Transcript Urinary Bladder (contn`d)

Anatomy
and
Physiology
Biology 2401
Chapter-26
The Urinary System
Urinary system - http://www.sumanasinc.com/webcontent/animations/content/kidney.html
http://www.biologymad.com/resources/kidney.swf
Urinary System Functions
• Three Functions of the Urinary System
1. Excretion
• Removal of organic wastes from body fluids
2. Elimination
• Discharge of waste products
3. Homeostatic regulation
• Regulates blood volume and blood pressure
• Regulates plasma ion concentrations
• Helps stabilize blood pH
• Conserves valuable nutrients
• Assists liver in detoxifying toxins
Anatomy of the Kidney
Blood supply of the Kidney
• Kidneys are highly vascular which is absolutely
necessary for their function.
Path of blood through the
kidney:
renal artery  segmental arteries
 interlobar arteries  arcuate
arteries  interlobular arteries 
afferent arterioles  glomerular
capillaries  efferent arterioles 
peritubular capillaries and vasa
recta  interlobular veins 
arcuate veins  interlobar veins
 segmental veins  renal vein
Blood supply of the Kidney
The Nephron https://highered.mheducation.com/sites/9834092339/student_view0/chapter50/
basic_renal_processes.html
• functional units of the kidneys.
• The nephron receives blood from the afferent
arteriole.
• The afferent arteriole supplies the glomerulus, an
intricate network of capillary with a unique
structure.
• From the glomerulus blood flows into the efferent
arteriole.
• The efferent arteriole flows into more capillaries, the
peritubular capillaries, and, in juxtamedullary
nephrons (see below), the vasa recta.
• Peritubular capillaries and vasa recta lead to the
venous drainage of the kidney.
Nephrons of the Kidney
Types of Nephrons
• Cortical nephron
(85%)
• Juxtamedullary
nephrons (15%)
Renal Corpuscle
• Bowman’s capsule and glomerulus.
• Filtration unit of the nephron
• The glomerulus is a condensed mass of capillaries which
allows substances to escape by filtration.
• Glomerular capillaries are surrounded by specialized cells
called podocytes.
• Podocytes form the inner (visceral) layer of Bowman's
capsule.
• Podocytes have processes called pedicels which
interdigitate to produce openings called filtration slits.
• Glomerular capillaries are fenestrated in order to allow
filtration.
• The outer (parietal) layer of Bowman's capsule consists of
epithelial cells with tight junctions and serves to contain
the filtrate in the capsular space.
Anatomy of the Renal Corpuscle
Filtration
Membrane
http://www.colorado.edu/intphys/C
lass/IPHY3430200/countercurrent_ct.swf
http://www.as.wvu.edu/~sraylman/
physiology/renal_processes1_ct.s
wf
Net Filtration Pressure
• Force that produces filtration.
• It is the result of the interaction of three forces:
– Glomerular blood hydrostatic pressure (GBHP)
tends to force fluids and electrolytes out of the
capillaries.
– Capsular hydrostatic pressure (CHP) opposes
GBHP.
– Blood colloid osmotic pressure (BCOP) is the pull
exerted on capsular fluid by blood proteins. It
opposes GBHP.
– Net filtration pressure is calculated as follows
• Net filtration pressure (NFP) is calculated as follows:
NFP = GBHP – CHP – BCOP
• In a normal kidney NFP is usually positive.
Glomerular Filtration Rate
• GFR is highly regulated because wastes and
undesirable substances must be removed from the
blood constantly, and blood volume must be constant
to allow adequate hydration of the body.
• GFR is very sensitive to changes in NFP, which is
sensitive to changes in blood pressure.
• The three mechanisms of GFR regulation are renal
autoregulation, neural regulation, and hormonal
regulation.
• These 3 mechanisms work together to maintain a
constant GFR.
Renal Autoregulation
• Renal autoregulation - Maintains GFR despite changes in local blood
pressure and blood flow by vasoconstriction/vasodilation of afferent arterioles,
efferent arterioles and glomerular capillaries
• Hormonal Control: causes the release of renin by macula
densa cells: Renin is involved in the Renin-AngiotensinAldosterone System (RAAS) mechanism:
– renin activates angiotensin 1 from angiotensinogen. Angiotensin 2 is
then activated form angiotensin 1.
– Angiotensin 2 causes constriction of the efferent arteriole increasing
glomerular hydrostatic pressure, and increasing GFR. Angiotensin 2
also causes the adrenal glands to release Aldosterone.
– Aldosterone increases the permeability of principal cells of the DCT
and collecting duct to Na+ and K+: Na+ is reabsorbed, K+ is secreted.
•
Autonomic Regulation of the GFR
– Mostly consists of sympathetic postganglionic fibers
– Sympathetic activation
• Constricts afferent arterioles
• Decreases GFR
• Slows filtrate production
Figure 26-11 The Response to a Reduction in the GFR
Autoregulation
Immediate local
response in the
kidney
Increased
glomerular
blood pressure
Dilation of
afferent arterioles
Contraction of
mesangial cells
Constriction of
efferent arterioles
if sufficient
HOMEOSTASIS
RESTORED
Normal
GFR
HOMEOSTASIS
DISTURBED
Decreased GFR
resulting in
decreased filtrate
and urine
production
HOMEOSTASIS
Start
Normal
glomerular
filtration rate
Tubular Reabsorption
• Substances are returned to the blood and
interstitial fluid.
• The major substances reabsorbed are water,
NaCl, glucose, and amino acids.
• Some of the urea, together with other salts are
also reabsorbed.
• Substances may be reabsorbed through the
cell (transcellular pathway), or they can pass
between cells (paracellular pathway) via
diffusion or via primary and secondary active
transport.
Transport Maximum
• Nutrients such as glucose and amino acid must be
completely reabsorbed from the PCT.
• In fact there is a limit to the rate of tubular
reabsorption called Transport maximum (Tm):
tubular cells can only add a limited number of
channel proteins or carrier molecules to their
plasma membrane.
• Consequently if the quantity of nutrients exceed the
transport capacity of the cells, the excess nutrients
are not reabsorbed and pass into urine.
• When blood glucose levels are very high such as in
diabetes mellitus, a large amount of glucose
passes into the filtrate.
Role of Sodium in Reabsorption
• Sodium ions are essential for reabsorption.
• Na+ / K+ ATPase pumps remove Na+ from the absorptive
cells and send them to the renal interstitium.
• This increases the osmolarity of the interstitium, and lowers
the osmolarity of the absorptive cells.
• Consequently cells absorb sodium, water, and other
substances via a variety of mechanisms depending on the
nature of the substances.
• Nearly all nutrients such as glucose and amino acids as well
as 65% of water are reabsorbed in the proximal convoluted
tubule.
• The remaining filtrate contains water, urea, sodium and other
electrolytes that will be reabsorbed later.
Reabsorption of Water
• Water is reabsorbed by osmosis.
• 65% of water reabsorption occurs from the PCT.
• The squamous cells of the thin descending limb of the loop of
Henle are permeable to water but not to sodium. Additionally the
filtrate is exposed to increasingly hypertonic medulla.
• These two phenomena force another 20% of absorbable water
out of the filtrate from the descending limb
• Reabsorption in this area is termed obligatory because it must
occur due to the osmolarity of the surrounding interstitial fluid.
• The cells of the ascending limb of the loop of Henle are
impermeable to water.
• When the filtrate enters the collecting duct it is once again
exposed to the hypertonicity of the deep medulla.
• However reabsorption of water from the collecting duct is
controlled by the hormone ADH (Antidiuretic Hormone).
Role of ADH in Reabsorption of
Water
• ADH is a hormone produced by the hypothalamus and released
from the posterior pituitary gland on command of the hypothalamus.
• Such a command is generated in response to high blood osmolarity
which occurs during water loss and dehydration from sweating,
vomiting, and also from lack of adequate hydration (low fluid
intake).
• ADH allows water to be reabsorbed from the collecting duct and not
lost in urine. The water is reabsorbed by osmosis driven by
medullary hypertonicity
• Principal cells of the collecting duct are responsible for water
reabsorption. They are sensitive to ADH, and respond to this
hormone by increasing the number of aquaporins in their plasma
membrane. This increases the reabsorption of water from the
collecting duct. The urine produced is very concentrated.
• Overhydration causes low blood osmolarity which slows the release
of ADH from the neurohypophysis. As a result a dilute urine is
produced.
• ADH deficiency causes the production of a large amount of dilute
urine, a condition called diabetes insipidus.
The Countercurrent Multiplier
• This mechanism works in the loop of Henle to increase water
reabsorption from the descending limb as a result of salt
reabsorption from the ascending limb.
• The fact that filtrate moves in opposite directions in the two limbs of
the loop amplifies the effect of transport from one limb on transport
from the other limb.
• In fact the cells of the ascending limb are permeable to NaCl but
impermeable to water: as a result NaCl is removed from the filtrate
and the osmolarity of the surrounding interstitium is increased.
• Since the osmolarity of the filtrate decreases as fluid flows up the
ascending limb, an osmolarity gradient is established: the medullary
region has a greater osmolarity than the cortical region.
• Cells of the descending limb are impermeable to NaCl but very
permeable to water: since the filtrate is moving down the
descending limb a large amount of water is removed from the
filtrate which becomes more and more concentrated.
Reabsorption of Urea
• Urea is a waste product of protein metabolism.
• It is passively reabsorbed from the nephron and
this contributes to keep the surrounding
interstitial fluid hypertonic, pulling water.
• This same urea will be filtered later and may in
fact be reabsorbed again in the collecting duct
• Overall, more urea passes into urine than is
reabsorbed causing a net loss of urea from the
body.
Secretion
• Secretion is the release of substances into the
filtrate by active transport.
• It is accomplished by the tubular cells.
• The substances secreted into the filtrate are
mainly derived from the blood in the peritubular
capillaries.
• Secretion occurs concurrently with reabsorption
although it is the third process we consider.
• It occurs in the proximal convoluted tubule, the
distal convoluted tubule, and the collecting duct.
Purposes of Secretion
• The first purpose of secretion is to eliminate any
remaining toxins and drugs which have not been
filtered: these substances flow from the peritubular
capillaries directly into the tubules of the nephron
and are passed into urine.
• The second purpose of secretion is to establish
electrolyte balance.
– Reabsorption of Na+ causes an imbalance of charges
that is corrected by the secretion of positively charged
ions such as K+.
– Negatively charged ions such as Cl- will either be
secreted or will diffuse down their electrochemical
gradient.
– Bicarbonate ions are always retained by blood because
they act as buffers.
Purposes of Secretion
(contn’d)
• The third purpose of secretion is acid / base balance.
This is achieved by managing Hydrogen ions (H+) and
Bicarbonate ions (HCO3-):
– Hydrogen ions are produced when carbon dioxide (CO2) and water
combine to form carbonic acid (H2CO3), and then carbonic acid
dissociates into Bicarbonate ion and Hydrogen ion (H+).
CO2 + H2O
H2CO3
HCO3- + H+
– Bicarbonate ions are retained as a buffer and exchanged for
chloride: this is called the chloride shift.
– Hydrogen ions can be secreted during moderately acidic
conditions.
– In more severe acidity conditions they reach their secretion limit,
called the tubular maximum. At that point H+ combines with amino
groups from certain amino acids. The resulting ammonium ions,
NH4+ are secreted.
– During extreme acidity they can also combine with phosphate
groups to form phosphoric acid which is secreted.
Summary of Reabsorption and
Secretion
Ureter, Urinary Bladder, and
Micturition Reflex
• From the collecting ducts urine passes
through the papillary ducts, the minor
calyces, the major calyces, and the renal
pelvis.
• The ureters are connected to the renal pelvis
and carry urine to the urinary bladder.
• Urine is stored in the urinary bladder.
• Micturition (urination) occurs when sphincters
of the urethra open to allow urine to flow out
of the body.
Ureters
• Ureters are connected to the renal pelvis of the
kidneys.
• Urine travels to the urinary bladder through the
ureters by peristalsis.
• Ureters connect to the lower posterior wall of the
urinary bladder.
• The adventitia of the ureters is made of fibrous
connective tissue.
• Ureters have two layers of smooth muscle in their
wall: a longitudinal layer (outermost) and a circular
layer (innermost).
• The lining of the ureters is made of transitional
epithelium which allows them to stretch and reduce
back pressure on the kidney.
Urinary Bladder
• The urinary is a hollow muscular and elastic organ
designed to store urine.
• It is located in the lower part of the pelvic cavity.
• The wall of the bladder is subdivided into 3 layers:
– The mucosa
– The detrusor muscle
– The adventitia
Urinary Bladder (contn’d)
Urinary Bladder (contn’d)
• The mucosa of the bladder is made of transitional
epithelium supported by a lamina propria.
– Transitional epithelium can be stretched
– Epithelial cells secrete mucus which serves as a coat against the
acidity of urine.
– The numerous rugae allow the bladder to expand and return to
its original shape.
• The Detrusor muscle has a peculiar structure: it is made
of two layers of longitudinal smooth muscle (outer and
inner layers), and a middle circular layer.
– The internal urethral sphincter (involuntary)is part of the detrusor
muscle.
– The external urethral sphincter (voluntary) is part of the
urogenital diaphragm
• The fibrous adventitia covers the bladder and is attached
to the visceral peritoneum.
Urethra
• The urethra is a tubular organ that allows drainage of
the urinary bladder.
• In females it is a short tubule. Its external orifice is
located within the vulva.
• In males the urethra is subdivided into 3 regions:
prostatic, membranous, and spongy regions.
• Near the bladder the urethra is lined with transitional
epithelium and near the external os it is stratified
squamous, while in the middle it is pseudostratified
columnar epithelium.
• Small mucous cells of the urethral mucosa secrete
mucous to protect the urethral lining from acidic
urine.
Male and Female Urethra
Micturition Reflex
• Micturition is urination
• Urine pressure stimulates receptors in the bladder
wall: this stimulus triggers a parasympathetic reflex
which causes detrusor muscle contractions and
relaxation of the internal urethral sphincter.
– The internal urethral sphincter (involuntary) is part of the
detrusor muscle.
• The need to urinate cannot be repressed but the
delivery of urine can be delayed by the external
urethral sphincter.
– This sphincter is made of skeletal muscle fibers from the
urogenital diaphragm. It is voluntary.
• When conditions are appropriate, additional
parasympathetic stimuli result in micturition and
voluntary stimuli relax the external sphincter.
Urinary Incontinence
• Absence of control of urination:
– Normal in children less than 3 years old
– Abnormal in teenagers and adults
• Children may be trained to develop control of
the external urethral sphincter.
• Adults may experience various disorders
ranging from paralysis, stress, traumatic
injury etc…