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The Excretory System
• Excretion is the removal of the waste
products of metabolism from living
organisms.
• The accumulation of these waste products
would be toxic if they were not eliminated.
• In plants and simple animals, waste
products are removed by diffusion. Plants,
for example, excrete O2, a product of
photosynthesis.
METABOLIC WASTE A BY-PRODUCT OF…
Water
Carbon dioxide
Dehydration synthesis
and cellular respiration
Cellular respiration
Salts
Neutralization reactions
Urea
Deamination (removal of
amino group from amino
acids
Nucleic acid breakdown
Uric acid
Organs of Excretion
• There are four major
organs involved in
excretion:
1. The lungs
2. The liver
3. The skin
4. The kidneys
THE LUNGS
• Cellular respiration occurs in every living cell in
your body.
• Carbon dioxide is produced as a waste
product. As the carbon dioxide accumulates in
body cells, it eventually diffuses out of the cells
and into the bloodstream, which eventually
circulates to the lungs.
• In the alveoli of the lungs, carbon dioxide
diffuses from the blood, into the lung tissue,
and then leaves the body every time we
exhale. Some water vapor also exits the body
during exhalation.
THE SKIN
• As you already know, sweat comes out of
pores in your skin.
• Sweat is a mixture of three metabolic wastes:
water, salts, & urea.
• So as you sweat, your body accomplishes
two things:
1) sweating has a cooling effect on the body,
2) metabolic wastes are excreted.
• The skin is made up
of two layers:
1. The thin
epidermis at the top
2. The thicker
dermis below. This
is where oil glands,
hair follicles, fatty
layers, nerves, and
sweat glands are
found.
• The sweat gland is a tubular structure
tangled with capillaries (the smallest of
blood vessels).
• This close association of tubes allows
wastes (water, salts & urea) to diffuse from
the blood & into the sweat gland.
• When body temperature rises, the fluid
(sweat) is released from the gland, travels
through the duct, and reaches the skin
surface through openings called pores.
THE LIVER
• The liver is a large, important organ. In fact
it is the largest internal organ in our
bodies. Its numerous functions make it
"part" of the circulatory, digestive, and
excretory systems.
• In excretion, the liver removes the NH2
from amino acids in proteins. This is called
deamination
• The by-product of deamination is ammonia,
a water-soluble gas.
• Ammonia is extremely toxic so it is
combined with CO2 to make urea.
• Urea is much less toxic and can dissolve in
the blood for excretion through the sweat or
through urine.
• Uric acid is formed by the breakdown of
nucleic acids in the liver.
THE KIDNEY
•
•
•
The kidney is the major organ of excretion.
The kidney is also a major regulatory organ.
Through the formation of urine, it is responsible
for the following:
1. Removal of organic wastes: urea, uric acid and
the breakdown products of hemoglobin and
hormones
2. Regulation of concentrations of important ions:
sodium, potassium, calcium, magnesium, sulfate
and phosphate ions
3. Regulation of pH balance of body: control
levels of H+, HCO3- and NH4+
4. Regulation of Red Blood Cell production.
The kidneys release erythropoietin, which
regulates the production of RBCs in the
bone marrow.
5. Regulation of blood pressure: regulation
of fluid volume of the body.
6. Limited control of blood glucose and
blood amino acid concentration: eliminate
excess amounts
Structure of the Kidney
• Renal Capsule: A smooth semitransparent
membrane that adheres tightly to the outer
surface of the kidney.
• Renal Cortex: The region of the kidney just
below the capsule. This part of the kidney is
rich is arterioles and venules.
• Renal Medulla: The region
below the cortex that is
segregated into triangular
regions. The triangular
regions are the renal
pyramids, which are striated
(or striped) in appearance
due to the collecting ducts
running through them.
• Renal Pelvis: A cavity
within the kidney that is
continuous with the ureter.
The pelvis has portions that
extend towards the renal
pyramids. These extensions
are called calyces.
The Nephron
• The functional unit of the kidney is the
nephron. Each kidney contains over one
million of these microscopic filters.
• About 20% of the total blood pumped by
the heart each minute will enter the kidneys
to undergo filtration. This is called the
filtration fraction
The nephron
Urine Formation
• Occurs in the nephron through three
processes:
1. Glomerular Filtration
2. Tubular Reabsorption
3. Tubular Secretion
Glomerular filtration
• The transfer of fluid and solutes from the
glomerular capillaries into Bowman’s capsule.
• Blood enters the glomerulus under pressure. This
causes water, small molecules (urea, glucose,
amino acids) and ions to filter through the
capillary walls into Bowman’s capsule.
• Large blood components (RBCs, WBCs, platelets
and proteins) cannot filter through.
• The fluid in the Bowman's capsule appears
very much like interstitial fluid without the
proteins. It is called the nephric filtrate
• The glomerular capillaries are substantially
more (100 to 1000X) leaky than regular
capillaries and have 2-3 times more
pressure than regular capillaries.
• Glomerular filtration rate is fairly constant.
(130 ml/min or 7.8 l/hr). This means that
about 190 L of filtrate is formed every 24
hours by both kidneys
Tubular Reabsorption
• Occurs in the Proximal convoluted tubule, the
Loop of Henle and the Distal convoluted Tubule.
• Materials which are required by the organism are
returned to the bloodstream: water, ions, glucose,
amino acids etc…
• The kidney does not work by a process of
identifying what is bad; rather it works by
identifying those things that are good for the body
• Much of the urea is lost
simply because the kidney
chooses not to recover it
after it has been filtered.
• Any small foreign molecule
that has entered our blood,
even if it has not existed in
human evolutionary history
(drugs, new pollutants, etc)
can be removed by the
kidney.
Proximal convoluted tubule
• The cells of the
tubule are lined
with microvilli.
Why?
• Reabsorption of
glucose, amino
acids, and most
inorganic salts
occurs here.
• Na+ ions are transported out of the
tubule by active transport, through
carrier molecules.
• Cl- ions and HCO3- ions follow by
charge attraction.
• As these solutes move out of the
tubule, they create an osmotic
gradient and water moves out of the
tubule and back into the blood,
through osmosis.
• About 80-85% of the water in the
filtrate is reabsorbed in the proximal
tubule.
• Glucose and amino acids attach to carrier
molecules and are transported out by active
transport.
• This requires a lot of energy so there are
many mitochondria in the cells of the
proximal tubule.
• There is a limit to the amount of sodium,
glucose and amino acids that can be
reabsorbed by the carrier molecules: the
threshold limit. When this limit is reached,
these substances are excreted in the urine.
• H+ ions are secreted into the
proximal tubule. This helps
regulate pH.
• About 50% of the urea that was
in the nephric filtrate is
reabsorbed in the tubule. This is
a passive mechanism. The rest
is excreted in the urine.
The loop of Henle
• The descending loop of Henle
is permeable to water. Water
is reabsorbed into the
peritubular capillaries by
osmosis.
• The filtrate decreases in
volume,but increases in
osmotic concentration.
• Salt (NaCl) becomes
concentrated in the filtrate as
the loop penetrates the inner
medulla of the kidney.
• The ascending loop of Henle is permeable
to salt but not to water. Sodium is actively
transported out of the filtrate and chlorine
follows by charge attraction.
• The volume of the filtrate does not change,
but the concentration decreases.
• The peritubular capillaries ensure a rich
blood supply for reabsorption
The Distal Convoluted Tubule
• More sodium is reclaimed by active transport, and
still more water follows by osmosis.
• Although 97% of the sodium has already been
removed, it is the last 3% that determines the final
balance of sodium.
• This determines the water content and blood
pressure in the body.
• The reabsorption of sodium in the distal tubule
and the collecting tubules is closely regulated,
chiefly by the action of the hormone aldosterone.
The Collecting Duct.
• The filtrate now flows
into the collecting duct,
which gathers fluid from
many nephrons.
• Urine flows from the
collecting ducts into the
renal pelvis to the ureters
and into the bladder.
• Water regulation occurs here.
• The plasma membranes of the cells of the
distal tubule and the collecting duct have
transmembrane channels made of a protein
called aquaporin.
• When these channels are open, water can
pass through very quickly. These water
channels are responsive to levels of
antidiuretic hormone (ADH).
Tubular secretion
• This is the third mechanism of excretion.
• Secretion occurs when substances are
transported from the blood directly into the
distal tubule.
• Hydrogen, potassium, and ammonium ions
are actively secreted into the tubule. This
helps to regulate pH.
• If the pH of the blood becomes too acid,
more H+ ions are secreted into the tubule.
• If the pH of the blood becomes too alkaline,
then secretion of H+ is reduced.
• The pH of urine can vary from 4.5 to 8.5
• Certain drugs, such as penicillin, are also
secreted into the tubule for excretion.
• Secretion occurs in the proximal tubule, the
distal tubule and the collecting duct.
• While we think of the kidney as an organ of
excretion, it is more than that.
• It does remove wastes, but it also removes
normal components of the blood that are
present in greater-than-normal
concentrations and reclaim these
components when they are present in the
blood in less-than-normal amounts.
• Thus the kidney continuously regulates the
chemical composition of the blood within
narrow limits. The kidney is one of the
major homeostatic devices of the body.
Urine Testing
• Color: Normal urine will vary from light
straw to amber color.
• The color of normal urine is due to a
pigment called urochrome, which is the
end product of hemoglobin breakdown:
• Hemoglobin  hematin  bilirubin
urochromogen  urochrome
• The following changes in colour have pathological
implications:
a. Milky: presence of pus, bacteria, fat.
b. Reddish amber: presence of urobilinogen or
porphyrin. Urobilinogen is produced in the
intestine by the action of bacteria on bile
pigments. Porphyrin may be evidence of liver
cirrhosis, jaundice, Addison's disease, and other
conditions.
c. Brownish yellow or green: presence of bile
pigments.
d. Red to smoky brown: presence of blood and
blood pigments.
• Carrots may cause increased yellow color
due to carotene, while beets cause
reddening and rhubarb may cause the urine
to become brown. These food items and
certain drugs may color the urine, yet have
no pathological significance.
Glucose
• glucose is not normally present in the urine
because all of it is usually reabsorbed from
the renal tubules into the blood.
• When the glucose concentration of the
filtrate is within the normal limits (70-110
mg per 100 ml), there is a sufficient number
of carrier molecules in the renal tubules to
transport all the glucose back into the blood.
• However, if the blood glucose level exceeds
its threshold (for glucose, about 180 mg per
100 ml), there will not be enough carrier
molecules to reabsorb all of the glucose.
• The untransported glucose will end up in
the urine and result in a condition known as
glycosuria.
• The main cause of glycosuria is diabetes
mellitus
Ketones (Ketonuria)
• Normally, no ketones are present in urine.
• Detectable levels of ketone may occur in
urine during physiological stress conditions
such as fasting, pregnancy, and frequent
strenuous exercise.
• When there is carbohydrate deprivation,
such as in starvation or high-protein diets,
the body relies increasingly on the
metabolism of fats for energy.
• This pattern is also seen in people with
diabetes mellitus, where the lack of insulin
prevents the body cells from utilizing the
large amounts of glucose available in the
blood.
• When the production of the intermediate
products of fatty acid metabolism (ketone
bodies) exceeds the ability of the body to
metabolize these compounds, they
accumulate in the blood and spill over into
the urine (ketonuria).
pH
• Freshly voided urine is usually acidic (around pH
6) but the normal range is between 4.8 and 7.5.
• The pH will vary with the time of day and diet.
High acidity is present in acidosis, fevers, and
high protein diets. Excess alkalinity may be due to
urine retention in the bladder, chronic cystitis,
anemia, and obstructing gastric ulcers.
Protein
• Since proteins are very large molecules, they are
not normally present in measurable amounts in the
glomerular filtrate or the urine.
• The detection of proteins in the urine, therefore,
may indicate that the permeability of the
glomerulus is abnormally increased.
• This may be caused by renal infections
(nephritis), or it may be caused by other diseases
that affect the kidney,such as diabetes mellitus,
jaundice, or hyperthyroidism.