HomeBiologyB2: Structures and Functions in Living OrganismsB2.10 Excretion and Osmoregulation

B2: Structures and Functions in Living Organisms

B2.1 Cell Structure and FunctionB2.2 Biological MoleculesB2.3 Enzymes - Biological CatalystsB2.4 Movement Across MembranesB2.5 PhotosynthesisB2.6 RespirationB2.7 Ventilation and Gas ExchangeB2.8 Circulatory SystemB2.9 Coordination and ResponseB2.10 Excretion and Osmoregulation
B2: Structures and Functions

Excretion and Osmoregulation

How kidneys remove waste and control water balance

Kidneys and urinary system illustration

Waste Removal

Filtering blood and maintaining water balance

What is Excretion?
Removing metabolic waste from the body

Excretion is the removal of toxic waste products made by metabolism. It's essential because waste products like urea and carbon dioxide are harmful if they build up.

Urea

Made in the liver from breakdown of excess amino acids (from proteins). Removed by kidneys in urine.

Carbon Dioxide

Made by respiration in all cells. Removed by lungs when we breathe out.

Water & Ions

Excess water and mineral ions removed by kidneys (urine) and skin (sweat).

Kidney Structure
Anatomy of the urinary system

Each kidney contains about one million tiny filtering units called nephrons. Blood enters through the renal artery, waste is filtered out, and clean blood leaves through the renal vein.

Kidney Parts
  • Cortex: outer layer, contains nephrons
  • Medulla: inner layer, collecting ducts
  • Pelvis: central cavity, urine collects
  • Ureter: tube carrying urine to bladder
Nephron Parts
  • Glomerulus: ball of capillaries
  • Bowman's capsule: cup around glomerulus
  • Tubule: long coiled tube
  • Collecting duct: final urine concentration
Three Stages of Urine Formation
How kidneys filter blood and make urine
1. Ultrafiltration

High blood pressure forces small molecules through tiny pores in glomerulus into Bowman's capsule.

Filtered: water, glucose, urea, ionsStay in blood: blood cells, proteins (too large)
2. Selective Reabsorption

Useful substances are reabsorbed from the tubule back into the blood. This happens by active transport and diffusion.

Reabsorbed: ALL glucose, most water, some ionsStay in tubule: urea, excess water & ions
3. Osmoregulation

ADH (Anti-Diuretic Hormone) from the pituitary gland controls how much water is reabsorbed in the collecting duct. This maintains water balance.

High ADH: more water reabsorbed → concentrated urine
Low ADH: less water reabsorbed → dilute urine
Interactive Nephron Simulator
Step through the three stages of urine formation
Stage 1 of 3
Ultrafiltration
Glomerulus → Bowman's Capsule
High blood pressure forces small molecules through pores
Filtered Into Capsule:
Water
Glucose
Urea
Ions (Na+, Cl-)
Remain In Blood:
Red blood cells
Proteins
Water
Glucose
Urea
Ions
Blood cells
Proteins
Kidney Failure
When kidneys stop working properly

If kidneys fail, toxic waste (especially urea) builds up in blood. Without treatment, this is fatal.

Dialysis

Machine filters blood artificially. Patient connected to machine 3-4 times per week for several hours.

Advantages: keeps patient alive, no surgery needed

Disadvantages: time-consuming, dietary restrictions, not a cure

Kidney Transplant

Healthy kidney from donor transplanted into patient. Works like natural kidney.

Advantages: better quality of life, no regular dialysis

Disadvantages: donor needed, risk of rejection, immunosuppressant drugs

Worked Example
Understanding ADH and urine concentration
Scenario 1: After Drinking Lots of Water
  1. Blood becomes diluted (low concentration)
  2. Brain detects this and pituitary releases LESS ADH
  3. Collecting duct walls become less permeable to water
  4. LESS water reabsorbed back into blood
  5. MORE water stays in tubule
  6. Result: Large volume of dilute (pale) urine
Scenario 2: After Sweating (Exercise/Hot Day)
  1. Lost water through sweat, blood becomes concentrated
  2. Brain detects this and pituitary releases MORE ADH
  3. Collecting duct walls become more permeable to water
  4. MORE water reabsorbed back into blood
  5. LESS water stays in tubule
  6. Result: Small volume of concentrated (dark) urine

Key Concept: ADH maintains water balance through negative feedback. The body constantly adjusts ADH levels to keep blood concentration stable.

Key Terms Flashcards
Click the card to reveal the definition
Term
Excretion
Card 1 of 12
Test Your Knowledge
Question 1 of 8
What is the main nitrogenous waste product removed by kidneys?