Kidney Anatomy and Physiology

Introduction

The kidneys are the body's ultimate multitaskers. They filter waste, balance fluids, control blood pressure, regulate electrolytes, and even help make red blood cells. Without them, survival isn't just tough—it’s impossible. If you're a nurse or dialysis technician, understanding kidney function isn’t optional. It’s essential.

In this chapter, we’ll break down kidney anatomy and physiology in a way that sticks. No fluff, no jargon overload—just straight-to-the-point, engaging content that makes you say, “Wow, I actually get this!”

Anatomy of the Kidney

Each person has two kidneys, shaped like giant kidney beans. They sit snugly against the back muscles, one on each side of the spine, just below the ribcage. Each kidney weighs about 150 grams and is roughly the size of a fist. Despite their small size, they handle about 180 liters of blood every single day!

External Structure

The kidney has several layers that protect and support its function:

Renal Capsule – A tough, fibrous outer layer that shields the kidney from injury and infection.

Adipose Tissue (Fat Layer) – Provides cushioning and insulation.

Renal Cortex – The outer region where filtration starts. Packed with blood vessels and filtering units.

Renal Medulla – The inner region, home to the nephrons’ tubules and collecting ducts.

Renal Pelvis – A funnel-shaped structure that collects urine and channels it into the ureter.

Internal Structure

Inside each kidney, you’ll find about a million nephrons—tiny filtering units that do the real work. Let’s break them down:

Glomerulus – A tangled ball of tiny capillaries where blood filtration begins. Think of it as a high-powered sieve that lets small molecules (like water, salt, and waste) pass through while keeping larger ones (like proteins and blood cells) inside the bloodstream.

Bowman’s Capsule – Surrounds the glomerulus, collecting the filtered fluid (now called filtrate) and directing it into the nephron’s tubules.

Proximal Convoluted Tubule (PCT) – The first section of the tubule system, where valuable substances like glucose, amino acids, and electrolytes get reabsorbed back into the blood.

Loop of Henle – A U-shaped structure that concentrates urine by reabsorbing water and salts.

Distal Convoluted Tubule (DCT) – Adjusts ion and pH balance before sending the filtrate to the collecting ducts.

Collecting Ducts – The final passageway for urine before it drains into the renal pelvis and exits via the ureter.

Blood Supply to the Kidney

Kidneys are thirsty organs—they receive 20–25% of the body's blood supply at any given time. Their intricate blood flow system allows precise filtration and reabsorption.

Renal Artery – Supplies oxygen-rich blood to the kidneys.

Afferent Arterioles – Bring blood into the glomerulus for filtration.

Glomerulus – Filters out waste, water, and small molecules while keeping essential proteins and cells.

Efferent Arterioles – Carry the remaining blood away from the glomerulus.

Peritubular Capillaries & Vasa Recta – Surround the nephron, allowing for exchange of nutrients, electrolytes, and water.

Renal Vein – Returns the purified blood to circulation.

Physiology of the Kidney

The kidneys are responsible for keeping the body’s internal environment stable. Let’s break down their functions step by step.

1. Filtration

Filtration happens in the glomerulus. Blood enters under high pressure, pushing plasma (minus large proteins and cells) into the Bowman’s capsule. This fluid, now called filtrate, moves through the nephron’s tubules.

2. Reabsorption

Not everything in the filtrate is waste. The body needs to reclaim vital substances like:

Glucose – 100% should be reabsorbed. If glucose appears in urine, it may indicate diabetes.

Water – Controlled by antidiuretic hormone (ADH) based on hydration needs.

Sodium & Potassium – Regulated to maintain blood pressure and nerve function.

Bicarbonate (HCO₃⁻) – Helps maintain blood pH balance.

3. Secretion

The nephron also actively secretes unwanted substances directly from blood into urine. This fine-tunes electrolyte levels and pH.

Hydrogen Ions (H⁺) – To prevent acidosis.

Potassium (K⁺) – Controlled by aldosterone to prevent dangerous imbalances.

Drugs & Toxins – Medications like penicillin and creatinine are secreted and eliminated in urine.

4. Urine Formation

After filtration, reabsorption, and secretion, what remains is urine—a concentrated mix of waste products. The urine flows through:

  1. Collecting ducts →

  2. Renal pelvis →

  3. Ureter →

  4. Bladder →

  5. Urethra → Goodbye, waste!

Key Functions of the Kidneys

1. Waste Removal

The kidneys remove urea, creatinine, and toxins to prevent harmful buildup.

2. Fluid Balance

They adjust urine concentration based on hydration levels. More water intake? More urine output. Dehydrated? Kidneys conserve water.

3. Electrolyte Regulation

They maintain sodium, potassium, calcium, and phosphate levels to keep nerves, muscles, and the heart functioning properly.

4. Acid-Base Balance

They regulate blood pH by balancing bicarbonate (HCO₃⁻) and hydrogen ions (H⁺). This keeps the body from becoming too acidic or too alkaline.

5. Blood Pressure Control

The kidneys release renin, an enzyme that helps control blood pressure by adjusting sodium and water retention.

6. Red Blood Cell Production

The kidneys produce erythropoietin (EPO), a hormone that stimulates the bone marrow to make red blood cells. When kidney function declines, anemia can develop.

7. Bone Health

They activate vitamin D, which is necessary for calcium absorption and strong bones. Without this, bones weaken, increasing fracture risk.

Why This Matters for Dialysis Professionals

Kidney function isn’t just an interesting topic—it’s the foundation of dialysis therapy. When kidneys fail, waste, fluid, and electrolytes accumulate dangerously. Dialysis replaces some of these functions, but it’s not a perfect substitute. Understanding kidney physiology helps nurses and technicians:

Assess fluid and electrolyte imbalances.

Recognise dialysis complications early.

Educate patients on kidney health and disease management.

Conclusion

The kidneys work tirelessly to keep the body running smoothly. They filter waste, balance fluids, regulate electrolytes, and even control blood pressure. When they fail, life becomes complicated—but with dialysis, we can help bridge the gap.