Chapter 12
Excretory System
12.1 The Kidney
A. Introduction:
 The kidneys play a major role in homeostasis.
 They have three main functions:
1. Remove Wastes
o excess protein in the diet is converted into carbohydrates in the
liver by a process called deamination
o During this process NH3 (ammonia – highly toxic…0.005 mg
can kill you) is removed from amino acids
o (in the liver) 2 molecules of NH3 combine with CO2 forming
urea…….(urea is 1000X less toxic than ammonia)
o Uric acid is also formed...but it is from the breakdown of DNA
(nucleic acids)… build up of uric acid is called gout
2. Balance Blood pH Kidneys remove H+ ions (acid) and other ions
from the blood keeping the pH in normal range (7.4)
3. Maintain Water Balance
o we can only survive a few days without water
~ 2L of water is lost per day from perspiration, exhaling,
urine
1% loss of H2O = thirst,
5% loss of H2O = pain, collapse,
10% loss of H2O = death…eg. heat exhaustion
o hormones such as ADH (antidiuretic hormone) and
aldosterone regulate water concentration in the blood
o ~ 180 L moves through kidneys in one day…we lose only
o 2 L therefore 178 L are reabsorbed!
B. Anatomy of the Excretory System
Activity: The Urinary System Handout (fill in structures)
1. Renal Blood Vessels
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o the renal arteries branch from the abdominal aorta and supply the
kidneys with oxygenated, unfiltered blood
o the renal arteries remove deoxygenated, filtered blood
2. Kidneys
o we have two kidneys which filter our blood….together they hold as
much as ¼ of our blood at any one time
o each kidney has a mass of about 500 g
o each kidney has about 1 million nephrons
o It is Made up of 3 layers:
a.) the cortex: outer layer of connective tissue
b.) the medulla: inner layer (beneath the cortex) containing the
nephrons
c.) the pelvis: hollow chamber joining kidney to ureter
3. Ureters
o wastes filtered through the nephrons move from the kidneys into 2
tubes called the ureters
4. Bladder
o both ureters lead to the bladder
o storage for urine
o holds about 200 mL of urine at 400mL stretch receptors signal the
brain that it is “time to go….”
after 600 mL is collected all voluntary control is lost!
5. Urethra
o urine leaves the bladder through the urethra
o sphincter muscles relax and urine is voided
o the urethra is longer in men than in women therefore women are
more susceptible to bladder infections because bacteria on the
outside of the body have a shorter distance to travel
Your Assignment: Activity; The Nephron Handout: fill in structures
Note: the cortex contains the afferent, efferent arterioles and the Bowman’s
capsule
C. The Nephron
 nephrons: are the functional units of the kidney (each kidney has about 1
million)
 Afferent arterioles supply the nephrons with blood and branch into
the capillary bed called the glomerulus (high pressure filter)
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 Blood leaves the glomerulus by the efferent arterioles and travels to
the peritubular capillaries (they wrap around the kidney tubule)
…..to the venule …then to the renal vein
 The glomerulus is surrounded by the Bowman’capsule (funnel-like
part) which tapers to the proximal tubules
 The proximal tubules connects to the loop of Henle ( in the medulla)
 The loop of Henle connects to the distal tubule which empties into the
collecting ducts …these ducts merge in the renal pelvis
Your Assignment: Page 380, 1-3
D. Formation of Urine
It depends on:
1. Filtration: the movement of fluid from the blood into the Bowman’s
capsule
2. Reabsorption: involves the transfer of essential solutes and water
from the nephron back to the blood
3. Secretion: involves the transport of materials from the blood into the
nephron
1. Filtration
o filtration rate: if the pressure  then filtration 
o blood enters nephron through the afferent arteriole (pressure is 2 kPa)
this arteriole branches into a capillary bed called the glomerulus (pressure
is 8 kPa) which acts like a high pressure filter
o large solutes such as plasma proteins, blood cells and platelets are
too large to filter through and move through the glomerulus into the
efferent arteriole dissolved solutes (eg. Na+, Ca2+, glucose, urea, uric
acid etc) pass through the glomerulus into the Bowman’s capsule
the glomerular now can move on to go through reabsorption
2. Reabsorption
o occurs from nephron into bloodstream
o it occurs till the threshold level is reached…the maximum amount of
a substance that can be moved across the nephron
o 600 mL of fluid moves through kidney per minute…
o about 120 mL of that is filtered into the nephron…and from that
only 1 mL of urine is made
o both active and passive transport occur
o glomerular filtrate (dissolved solutes) travel down the proximal tubule
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o most reabsorption (85%) occurs in the proximal tubule
o contains a rich supply of mitochondria (for energy) and microvilli to
greatly increase surface area
o as the filtrate moves through the proximal tubule, Loop of Henle and
distal tubule, water and certain ions must be reabsorbed back into the
bloodstream to maintain proper water and ion balance
o Na+ions , glucose and amino acids are actively transported out of the
nephron by carrier molecules and into the peritubular capillaries
(bloodstream)
o in the descending loop, the filtrate is hypertonic therefore water
diffuses out of nephron into bloodstream (passive transport)
o once the filtrate reaches the ascending loop, the filtrate is hypotonic…
more Na+ is actively transported out to keep water from moving the
wrong way
o Cl and HCO3 ions follow the Na+ ions due to charge attraction
(passive transport)
o there is a limit to the energy available for active transport so some
NaCl and other solutes in excess stay in the urine
o once the ions move out an osmotic gradient is set up so water diffuses
from tubules and Loop of Henle into bloodstream
o filtrate becomes more concentrated as it moves along towards the
distal tubule
3. Secretion
o movement of wastes from the bloodstream into nephron
o left over wastes such as urea, histamine, NH3, K+ ions, H+ ions,
HCO3- ions, minerals, drugs such as penicillin etc. move from the
blood into the distal tubule of the nephron
o usually active transport
o urine moves into the collecting ducts which lead to the renal pelvis
then on to the ureters and bladder
o composition of urine:
water (95%)
urea
uric acid
trace amino acids
electrolytes: Na+, K+, Cl-, SO42-, PO43
excess vitamins, minerals, etc
o pH of urine ranges from 4 to 5 but can get as high as 8.5
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Remains in Blood
Filters into Capsule
Reabsorbed
RBC
Water
Water (most of)
WBC
Amino acids
Amino acids (all)
Platelets
Glucose
Glucose (all)
Fats
Salts
Salts (some)
Blood Protein
Urea, uric acid
Urea, uric acid (none)
Component
Plasma
Filtrate
Urine
Urea
0.03
0.03
2.0
Uric Acid
0.004
0.004
0.05
Glucose
0.10
0.10
0 to trace
Amino Acid
0.05
0.05
-----
Salt
0.72
0.72
~1.5
Protein
8.00
-----
----
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Summary
 All the glucose and amino acids get reabsorbed
 Some salts get reabsorbed
 No urea or uric acid get reabsorbed
 Proteins Never get filtered
Summary:
Filtration:
o glomerulus and Bowman’s capsule
Passive transport
Reabsorption
o proximal tubule (mainly), loop of Henle
active and passive transport
Secretion
o distal tubule
o mainly active transport
pH Balance
 the kidneys help maintain blood pH balance between 7.3 and 7.5
 carbon dioxide from the peritubular capillaries is transported to the
cells that line the nephron.. where it combines with water
 H+ and HCO3- ions are produced…. the HCO3- ions diffuse back to the
blood (buffering the blood)
Your Assignment: Page 386, 1-9
12.2. Kidney Dysfunction
Diabetes Mellitus (there are three types: Type 1, Type 2, Gestational )
 Caused by inadequate insulin
 Amount of glucose in blood increases therefore the amount of glucose
in the filtrate increases
 There is a limit to the energy that is available for active transport of
glucose back into the bloodstream
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 Excess glucose stays into the nephron and decreases the concentration
gradient therefore there is less H20 reabsorbed
 Urine volume increases and there is glucose in urine
Diabetes Insipidus
 Caused by the destruction of hypothalamus cells or of the nerves that
lead to the pituitary gland
 No ADH is released
 There is a huge increase in the amount of urine that is produced
because of no re-absorption from ADH
Bright’s Disease (nephritis)
 Many causes
 Blood vessels in the glomerulus become inflamed or are destroyed
 Glomerulus becomes permeable to plasma proteins and possibly blood
cells
 Body cannot reabsorb these large solutes (no mechanism)
 Concentration gradient decreases therefore there is less water
reabsorbed
 Urine in put increases (because of excess water)
Kidney stones
 They are caused by the precipitation of mineral solutes from the blood
 There are two types: acidic and alkaline
 They can lodge in the renal pelvis or urethra
 Tissues can be torn…. Very painful!
 Lithotripsy uses shock waves break the stones up into smaller pieces
so that they can be passed
 If the stones are too big = surgery
Dialysis Technology
Dialysis
 the proper functioning of our kidneys is necessary for life
 people who have malfunctioning kidneys undergo dialysis
 the person's blood is transferred from their body through a dialysis
machine that behaves like a mechanical kidney (peritoneal dialysis
and hemodialysis)
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 the dialysis tubing is semipermeable…. large solutes won’t pass
through but small solutes (glucose, urea, H+, uric acid, K+, other
electrolytes etc) can
 the solution surrounding the tubing has a lower concentration of the
small dissolved solutes
 the dissolved solutes diffuse from the tubing into the dialysis machine
then the “filtered” blood is returned to the body
 the best option is a kidney transplant
 There are two types of Dialysis
Hemodialysis:
 A dialysis mimics the action of the nephron.
 A person has a surgical procedure to create access, a shunt is put in
for the needles and tubing which connects the circulatory system to
the machine
 Blood is pumped through a series of tubes that are bathed in solutes.
 Glucose and a mixture of salts set up a concentration gradient.
Because the dialysis fluids have no urea, the solute always moves
from the blood into the dialysis fluid
 By continually flushing this fluid and replacing it urea is removed
along with other wastes
Peritoneal dialysis
 Is done through the lining of the abdominal cavity (peritoneal
membrane)
 A catheter (thin tube) is inserted.
 2L of Dialysate (a solution) is pumped into the abdominal cavity
through the catheter and remains in the abdomen cavity for 4 to 6
hours (The membranes in the cavity filter wastes)
 The fluid is then drained using the catheter.
 The process begins again
 The only function that a dialysis machine can not do is ACTIVE
TRANSPORT!
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Kidney Transplant
 Involves placing a new kidney and the ureter in the lower abdomen
near the groin
 It takes 2-4 hours
 Old kidneys are not usually removed
 Sometimes dialysis will be needed until the new kidney can fully
function
Water Balance and ADH
 More water intake = increased urine
 Less water intake = decreased urine
 Nervous system and endocrine system balance fluid levels
1. Regulation of ADH
 The function of ADH (antidiuretic hormone) is to increase H2O
reabsorption into the bloodstream
 nerve cells in the hypothalamus (in the brain) produce ADH
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 it is secreted to the pituitary gland at the base of the brain where it is
stored until needed
 osmoreceptors in the hypothalamus detect changes in osmotic (H2O)
pressure in the bloodstream
 as H2O loss occurs H2O moves from the tissues into the bloodstream
which increases the blood’s osmotic pressure
 the cells of the cells of the hypothalamus shrink causing a nerve
message to be sent to the pituitary gland stimulating the release of
ADH and triggering the thirst response
 the kidneys then reabsorb more H2O and produce a more concentrated
urine (dark yellow)
2. Effect of ADH on Nephron
 proximal tubule and descending loop of Henle are permeable to
H2O
 ascending loop of Henle, distal tubule and collecting duct are
impermeable to H2O
 If there is no ADH the 15% H2O that didn’t get reabsorbed in the
proximal tubule is lost in the urine
 ADH causes the distal tubule and collecting duct to become
permeable to H2O therefore decreasing the amount of H2O lost in
urine
 Cold weather, caffeine and alcohol all inhibit the release of ADH so
that reabsorption of the H2O can’t take place and urine volume is
increased
Blood Pressure and Aldosterone
 The kidneys help regulate blood pressure
 When blood pressure is low:
 A structure near the glomerulus secretes rennin
1. The adrenal glands release aldosterone
2. The function of aldosterone hormone is to increase blood
pressure by increasing the reabsorption of H2O
 blood volume can regulate blood pressure… as volume decreases
blood pressure decreases
 aldosterone is produced in the adrenal gland which is located above
the kidney
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 blood pressure in the kidneys is monitored by the osmoreceptors in
the juxtaglomerular apparatus (cells located in the afferent arteriole
near the glomerulus)
 when BP drops the cells in the juxtaglomerular apparatus release
rennin which converts angiotensinogen (a plasma protein secreted by
the liver) to angiotensin (active form)
 angiotensin constricts blood vessels and causes aldosterone to be
released (both of which increase BP)
 aldosterone is released and causes an increase in Na+ reabsorption
from the ascending loop, distal tubule and collecting duct (nephron)
 as more NaCl leaves the nephron (ascending loop), the osmotic
gradient increases
 with a higher concentration of H2O in the nephron H2O moves into
the bloodstream by osmosis therefore increasing blood volume and
BP
Your Assignment: Page 396, 1-19
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