WJEC A-Level Biology: Notes on Homeostasis and Kidneys (Unit 3.7)

Kidney Structure and Homeostasis Basics

Homeostasis is your body's way of maintaining a constant internal environment, no matter what's happening outside. Think of it like a really sophisticated thermostat that controls not just temperature, but water levels, salt concentration, and waste removal too.

Your kidneys have two main regions doing different jobs. The cortex (outer layer) handles the initial filtering and reabsorption of useful stuff like glucose and water. The medulla (inner layer) is where your urine gets concentrated and osmoregulation (water balance control) happens.

Each kidney contains millions of tiny filtering units called nephrons. These are perfectly designed for their job with loads of surface area thanks to microvilli, plenty of mitochondria for energy, and tight connections to prevent any leakage.

Key Point: Negative feedback keeps everything balanced - when something changes, your body automatically reverses that change to get back to normal.

Ultrafiltration and Selective Reabsorption

The glomerulus is basically a high-pressure filter system that's incredibly clever. Blood enters through a wide tube (afferent arteriole) but leaves through a narrower one (efferent arteriole), creating massive pressure that forces filtration.

This three-layer filter lets small molecules like water and glucose through whilst keeping the big stuff (red blood cells, proteins) in your blood where they belong. Podocytes wrap around the capillaries like octopus tentacles, making the whole system super efficient.

In the proximal convoluted tubule (PCT), your body reclaims everything useful. Water moves back into blood by osmosis, whilst glucose hitches a ride with sodium through special protein channels. This cotransport mechanism is brilliant because it uses less energy than pumping glucose directly.

The loop of Henle creates a concentration gradient that's essential for water conservation. The descending limb loses water, whilst the ascending limb pumps out salt, making the whole system work like a biological desalination plant.

Remember: No glucose should appear in healthy urine - if it does, that's a sign of diabetes.

ADH and Water Balance Control

Antidiuretic hormone (ADH) is your body's water-saving superhero. When your blood gets too concentrated, your hypothalamus detects this and triggers ADH release from the pituitary gland.

ADH travels to your kidneys and opens more aquaporins (water channels) in the collecting duct walls. This means more water gets reabsorbed back into your blood instead of being lost as urine - pretty neat negative feedback in action.

When your kidneys fail, things go wrong fast. Kidney failure leads to waste product build-up and dangerous changes in water and salt balance. High-protein diets can worsen problems by creating more uric acid and kidney stones.

Excess salt consumption puts extra strain on your kidneys by increasing blood pressure and forcing them to work harder during ultrafiltration.

Health Tip: Staying hydrated and limiting processed foods helps keep your kidneys happy and functioning properly.

Treatment Options and Evolutionary Adaptations

Kidney transplants work best when donors are close relatives because similar antigens reduce rejection risk. Recipients need immunosuppressants to stop their immune system attacking the new kidney.

Haemodialysis uses countercurrent flow - your blood and cleaning fluid flow in opposite directions across a membrane. This maximises waste removal whilst keeping useful molecules in your blood. Peritoneal dialysis uses your abdomen's natural membrane as the filter instead.

Different animals have evolved brilliant solutions for waste removal. Aquatic animals produce highly toxic ammonia but dilute it quickly in water. Birds and insects make uric acid crystals that need hardly any water. Mammals like us produce urea as a compromise between toxicity and water conservation.

Desert animals have extra-long loops of Henle for maximum water reabsorption, whilst aquatic animals have shorter ones since water conservation isn't their priority.

Evolution Insight: Your kidney structure reflects your evolutionary history - longer loops mean better water conservation for surviving in dry environments.