dr Dedy
-
Upload
ferry-prawira -
Category
Documents
-
view
228 -
download
0
Transcript of dr Dedy
Basic Physiology
The Urinary System
• Overview of Functions
• Glomerular Filtration
• The Nephron
• Hormonal Control
• Renin-Angiotensin System
• Micturition Reflex
Overview of Functions
• The basic function of the urinary system is
the formation of urine.
• What is urine:
– Filtrate of blood plasma
– Vehicle for secretion of waste products:
• Filtered: drugs, pesticides, food additives.
• Secreted: ammonia, potassium, hydrogen ions.
– High salt concentration (4 x more concentrated
than plasma).
Overview of Functions
• High salt concentration results from
reabsorption of water.
• Ability to recycle water is a major
advantage to land-based organisms
such as humans (amniotes).
– able to go without water for extended
periods of time.
Overview of Functions
• The kidneys therefore have two major
obvious roles:
– Elimination of waste products
– Reabsorption of nutrients/water.
Overview of Functions
• But there are other less obvious but important functions:
– Regulation of blood pressure
– Regulation of fluid composition
– Regulation of fluid pH
– Hormone secretion:
• Renin and erythropoietin
– Activation of vitamin D
twelfth rib
ureters
urinary bladder
SECTION THROUGH A KIDNEY
medulla
cortex
capsule
ureter
renalpyramids
BLOOD VESSELS IN THE KIDNEY
renal artery
renal vein
interlobularartery and vein
arcuateartery and vein
interlobar artery
FEATURES IN A RENAL PYRAMID
interlobulararteries and
veinscortex
medulla
interlobarartery and
vein
arcuateartery and
vein
NEPHRONS
medulla
cortex
loop ofHenle
collectingtubule
corpuscles
RENAL CORPUSCLE
juxtaglomerularapparatus
sphincterof afferentarteriole
glomeruluscovered withpodocytes
outer layer ofBowman’scapsule
capsularspace
Components:-
1. Afferent arteriole - carries blood to the glomerulus
2. Glomerulus - a tuft of capillaries that filters a protein
free plasma into the Bowman’s capsule
3. Bowman’s capsule - collects the glomerular filtrate
180 Litres are filtered each day!!!
Glomerula Filtration
GLOMERULAR CAPILLARIES
The glomerular capillaries are fenestrated (riddled with pores)
podocyte
pedicelof podocyte
filtration slitsin betweenpedicels
ELECTRON MICROGRAPH OF GLOMERULAR SURFACE
Covering the capillaries are podocytes (the visceral layer)
GLOMERULAR FILTRATION
blood solutemolecules
basement lamina
pedicel
filtration slits
fenestration
capsular space
BLOOD HYDROSTATIC PRESSURE
Hydrostatic pressure forces fluid out of the glomerulus.
Normal blood pressure in the glomerulus is around 55 mm Hg.
This is the major force driving glomerular filtration.
BLOOD OSMOTIC PRESSURE
The osmotic (oncotic) pressure is due to plasma proteins and
attracts water into the glomerulus.
Normally about 30 mm Hg
HYDROSTATIC PRESSURE IN THE BOWMAN’S CAPSULE
Hydrostatic pressure forces fluid out of the Bowman’s capsule.
Normal pressure in the capsular space is around 15 mm Hg.
NET FILTRATION PRESSURE
Driving pressure minus opposing pressures = net filtration pressure
= 55 - (30 + 15) = 10
GLOMERULAR FILTRATION RATE
- kidneys normally receive 20 to 25% of cardiac output
- average GFR is about 125 ml/minute (180 litres urine/day)
- GFR is dependent on the diameter of the afferent arteriole
Diameter is under sympathetic control
- Vasoconstriction of the afferent arterioles decreases
GFR
- Therefore baroreceptors and mean arterial blood pressure will
affect GFR
ARTERIOLE DIAMETER CONTROLS GLOMERULAR PRESSURE
1. Smooth muscle tends to contract when it is stretched-autoregulation
2. Smooth muscle is under control of the sympathetic system- receptors
3. Smooth muscle is influenced by chemicals (from the macula densa)α
Glomerular hydrostatic pressure remains relatively constant - 55 mm Hg
sphincterof afferentarteriole
The Nephron
• Basic functional unit of the kidney.
• Consists of:
• Bowmans Capsule.
• Proximal convoluted tubule
• Site of most reabsorption.
• Loop of Henle
• Site of “Counter-current Multiplier System”
• Sodium/water reabsorption
• Distal convoluted tubule
• Adjustment of extracellular sodium levels
• Enter into collecting ducts
• Formation of concentrated urine by water reabsorption
proximalconvoluted
tubule
ascending loop
descending loop
distalconvolutedtubule
collectingtubule
THE NEPHRON- TUBULAR COMPONENTS
peritubular
capillary
network
vasa recta
CAPILLARIES AROUND THE NEPHRON
TUBULAR REABSORPTION
Some substances are not reabsorbed or are reabsorbed incompletely
because they lack carriers or do not pass through the membrane
TUBULAR REABSORPTION
Most takes place in the proximal tubule.
In general waste products are not reabsorbed
- water 99% reabsorbed (180 litres → 1.8 litres urine)
- sodium 99.5% reabsorbed
- glucose 100% reabsorbed
- aminoacids 100% reabsorbed
- water soluble vitamins 100% reabsorbed
- urea 44% reabsorbed
Two types of uptake by the epithelial cells
1. active transport
2. passive transport
TUBULAR SECRETION
Some substances such as hydrogen and potassium ions
are secreted into the urinary filtrate
Water soluble drugs such as penicillin and morphine
are also secreted
The secretory systems also rids the body of pesticides
and food additives
Ammonia diffuses into the urine
THE LOOP OF HENLE
THE LOOP OF HENLE
DIFFERENCES IN THE NEPHRON LOOP
The descending limb:-
1. Highly permeable
to water
2. Relatively
impermeable to
sodium
The ascending limb:-
1. Impermeable
to water
2. Actively transports
sodium out of the
filtrate
OSMOTIC GRADIENT
In the medulla there is a vertical osmolality gradient.
Osmolality is greatest in the apices of the pyramids
ESTABLISHING THE OSMOTIC GRADIENT 1
(“COUNTERCURRENT MULTIPLIER SYSTEM”)
300
300
300
300
300
300
300
300
descendinglimb
ascendinglimb
Filtrate entering the loop of Henle is iso-osmotic with plasma
ESTABLISHING THE OSMOTIC GRADIENT 2
Salt is pumped out of the ascending limb and
water leaves the descending limb passively until
the osmotic difference is 200 mosm/L
NaCl
NaCl
NaCl
NaClwater
water
water
water400
400
400
400
400
400
400
400
ESTABLISHING THE OSMOTIC GRADIENT 3
More filtrate comes in from the proximal convoluted tubule
ESTABLISHING THE OSMOTIC GRADIENT 4
NaCl
NaCl
NaCl
NaClwater
water
water
water350
350
350
350
500
500
500
500
The 200 mosm/L gradient is re-established at each level
ESTABLISHING THE OSMOTIC GRADIENT 5
More fluid enters from the proximal tubule
ESTABLISHING THE OSMOTIC GRADIENT 6
The 200 mosm/L gradient is re-established at each level
NaCl
NaCl
NaCl
NaClwater
water
water
water325
325
425
425
425
425
600
600
THE FINAL OSMOTIC GRADIENT
300
450
600
750
900
1050
1200
1200
The final osmotic gradient is maintained. Blood flow
through the medulla is specialised so that osmolytes
forming the gradient are not removed.
“Countercurrent Multiplication System”
• Summary:
– “Countercurrent” refers to opposite directions of flow within the descending and ascending loop of Henle.
– “Multiplication” refers to the multiplied increase in osmolarity towards apex of medullary pyramids as filtrate continues to flow into nephron.
“Countercurrent Multiplication System”
• Summary:
– Results in the formation of an osmotic
gradient.
– Enables formation of a hypotonic filtrate
by the nephron.
– Assists of osmosis of water into the
ascending limb (loop of Henle) and into
collecting ducts (requires ADH).
vasa recta
1200
300
600
Vasa recta follow the loops of Henle into the medulla.
Blood flow through these capillaries is sluggish.
VASA RECTA
Vasa recta are also freely permeable to salt and water.
As blood flows slowly there is ample time for diffusion to occur.
SALT AND WATER EXCHANGE IN VASA RECTA
CONCENTRATING URINE
Urine enters the
distal convoluted
tubule with an
osmolality of
100 mosm/L
CONCENTRATING URINE
Urine passes down
the collecting
tubule back into
the medulla through
a concentration
gradient
300
450
600
750
900
1050
1200
1200
ANTIDIURETIC HORMONE
– Osmoreceptors in hypothalamus monitor osmotic pressure
of extracellular fluid
– ADH is released when osmotic pressure increases
– Produced in hypothalamus
– Secreted from posterior pituitary
– Increases reabsorption of water in distal tubule and
collecting tubule (opening of channels in the cells)
ANTIDIURETIC HORMONE AND TISSUE OSMOLALITY
The collecting tubules are normally impermeable to water.
ADH causes water channels to be inserted in the plasma membrane
ANTIDIURETIC HORMONE AND COLLECTING TUBULES
EFFECT OF ADH
The distal convoluted tubule and the collecting tubules are
normally impermeable to water.
- ADH causes them to become permeable
- Water will then move into the medullary fluid by osmosis
- The urine becomes more concentrated
- Maximum urine concentration is 1200 mosm/L
- Minimum urine concentration is 100 mosm/L
REABSORBING MORE WATER
SODIUM BALANCE - SENSORS
juxtaglomerularapparatus
maculadensa cells
juxtaglomerularcells
Macula densa cells are part of the distal convoluted tubule and are
osmoreceptors. JG cells are smooth muscle and contain renin granules.
SODIUM BALANCE - EFFECTORS
Fine tuning of sodium balance occurs in the distal convoluted
tubules and collecting ducts. Sodium is exchanged for potassium
REABSORBING MORE SODIUM
Aldosterone targets the DCT and collecting ducts:-
sodium channels opened
more sodium transporters synthesised
more potassium channels synthesised
An indirect effect of aldosterone is the promotion of water
absorption because water follows sodium if it can.
REABSORBING MORE SODIUM
MAINTAINING SODIUM BALANCE
THROUGH THE RENIN-ANGIOTENSIN SYSTEM
ANGIOTENSINOGEN ANGIOTENSIN I ANGIOTENSIN II
RENIN
CONVERTING
ENZYME
ARTERIOLE
VASOCONSTRICTION
liver kidney lungs
adrenalcortex
kidney
ALDOSTERONE
ALDOSTERONE PRODUCTION
– Osmoreceptors in the juxtaglomerular apparatus monitor
the volume of extracellular fluid
– Renin is released when the volume decreases
– Renin acts on angiotensinogen
– Angiotensin I is formed in the blood stream
– Angiotensin II is formed in the lungs
– Aldosterone is released from the adrenal cortex
– Sodium is reabsorbed by the distal tubules
– Water follows by osmosis
Increased blood volume causes
increased atrial stretch
atrial myocardial cells stretch
and release
atrial natriuretic peptide (factor)
hypothalamus kidney adrenal cortex
inhibits ADH decreases renininhibits
aldosterone
increases NaCl and water excretion
EXCRETING MORE SODIUM
ERYTHROPOIETIN
peritubular
capillary
network
Interstitial cells of the kidney peritubular capillary bed produce EPO
OXYGEN, EPO AND THE KIDNEY
EPO is also made, to a lesser extent, by cells in the liver.
ACID AND THE KIDNEY
pH homeostasis depends on buffers, the lungs and the kidneys
fatty acids
amino acids
carbon dioxide
lactic acid
Plasma pH 7.35 - 7.45
Buffers (proteins, haemoglobin
bicarbonate and phosphate)
carbon dioxide
lactic acidacid
diet
metabolism
ventilation
renal
acid input
acid output
BLADDER
MICTURITION REFLEX
Bladder fills
Stretch receptors
Parasympathetic
nerve
Bladder contractsInternal urethral
sphincter opens
Only the external urethral sphincter is controlled voluntarily
+
+
Spinal Cord