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Excretion
Syllabus links
3.4.5 Plant Excretion
The role of leaves as excretory organs of
plants.
3.4.6 The Excretory System in the
Human
Role of the excretory system in
homeostasis. Function, location and
excretory products of the lungs, skin, and
urinary system.
Excretion
“Excretion is the
removal from the
body of the toxic
waste products of
metabolism”
Excretion in plants
• Excretion is a term normally associated with animals
• The equivalent term in plants is the loss of waste
(gases)
Excretion and homeostasis
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Excretion helps in homeostasis as follows:
Removes excess water, salts and wastes from
the body
By controlling the salt/water balance it acts to
regulate osmosis
Organs of excretion in humans:
Skin (excretes salts and water as sweat)
Lungs (excrete carbon-dioxide and water)
Kidneys (excrete water, salts, and urea in the
form of urine)
Syllabus links
3.4.6 The Excretory System in the
Human
Macrostructure and basis function of the
urinary excretory system in humans
(kidney, ureters, urinary bladder, and
urethra).
Urinary System
Kidneys
Functions
• Excretion:
Nitrogenous wastes
(urea, uric acid), excess
salts, excess water
• Osmoregulation:
Maintaining the blood at a
suitable constant
concentration
• Homeostasis:
Maintaining a suitable
constant internal
environment to sustain
efficient metabolism
Syllabus links
3.4.6 The Excretory System in the
Human
Role of the kidney in regulating body
fluids. Identification of the site of filtration.
Reabsorption in the cortex, in the medulla
and renal pelvis.
Description of the pathway of urine from
the kidney to the urethra.
Kidney Structure LS
Syllabus links
H.3.4.8 The Nephron as a Unit of Kidney Function
The nephron structure and its associated blood supply.
Formation of urine: Bowman’s capsule, passage of
glomerular filtrate through the proximal convoluted
tubule, where reabsorption of required body substances
takes place – glucose, amino acids, some salts and
water reabsorbed into the blood by osmosis, diffusion,
and active transport. More water reabsorbed in the Loop
of Henle and the distal convoluted tube. Urine passes
into the pelvis of the kidney and to the bladder for
storage. Reabsorption of water in the collecting duct is
under hormonal influence (ADH). Its action depends on
the water content of the blood. (No further details
required).
Nephron
• The nephron or renal
tubule is the functional
unit of the kidney
• The nephron has a
number of functionally
distinct parts
• Each human kidney has
about one million
nephrons
• Urine is manufactured by
the nephrons
• Production of Urine:
Filtration and Selective
Reabsorption
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Details of Urine Formation Glomerular filtrate
Filtration
composition is water,
glucose, amino acids,
vitamins, salts, urea, and
uric acid
About 20% of the plasma
volume passes out of the
glomerulus
The filtration is much
higher than expected
The blood pressure is
unusually high in the
glomerulus
The blood pressure is
generated by the
pumping action of the
heart
The high blood pressure in the
glomerulus is due to:
• The efferent arteriole is narrower than the afferent
arteriole creating a high pressure
• The surface area in the capillaries is large increasing the
area for filtration
• The walls of the glomerular capillaries are more porous
than normal capillaries
• The Bowman’s capsule is only 1 cell thick
• The higher than normal filtration at the glomerular
capillaries is known as ultrafiltration
• 180 litres of glomerular filtrate are formed every 24 hours
– 4 to 5 times the fluid content of the body!!
Proximal Convoluted Tubule
(PCT) - Reabsorption
• Total reabsorption of
glucose and amino acids
• Four fifths of the salts and
water are reabsorbed
• Glucose, amino acids and
salts are reabsorbed by
active transport
• Water is reabsorbed by
osmosis
• The cells lining the PCT
are rich in mitochondria,
which supply the ATP for
active transport
•
The Loop of Henle This structure allows the
Reabsorption
kidney to reabsorb extra
water in times of water
stress
• As a result it is possible for
the kidney to produce
hypertonic urine, i.e., more
concentrated than blood
plasma
• A Loop of Henle is only
present in mammals and
birds — the only animals
able to produce hypertonic
urine
• About 5% of the water from
the glomerular filtrate is
reabsorbed from the Loop
of Henle by osmosis
• Reabsorption of water is
by osmosis
• The amount varies
depending on the need of
the body
• Water reabsorption by the
DCT is under the
influence of ADH
(antidiuretic hormone)
• Reabsorption of salt is by
active transport
• The amount of salt
reabsorbed depends on
the needs of the body
• The role of the DCT is
crucial in osmoregulation
• Osmoregulation is a
major process in
homeostasis
Distal Convoluted
Tubule
(DCT) Reabsorption
Secretion
• Some substances
pass from the blood
to the nephron
• K+, H+ and NH4+ ions
• These ions are
secreted by a
combination of
diffusion and active
transport
Summary of nephron functions
Location
% Water
reabsorbed
Proximal tubule 80
Salts
reabsorbed
Most
Descending
5
limb of loop of
Henle
Ascending limb 0
of loop of Henle
Distal tubule
10
0
Collecting duct
None
4.9
Some
Some
Summary of the main functions
of the nephron
Osmoregulation
“Blood concentration
is kept in check by
varying the amount of
water and salt
reabsorbed by the
kidneys nephrons”
• Blood Concentration Rising
• Cause: salty food, water loss due to sweating,
inadequate water intake
• Response: increases water reabsorption,
decreases salt reabsorption
• Blood Concentration Falling
• Cause: excessive water intake, cold weather
(sweating less than usual), diet very low on salt
• Response: decreases water reabsorption,
increases salt reabsorption
• Note: the greater the excess protein in the diet
the greater is the urea content of the urine
Regulation of Body Fluids by the
Kidney
• The kidney maintains
the blood at the
correct composition
and concentration by
excretion and
osmoregulation
• As a result all the
other body fluids are
kept at optimum
condition i.e. tissue
fluid and cell
cytoplasm
Control of
urine
volume