Summary Biology A2 level 'homeostasis and the kidney' notes (WJEC)
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Homeostasis and the kidney
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WJEC
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WJEC Biology for A2: Study and Revision Guide
Biology A2 level 3.7 homeostasis and the kidney. These notes were formulated using class notes, the WJEC study guide and past papers. These notes helped me to achieve an A* at A level biology!
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Homeostasis and the kidney
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3.7 Homeostasis and the kidney
Homeostasis
● The maintenance of an internal environment within tolerable limits, by negative feedback
● Negative feedback
○ The mechanism by which the body reverses the direction of change in a system
to restore the set point
○ Keeps physical parameters constant / within a dynamic equilibrium e.g.
temperature at 37℃
○ Involves the following cycle;
1. Input - a change away from the set point e.g. rise in core body temp
2. Receptor - a sensor that detects change from the set point e.g.
temperature receptors
3. Control centre - detects signals from receptors and coordinates a
response via effectors e.g. hypothalamus
4. Effector - brings about changes to return the body to set point (either a
muscle or gland) e.g. release more sweat from skin glands
5. Output - corrective procedure e.g. evaporation of sweat cools skin
Excretion
● Excretion is the removal of metabolic waste (waste made by the body) from the body
○ Mammals excrete compounds from;
■ Lungs - CO 2 & H2O from respiration
■ Skin - urea in sweat
■ Liver - bile pigments in faeces
■ Kidneys - urea, creatinine and uric acid in urine
○ Excess amino acids are deaminated in the liver
■ In the liver
■ Amine group (-NH2) is removed
(deamination), converted to ammonia, then
into urea
■ Urea is removed from the body by the kidney
■ Remaining organic acid can be used in
respiration or converted to lipids or glucose
■ Egestion is the removal of undigested material from a cell/system
, The kidney
● 2 main functions
○ Excretion of nitrogenous waste from the body e.g. urea
○ Osmoregulation - the control of the water potential of body fluids e.g. blood
● Humans have 2 kidneys
○ The renal artery supplies blood containing O2, & waste
○ Filtered blood returns to the general circulation via the renal vein
○ Each kidney has 1 million nephrons, which are 30mm long
■ Nephrons are the functional units of the kidney
■ Processes occurring in the nephron;
● Ultrafiltration
● Selective reabsorption
● Osmoregulation
○ Capillary network surround the convoluted tubules and loop of Henle - allow
substances to be absorbed into blood
■ Capillaries surrounding the loop of Henle are called ‘vasa recta’
○ Urine
■ Is made up of excess water and solutes (inc. urea)
■ Drains into the collecting duct and pelvis, which empty into the ureter
■ Each ureter connects to the bladder
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