AQA GCSE Biology (8461) — Homeostasis
Have you ever used a sieve to drain cooked pasta? The water falls through the holes, but the pasta is held back because it is too large. The kidneys work in a very similar way, using a microscopic filter to clean your blood.
Filtration happens in the nephrons, which are the tiny functional units of the kidney. Specifically, it occurs where highly branched capillary networks meet the start of the kidney tubule.
Blood enters the kidney under high pressure. This pressure forces water and small molecules out of the blood capillaries and into the kidney tubule, forming a liquid called the filtrate. This process is non-selective; it is based purely on whether a molecule is small enough to fit through the filter pores (which have a radius of roughly ).
Small molecules that pass into the filtrate include water, glucose, ions (such as sodium and potassium), and urea. Large molecules cannot pass through the pores and remain in the blood. These include proteins and blood cells (red blood cells, white blood cells, and platelets).
Data Interpretation: Blood Plasma vs. Kidney Filtrate Small molecules will have the exact same concentration in the newly formed filtrate as they do in the blood plasma because filtration is non-selective based on size.
| Substance | Concentration in Blood Plasma (g/dm³) |
|---|
| Concentration in Filtrate (g/dm³) |
|---|
| Reason |
|---|
| Proteins | 80.0 | 0.0 | Too large to pass through filter pores. |
| Glucose | 1.0 | 1.0 | Small molecule; completely passes into filtrate. |
| Ions | 7.0 | 7.0 | Small molecule; completely passes into filtrate. |
| Urea | 0.3 | 0.3 | Small molecule; completely passes into filtrate. |
Once the filtrate is formed, the body needs to recover useful substances before they are permanently lost in urine. This step-by-step recovery process is called selective reabsorption.
As the filtrate flows through the kidney tubules, specific substances are moved from the tubule back into the blood capillaries:
We can calculate the percentage of any substance that has been reabsorbed using this formula:
Worked Example:
A healthy person filters of sodium ions into their kidney tubules each day, but only is lost in their urine. Calculate the percentage of sodium ions reabsorbed.
(Warning: Only ever use the amount of a substance (e.g., in grams) in this formula. Do not use concentration values from the table below! Because water is heavily reabsorbed by the kidneys, the concentration of substances like ions and urea actually increases in the urine, even though the total amount of these substances is reduced.)
Data Interpretation: Blood Plasma vs. Urine The table below shows how concentrations change as the kidney filters and reabsorbs substances.
| Substance | Concentration in Plasma (g/dm³) | Concentration in Urine (g/dm³) | Reason |
|---|---|---|---|
| Glucose | 1.0 | 0.0 | All is reabsorbed back into the blood. |
| Ions | 7.0 | 15.0 | Some are reabsorbed; the rest is concentrated in urine. |
| Urea | 0.3 | 18.0 | None is reabsorbed; it concentrates as water is removed. |
| Protein | 80.0 | 0.0 | Too large to be filtered initially. |
Why is your urine sometimes dark yellow and other times pale and clear? This happens because your body is tightly controlling its blood water levels through a process called osmoregulation.
If blood water levels are not controlled, body cells will gain or lose too much water by osmosis. This causes cells to either shrivel or burst, preventing them from functioning efficiently. To prevent this, the brain monitors water levels and coordinates a negative feedback response using ADH (Antidiuretic Hormone).
ADH is produced and released by the pituitary gland into the bloodstream. When it reaches its target organ (the kidneys), it increases the permeability of the kidney tubules, allowing them to reabsorb more water.
The Negative Feedback Loops:
Students often write that urea 'diffuses' out of the blood during filtration. You must state that it is 'forced out by high pressure', as filtration is a mass flow process.
When answering questions about selective reabsorption, examiners specifically look for the words 'all' and 'some' — you must state that ALL glucose is reabsorbed, but only SOME water and ions are reabsorbed.
Always clearly state the direction of movement when describing reabsorption: useful substances move 'from the tubule back into the blood'. Reversing this direction will cost you marks.
For Higher Tier questions on ADH, 'permeability' is the most crucial keyword. You must explicitly state that ADH changes the permeability of the kidney tubules to water.
If an exam question shows glucose present in the urine, it indicates that the blood glucose concentration was too high for the kidney tubules to reabsorb all of it (often an indicator of diabetes).
When calculating the percentage of a substance reabsorbed, always use the total amount (e.g., in grams) rather than concentration. Water reabsorption causes the concentration of some substances to increase in urine, which would give you a mathematically impossible negative percentage if used in the formula!
Nephrons
The microscopic functional units of the kidney where filtration and selective reabsorption take place.
Kidney tubule
The tube inside the kidney where filtrate is collected and from which useful substances are selectively reabsorbed back into the blood.
Filtrate
The liquid formed in the kidney tubule immediately after high-pressure filtration, containing small molecules like water, glucose, ions, and urea.
Glucose
A small, useful sugar molecule that is completely filtered out of the blood and then 100% selectively reabsorbed via active transport.
Ions
Charged particles (such as sodium and potassium) that are filtered and partially reabsorbed depending on the body's requirements.
Urea
A toxic waste product formed in the liver from the breakdown of excess amino acids, which is filtered by the kidneys and excreted in urine.
Proteins
Large molecules in the blood plasma that are too big to pass through the kidney filter pores during filtration.
Blood cells
Components of the blood (red cells, white cells, platelets) that are too large to be filtered into the kidney tubules.
Urine
The final liquid waste produced by the kidneys, containing urea, excess water, and excess ions.
Selective reabsorption
The process by which specific useful molecules (all glucose, some ions, some water) are taken back into the blood from the kidney tubule.
Active transport
The movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient) requiring energy from respiration.
Osmosis
The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Diffusion
The spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration.
Osmoregulation
The homeostatic control of water and ion concentrations in the blood to prevent cells from bursting or shrivelling.
ADH
Antidiuretic Hormone; a hormone released by the pituitary gland that increases the permeability of the kidney tubules to water.
Pituitary gland
The 'master gland' located in the brain that releases ADH into the bloodstream in response to changes in blood water concentration.
Permeability
The extent to which the walls of the kidney tubules allow water to pass through them and back into the surrounding blood.
Negative feedback
A control mechanism that restores a system to its normal level by triggering a response that is the exact opposite of the detected change.
Put your knowledge into practice — try past paper questions for Biology
Nephrons
The microscopic functional units of the kidney where filtration and selective reabsorption take place.
Kidney tubule
The tube inside the kidney where filtrate is collected and from which useful substances are selectively reabsorbed back into the blood.
Filtrate
The liquid formed in the kidney tubule immediately after high-pressure filtration, containing small molecules like water, glucose, ions, and urea.
Glucose
A small, useful sugar molecule that is completely filtered out of the blood and then 100% selectively reabsorbed via active transport.
Ions
Charged particles (such as sodium and potassium) that are filtered and partially reabsorbed depending on the body's requirements.
Urea
A toxic waste product formed in the liver from the breakdown of excess amino acids, which is filtered by the kidneys and excreted in urine.
Proteins
Large molecules in the blood plasma that are too big to pass through the kidney filter pores during filtration.
Blood cells
Components of the blood (red cells, white cells, platelets) that are too large to be filtered into the kidney tubules.
Urine
The final liquid waste produced by the kidneys, containing urea, excess water, and excess ions.
Selective reabsorption
The process by which specific useful molecules (all glucose, some ions, some water) are taken back into the blood from the kidney tubule.
Active transport
The movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient) requiring energy from respiration.
Osmosis
The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Diffusion
The spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration.
Osmoregulation
The homeostatic control of water and ion concentrations in the blood to prevent cells from bursting or shrivelling.
ADH
Antidiuretic Hormone; a hormone released by the pituitary gland that increases the permeability of the kidney tubules to water.
Pituitary gland
The 'master gland' located in the brain that releases ADH into the bloodstream in response to changes in blood water concentration.
Permeability
The extent to which the walls of the kidney tubules allow water to pass through them and back into the surrounding blood.
Negative feedback
A control mechanism that restores a system to its normal level by triggering a response that is the exact opposite of the detected change.
