Cambridge International AS and A Level Biology Coursebook with CD-ROM
This section of my summarised notes covers the topic 'Homeostasis' in CIE 9700 Biology. These are simple, stylised note pack which are perfect for A2 biology revision. They cover the topic in the CIE syllabus with needed detail to ensure you get the perfect score.
Juggling 4 different A-level sub...
Homeostasis
↬ Involves a receptor detecting a stimuli from an
external or internal environment change and then Factor rises
sending signals to central control in brain or spinal cord. above set Factor falls
This information is considered the input and the output point below set point
is effector carrying out corrective actions to correct the
conditions back to set point/optimum referred to as Effectors act Effectors ac
negative feedback. to increase to decrease
factor factor
The two coordination systems needed to carry out
homeostatic mechanisms:
Nervous system: information as electrical impulses transmit along neurones
Endocrine system: uses chemical messengers (hormones) acting as long distant cell signalling by
travelling in the blood.
Thermoregulation
↬ Control of body temperature using both nervous and endocrine system.
• Hypothalamus, spinal cord contains thermoreceptors cells that continually monitoring the core
temperature of the blood flowing through it.
• If thermoreceptors in the skin detects change in core temperature of surroundings, impulses are sent
that activate the following physiological responses.
In hot environment In cold environment
⇾ Vasodilation: arterioles muscles relax to allow ⇾ Vasoconstriction: arteriole muscles contract
more heat loss from bloods flow to narrow lumens and minimise heat loss
⇾ Sweat glands secrete sweat → evaporative ⇾ Shivering: skeletal muscles contraction
cooling ⇾ Hair erector muscles contract to raise hairs
⇾ Hair erector muscles relax to lie hairs flat and increase depth of fur
reducing layers of insulation ⇾ Adrenal glands secrete adrenaline and
thyroid glands increase thyroxine secretion
to overall increase liver heat production
Excretion
• Removal of toxic products of metabolic reactions is referred to as excretion
• Deamination: breakdown of excess amino acids in the liver by the removal of the amine group
Deamination • Ammonia is very soluble and highly toxic and
build up causes severe damage. The urea cycle
combine CO2 and ammonia to form urea - the main
nitrogenous excretory product
• Creatine made in the liver is used in muscle as
creatine phosphate where it acts as an energy store.
Urea Formation Some is converted to creatinine and excreted
• Uric acid is made from breakdown of nucleotide
purines
SxTeri Notes Page 13
, Kidneys
⇾ Kidneys receive bloods from the renal artery and returns it through the renal vein
⇾ Glomerulus in nephron is supplied with a branch of renal artery called the afferent arteriole and then
exit through the efferent arteriole.
Ultrafiltration
• The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by:
- Endothelium (have more gaps than typical squamous capillary)
- Podocytes (gaps between the projections of epithelial cells)
- Basement membrane (made of collagen and glycoproteins)
• 2 cell layers allows dissolved substances to pass but BM acts as filter to large proteins (>69000 Mr)
• NB: holes can also be referred to as fenestrations
• Glomerular filtrate therefore does not contain blood cells
Factors affecting glomerular filtrate rate
Difference in water potential: water potential is higher in
glomerulus due to the diameter of afferent arteriole being
wider than efferent causing a build up of pressure in
glomerulus
Concentration of solutes: higher in blood plasma of
capillaries than the filtrate due to filtration of plasma
proteins. This makes water potential of capillaries lower
than the filtrate.
However, the effect of difference in pressure outweighs the
solute difference so the water potential of blood plasma in
higher in glomerulus than the capsule. So water moves down
the water potential gradient into Bowman’s capsule.
SxTeri Notes Page 14
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