2. Importance of homeostasis
• In both animals and plants, chemical
messengers called hormones/plant
growth regulators help to transfer
information from one part to another and
so achieve coordination
• In many animals, nerves transfer
information in the form of electrical
impulses
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3. Homeostasis
• Maintaining a constant environment for the cells within
the body
• Many features of the environment affect the functioning
of the cell:
– Temperature – low temperatures slow metabolic rates/high
temperatures cause denaturation of proteins
– Amount of water – lack of water in tissue fluid causes water to
be drawn out of cells by osmosis, causing metabolic reactions in
the cell to slow or stop/too much water entering cell may cause it
to swell or burst
– Amount of glucose – lack of it causes respiration to slow or
stop (no energy source)/too much glucose may draw water out
of the cell by osmosis
• Homeostatic mechanisms work by controlling the
composition of blood, which controls the composition of
tissue fluid
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4. • Most control mechanisms use a negative
feedback control loop (involving a
receptor /sensor and an effector )
• Input : receptor picks up information about the
parameter being regulated
• Output : action by the effector
• Continuous monitoring of the parameter by the
receptor produces continuous adjustments of the
output, which keep the parameter oscillating
around a particular ‘ideal’ level, or set point.
• A rise in the parameter results in something
happening that makes the parameter fall
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6. Excretion
• The removal of toxic or excess products of
metabolism from the body
• Two main excretory products are carbon
dioxide and urea
• Urea produced in the liver (from excess
amino acids) and is transported from the
liver to the kidneys, in solution in blood
plasma
• The kidneys remove urea from the blood
and excrete it, dissolved in water, as
urine
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7. • Deamination
– The breakdown of excess amino acids in the liver, by
the removal of the amine group; amine and eventually
urea are formed from the amine group
– Urea is the main nitrogenous excretory product
of humans other than creatinine and uric acid
– Creatine is made in the liver from certain amino acids,
used in the muscles (creatine phosphate) where it
acts as an energy store and some converted to
creatinine and excreted
– Uric acid is made from the breakdown of nucleic acids
– Urea made in liver passes from liver cells into blood
plasma. As blood passes through kidneys, the urea is
extracted and excreted
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10. Ultrafiltration
• Involves filtering small molecules (urea) out of
the blood and into the renal capsule, from here
they flow along the nephron towards the ureter
• Blood in the glomerular capillaries is separated
from the lumen of the renal capsule by two cell
layers and a basement membrane
• Capillary endothelium – more gaps than other
capillaries
• Basement membrane – made up of a network
of collagen and glycoproteins; stops large
protein molecules and blood cells from getting
through (filter)
• Epithelial cells – make up the wall of the renal
capsule; have podocytes
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12. • Factors affecting glomerular filtration rate
– Glomerular filtration rate : rate at which fluid
seeps from the blood in the glomerular capillaries into
the renal capsule(125 cm3min-1 in humans)
– Determined by the differences in water potential
between contents of the glomerular capillaries and the
renal capsule
– Afferent arteriole is wider than the efferent arteriole
causing a ‘traffic jam’ inside the glomerulus; blood
pressure rises and so raising the water potential as
well
– Concentration of solutes in blood plasma in the
capillaries is higher than the concentration of solutes
inside the renal capsule (plasma protein still remain)
– Overall, the effect of difference in pressure outweighs
the effect of the differences in solute concentration so
water move down water potential gradient from the
blood into capsule
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13. Reabsorption
• Involves taking back any useful molecules from the fluid
in the nephron as it flows along
• Reabsorption in the proximal convoluted tubule
– Many of the substances in the filtrate (identical to blood plasma
except large protein molecules) need to be kept in the body, so
they are reabsorbed into the blood as the fluid passes along the
nephron (selective reabsorption )
– Na+ transport
– All glucose in glomerular filtrate is transported out of the
proximal convulated tubule and into blood (amino acids ,
vitamins , sodium and chloride ions are actively reabsorbed)
– 65% of water in the filtrate is reabsorbed as water can move
freely out of the filtrate, through the walls of the tubule and into
the blood by osmosis
– About half the urea in the filtrate is reabsorbed by diffusing
passively through the wall into the blood
– Uric acid and creatinine are not reabsorbed
– Creatinine is actively secreted by the cells of the proximal
convulated tubule into its lumen
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15. • Reabsorption in the loop of Henle and
collecting duct
– The function of the loop of Henle is to create a very high
concentration of salts in the tissue fluid in the medulla of the
kidney
– This allows a lot of water to be reabsorbed from the fluid in the
collecting duct as it flows through the medulla
– The loop of Henle allows water to be conserved in the body
rather than lost in urine
– Counter-current multiplier : an arrangement in which fluid in
adjacent tubes flows in opposite directions, allowing relatively
large differences in concentration to be built up
– Collecting duct runs down into medulla where the solute
concentration of the tissue fluid is very high
– Water moves out of collecting duct by osmosis until the water
potential of urine is the same as the water potential of the tissue
fluid in the medulla
– The degree to which this happens is controlled by antidiuretic
hormone (ADH )
– The longer the loop of Henle, the greater the concentration that
can be built up in the medulla and the greater the concentration
of the urine which can be produced
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17. • Reabsorption in the distal
convoluted tubule and collecting
duct
– First part behaves in the same way as the
ascending limb of the loop of Henle and
second part as the collecting duct
– In distal convoluted tubule and collecting duct,
sodium ions are actively pumped from the
fluid in the tubule into the tissue fluid, from
where they pass into the blood
– Potassium ions are actively transported into
the tubule
– The rate at which these 2 ions are moved into
and out of the fluid in nephron can be varied
and helps regulate the amount of these ions
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18. Control of water and
metabolic wastes
• Osmoreceptor, the hypothalamus and
ADH
– Osmoregulation : the control of the water content of
the fluids in the body/regulating the concentration of
water in body fluids (kidney)
– In osmoregulation in mammals, the receptor is cells in
the hypothalamus (osmoreceptors ), and the
effectors are the pituitary gland and the walls of the
distal convulated tubules
– Nerve cells produce a chemical called antidiuretic
hormone (ADH – polypeptide of 9 amino acids)
– ADH passed along to the endings in the posterior lobe
of the pituitary gland
– Action potentials from stimulation by osmoreceptor
cells causes ADH to be released from endings into
blood in capillaries in the posterior pituitary glandALBIO9700/2006JK
19. • How ADH affects the kidneys
– ADH acts on the plasma membranes of the cells
making up the walls of the collecting ducts, making
them more permeable to water than usual
– This change in permeability is brought about by
increasing the number of water-permeable channels
in the plasma membrane
– As the fluid flows down through the collecting duct,
water is free to move out of the tubule and into the
tissue fluid and it does so because this region of the
kidney contains a high concentration of salts
– Secretion of ADH caused the increased reabsorption
of water into the blood
– Diuresis: production of dilute urine (antidiuretic
hormone stops production of dilute urine)
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20. • Negative feedback in the control of
water content
– When blood water content rises, the
osmoreceptor are no longer stimulated and
stop stimulating their neighboring nerve cells.
So ADH secretion slows down
– The collecting duct cells do not respond
immediately to the reduction in ADH secretion
by the posterior pituitary gland
– It takes some time for the ADH already in the
blood to be broken down
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