Homeostasis, hormones and the kidney — Edexcel International A Level Biology
Negative and positive feedback, homeostasis and thermoregulation, hormones switching genes, and the kidney's role in excretion and osmoregulation.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Negative feedback is a control mechanism in which a change in a factor is detected and corrected, returning it towards its set point, so the factor stays within narrow limits. In positive feedback, a change leads to responses that increase the change further, moving the system away from its starting point.
Homeostasis is the maintenance of a stable internal environment within narrow limits, a state of dynamic equilibrium. It keeps conditions suitable for enzymes and cells, including during exercise. The hypothalamus detects changes in blood temperature and coordinates responses such as sweating, vasodilation, vasoconstriction and shivering.
Genes are switched on and off by transcription factors, which are proteins that bind to specific DNA sequences and increase or decrease transcription. Steroid hormones are lipid-soluble, enter the cell and bind to intracellular receptors that act as transcription factors. Peptide hormones bind to receptors on the cell surface and act extracellularly through a second messenger.
The mammalian kidney has an outer cortex, an inner medulla and a pelvis leading to the ureter. Each kidney contains many nephrons: a Bowman's capsule containing a glomerulus, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule and a collecting duct. Blood arrives in the renal artery and leaves in the renal vein.
Excess amino acids cannot be stored, so in the liver the amino group is removed by deamination and converted to urea, which is less toxic. Urea is carried in the blood to the kidneys, where ultrafiltration at the glomerulus forces small molecules, including urea, through the capillary wall, basement membrane and podocytes into Bowman's capsule.
In the proximal convoluted tubule, glucose, amino acids and ions are selectively reabsorbed by co-transport and active transport, followed by water by osmosis. The loop of Henle acts as a countercurrent multiplier: ions pumped out of the ascending limb create a low water potential in the medulla, so more water is reabsorbed from the filtrate.
Osmoreceptors in the hypothalamus detect changes in the concentration of blood plasma. A rise stimulates the posterior pituitary gland to release more antidiuretic hormone (ADH), which makes the collecting ducts more permeable to water, so more is reabsorbed. This returns the plasma concentration and blood volume to normal by negative feedback.
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Quick check questions
What is negative feedback?
Show answer
A change is detected and the response reverses it, returning the factor towards its set point.
Which part of the brain contains the thermoreceptors that detect core temperature?
Show answer
The hypothalamus.
What forces fluid out of the glomerulus?
Show answer
High hydrostatic pressure, caused by the afferent arteriole being wider than the efferent arteriole.
Which limb of the loop of Henle is impermeable to water?
Show answer
The ascending limb.
What is the effect of ADH on the collecting duct?
Show answer
It makes the walls more permeable to water by inserting aquaporins, so more water is reabsorbed.
Frequently asked questions
What is the difference between negative and positive feedback?
In negative feedback the response reverses the change, so a factor such as temperature stays near its set point. In positive feedback the response increases the change and moves the factor further away, as with oxytocin in childbirth. Homeostasis relies on negative feedback.
How does the body regulate temperature during exercise?
Respiration in muscles releases heat, raising the temperature of the blood. Thermoreceptors in the hypothalamus detect this and trigger vasodilation of skin arterioles and sweating. These transfer heat to the surroundings, so core temperature returns to the set point by negative feedback.
How does the kidney filter blood in ultrafiltration?
The afferent arteriole is wider than the efferent arteriole, so blood in the glomerulus is at high pressure. Water and small solutes are forced through pores in the capillary endothelium, the basement membrane and the podocytes into the Bowman's capsule, while blood cells and large proteins stay in the blood.
How does the loop of Henle make urine concentrated?
The loop of Henle is a countercurrent multiplier. Ions are pumped out of the ascending limb into the medulla, giving it a low water potential. Water then leaves the descending limb and the collecting duct by osmosis, so more water is reabsorbed and urine can be concentrated.
How does ADH control the water content of the blood?
When osmoreceptors in the hypothalamus detect a low water potential, the posterior pituitary releases more ADH. ADH makes the collecting duct more permeable by adding aquaporins, so more water is reabsorbed and a small volume of concentrated urine is made. The opposite happens when blood is dilute.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026