes with RBF in linearity.
3. Hydrostatic pressure in glomerulus – If PG increases GFR also increases.
PG increases in afferent arteriolar dilatation and efferent arteriolar constriction
PG decreases with decreased arterial pressure, afferent arteriole constriction and
efferent arteriole dilatation.
4. Oncotic pressure in Glomerulus (πG) - Increase in πG results in decrease in GFR
Increase in πG is seen in hyperproteinmemia e.g. dehydration. Decrease in πG is
seen in hypoproteinemia e.g. severe anaemia and nephrotic syndrome
5. Hydrostatic pressure in Bowman’s space (PB) - With increase in PB, GFR
decreases e.g.obstruction to urine flow i.e. in ureteric calculus.
6. Membrane permeability – permeability of glomerular membrane increases in its
infection as in glomerular nephritis and decreases when it becomes thick as in
diabetes mellitus.
7. Surface area of filtration membrane – Greater the surface area more will be the
GFR while small surface area reduces GFR.
vii) Regulation of GFR: Since the same regulatory mechanism affects both GER and RBF
so regulation of RBF is also discussed with regulation of GER
Before discussing various regulatory mechanisms, it is essential to understand the
relationship between changes in afferent arteriole and efferent arteriole with RBF and GFR
(Fig.11).
1. Constriction of afferent arteriole decreases both RBF and GFR without charges in
filtration fraction.
2. Dilatation of the afferent arteriole increases both RBF and GFR without charges in
filtration fraction.
3. Constriction of efferent arteriole decreases the RBF but increases GFR so also
filtration fraction.
4. Dilatation of efferent arteriole increases RBF and Decreases GFR and filtration
fraction.
The mechanisms that regulates RBF and GFR are:
•
Autoregulation,
•
Hormonal regulation, and
•
Nervous regulation.
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Fig. 11: Relation of afferent and efferent arteriole diameter with RBF and GFR
1. Autoregulation of GFR and RBF
The renal blood flow (RBF) and thus the glomerular filtration rate (GFR) remain constant
over a wide range of renal arterial pressures, 80-200mm Hg (Fig 12). This occurs due to
an intrarenal mechanism known as autoregulation.
Fig. 12: Autoregulation of RBF and GFR
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Mechanism of autoregulation: Autoregulation has been observed to persist after renal
denervation, in the isolated perfused kidney, in the transplanted kidney, after adrenal
demedulation, and even in the absence of erythrocytes thus ruling out the role of all
these factors. Two mechanisms are considered responsible for autoregulation of RBF
and GFR:
i) Myogenic mechanism: It is related to an intrinsic property of vascular smooth
muscle, the tendency to contract when it is stretched. Thus, when renal arterial
pressure is raised, the afferent arterioles are stretched and with the result they
constrict. Increase in vascular resistance offsets the effect of increased arterial
pressure and thereby maintains a constant RBF and GFR.
ii) Tubuloglomerular feedback (TGF) mechanism: Tubulo-glomerular feedback
(TGF) mechanism is based on the NaCl concentration of tubular fluid. It involves a
feedback loop which operates as (Fig 13)
Fig 13: Tubuloglomerular feedback mechanism
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•
•
•
•
•
Changes in the GFR cause change in NaCl concentration of fluid in the
loop of Henle.
Changes in the NaCl concentration are sensed by the macula densa cells
and converted into a signal.
The signal from the macula densa cells changes the vascular resistance in
afferent arterioles.
Increase in GFR causes increase concentration of NaCl load in DCT,
which is sensed by macula densa cells and they produce afferent arteriolar
constriction. This results in decrease of GFR and decrease load of NaCl in
DCT.
Conversely signals obtained due to decreased NaCl causes vasodilatation
of afferent arterioles and so on.
The effector mechanism responsible for vasoconstriction and
vasodilatation is not exactly known. Perhaps, adenosine triphosphate
(ATP), which selectively constricts the afferent arterioles and metabolites
of arachidonic acid may contribute to TGF mechanism.
Physiological significance: Autoregulation of GFR and RBF is an effective mechanism
for uncoupling renal function from fluctuation in arterial pressure and maintain fluid and
electrolytic balance.
Special features of autoregulatory mechanisms
• Autoregulation mechanisms do not work when the mean arterial blood
pressure falls below 80 mm Hg or increases more than 200 mm Hg
• Autoregulation is not a perfect mechanism.
• Several hormones and other factors can change RBF and GFR in spite of
autoregulatory mechanisms.
2. Hormonal regulation: Both GFR and RBF are also influenced by hormones as described
in Table 3.
Table 3: Hormones influencing RBF and GFR
Decrease in GFR &RBF
by Vasoconstrictors
Increase in GFR & RBF by
Vasodilators
Nor epinephrine
Prostaglandins (no effect on
GFR)
Nitric oxide
Bradykinin
Dopamine
Histamine
Angiotensin II
Endothelin
3. Nervous regulation: Like most blood vessels of the body, kidneys are supplied by
sympathetic fibers from T12 - L2 segments. At rest, sympathetic stimulation is
moderately low, the afferent and efferent arterioles are dialated and renal autoregulation,
of GFR prevails. With moderate sympathetic stimulation, both afferent and efferent
arteriole constrict to the same degree, decreasing GFR and RBF only slightly. With
greater sympathetic stimulation, as occurs during exercise and hemorrhage,
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vasoconstrictuion of afferent arteriole predominate, as a result, blood flow into
glomerular capilllaries is greatly decreased and GFR drops.
vii.
Measurement of GFR - GFR can be measured by renal clearance of inulin, urea
and creatinine.
Tubular transport process
About 180 L of glomerular filtrate is formed per day and only 1.5 L is excreted in the
form of urine i.e. 99% of filtered water and many useful substances are reabsorbed. The
different segments of tubule like PCT, Loop of Henle, DCT and Collecting duct
determines the composition and volume of urine by the process of selective reabsorption
and selective secretion of water and solutes (Table 4). For understanding, tubular
transport can be studied in the following subheadings:
A. General consideration:
1.
Transport pathways and mechanisms across cell membranes.
2.
Selective reabsorption
3.
Pattern of renal handling
4.
Quantification of renal tubular transport.
B. Transport across different segments of nephrons.
C. Renal handling of some common solutes and water.
Table 4: Substances filtered, reabsorbed, secreted nad excreted in the urine
Substances
Filtered Reabsorbed Secreted Excreted Percentage
2000
Na+ (mEq)
+
720
K (mEq)
18000
Cl+- (mEq)
4900
HCO3 (mEq)
++
540
Ca (mEq)
800
Glucose (mmol)
870
Urea(mmol)
12
Creatinine (mmol)
50
Uric acid (mmol)
Total solute (mosmol) 54000
180
Water (L)
25850
620
17850
4900
530
800
435
1
49
53400
178.5
1
100
-
150
100
150
10
0
435
12
5
700
1.5
99.1
86.1
99.2
100
98.2
100
50
98
98.9
99.2
A. General Consideration
1. Transport pathways and mechanisms across Tubular cell membrane
In renal tubule, substances can be transported by two pathways (Fig. 14):
(i)
Paracellular pathway: Transport between the cells through tight junctions..
(ii)
Transcellular pathway: Transport across the cell. It is a two-step process.
The substances can be transported by a) Active transport process and b) Passive transport
process– Diffusion, facilitated diffusion and solvent drag. For reabsorption by
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transcellular pathway, a substance first has to pass from tubular lumen into the tubular
epithelial cell and then from cell into the lateral intercellular space. The opposite happens
if the substance has to be secreted by transcellular pathway. The water moves across the
cell passively while solutes moves both actively and passively. For these mechanisms
various pumps, carriers and channels are present (Table 5).
Fig. 14: Reabsorption pathway
Table 5: Various transport mechanisms in a nephron
Pumps
Carrier
Symporter
3Na+2K+ ATPase
3 H+- ATPase
H+-K+ ATPase
Ca++-ATPase
Na+-Glucose
Na+-Amino-acid
2Na+- HPO4Na+-3HCO3Na+-2Cl-K+
K+-Cl-
Channel
Antiporter
Na+ - H+
Na+ - NH4+
Na+-Ca++
Cl- - HCO3-
Na+
K+
ClCa++
2. Selective reabsorption
The tubular cell reabsorbs the substances present in glomerular filtrate according to the
need of the body. So the tubular reabsorption is selective reabsorption, the various
substances are classified into three categories:
i)
High threshold substance – The substances like glucose, amino acids and
vitamins are completely reabsorbed and do not appear in the urine. These
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ii)
iii)
substances can reappear in urine only when their concentration in plasma
is very high or in renal disease when they are not re-absorbed.
Low threshold substance – The substances such as urea, uric acid and
phosphate are reabsorbed to little extent. These appear in urine even under
normal conditions.
Non-threshold substance – The metabolic end products like creatinine
are not reabsorbed and are excreted in urine irrespective of their
concentration in plasma.
3. Pattern of renal handling
A substance may be handled in different ways while passing through nephrons (Fig. 15):
i) By glomerular filtration only, e.g. Inulin
ii) By glomerular filtration plus partial reabsorption, e.g. urea.
iii) By glomerular filtration plus complete reabsorption, e. g. Glucose
iv) By glomerular filtration plus partial secretion, e.g. PAH
v) By glomerular filtration plus tubular reabsorption and secretion, e.g. K+.
vi) By only secretion, e.g. organic compound bound to proteins.
4. Quantification of renal tubular transport
i) Filtered load : Amount of substance entering the tubule through glomerular filtration
per unit time. It is denoted as Fo.
Fo = GFR x Px
ii) Excretion rate :It is the amount of substance that appear in the urine per unit time. It
is denoted as Eo.
Eo = V x Ux
V - Urine flow rate
Ux - Concentration of substance in the urine
iii) Reabsorpton rate : It is the rate at which a particular substance is reabsorbed. It is
labeled as R.
R = Fo - Eo
iv) Secretion rate : It is the rate at which substance is secreted from peritubular
capillaries into the tubule. It is denoted as S.
S = Eo - Fo
v) Tubular transport maximum: Like transport system elsewhere in the body, renal
active transport system have a maximal rate or Transport maximum (Tm) at which they
can transport particular solute. The rate of transport of any substance depends upon the
rate at which this specific transport system operates. The transport system, in turn,
depends upon the carrier substance or enzyme. So the maximum rate at which a
substance is reabsorbed or secreted is called as tubular transport maximum or Tm for
that solute. Thus the amount of a particular substance transported is proportionate to the
concentration of that solute but after certain limit, when that carrier becomes saturated
then no amount of solute is transported. This is called as (Tm) for that solute. Reabsorbed
substances that have (Tm) are glucose, amino acids, uric acid, albumin etc. Secreted
substances that have (Tm) are PAH, penicillin, certain diuretics, vitamins etc.
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A
C
B
D
Fig. 15: Tubular transport in PCT – A. Sodium glucose transport, B. Bicarbonate
reabsorption, C. Sodium hydrogen exchange, D. Passive reabsorption of Cl-, K+,Ca++,
Mg++, Urea and water
B. Transport across different segments of nephrons
Reabsorption and secretion in the proximal convoluted tubule
The largest amount of solute and water reabsorption occurs in PCT (Fig. 16).
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Fig. 16: Tubular transport in LH
Substances reabsorbed: Approximately 67% of the filtered water, Na+, Cl-, K+ and other
solutes like Ca++, Mg++ and urea are reabsorbed from here. Glucose, amino acids and
vitamins are 100% reabsorbed but reabsorption of HCO3– is 80-90%.
Substances secreted: H+, PAH, drugs, creatinine etc are secreted in this segment.
Various pumps and carriers: On apical surface – Na- glucose/ amino acid transporter
(Symport) Na+- H+ exchange (Antiport)
On basolateral surface: Na- K ATPase
Mechanism
i) Normally, filtered glucose, amino acids, lactic acid, water-soluble vitamins, and other
nutrients are not lost in the urine. Rather, they are completely reabsorbed in the first half of
the proximal convoluted tubule (PCT) by Na+ symporters located in the apical membrane.
ii) Another secondary active transport process, the Na+ /H+ antiporters, carry filtered Na+
down its concentration gradient into a PCT cell in the exchange of H+ which is secreted into
tubular fluid. PCT cells produce the H+ in the following way: Carbon dioxide (CO2) diffuses
from peritubular blood or tubular fluid or is produced by metabolic reactions within the cells.
As also occurs in red blood cells the enzyme carbonic anhydrase (CA) catalyzes the reaction
of CO2 with water (H2O) to form carbonic acid (H2CO3), which then dissociates into H+ and
HCO3- . The HCO3 diffuses in the blood while H+ is secreted into the fluid within the lumen
of the proximal convoluted tubule and it reacts with filtered HCO3- to form H2CO3, which
dissociates into CO2 and H2O. Carbon dioxide then diffuses into the tubule cells and joins
with H2O to from H2O3. Thus, every H+ secreted into the tubular fluid of the proximal
convoluted tubule causes reabsorption of a filtered HCO-3.
iii) Besides achieving reabsorption of sodium ions, the Na+ symporters promote osmosis of
water and passive reabsorption of other solutes. As water leaves the tubular fluid, the
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concentration of the remaining filtered solutes increase. In the second half of the PCT,
electrochemical gradients for Cl- , K+ , Ca2+ , Mg2+ , and urea promote their passive diffusion
into peritubular capillaries via both paracellular and transcellular routes. Among these ions,
Cl- is present in the highest concentration. Diffusion of negatively charged Cl- into interstitial
fluid via the paracellular route makes interstitial fluid electrically more negative than the
tubular fluid. This negativity promotes passive paracellular reabsorption of cations, such as
K+, Ca2+, and Mg2+.
iv) Each reabsorbed solute increases the osmolarity, first inside the tubule cell, then in
interstitial fluid, and finally in the blood. Water thus moves rapidly from the tubular fluid, via
both the paracellular and transcellular routes, into the peritubular capillaries and restores
osmotic balance. In other words, reabsorption of the solutes creates osmosis. Cells lining that
promote the reabsorption of water via osmosis. Cells lining the proximal convoluted tubule
and the descending limb of the loop of Henle are especially permeable to water because they
have many molecules of aquaporin–1. This integral protein in the plasma membrane is a
water channel that greatly increases the rate of water movement across the apical and
basolateral membranes.
Transport process in the loop of Henle
Because the proximal convoluted tubules reabsorb about 67% of the filtered water (about
80mL/min), fluid enters the next part of the nephrons, the loop of Henle, at a rate of 40 – 45
mL/min. The chemical composition of the tubular fluid now is quite different from that of
glomerular filtrate because glucose, amino acids, and other nutrients are no longer present.
However, the osmolarity of the tubular fluid is still close to the osmolarity of blood
(~300mosmol/L) as reabsorption of water by osmosis keeps pace with reabsorption of solutes
all along the proximal convoluted tubule.
The LH reabsorbs about 20-30% of the filtered Na+, K+, Ca2+; 10 – 20% of the filtered HCO3; 35% of the filtered Cl-; and 15% of the filtered water (Fig. 17).
Fig. 17: Tubular transport in late DCT and CD
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Characterstics
Pump: Na +2Cl – K+ symport is present on the apical membrane and Na +- K + ATPase on the
basolateral surface.
DTS: the membrane of it is highly permeable to water but moderately to solutes. Water is
reabsorbed passively along with diffusion of Na+ from interstitial space into the tubular
lumen. Therefore this limb is also called as concentrating segment.
ATS: Passive reabsorption of Na+ and Cl- occurs in this region.
TAL: It is impermeable to water but permeable to solutes. Half of the Na+ is reabsorbed
actively through transcellular route and half of the Na+ is reabsorbed passively by
paracellular pathway. Since there is no reabsorption of water and only solutes are reabsorbed
therefore this limb is also called as diluting segment.
Mechanism
i) The apical membrane of cells in the thick ascending limb of the LH has Na+ - K+ - 2Clsymporters that simultaneously reclaim one Na+, one K+, and two Cl- from the fluid in the
tubular lumen. Na+ that is actively transported into interstitial fluid at the base and sides of
the cell diffuses into the vasa recta. Cl- moves through leakage channels in the basolateral
membrane. Because many K+ leakage channels are present in the apical membrane, most K+
brought in by the symporters moves down its concentration gradient back into the tubular
fluid. Thus, the main effect of the Na+ - K+ -2Cl- symporters is reabsorption of Na+ and Cl-.
ii) The movement of positively charged K+ into the tubular fluid through the apical
membrane channels leaves the interstitial fluid and blood with a negative charge relative to
fluid in the ascending limb of the LH. This relative negativity promotes reabsorption of
cations – Na+, K+, Ca2+ and Mg2+ - via the paracellular route.
Since ions but not water molecules are reabsorbed from the TAL, the osmolarity of the
tubular fluid progressively decreases as fluid flows towards the end of the ascending limb
(~100 mosmol/L).
Transport process in the distal convoluted tubule
Filtered NaCl (7%) and water (8-17%) are reabsorbed while K+ and H+ are secreted in this
segment.
Characterstics
Early DCT – Reabsorbs Na+, Cl-, and Ca++ but is impermeable to water like ALH.
Pump: Na+ - Cl- symporter in apical membrane and Na+- K+ ATPase in the basolateral
membrane.
Fluid enters the distal convoluted tubules (DCT) at a rate of about 25mL/min because 80% of
the filtered water has now been reabsorbed. As fluid flows along the DCT, reabsorption of
Na+ and Cl- continues by means of Na+ -Cl- Symporters in the apical membranes. Sodium –
potassium pumps and Cl- leakage channels in the basolateral membranes then permit
reabsorption of Na+ and Cl- into the peritubular capillaries. The DCT also is the major site
where parathyroid hormone (PTH) stimulates reabsorption of Ca+.
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Transport process in the late DCT and collecting duct
By the time fluids reach the end of the distal convoluted tubule, 90- 95% of the filtered
solutes and water have returned to the blood stream. As described earlier late DCT and
collecting duct have 2 types of cell:
Principal cell - It causes reabsorption of (i) Na+ actively across basolateral surface with help
of Na +K+ ATPase and passively across apical membrane, ii) Cl- passively through
paracellular route, iii) water only in presence of ADH. iii) It causes secretion of K+ via Na +K + ATPase across basolateral membrane and which diffuses passively across apical
membrane.
Intercalated cell - It reabsorbs K+ by H+ - K+ ATPase in the apical membrane and secretes
H+ by H+ ATPase.
Note: Aldosterone hormone increases Na+ reabsorption and K+ excretion from principal cell
while increases H+ secretion by intercalated cell by stimulating H+ ATPase.
In contrast to earlier segments of the nephrons, Na+ passes through the apical membrane of
principal cells via Na+ leakage channels rather than by means of symporters or antiporters.
The concentration of Na+ in the cytosol remains low, as usual, because the sodiumpotassium pumps actively transport Na+ across the basolateral membranes. Then Na+
passively diffuses into the peritubular capillaries from the interstitial spaces around the tubule
cells (Fig. 18).
Normally, transcellular and paracellular reabsorption in the proximal convoluted tubule and
loop of Henle return most of the filtered K+ blood stream. To adjust for varying dietary intake
of potassium and to maintain a stable level of K+ into body fluids, principal cells, secrete a
variable amount of K+ because the basolateral sodium-potassium pumps continually bring K+
into principal cells, the intracellular concentration of K+ remains high. K+ leakage channels
are present in both the apical and basolateral membranes. Thus, some K+ diffuses down its
concentration is very low. This secretion mechanism is the main source of K+ excreted in the
urine.
Hormonal regulation of tubular reabsorption and tubular secretion
Four hormones affect the extent of Na+, Cl-, and water reabsorption as well as K+ secretion
by the renal tubules. The most important hormonal regulators of electrolyte reabsorption and
secretion are angiotensin II, ADH and alosterone. Atrial natriuretic peptide (ANP) plays a
minor role in inhibiting both electrolyte and water reabsorption.
Renin- Angiotensin-Aldosterone System
When blood volume and blood pressure falls, the walls of the afferent arterioles are stretched
less, and the juxtaglomerular cells secrete the enzyme renin into the blood. Sympathetic
stimulation also directly stimulates release of renin from juxtaglomerular cells. Renin converts
angiotensinogen into Ag I. Angiotensin converting enzyme (ACE) converts angiotensin Ag I to
Ag II. Angiotensin II affects renal physiology in the following ways:
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Fig. 18: Water reabsorption under the effect of AD
i) It causes vasoconstriction of the afferent arterioles leading to decrease GFR.
ii) It enhances reabsorption of Na+, Cl-, and water in the proximal convoluted tubule by
stimulating the activity of Na+ /H+ antiporters.
iii) It stimulates release of aldosterone hormone from adrenal cortex that in turn, stimulates
the principal cells in the collecting ducts to reabsorb more Na+ and Cl- and secrete more
K+.
All these three mechanisms ultimately result in increase in blood volume.
Antidiuretic hormone
Antidiuretic hormone (ADH or Vasopressin) is released by the posterior pituitary. It regulates
facultative water reabsorption by increasing the water permeability of principal cells in the last
26
part of the distal convoluted tubule and throughout the collecting duct. In the absence of ADH,
the apical membranes of principal cells have a very low permeability to water. Within principal
cells are tiny vesicles containing many copies of a water channel protein known as aquaporin
–2. ADH stimulates insertion of the aquaporin-2 containing vesicles into the apical
membranes via exocytosis. As a result, the water permeability of the principal cell’s apical
membrane increases, and water molecules move more rapidly from the tubular fluid into the
cells. Because the basolateral membranes are always relatively permeable to water, water
molecules then move rapidly into the blood. The kidneys can produce as little as 400-500 mL
of very concentrated urine each day when ADH concentration is maximal, for instance during
severe dehydration. When ADH level declines, the aquaporin-2 channels are removed from the
apical membrane via endocytosis. The kidneys produce a large volume of dilute urine when
ADH level is low.
A negative feedback system involving ADH regulates facultative water reabsorption (Fig.
18). When the osmolarity or osmotic pressure of plasma and interstitial fluid increase – that
is, when water concentration decreases by as little as 1%, osmoreceptors in the
hypothalamus detect the change. Their nerve impulses stimulate secretion of more ADH into
the blood, and the principal cells become more permeable to water. As facultative water
reabsorption increases, plasma osmolarity decreases to normal. A second powerful stimulus
for ADH secretion is a decrease in blood volume, as occurs in hemorrhage or severe
dehydration. In the pathological absence of ADH activity, a condition known as diabetes
insipidus, person may excrete up to 20 liters of very dilute urine daily.
Atrial natriuretic peptide
A large increase in blood volume promotes release of atrial natriuretic peptide (ANP) from
the heart. Although the importance of ANP in normal regulation of tubular function is
unclear, it can inhibit reabsorption of Na+ and water in the proximal convoluted tubule and
collecting duct. ANP also suppresses the secretion of aldosterone and ADH. These effects
increase the excretion of Na+ in urine (natriuresis) and increase urine output (diuresis),
which decreases blood volume and blood pressure.
Transport of various substances across different segments of nephrons is summarized in table
Renal handling of some common solutes and water
1. Renal handling of sodium and water
Reabsorption of sodium
Percentage reabsorption of the filtered sodium in different segments of the renal tubule is
67% from PCT, 20% from LH (mainly from TAL), 7% from DCT and 5% from CD. A
mechanism of reabsorption is described in detail above in the subsection on transport across
different segments of renal tubule.
Sodium recycling
The sodium, which is actively pumped out from the thick ascending limb (TAL) into the
outer medullary interstitium, mostly enters the outer medullary descending thin segments
(DTS). This results in recycling of Na+ in the long loops of the juxtamedullary nephrons. The
recycling causes accumulation of Na+ in the interstitium of the renal medulla.
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Reabsorption of water
Water is absorbed passively by osmosis in response to a transtubular osmotic gradient. Rapid
diffusion of water across the cell membrane occurs through water channels made up to
proteins called Aquaporins. Different types of aquaporins are aquaporin-1, 2, 5 and 9.
Mostly these are present in kidneys. Other sites are leucocytes, liver, lung and lacrimal gland.
Renal handling of water by different segments of renal tubule is as follows:
From PCT: Passive reabsorption (67 %).
From Loop of Henle:
Descending thin segment (DTS): Passive reabsorption (15 %)
Ascending thin segment (ATS): Impermeable
Thick ascending limb (TAL): Impermeable
From Distal tubule and collecting duct: (8 to 17 %)
Early DCT: Impermeable
CCD: Reabsorption is ADH dependent
OMCD: Reabsorption is ADH dependent
IMCD: Reabsorption is ADH dependent
Obligatory and facultative reabsorption of water
Out of the total amount of water reabsorbed from the nephron, 80% of the reabsorption is
compulsory i.e. about 80 % of the filtered water is always reabsorbed, irrespective of the
body water balance. This reabsorption occurs by osmosis in response to a transtubular
osmotic gradient and is called obligatory (must occur) reabsorption. About 67% of
obligatory reabsorption occurs in the proximal tubules, and about 15-18% of obligatory
reabsorption occurs in the descending thin segment of loop of Henle. The remaining 15-18%
of the filtered water may or may not be absorbed depending upon the body water balance. It
is called facultative (optional) reabsorption. Facultative reabsorption of water occurs from
the late DCT and collecting tubule and is under the control of ADH.
Regulation of NaCl and water absorption
i) Hormonal regulation. Various hormones including angiotenisn II, aldosterone, ADH,
ANP, urodilatin epinephrine and nor epinephrine (released from sympathetic nerves)
and dopamine, regulates NaCl reabsorption (already discussed)
ii) ADH is the only major hormone that directly regulates the amount of water excreted
by kidney.
iii) Role of starting forces. Although Na+ reabsorption is an active process, it is affected
by the passive starling forces operating between the intercellular spaces and the
peritubular capillaries in the proximal tubule.
Glomerulotubular balance (GTB) – As GFR increases, the tubular reabsorption of solutes
and water increases in PCT due to glomerulotubular balance (GTB). This mechanism in the
proximal tubule maintains reabsorption at a constant fraction (2/3 or 67 % of the filtered Na+
and H2O). It is because of oncotic pressure in the peritubular capillaries. When GFR
increases, more amount of plasma proteins accumulate in the glomerulus. Consequently, the
oncotic pressure increases in the blood by the time it reaches to the efferent arteriole and
peritubular capillaries. The elevated pressure in the capillaries increases reabsorption of
sodium and water from PCT.
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iv) Effects of ECF volume on proximal tubular reabsorption.
a. ECF volume contraction increases reabsorption.
b. ECF volume expansion decreases reabsorption.
2. Renal handling of glucose
Glucose is freely filtered into glomerular filtrate. Filtration load (Fo) increase is direcly
proportional to the plasma glucose concentration (PG). All filtered glucose is completely
reabsorbed from PCT. The transport mechanism occurs in two steps (Fig. 15A):
i) From lumen into cell via secondary active transport processes– Glucose binds
with carrier protein, Sodium – Glucose transporter (SGLUT), located at the apical
membrane. Transport of Na+ down its electrochemical gradient with the help of this
carrier protein, into the cell liberates energy that is utilized to transport glucose into
the cell against concentration gradient.
ii) From cell into the intercellular space via facilitated diffusion - The carrier for this
transport is called GLUT-2 and GLUT-1
Characteristic features of glucose reabsorption
i) Glucose is reabsorbed by transport maximum process i.e. depend upon the number of Na+ glucose transporter.
ii) The glucose reabsorbtion and excretion processes are function of the plasma glucose
titration curve (Fig. 19):
a)
Increase in the PG result in progressive linear increase in the filtered load.
b)
At low PG glucose reabsorption is 100% i.e. no amount of glucose appears in
urine. In this region, line of reabsorption is same as that of filtration.
c)
When PG increase above 180-200 mg the glucose reabsorption is incomplete
and glucose appears in urine (Glycosuria). This PG at which glucose appears in
urine is called as Renal Threshold for glucose, which is 200 mg % of arterial
plasma and 180 mg% of venous plasma.
d)
Transport maximum Tm refers to the plasma concentration at which carrier
proteins are fully saturated. As shown in figure, Tm for glucose is 375 mg/min
i.e. after this PG level reabsorption rate become constant and is independent of
PG. Thus, beyond TmG all the additional glucose is excreted in the urine. And
now the urinary excretion rate increases in linearity with PG.
Splay: It represents the excretion of glucose in urine before the TMG is fully
achieved. It is between PG 180mg % and 350 mg% i.e. the predicted renal
threshold for glucose is 300mg. However actual curve obtained is rounded rather
than sharply angulated predicted curve.
Normally their filtered load is 100mg/min. PG is 80mg %. So if glucose is filtered at
therate of 375mg/min then PG will be 80 x 375/100 = 300 mg %. Therefore predicated
renal threshold for glucose would be 300 mg %, however, the actual renal threshold is
200 mg % of arterial plasma or 180 mg % of venous plasma.
The causes for the splay may be:
i)
Helerogenecity in glomerular size.
ii)
Variability in TMG of nephrons.
3. Renal handling of proteins, peptides and amino acids
Like glucose, peptides and amino acids filter across glomerular membrane and are 100 %
reabsorbed while proteins are not filtered. The small amount of protein that is present in the
29
urine usually comes from the tubular shedding. Normally up to 150 mg of proteins are
excreted in urine in a day. Reabsorption of amino acids is via secondary active transport
process and facilitates diffusion just like that of glucose.
Fig. 19: Glucose titration curve
4. Renal handling of urea
Urea is freely filtered into the glomerular filtrate. The amount of urea filtered by glomerular
capillaries varies with protein intake. PCT reabsorbs 5% of filtered urea passively. DTS and
ATS, CD, OMCD are totally impermeable to urea. IMCD is permeable to urea so reabsorbs
large amount of urea in the effect of ADH.
Urea recycling: Recycling of urea plays important role in concentration of medullary
interistitium Following steps occur:
i)
Concentration of urea in collecting duct- As the nephrons beyond TAL till
OMCD is impermeable to urea, therefore as water is reabsorbed from CD &
OMCD, the urea gets more and more concentrated.
ii)
With the result massive and rapid reabsorption of urea occurs from IMCD into
medullary interstitium.
iii)
From medullary interstitium, most of urea enters vasa recta and is carried upward
towards renal cortex by ascending limb of vasa recta.
iv) From the renal cortical interstitium, urea is secreted into the PCT of cortical
nephrons. Some of the urea also enters the thin segment of the long loop of JM
nephrons. In this way urea is again carried back to the IMCD from where it
diffuses out again resulting in a constant recyling
5. Renal handing of uric acid
Urate is freely filtered by the glomerular capillaries. Tubular transport is exclusively limited
to PCT. Early PCT reabsorbs 95 % of filtered uric acid, mid PCT secrete moderate amount of
uric acid equivalent to 50 % of the GF while late PCT reabsorbs moderate amount of uric
acid, equivalent to 40 % of GF, called as post secretary reabsorption.
30
Mechanism – Two mechanisms are involved in the transport of uric acid:
1. Passive reabsorption through paracellular pathway.
2. Secondary active transport through transcellular pathways involves two steps –
a)
Across apical membrane it enters the cell by counter transport with
intracellular ions like Cl-, H CO3- etc. The carrier protein is called urate
transport protein.
b)
Across basolateral membrane, the urate moves out using another anion
exchanger.
Suggested Readings
1.
2.
3.
Text Book of Medical Physiology by AC Guyton
Fundamentals of Anatomy and Physiolgy by Martini
Text Book of Physiology by AK Jain
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