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Homeostasis
Homeostasis; the maintenance of a constant internal environment despite fluctuations in both the
body’s activities and the external environment
It is important as various body systems work cooperatively to ensure that the composition of blood
and tissue fluid remains within tolerable limits
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Enzymes control
cellular biochemical reactions; their activity is affected by fluctuating pH and temperature
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It involves deviation of a factor from normal value or set point,
receptors that detect any deviations from the norm, and effectors that are activated by receptors
and correct the deviation by mechanisms that restore the norm
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E
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in child
birth oxytocin hormone is released and the increased contractions causes more release so there’s
more contractions until the baby is born
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g
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Dynamic equilibrium; a system in a steady state since the forward and backward reactions occur at
the same rate
Homeostasis examples:
 Control of blood osmotic potential
 Control of blood glucose level
 Control of blood pH
 Control of body core temperature
 Control of ion concentrations in the blood
Ecological advantages; allows animals to adapt to a changing environment, especially temperature
changes, keeping body temperature at optimum for body cells and enzymes to function
The kidney
We have 2 kidneys, which have 3 areas; the cortex, medulla, and the region where the ureter joins
the pelvis
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Liver salvages this energy by removing
nitrogen atoms from the amino acids, excreting these as urea
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Keto acid that remains may become a
carbohydrate- used in respiration or stored as fat
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Urea passes from liver cells into blood plasma
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As blood passes through the kidneys, urea is extracted and excreted
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Layer 1 is endothelial cells of the capillary in the glomerulus which has
more gaps than other capillaries
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It is permeable to small molecules
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The gaps in the endothelium and epithelium make it easy for solutes in the blood plasma to
get through from the blood into the capsule
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The glomerular filtrate is identical to blood plasma but without the plasma proteins
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Blood pressure in the glomerulus is
relatively high
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Most of the pressure forcing the molecules through the filter comes from the hydrostatic
pressure of the blood in the glomerular capillaries, amplified by the pressure in the
capillaries
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The basal
membranes of the cells lining the pct actively transport ions out of the cell
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The ions can’t diffuse freely through the membrane so have to use carrier proteins in the
membrane
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g
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All the glucose is transported out of the pct into the blood
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The
uptake of these ions should decrease the solute concentration of the filtrate
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As substances move into
the cells surrounding the tubule, water follows by osmosis, keeping the overall filtrate
concentration the same
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65% of water in filtrate is reabsorbed in the pct and half of the urea by passive
diffusion
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The cell walls in the pct have microvilli providing a large surface area and numerous
mitochondria providing ATP for the active transport of glucose and salts
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Reabsorption in the loop of Henle
1
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3
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5
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This raises the concentration of Na+ and Cl- in the tissue fluid
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Loss of water concentrates Na+ and Cl- in the descending limb
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The descending limb is permeable to sodium ions and water
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Fluid is relatively dilute as it leaves the ascending limb
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The concentration of salt in the tissue fluid can be 4x greater than in the rest of the body
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Distal convoluted tubule and the collecting duct
o
o

o
o

The water potential of the urine becomes the same as that of the tissue fluid in the medulla
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The tissues in the deeper layer of the medulla contain very concentrated solution of ions
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The first part of the distal convoluted tubule behaves the same as the ascending loop of
Henle, the second part of the dct behaves the same as the collecting duct
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Potassium ions are
actively transported to the tubule
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Osmoregulation


The homeostatic control of body water

Mammals must maintain a balance between water gain and loss
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Most water is lost by urine as well as by sweating, keeping exchange surfaces
moist, and lost in faeces
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There also needs to be an effector carrying out an action to bring the condition
back to normal if it deviates too far
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The walls of the collecting ducts act as effectors
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This hormonal
effect, with hypertonic interstitial fluids built up by the loop of Henle in the medulla,
determine if hypotonic or hypertonic urine is released from the kidney
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Negative feedback restores normal
osmotic concentration if blood is diluted or becomes more concentrated
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The concentration of sodium chloride in the blood is
an indirect indication of the volume of water in the body
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If water content is low, a
fall in water potential is detected by osmoreceptors in the hypothalamus and causes nerve
impulses passing to the posterior pituitary gland, which releases ADH in the blood stream
causing these effects
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Ammonia diffuses across the gills and is quickly diluted to non-toxic levels
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Large energy
cost of its production but very little water needed for its excretion
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Mammals excrete urea
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The longer the loop of Henle, the greater the water potential in the medulla, enabling more water to
be reabsorbed so more concentrated urine
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Desert
animals live on metabolic water, produced from the breakdown of food in respiration in cells
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