Excretion• Physiology of ultrafiltration,
reabsorption, tubular secretion
• Counter current theory
• Regulation of urine formation
• Method of urine formation
• Nitrogenous waste
• Regulation of acid0base balance
• Single Cell organism: waste discharge directly via cell surface
• Aquatic animal: amonia
• Terrestrial animal: Urea , Uric acd
Nitrogen is excreted as ammonia,
urea or uric acid
The ammonia, produced from the deamination of amino acids, enters the
ornithine cycle where it is converted into urea
flatworm earthworm
Fig. 49.17(TE Art)
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Glomerulus
Renal cortex
Renal medulla
Bowman'scapsule Proximal
convoluted tubule
Descending limb of loop of Henle
Loop of Henle
Distalconvolutedtubule
Ascending limb of loop of Henle
Collecting duct
To ureter
Vasa recta
Peritubularcapillaries
Blood Supply of a Nephron
– The glomerulus receives blood from a fairly large afferent
arteriole and passes it to a smaller efferent arteriole.
– The efferent arteriole gives rise to the peritubular capillary
system, which surrounds the renal tubule.
Juxtaglomerular Apparatus
– At the point of contact between the afferent and efferent
arterioles and the distal convoluted tubule, the epithelial cells
of the distal tubule form the macula densa.
– Near the macula densa on the afferent arteriole are smooth
muscle cells called juxtaglomerular cells.
– The macula densa together with the juxtaglomerular cells
make up the juxtaglomerular apparatus.
Copyright © 2006 by Elsevier, Inc.
Structure of
the juxtaglomerular
apparatus: macula
densa
Figure 26-17;
Guyton and Hall
Copyright 2009, John Wiley & Sons, Inc.
Nephron• Parts:
– Glomerular apparatus
– Proximal tubule
– Loop of Henle
– Distal tubule
– Collecting ducts
• Types of nephrons:
– Cortical nephrons (glomerularapparatus belong the surface andLoop of Henle only to the outer partof the medulla)
– Intermedial nephrons (in themiddle)
– Juxtamedullary nephrons(glomerular apparatus deep in cortexnear the medulla and Loop of Henleis going deep to the inner part of themedulla)
Function of urinary system• Keeping homeostasis
• Keeping acid-base balance
• Secretion (rennin, kallikrein, erytropoetin)
• The kidneys also help control the rate of red blood cell formation by secreting erythropoietin, and regulate blood pressure by secreting renin.
• The kidneys function to regulate the volume, composition, and pH of body fluids and remove metabolic wastes from the blood in the process.
• Excretion of metabolic waste products: urea, creatinine, bilirubin,
hydrogen
• Excretion of foreign chemicals: drugs, toxins, pesticides, food additives
• Secretion, metabolism, and excretion of hormones
- renal erythropoetic factor
- 1,25 dihydroxycholecalciferol (Vitamin D)
- Renin
• Gluconeogenesis: glucose synthesis from amino acids
• Control of arterial pressure
• Regulation of water & electrolyte excretion
RENAL PHYSIOLOGY
Ultrafiltration – filteringblood under pressure
Selective reabsorption –reabsorbing the usefulsubstances
Production of an iongradient in the medulla – toallow production ofhypertonic urine if necessary
Adjustment of the water andion content of the blood – tomaintain homestasis
.
ULTRAFILTRATION
NET FILTRATION PRESSURE (NFP)
=GBHP – CHP – BCOP
= 55 mmHg 15 mmHg 30 mmHg
= 10 mmHg
GLOMERULAR BLOOD
HYDROSTATIC PRESSURE
(GBHP) = 55 mmHg
Capsular
space
Glomerular
(Bowman's)
capsule
Efferent
arteriole
Afferent arteriole
1
Proximal convoluted tubule
NET FILTRATION PRESSURE (NFP)
=GBHP – CHP – BCOP
= 55 mmHg 15 mmHg 30 mmHg
= 10 mmHg
CAPSULAR HYDROSTATIC
PRESSURE (CHP) = 15 mmHg
GLOMERULAR BLOOD
HYDROSTATIC PRESSURE
(GBHP) = 55 mmHg
Capsular
space
Glomerular
(Bowman's)
capsule
Efferent
arteriole
Afferent arteriole
12
Proximal convoluted tubule
NET FILTRATION PRESSURE (NFP)
=GBHP – CHP – BCOP
= 55 mmHg 15 mmHg 30 mmHg
= 10 mmHg
BLOOD COLLOID
OSMOTIC PRESSURE
(BCOP) = 30 mmHg
CAPSULAR HYDROSTATIC
PRESSURE (CHP) = 15 mmHg
GLOMERULAR BLOOD
HYDROSTATIC PRESSURE
(GBHP) = 55 mmHg
Capsular
space
Glomerular
(Bowman's)
capsule
Efferent
arteriole
Afferent arteriole
12
3
Proximal convoluted tubule
ultrafiltration occurs at the barrier between the blood and the filtrate in
the renal capsule or Bowman's capsule in the kidneys. The Bowman's
capsule contains a dense capillary network called the glomerulus. Blood
flows into these capillaries through the afferent arteriole and leaves
through the efferent arteriole.
The high hydrostatic pressure forces small molecules such as water,
glucose, amino acids, sodium chloride and urea through the filter, from
the blood in the glomerular capsule across the basement membrane of the
Bowman's capsule and into the nephron. This process is called
ultrafiltration. The fluid filtered in this way is called glomerular filtrate.
Glomerular pressure is about 75 millimeters of mercury (10 kPa). It is
opposed by osmotic pressure (30 mmHg, 4.0 kPa) and hydrostatic
pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This
difference in pressure is called effective pressure (25 mmHg, 3.3 kPa).
Physiology of ultrafiltration:
Movement of useful substances (water, salts, glucose)
back into the capillaries.
Filtrate from glomerular capsule flows into the renal
tubule.
Substances are reabsorbed into the surrounding
capillaries.
Sodium is actively pumped out of the epithelial cells of
the renal tubule and diffuses into capillaries.
The increased sodium in the interstitial space creates
high osmotic pressure, which draws water out of the
tubule by osmosis.
REABSORPTION
Physiology of Reabsorption:
Glucose, various amino acid and water are removed from the tubular
fluid and transported into the blood. It is called reabsorption (and not
absorption) because these substances have already been absorbed once
(particularly in the intestines).
Reabsorption is a two-step process beginning with the active or passive
extraction of substances from the tubule fluid into the renal interstitium
(the connective tissue that surrounds the nephrons), and then the
transport of these substances from the interstitium into the bloodstream.
These transport processes are driven by Starling forces, diffusion, and
active transport.
Some key regulatory hormones for reabsorption include:
❖ aldosterone, which stimulates active sodium reabsorption (and
water as a result).
❖ antidiuretic hormone, which stimulates passive water reabsorption.
❖ both hormones exert their effects principally on the collecting ducts.
Tubular reabsorption
Tubular secretion
At the same time that the “good”
substances are being reabsorbed,
wastes (urea) still in the blood are
actively secreted from the capillaries.
SECRETION
Method of Urine formation
Urine Formation
• Proximal convoluted tubule and Peritubularcapillary
• Na+ goes down gradient and brings glucose, amino acids, etc. back into blood stream (cotransport).
• Reabsorbs about 65% of filtrate.
Urine Formation
• Descending limb
• Goes into medulla - increasing salt gradient
• Water leaves
• Fluid concentrates
• Ascending limb
• Goes up toward cortex - decreasing salt gradient
• Na+ pumped out
• Fluid relatively diluted
Countercurrent Multiplication
in the Nephron Loop
Nephron Loop
Collecting duct
COUNTERCURRENT
vs
COUNTERCURRENT EXCHANGER
Tow function types of countercurrent systems are recognized:
Active system and passive system
• The active system are countercurrent multiplier.
• The passive system are countercurrent exchanger.
(countercurrent diffusion exchanger)
In active system metabolic energy is used within countercurrent
system Itself to induce to flux of commodities into or out of the
fluid stream (energy is used to transport NaCl out of the
ascending limb)
The loop of Henle multiplies the sodium concentration within medulla by
retaining the new sodium ions coming from the glomerular filtrate. It is
called Counter Current Multiplier
Urine Composition
– Urine composition varies from time to time and reflects the amounts of
water and solutes that the kidneys eliminate to maintain homeostasis.
– Urine is 95% water, and also contains urea, uric acid, a trace of amino
acids, and electrolytes.
Urine Elimination
• After forming in the nephrons, urine passes from the collecting ducts
to the renal papillae, then to the minor and major calyces, and out the
renal pelvis to the ureters, urinary bladder, and finally to the urethra,
which conveys urine to the outside.
Regulation of Urine Concentration and
Volume
– Most of the sodium ions are reabsorbed before the urine is excreted
under the direction of the hormone, aldosterone
– Normally the distal convoluted tubule and collecting duct are
impermeable to water unless the hormone ADH is present.
- A countercurrent multiplier system is a mechanism that expends
energy to create a concentration gradient. It can refer to the process that is
underlying the process of urine concentration, that is, the production
of hyperosmotic urine by the mammalian kidney
Renal auto control: myogenic, Tuboglomerular
Neuronal control:
Hormonal control:
Copyright © 2006 by Elsevier, Inc.
1. Sympathetic Nervous System
RA + RE GFR + RBF
Control of Glomerular Filtration
3. Angiotensin II
RE GFR + RBF
(prevents a decrease in GFR)
2. Catecholamines ( norepinephrine)
RA + RE GFR + RBF
(Renal Afferent, Renal Efferent, Glomerulus Filtration Rate, Renal Blood Flow)
Copyright © 2006 by Elsevier, Inc.
Control of Glomerular Filtration
5. Endothelial-Derived Nitric Oxide (EDRF)
RA + RE GFR + RBF
4. Prostaglandins
RA + RE GFR + RBF
6. Endothelin
RA + RE GFR + RBF
Copyright © 2006 by Elsevier, Inc.
Control of Glomerular Filtration
7. Autoregulation of GFR and Renal Blood Flow
• Myogenic Mechanism
• Macula Densa Feedback
(tubuloglomerular feedback)
• Angiotensin II ( contributes to GFR but
not RBF autoregulation)
Copyright © 2006 by Elsevier, Inc.
Myogenic Mechanism
Stretch of
Blood Vessel
Cell Ca++
Permeability
Arterial
Pressure
Intracell. Ca++Blood Flow Vascular
Resistance
Copyright © 2006 by Elsevier, Inc.
Macula densa
feedback
mechanism
for GFR
autoregulation
Figure 26-18;
Guyton and Hall
Copyright © 2006 by Elsevier, Inc.
Regulation of GFR by Ang II
GFR Renin
AngII
Efferent Arteriolar
Resistance
Macula Densa NaCl
BloodPressure
When the sodium chloride concentration in the tubular fluid decreases, the macula densa senses
these changes and causes the juxtaglomerular cells to secrete renin.
The heart can also increase filtration rate when blood volume is high.
Glomerular filtration rate is relatively constant, although sympathetic impulses may
decrease the rate of filtration.
ACID-BASE REGULATION
Maintenance of an acceptable pH range in the
extracellular fluids is accomplished by three
mechanisms:
1) Chemical Buffers (blood)
React very rapidly
(less than a second)
2) Respiratory Regulation (lung)
Reacts rapidly (seconds to minutes)
3) Renal Regulation (kidney)
Reacts slowly (minutes to hours)
Kidneys role in acids and bases control• Bicarbonate is filtered and enters nephron at Bowman’s capsule
• Proximal convoluted tubule
– Can reabsorb all bicarbonate (when need it to neutralize excessive acids in body)
OR
– Can reabsorb some or NONE of the bicarbonate (have too much base in body and it needs to be eliminated)
How can the kidneys control acids and
bases?
• Acidosis
• Intercalated cells
– Secrete excessive hydrogen
– Secreted hydrogen binds to buffers in the lumen (ammonia and phosphate bases)
– Secretion of hydrogen leads to formation of bicarbonate
HPO4-
NH3