Homeostasis, hormones and the kidney: exam questions

Answer: C – the loop of Henle (the glomerulus, Bowman's capsule and proximal tubule are in the cortex).

• negative feedback: a change is detected and the response reverses / corrects it
• positive feedback: the response increases / amplifies the change
• negative feedback keeps the factor within narrow limits / around a set point
• so conditions remain suitable for enzymes / cells
• Max 4

• thermoreceptors in the hypothalamus detect the rise in temperature of the blood
• hypothalamus sends impulses to effectors
• sweat glands secrete sweat; evaporation of water takes heat from the skin
• vasodilation of skin arterioles, more blood near the surface and more heat lost by radiation
• temperature falls to normal, so response reduced (negative feedback)
• Max 4

• steroid is lipid-soluble and passes through the cell surface membrane
• binds to a receptor inside the cell; the complex acts as a transcription factor binding to DNA
• peptide hormone cannot cross the membrane; binds to a receptor on the cell surface
• activates a second messenger / cascade in the cell, which activates a transcription factor or enzyme
• similarity: both act through transcription factors that switch genes on or off, changing the proteins made
• Max 4

• excess amino acids are deaminated in the liver; amino group removed
• ammonia formed is converted to urea (ornithine cycle) which is less toxic
• urea carried in blood plasma to the kidney
• high pressure in glomerulus because afferent arteriole is wider than efferent arteriole
• urea and small molecules are filtered through capillary endothelium, basement membrane and podocytes into the Bowman's capsule
• Max 5

• protein absent from filtrate because too large to pass through the basement membrane
• glucose is the same in plasma and filtrate because it is small and filtered freely
• no glucose in urine because all of it is reabsorbed in the proximal tubule by co-transport / active transport
• urea is much more concentrated in urine because water is reabsorbed and urea is not (all) reabsorbed

• sodium and chloride ions are actively transported out of the ascending limb into the medulla
• ascending limb is impermeable to water, so water cannot follow
• medulla tissue fluid has low water potential
• descending limb is permeable to water, so water leaves by osmosis into the medulla
• filtrate is most concentrated at the bottom of the loop; fluid flows in opposite directions, maintaining a steep gradient along the whole length
• water then leaves the collecting duct by osmosis (when permeable), and is carried away by the blood capillaries
• Max 6

• blood plasma becomes more concentrated / lower water potential
• detected by osmoreceptors in the hypothalamus
• posterior pituitary gland releases more ADH
• ADH makes collecting duct walls more permeable to water (more aquaporins)
• more water reabsorbed by osmosis, so a small volume of concentrated urine is produced
• plasma concentration and blood volume return to normal, so less ADH is released
• Max 5