What is the primary difference in composition between blood in the glomerulus and glomerular filtrate?

Blood contains large proteins and cells, whereas filtrate contains only small molecules like water, glucose, urea, and ions. This difference exists because the basement membrane acts as a size-selective sieve that prevents large entities from leaving the capillaries.

How does the effect of high ADH levels on urine compare to the effect of low ADH levels?

High ADH levels increase the permeability of the collecting duct, leading to more water reabsorption and concentrated urine. Low ADH levels result in less permeable walls, less reabsorption, and a larger volume of dilute urine.

How does the mechanism of ultrafiltration differ from the mechanism of selective reabsorption?

Ultrafiltration is a passive process driven by hydrostatic blood pressure in the glomerulus. Selective reabsorption is an active process requiring metabolic energy ($ATP$) to move glucose against its concentration gradient in the PCT.

Why is it incorrect to assume that glucose is excreted in the urine of a healthy individual?

In a healthy person, $100\%$ of filtered glucose is selectively reabsorbed back into the blood in the proximal convoluted tubule. If glucose appears in urine, it usually indicates that blood sugar levels have exceeded the nephron's reabsorption capacity, often due to diabetes.

What is the misconception regarding the role of diffusion in ultrafiltration?

Ultrafiltration is not driven by diffusion (movement down a concentration gradient) but by high hydrostatic pressure. The difference in vessel diameters forces small molecules through the filter regardless of their concentration inside or outside the capillaries.

Why is the presence of protein in urine a sign of kidney damage rather than a failure of reabsorption?

Proteins should never enter the nephron because they are too large to pass through the glomerular filter during ultrafiltration. If protein is found in urine, it implies the filtration membrane itself is damaged, allowing large molecules to leak into the tubule system.

What is the definition and role of the Antidiuretic Hormone (ADH)?

ADH is a hormone produced by the pituitary gland that regulates water balance by changing the permeability of the collecting ducts. By increasing permeability, it allows more water to be reabsorbed into the blood via osmosis when the body is dehydrated.

Define Selective Reabsorption in the context of the nephron.

It is the process occurring in the proximal convoluted tubule where useful substances, primarily glucose, are moved from the filtrate back into the bloodstream. This process is 'selective' because it specifically targets nutrients while allowing metabolic wastes like urea to remain in the tubule.

What is Glomerular Filtrate?

Glomerular filtrate is the fluid that has been forced out of the glomerulus into the Bowman's capsule. It contains water, glucose, urea, and ions, but lacks the proteins and blood cells found in the original blood supply.

What is the functional purpose of the nephron?

The nephron serves as the functional unit of the kidney, responsible for filtering the blood to remove metabolic waste. It simultaneously performs osmoregulation and nutrient conservation to maintain the body's internal environment.

Nephron Function | Pearson Edexcel IGCSE Biology

2. Structure & Function in Living Organisms

Nephron Function

Summary

Nephron function is the multi-stage process by which the kidneys filter metabolic waste from the blood while carefully regulating water and nutrient balance. Through ultrafiltration, selective reabsorption, and hormonally-controlled water recovery, the nephron transforms blood plasma into urine, maintaining the body's internal chemical stability.

1. Definition & Core Concepts

The Functional Unit: The nephron is the microscopic structural and functional unit of the kidney, with approximately one million units per kidney working to process blood. Its primary purpose is to remove metabolic wastes like urea while ensuring that essential molecules like glucose and water are retained according to the body's needs.
Three-Stage Process: Urine formation is not a single event but a sequence involving ultrafiltration in the Bowman's capsule, selective reabsorption in the proximal convoluted tubule (PCT), and regulated water reabsorption in the collecting duct. This staggered approach allows for high-volume filtration followed by precise recovery of vital substances.
Glomerular Filtrate: This is the liquid that enters the nephron after initial filtration, consisting of water, glucose, urea, and ions. It is fundamentally different from blood because it lacks large proteins and blood cells, serving as the raw material for the subsequent processing stages.

Diagram showing the high-pressure filtration mechanism in the Glomerulus and Bowman's capsule.

2. Ultrafiltration: The Pressure-Driven Sieve

Pressure Gradient: Ultrafiltration is driven by high blood pressure in the glomerulus, created because the afferent arteriole (entry) is significantly wider than the efferent arteriole (exit). This bottleneck effect forces fluid and small solutes through the capillary walls and into the Bowman's capsule.
Size-Selective Barrier: The basement membrane acts as a molecular sieve, allowing only molecules with a small molecular mass to pass through while retaining larger entities. Consequently, blood cells and plasma proteins remain in the blood, while water, glucose, urea, and mineral ions enter the nephron as filtrate.
Filtrate Composition: The resulting fluid is nearly identical to blood plasma minus the proteins, making it a concentrated source of both waste and nutrients. This initial lack of discrimination is why the subsequent 'selective' stages are biologically necessary for survival.

3. Selective Reabsorption: Nutrient Recovery

4. Water Reabsorption: The Role of ADH

5. Key Distinctions: Filtrate vs. Blood vs. Urine

6. Exam Strategy & Tips