Excretory System
Excretion
is the process by which the body rids itself
of metabolic wastes.
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The lungs eliminate carbon dioxide
The large intestine eliminates toxic digestive
waste
The liver changes toxin and products of
protein metabolism into soluble compounds
that the kidney can collect and eliminate.
The role of the liver:
Excess protein is converted to carbohydrates by a
process called deamination. This involves the
removal of an amino group from amino acids to
produce a carbohydrate and ammonia (a toxic gas).
Two molecules of toxic ammonia react with carbon
dioxide to produce urea. Urea is about 100,000 x
less toxic than ammonia and can be safely
transported through the bloodstream.
Nucleic acids are also broken down into uric acid.
Excess uric acid can cause kidney stones or gout.
All of the waste products made by the liver travel
through the blood and are then filtered by the kidney.
The role of the kidneys:
1. Main role: Removal of poisonous
nitrogenous wastes
2. Maintenance of blood pH
3. Maintenance of water balance
4. Maintain blood pressure
Urinary System
Blood is carried to the paired kidneys from
renal arteries that branch off the aorta.
The kidney’s can hold as much as 25% of
the entire blood supply at any given time.
Wastes are then filtered by the kidneys
and taken to the urinary bladder via the
ureters.
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Urinary System Anatomy
A sphincter muscle at the base of the urinary
bladder acts as a valve and released stored urine
through the urethra when the muscles relax. The
signal to urinate is relayed to the brain when the
bladder is about 200 mL full of urine. The liquid in
the bladder cause the walls to stretch, stimulating
receptors. At 400 mL volume, the signal will be of
increasing strength and urgency, and at a 600 mL
volume, the sphincter muscles relax, voluntary
bladder control is lost and urination occurs.
Howstuffworks "Urination"
Remember, that when you drink fluids, the fluid
goes to your stomach first, and is then absorbed
by the body. The loss of bladder control only
occurs when much more than 600 mL is
processed, absorbed, filtered and finally
collected in the bladder.
The bottom line:
You won’t lose bladder control from drinking a
Big Gulp or Slurpee. Input doesn’t equal output
when it comes to excretion. Your body retains
much of the fluid it consumes for normal
processes.
The Kidneys
Cortex
A cross section of the
kidney reveals the
anatomy of three
major structures:
1. The cortex
2. The medulla
3. The renal pelvis
Renal
Pelvis
Medulla
Detailed Kidney Structure
A cross-section of the kidney reveals the
anatomy of three major structures: cortex,
medulla, and renal pelvis
ureters – tubes that conduct urine from the
kidneys to the bladder
renal artery - delivers blood to the kidney
renal vein – sends blood back to the body
renal calyces – outer extensions of the renal
pelvis that filter blood
cortex – outer layer of connective tissue
medulla – inner layer beneath the cortex, hold
the major part of the nephron
renal pelvis – hollow chamber that joins the
kidney with the ureter
The Nephron
Blood Pathway:
The nephron is the functional unit of the kidney.
There are about 1 million of these slender
tubules in the kidney. These tubules are
supplied with blood from the afferent arterioles
which branch from the renal artery. They lead
into a high pressure capillary bed called the
glomerulus. This is where filtration occurs.
Blood leaves the glomerulus via the efferent
arterioles and is carried to a capillary network,
called the peritubular capillaries, that wraps
around the kidney tubule. Blood is then
transferred to a renal vein. Then back to the
venous blood system
http://www.wisconline.com/objects/AP2504/AP2504.swf
Filtrate Pathway:
The glomerulus is surrounded by a cup-like
portion of the nephron called Bowman’s
capsule. The cortex contains Bowman’s
capsule, afferent and efferent arterioles. Fluids
to be processed into urine enter Bowman’s
capsule from the glomerulus. Next, fluids move
through the proximal/distal tubules and then
into the collecting ducts which collect urine
from many different nephrons and merge into
the renal pelvis.
kidney patient guide - healthy kidney
Urine Formation- Removal of Wastes
Formation of urine depends on filtration, re-absorption and
secretion.
Filtration – is accomplished by the movement of fluid into
Bowman’s capsule. Blood moves through the afferent
arteriole to the glomerulus and small dissolved solutes
(glucose, aa, urea, uric acid, ammonia, and salts) and
water pass into Bowman’s capsule (much lower
pressure). The pressure gradient allows only some
substances to be filtered. Red blood cells, plasma
proteins, white blood cells, platelets are too large to
pass through.
Nephron Information Center
Re-absorption – involves the transfer of essential
solutes (glucose, aa, vitamins, salts, K+,and
H+) and water from the nephron, back into the
blood. This is very important in maintaining
the body’s water balance. About 85% of
filtrate is re-absorbed. (Remember the Big
Gulp: If you drink a 1L Big Gulp, about 850 mL
of the fluid is reabsorbed, and only 150 mL is
collected in the bladder. You don’t even feel
like you have to “go” yet.)
-Re-absorption mostly occurs in the proximal
tubule, with some minor solutes being reabsorbed in the distal tubule
-Most water re-absorption occurs in the
descending limb of the Loop of Henle
Steps involved in re-absorption:
1. Na+ is actively transported out of the nephron.
2. Cl- and HCO3- follow Na+ by charge
attraction.
3. The resulting osmotic gradient draws water
from the nephron into the blood. (Remember,
water follows salt)
4. An additional osmotic gradient is created by
plasma proteins that stay in the blood. (They
were too big to pass through Bowman’s capsule)
5. Urea and uric acid may diffuse out as well, but
will be reabsorbed later
Re-absorption occurs
until a threshold level
of substance is
reached. Excess
amounts of glucose and
salts in the blood will
not be re-absorbed and
will be excreted in the
urine. This is why urine
tests can tell us about
our blood chemistry.
Tubular Secretion – involves the movement of
materials from the blood back into the
nephron. Nitrogen containing wastes (urea,
uric acid), histamines, excess H+ (that
regulate pH), minerals, drugs, penicillin, etc.
are all removed from the body by the cells in
the distal tubule, which actively transports
these substances back to the nephron. The
distal tubule contains loads of mitochondria to
keep up with this energy demand.
http://cpharm.vetmed.vt.edu/VM8314/Nephr
onMovie.swf
Regulating Water Balance
Antidiuretic Hormone
(ADH) helps regulate the
osmotic pressure of body
fluids by causing the kidneys
to increase water reabsorption, producing more
concentrated urine. ADH
makes the distal tubules
more permeable to the last
15% of water can be reabsorbed into the blood.
Special nerve receptors called
osmoreceptors located in the
hypothalamus in the brain
detect changes in the osmotic
pressure of the blood and
stimulate or inhibit the secretion
of ADH. These receptors also
elicit the thirst response,
inspiring you to increase fluid
consumption. Substances such
as alcohol and caffeine
decrease the release of ADH,
resulting in increased urine
output and dehydration.
Blood Pressure
The kidneys regulate blood pressure by
regulating the amount of fluid in the blood. More
fluid means higher pressure. The hormone
aldosterone, acts on the nephrons to increase
sodium (salt) re-absorption from the distal
tubule, back into the blood. Chloride ions and
water will follow, causing the blood volume to
increase. Aldosterone is secreted by the
adrenal cortex, just above the kidney. A drop in
blood pressure is detected by the
juxtaglomerular apparatus, located near the
glomerulus. This causes the release of liver
proteins, angiotensinogen and rennin, which
stimulate the release of aldosterone from the
adrenal gland.
http://www.wisconline.com/objects/AP2204/AP2204.swf
pH Balance
Despite the variety of foods and fluids with
different pH levels we consume, our bodies
maintain a relatively constant pH between about
7.3 and 7.5. A relatively stable pH is maintained
by a buffer system that absorbs excess H+ ions
or basic ions. Bicarbonate ions (HCO3-) are key
components of this system:
HCO3- + H+ === H2CO3 === H2O + CO2
Bicarbonate ions in the blood remove excess H+ ions,
but the buffer must be restored for this system to
continue working indefinitely. The kidneys reverse this
reaction. Carbon dioxide is actively transported from the
peritubular capillary and combines with water to produce
HCO3- and H+ ions. The bicarbonate ion diffuses back
into the blood and the H+ ions combine with phosphate
or ammonia and are excreted in the filtrate.
The pH balance is mostly controlled by the distal tubule
of the nephron
Acid-Base Balance
Kidney Dysfunction
Diabetes Mellitis – is caused by the
inadequate secretion of insulin from the
pancreas. Without insulin, blood glucose
levels are extremely high, and excess
glucose remains in the nephron. The high
osmotic gradient prevents water reabsorption and increases urine production.
Diabetes Insipidus – is caused by
inadequate production of Anti-Diuretic
Hormone (ADH). Without ADH, urine input
increases dramatically, as much as 20 L per
day and the patient will be extremely thirsty.
Nephritis – ‘Bright’s Diseases’ is inflammation of
the nephrons which can have a variety of causes.
Protein in the urine is a common symptom of
nephritis. The osmotic gradient also causes an
increase in urine production. Nephritis can lead to
irreversible kidney damage and eventual kidney
failure.
Kidney Stones – are caused by
the precipitation of mineral solutes
form the blood. The stones lodge in the
renal pelvis or the ureter, causing major pain and
bleeding. Stones can be removed by surgery or by
using ultrasonic waves that blast the stones into
smaller fragments.
Dialysis – is used for patients whose kidneys no
longer function properly.
Hemodialysis – a machine is connected to the
patient’s circulatory system by a vein. Blood is
pumped through a series of tubes submerged in
solutes that removed waste from the blood.
kidney patient guide - Haemodialysis animation
Peritoneal Dialysis – is done through the lining
of the abdominal cavity. A catheter tube is
inserted and solution is fed into the abdominal
cavity for two to six hours. This fluid collects
wastes from the body and is drained from the
catheter when the process is complete.
kidney patient guide - How Peritoneal Dialysis
works animation
Kidney Transplant
kidney patient guide - Kidney Transplant
animation