Biology HL Help - The Kidney

[armonteb]

Please and thank you.

1. Explain how the collecting ducts can alter the volume of urine produced by the kidney. [5]

2. Blood plasma, glomerular filtrate, and urine have different concentrations of solutes, such as glucose, protein and urea. Explain the processes occurring in the kidney that cause the differences in the concentrations of these solutes between blood plasma, glomerular filtrate and urine. [8]

[Lero]

Explain how the collecting ducts can alter the volume of urine produced by the kidney.

ADH is secreted when the solute concentration of the blood is too high/OWTTE/converse;

ADH makes the collecting duct more permeable to water / when not secreted the collecting duct is less permeable to water;

(causes) more aquaporins in the (membranes of cells in the) collecting duct;

collecting duct passes through medulla;

increasing salt concentration of medulla / hypertonic medulla;

leading to osmosis / more water is reabsorbed (from the collecting duct);

so volume of urine is less / urine more concentrated;

(without ADH) higher flow rates so less time for water reabsorbtion;

(without ADH) dilute / large volume of urine is produced;

If the solute concentration of the blood (outside the collecting duct) is too high (hypertonic) then the hormone ADH is secreted from the posterior pituitary gland. This hormone makes the collecting duct of the kidney more permeable to water, without this hormone, the collecting duct is less permeable to water. The action of this hormone causes more aquaporins in the membrane of cells in the collecting duct to activate. Also worth noting that the collecting duct is positioned throughout the junction from the cortex to the medulla, it passes through both sections. The hypertonic medulla with high concentration of solutes leads to osmosis and water is reabsorbed from the collecting duct into the blood and this results in less volume and higher concentrated urine. Without the action of ADH water is not reabsorbed into the blood and the urine is dilute and high in volume.

I have provided the marking scheme for the question and an example of a structured extended response to the specific question.

Edited May 10, 2013 by Hus

[Sandwich]

2. Blood plasma, glomerular filtrate, and urine have different concentrations of solutes, such as glucose, protein and urea. Explain the processes occurring in the kidney that cause the differences in the concentrations of these solutes between blood plasma, glomerular filtrate and urine. [8]

The kidney basically goes through a cycle known as:

1. Filter all of the things out of the blood (EXCEPT for big molecules like proteins - hence 'filtering')

2. Re-absorb and/or excrete further all of the important things in the tubules and collecting ducts (so we re-absorb glucose and actually also some urea in order to create a hyperosmolar atmosphere in the medulla, allowing H2O to therefore move back out of the urine and into the body - as well as a whole lot of other things. We can also excrete some of the urea back out.)

3. Urine is what is left at the end.

Diabetic people have SO MUCH blood glucose that it overwhelms the glucose transporters in the kidney so you can get blood in the urine. Damaged kidneys (which can also happens in diabetes, incidentally) will fail to filter properly and you can accidentally end up with protein in the urine. I find that knowing what happens when these processes go wrong helps you to remember how the processes work in the first place, which is why I mention this.

From that basic knowledge you can work out what's going to be where. You know that blood plasma (which is basically just = blood) is the stuff which gets filtered, and you know that proteins in a healthy person will not be filtered. So blood has glucose, proteins and urea.

Once it's been filtered out and becomes glomerular filtrate, it undergoes the process of reabsorption and secretion (which is very complicated but you only really need to know that glucose is re-absorbed!). So glomerular filtrate will contain glucose and urea, but no protein as this wasn't allowed through the filter from the blood.

Urine - well all the glucose was hopefully taken back out of the glomerular filtrate as it travelled along so it will contain urea - but no protein and in a healthy person, no glucose.

That's probably not a textbook answer (or indeed a markscheme answer as given above!) but hopefully it helps you understand it

Filtration --> Reabsorption/Excretion in a specific manner --> Urine