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Excretion

Kidney function, nephrons, waste removal

What Is Excretion?

Removal of metabolic waste products from body

CO₂: From respiration. Removed via lungs (breathing).

Urea: From amino acid deamination (liver). Excess amino acids broken down, amino group removed → urea (less toxic than ammonia) → dissolved in blood → filtered by kidneys

NOT egestion: Egestion = undigested food from gut (feces). Excretion = metabolic waste (urine)

Kidney Structure

Cortex: Outer region. Contains Bowman's capsules & proximal tubules.

Medulla: Inner region. Contains loops of Henle & collecting ducts. Concentrated urine here.

Pelvis: Funnel. Collects urine → ureter → bladder

Ureter: Carries urine to bladder. Peristalsis.

Urethra: Carries urine out of body.

The Nephron — Functional Unit of Kidney

Bowman's capsule: Cup surrounding glomerulus. Receives filtrate. Epithelium with many mitochondria (active transport).

Glomerulus: Capillary knot. High blood pressure → ultrafiltration.

Proximal convoluted tubule: Selective reabsorption of glucose, amino acids, water, ions. Mitochondria-rich for active transport.

Loop of Henle: Concentrates urine. Countercurrent multiplier.

Distal convoluted tubule & collecting duct: Fine-tune water reabsorption (ADH controls). Urea mostly stays in urine.

Ultrafiltration — First Stage

Location: Bowman's capsule / glomerulus

Process: High blood pressure in glomerulus → forces small molecules through capillary wall → filters into Bowman's capsule

Filtered OUT: Water, glucose, amino acids, ions, urea, excess of small molecules

NOT filtered: Large proteins, RBCs, WBCs (too large for pores)

Filtrate = dilute mixture of useful + waste substances

Selective Reabsorption — Second Stage

Location: Proximal convoluted tubule (mainly)

Glucose: ALL reabsorbed (active transport). Useful for energy.

Amino acids: ALL reabsorbed (active transport). For protein synthesis.

Water & ions: Selectively reabsorbed. Amount depends on ADH (water) & aldosterone (ions).

Urea: Mostly stays (some reabsorption). Most excreted.

Uses ATP (energy) for active transport. Cells have many mitochondria.

ADH — Controlling Water Reabsorption

ADH (antidiuretic hormone): From pituitary gland. Increases water permeability of collecting duct.

Low blood water (high osmolarity): ADH ↑ → more water reabsorbed → less urine (concentrated). Conserves water.

High blood water (low osmolarity): ADH ↓ → less water reabsorbed → more urine (dilute). Removes excess water.

Negative feedback: maintains constant blood osmolarity & water balance

Dialysis — Artificial Kidney

For kidney failure: Kidneys can't filter waste → urea & other toxins accumulate → dialysis cleans blood

Principle: Partially permeable membrane. Blood side: high urea. Dialysate side: no urea. Urea diffuses out.

Process: Blood from artery → passes alongside dialysate → waste diffuses out, glucose/ions diffuse in if needed → blood returns to vein

Need 3-4 hours, 3× per week OR transplant (better if available)

Exam Tips

✓ Excretion ≠ egestion (urine ≠ feces)

✓ Ultrafiltration: high pressure, small molecules filtered

✓ Selective reabsorption: glucose & amino acids ALL reabsorbed (active transport)

✓ ADH increases water reabsorption → less urine

✓ Dialysis for kidney failure (partially permeable membrane)

Key Takeaways

✅ Excretion: removal of metabolic waste (CO₂, urea)

✅ Kidney filters blood in three stages

✅ Ultrafiltration: high pressure forces small molecules out

✅ Selective reabsorption: glucose, amino acids, water, ions back into blood

✅ ADH controls water reabsorption (maintains water balance)

✅ Dialysis: artificial kidney for kidney failure

Keep it clean! Your kidneys filter all day! 🫘✓