Edexcel GCSE Biology (1BI0) — Animal Coordination, Control and Homeostasis
During ultrafiltration, glucose is filtered out of the blood into the nephron, yet a healthy person loses zero grams of it in their urine. This complete recovery is achieved through selective reabsorption. This process occurs exclusively in the Proximal Convoluted Tubule (PCT), which is the very first coiled section of the nephron located in the kidney cortex.
Because glucose must be fully recovered, it is moved from the nephron filtrate (inside the tubule lumen) back into the surrounding blood capillaries. This movement occurs against its concentration gradient. Consequently, it relies entirely on active transport rather than passive diffusion.
The cells lining the PCT possess specific structural adaptations to ensure 100% of the filtered glucose is reabsorbed. They feature a brush border of microvilli that vastly increases the surface area available for transport. Furthermore, these cells contain a high density of mitochondria to perform aerobic respiration, supplying the abundant ATP needed to power the active transport mechanisms.
In individuals with untreated diabetes, blood glucose levels become abnormally high. When this heavily glucose-laden filtrate enters the PCT, the transport proteins become overwhelmed and saturated. Because the proteins cannot transport the excess glucose fast enough, the unabsorbed glucose is left behind in the filtrate and ultimately excreted in the urine.
After a heavy workout on a hot summer afternoon, you will likely notice that your body produces much less urine than usual. This is a direct result of osmoregulation, a negative feedback loop designed to maintain a stable water potential within your blood. The control centre for this process is the hypothalamus, which contains osmoreceptors that detect tiny changes in blood water potential.
When your body is dehydrated, the water potential of your blood drops. The hypothalamus detects this decrease and signals the pituitary gland to release more Antidiuretic Hormone (ADH) into the bloodstream. ADH travels through the blood until it reaches its specific target organ: the kidneys.
Once in the kidneys, ADH binds to the walls of the collecting duct, causing a significant increase in its permeability to water. Because the walls are now highly permeable, a larger volume of water is drawn out of the filtrate and back into the blood capillaries via osmosis. This conservation of water results in a very small volume of dark, highly concentrated urine.
Conversely, if you are over-hydrated, your blood water potential rises. The pituitary gland responds by releasing less ADH, making the collecting duct less permeable. Consequently, less water is reabsorbed by osmosis, leaving excess water in the tubule to form a large volume of pale, dilute urine.
At the Bowman's capsule, a patient's filtrate contains of urea per . By the time the filtrate reaches the end of the collecting duct, water reabsorption has reduced the volume of fluid containing that same mass of urea to only . Calculate the final concentration of urea in .
Step 1: Identify the formula for concentration.
Step 2: Substitute the values for the final fluid to find the concentration per .
Step 3: Scale the concentration to .
(Explanation: Even though no extra urea was added, the concentration of the waste product increases from to because water is selectively reabsorbed out of the collecting duct via osmosis, leaving the urea behind in a much smaller volume of fluid.)
Students often refer to water 'concentration' when discussing osmoregulation — you must always use the precise scientific term 'water potential' to secure the marks.
In 4-6 mark questions explaining ADH action, examiners expect a strict causal sequence: state the sensor (hypothalamus), the hormone source (pituitary), the target (collecting duct), the mechanism (permeability/osmosis), and the final result (urine volume/concentration).
When identifying where glucose is reabsorbed, mark schemes strictly require the full name 'Proximal Convoluted Tubule' — vague answers like 'the tubule' or 'the kidney' will not be awarded marks.
Always specify the direction of movement explicitly in your explanations by stating that glucose or water moves 'from the filtrate back into the blood'.
Selective reabsorption
The process in the kidney where specific useful substances, such as glucose and water, are taken back into the blood from the glomerular filtrate.
Proximal Convoluted Tubule (PCT)
The portion of the nephron immediately following the Bowman's capsule where the vast majority of selective reabsorption, including 100% of glucose, takes place.
Active transport
The movement of substances against a concentration gradient across a cell membrane, a process that requires energy in the form of ATP from respiration.
Osmoregulation
The vital process of maintaining a constant water potential and ion concentration within the blood and body fluids.
Water potential
A measure of the tendency of water molecules to move from one area to another; osmoreceptors in the hypothalamus monitor this in the blood.
Antidiuretic Hormone (ADH)
A hormone produced in the hypothalamus and released by the pituitary gland that controls the permeability of the collecting duct to regulate water reabsorption.
Collecting duct
The final segment of the nephron where the ultimate concentration of urine is determined through the action of ADH.
Permeability
The property of a membrane that determines how easily liquids or gases can pass through it.
Osmosis
The movement of water out of the filtrate and back into the blood capillaries, driven by a difference in water potential.
Put your knowledge into practice — try past paper questions for Biology
Selective reabsorption
The process in the kidney where specific useful substances, such as glucose and water, are taken back into the blood from the glomerular filtrate.
Proximal Convoluted Tubule (PCT)
The portion of the nephron immediately following the Bowman's capsule where the vast majority of selective reabsorption, including 100% of glucose, takes place.
Active transport
The movement of substances against a concentration gradient across a cell membrane, a process that requires energy in the form of ATP from respiration.
Osmoregulation
The vital process of maintaining a constant water potential and ion concentration within the blood and body fluids.
Water potential
A measure of the tendency of water molecules to move from one area to another; osmoreceptors in the hypothalamus monitor this in the blood.
Antidiuretic Hormone (ADH)
A hormone produced in the hypothalamus and released by the pituitary gland that controls the permeability of the collecting duct to regulate water reabsorption.
Collecting duct
The final segment of the nephron where the ultimate concentration of urine is determined through the action of ADH.
Permeability
The property of a membrane that determines how easily liquids or gases can pass through it.
Osmosis
The movement of water out of the filtrate and back into the blood capillaries, driven by a difference in water potential.
