• If you knew there was poison hidden in your house,
you would surely do everything possible to find and
remove that poison.
• If you didn't, you and your family would slowly die.
• How would you find and then
remove it?
• You would probably figure out
a system of searching and
removing.
• That is what the excretory
system does!!
1. Filters the blood
2. Removes wastes from the blood
3. Keeps the amount of water and salts in the
blood constant
4. Helps to regulate Blood Pressure
Humans will die if they lose about
12% of their body water.
The threat of desiccation is the most
important problem confronting
terrestrial life.
How do we control our loss of water?
1. Multiple layers of dead, keratinized skin cells
2. Drink and eat moist foods
3. Nervous and hormonal mechanisms control thirst
4. Behaviour: (ie: stay out of intense heat)
5. Kidneys and other excretory organs help conserve water.
Water loss can be considerable: a
person in a 38oC temperature will
lose 1 Litre of H2O/hour.
We acquire most of our water in our
food and drink, and obtain a smaller
amount by dehydration synthesis
and as a by-product of cellular
respiration.
We lose water by urinating,
defecating, sweating, and breathing.
The Excretory System is made up of the kidneys, liver,
lungs, and skin. Each plays a role in excretion.
Skin: sweat with water, salts, heat and some urea
Lungs: excretes carbon dioxide, water, and heat
Liver: gluconeogenesis – makes urea as a byproduct
Intestines: excretes certain heavy metals
Kidneys: the main excretory organ. Excrete toxins,
drugs, wastes (urea…), and water.
KIDNEY
Kidney:
• Bean-shaped
• 10 cm long
• Lower, dorsal part
of the abdomen
•Blood enters the
kidney via the renal
artery, is cleaned,
and leaves the
kidney in the renal
vein
The kidneys
account for
less than 1%
of the weight
of the human
body, but
they receive
about 20% of
the blood
pumped with
each
heartbeat.
Renal cortex
Renal cortex
renal medulla
Renal cortex
renal medulla
renal pelvis
Renal cortex
renal medulla
renal pelvis
Renal pyramid
Renal cortex
renal medulla
renal pelvis
Renal pyramid
Ureter
CROSS SECTION OF KIDNEY
KIDNEY
Ureter:
Waste fluid made
in the kidney
exits through a
duct called a
ureter.
Moves the urine
to the bladder
via peristalsis
URETER
KIDNEY
Bladder:
stores the urine
An average
bladder can hold
a maximum of
1.5 to 2 cups of
urine.
When full, the
BLADDER
sphincter muscles
control the release
of urine from the
bladder
URETER
KIDNEY
Urethra: when the
sphincter relaxes, the
urethra carries the urine
outside the body.
20 cm in males
URETER
4 cm in females
BLADDER
Females are more prone
to infections of the
urinary system.
URETHRA
The kidneys perform a number of homeostatic functions, as they
are the chief regulators of our internal environment:
1. Regulates blood volume and osmotic balance by excreting or
conserving water as the situation demands.
2. Regulates the ionic balance of the blood by controlling the
excretion of inorganic salts (especially sodium).
3. Regulates Blood pH by excreting excess acids or base
4. Excretes toxic metabolic by-products such as urea, ammonia,
uric acid, and creatine (a product of muscle activity).
Nitrogen wastes are a by-product of protein metabolism.
When proteins are turned into glucose during
gluconeogenesis, ammonia is made.
Because ammonia is very toxic and must be diluted as it
travels around the body, however, terrestrial animals usually
need to conserve water.
What do we do? The liver converts the ammonia
to urea, which is then transported to the kidneys,
where it is concentrated and excreted out of the
body as urine. This takes a lot of energy.
The functional unit of the kidney is
the NEPHRON, which consists of a
renal tubule and its associated
blood vessels.
Each kidney contains
approximately 1 million nephrons,
which represents approximately
80km of tubules.
Water, urea, salts, and other small molecules
in the blood flow from the capillaries into the
renal tubules, where the fluid is now called
FILTRATE.
The epithelial cells that line the renal tubule
adds and removes things from the filtrate to
eventually form URINE.
From the 1100 to 2000L of blood that flows
through the human kidneys each day, the
nephron processes about 180L of filtrate, but
excretes only ~1.5L of urine.
The rest of the filtrate, including ~99% of the
water, is reabsorbed into the blood.
filtrate
blood
1. Bowman’s Capsule: this cup shaped receptacle is the blind end
of the renal tubule, which receives filtrate from the blood. It
encloses a ball of capillaries called the glomerulus.
2. The proximal convoluted tubule (PCT): most of the important
things (nutrients, water, salt…) are reabsorbed from the PCT.
3. The ascending and descending loop of Henle: a lot of salt and
water are reabsorbed from Loop of Henle.
water
water
salt
salt
4. The distal convoluted tubule (DCT): the blood dumps things it
wants to get rid of into the DCT so it can be removed in the urine.
acids
drugs
5. The collecting duct collects filtrate from many tubules and pass
the urine into the renal pelvis. Water and urea are also reabsorbed
here.
water
urea
1. Afferent Arteriole: this arteriole enters the bowman’s capsule
from the renal artery .
Renal artery
Renal vein
2. Glomerulus: the capillaries of the glomerulus are porous and
very twisted up. They sit within the Bowman’s capsule.
Renal artery
Renal vein
Bowman’s
Capsule
3. Efferent Arteriole: this arteriole leaves the bowman’s capsule.
The blood is very hypertonic as most of the plasma has left the
blood. The blood carries RBC, WBC, platelets and blood proteins.
Renal artery
Renal vein
4. Peritubular Capillaires: these are the capillaries that surround
the Proximal and Distal convoluted tubules.
Renal artery
Renal vein
5. Vasa Recta: these are the capillaries that surround the Loop of
Henle.
Renal artery
Renal vein
The Nephron has
4 main functions:
1. Pressure Filtration: the blood pressure forces water and
solutes through the pores of the glomerulus into the Bowman’s
capsule.
The holes in the glomerulus are small, so they are permeable to
water and small solutes, but not to blood cells or blood proteins.
The filtrate contains a mixture of solutes such as salts, nutrients,
water and other small molecules.
At this point, the concentration of substances in the blood plasma
is equal to that of the filtrate.
salts, nutrients (glucose, fatty acids,
glycerol, nucleotides, amino acids),
water, hormones, antibodies, water
soluble vitamins, penicillin and other
drugs, histamines, nitrogenous wastes
(urea, uric acid…) and other small
molecules.
salts, nutrients (glucose, fatty acids,
glycerol, nucleotides, amino acids),
water, hormones, antibodies, water
soluble vitamins, penicillin and other
drugs, histamines, nitrogenous wastes
(urea, uric acid…) and other small
molecules.
Composition of Plasma, and Glomerular Filtrate
Urea
Glucose
Amino acids
Water
Plasma
0.03
0.10
0.05
52.0
Filtrate
0.03
0.10
0.05
52.0
Salts (ie: Na+, Cl-)
0.9
0.9
Protein
RBC, WBC, Platelets
8.0
Lots
None
None
2. Selective Reabsorption: since filtration is nonselective, the
body MUST make sure that the important molecules are
returned to the blood plasma as soon as possible.
Nearly all of the nutrients
(esp. sugar), salts and
water are reabsorbed.
This happens at the PCT
and it is a very selective
process. There are a lot
of mitochondria in the
cells of the PCT.
Why? This costs
ENERGY!!
WHAT IS REABSORBED? : 70% of the water, and 75% of the
salts, glucose, amino acids, and other nutrients move back into
the blood.
The positive sodium ions are actively transported into the
interstitial fluid and the negative chlorine ions follow passively,
and finally, water follows by osmosis.
The glucose, amino acids, and other
nutrients are actively transported
into the blood.
Lots of ATP required!!
Recall that osmosis is the
movement of water (from
[High] to [Low])across a
selectively permeable
membrane.
This movement occurs
whenever two solutions
separated by a membrane
differ in total solute
concentration or osmolarity.
When two solutions differ:
The one with the greater concentration
solutes (salt, urea) is HYPEROSMOTIC
The one with the more dilute
solution is HYPOOSMOTIC
Hyperosmotic
of
Hypo-osmotic
(watery)
Hyperosmotic
Remember:
Water always moves towards a hyperosmotic environment.
SALT SUCKS!!
Changes in the body fluids are controlled by a variety of regulatory
mechanisms, usually involving negative feedback.
The control of water gain/loss is called OSMOREGULATION.
3. Water Reabsorption: water reabsorption occurs mainly at the
loop of henle and in the collecting duct.
The nephron pumps out salt and urea in these areas to make the
blood and medulla extremely hyperosmotic.
Water then moves back in
to the blood by osmosis.
It is important to note that
the blood in the vasa recta
moves in the OPPOSITE
DIRECTION that the filtrate
is moving.
This allows the nephron to
create a hyperosmotic
environment around the
descending loop of henle.
So we’ll talk about the
ascending Loop of Henle 1st.
The ASCENDING LOOP OF HENLE:
This part of the loop of henle is NOT
PERMEABLE to water, but the salt can move.
So, the NaCl passively and actively moves out
of the tubule and into the blood and the
medulla of the kidney.
This creates a very HYPERTONIC solution in
the blood.
As the filtrate moves towards the DCT, most
of the salt has left and the filtrate becomes
more and more dilute.
The DESCENDING LOOP OF HENLE:
The loop of henle continues the reabsorption of
water as the filtrate moves from the cortex and
down into the concentrated medulla.
The epithelium of this portion of the renal
tubule is permeable to water, but not very
permeable to salt and other solutes.
Lots of WATER moves into the HYPERTONIC
blood (created at the ascending loop) by
osmosis as it follows the salt.
As a result of this great loss of water, the filtrate
becomes more and more concentrated.
The COLLECTING DUCT:
Water is also reabsorbed here.
The epithelium of this duct is permeable to
water and urea.
As the duct moves down into the VERY
HYPERTONIC medulla, the filtrate loses
more and more water by osmosis as it
moves back into the blood.
This loss of water concentrates the urea in
the filtrate, and because there is so much
UREA in the collecting duct, some of it
diffuses down its concentration gradient
and back into the blood and medulla.
The kidney is made up of two sections: the cortex and the medulla.
The medulla is HYPEROSMOTIC as it has high concentrations of the
solutes NaCl and urea.
renal medulla
HYPERTONIC
renal cortex
4. Secretion: if the blood still needs to rid itself of other ions and
waste products (hydrogen ions, ammonia, potassium ions), it will
secrete them into the Distal Convoluted Tubule (DCT).
This is a very selective process that involves both active and passive
transport. Lots of mitochondria in the DCT too! ATP required!!!
REVIEW OF NEPHRON:
http://www.youtube.com/watch?v=glu0dzK4dbU
Composition of Plasma, Glomerular Filtrate, and Urine (g/100 ml of fluid).
Plasma
Filtrate Final Urine % Reclaimed
Urea
Glucose
Amino acids
Water
0.03
0.10
0.05
52.0
0.03
0.10
0.05
52.0
Salts (ie: Na+, Cl-)
0.9
0.9
Protein
RBC, WBC, Platelets
8.0
Lots
None
None
1.8
None
None
1.0
50%
100%
100%
99%
< 0.9-3.6 99.5%
None
None
None
None
If the blood is ACIDIC, 2 things will happen:
HCO3H+
1. The bicarbonate ions are actively transported back into the blood.
The HCO3- is an important buffer which will join with H+ to remove it
from the solution.
2. The blood will also secrete hydrogen ions into the filtrate so we will
pee it out.
If you are dehydrated, the kidneys can excrete a small,
concentrated volume of urine and reabsorb the majority
of water from the filtrate. You will have very little dark
yellow urine.
If you have consumed an excessive amount of fluid, the
kidneys can excrete a large, dilute volume of urine, with
very little water being reabsorbed from the filtrate. You
will have lots of pale yellow urine.
How do we maintain the proper amount of water in our
bodies? It is controlled homeostatically with 2 hormones
are ADH and Aldosterone.
FIRST HORMONE: ANTIDIURETIC HORMONE (ADH)
This hormone controls water reabsorption with a negative
feedback cycle.
The hypothalamus makes ADH and it is stored and released from
the posterior pituitary gland.
It is released in response to an
increase in the osmolarity of the
blood (very concentrated blood) when
you are dehydrated (inadequate intake
of water).
When you have low blood volume (high osmolarity), the
osmoreceptor cells in the hypothalamus triggers the posterior
pituitary to release ADH.
ADH increases the permeability of the DCT and collecting ducts to
water.
This causes more water to be reabsorbed and ultimately increases
blood volume (amount of blood).
At the same time, the removal of water from the filtrate increases
the concentration of the urine. You urinate less and it will be dark
yellow.
When you have high blood volume and low osmolarity, sensors in
the heart signal the hypothalamus to cause a reduction of the
amounts of ADH in the blood.
This decreases the amount of water that is reabsorbed into the
blood, and large quantities of a more dilute urine are produced.
This is a negative feedback cycle, because as the osmolarity of the
blood is reduced (blood volume increases) less ADH will be
secreted.
Alcohol can perturb water balance by inhibiting the release of
ADH, causing excessive loss of water in the urine and dehydrating
the body.
This leads to the symptoms of a hangover. Which is why you
should always drink a lot of water when drinking alcohol. This
keeps you hydrated.
Second Hormone: ALDOSTERONE
There is tissue surrounding the afferent arteriole called the
juxtaglomerular apparatus (JGA) which measures Blood Pressure.
• When the BP drops, or if the [Na+ ] in
the blood is too low (due to diarrhea, a
severe injury…), the JGA releases an
enzyme called RENIN.
• The liver makes a protein called
ANGIOTENSIN which is always
circulating around the blood.
• RENIN activates angiotensin.
The active form of ANGIOTENSIN does two things:
1. It stimulates the adrenal gland to release the
hormone ALDOSTERONE.
Aldosterone tells the DCT to reabsorb lots of
Na+, and water follows the sodium by
osmosis. This increases the amount of
water in the blood, which increases the BP.
2. It also causes VASOCONTRICTION of the
arterioles, which also raises blood pressure.
Low blood pressure is bad
because there is not enough
pressure to filter the blood
properly. This makes the kidneys
less efficient at ridding the body
of drugs and toxins.
High blood pressure is bad
because the glomerulus can burst
and then we would see blood and
blood proteins in the urine.