The Bio Brief

What is Excretion?

• Excretion is the process of removing harmful metabolic wastes from the body.
• These wastes are mainly nitrogenous wastes like:
  • Urea
  • Uric acid
• Excretion is important to keep the body healthy.

 Excretion in Different Organisms

• Unicellular  
  → Remove wastes by simple diffusion through body surface.
• Multicellular
  → Use specialised organs for excretion.

EXCRETION IN HUMAN BEINGS

Human Excretory System (Parts)

Kidneys → Ureters → Urinary Bladder → Urethra

Functions of Parts:

• Kidneys: Filter blood and form urine
• Ureters: Carry urine to bladder
• Urinary bladder: Stores urine
• Urethra: Releases urine outside the body

 How is Urine Formed?

Basic Unit of Kidney → Nephron

Each kidney has millions of nephrons

Structure of Nephron (Simplified)

Blood Capillaries

      ↓

Bowman’s Capsule

      ↓

Kidney Tubule

Steps of Urine Formation (Flow Chart)

Blood enters kidney

        ↓

Filtration in Bowman’s capsule

        ↓

Useful substances reabsorbed

        ↓

Waste + extra water forms urine

Important Points:

• Filtered substances: Urea, salts, water
• Reabsorbed substances:
  • Glucose
  • Amino acids
  • Most water
• Amount of water reabsorbed depends on:
  • Body’s water level
  • Amount of waste present

Storage and Removal of Urine

Kidney → Ureter → Urinary Bladder → Urethra

• Urinary bladder is muscular
• Controlled by nervous system
• Humans can control urination

ARTIFICIAL KIDNEY (HEMODIALYSIS)

When is it needed?

• Kidney failure due to:
  • Infection
  • Injury
  • Poor blood supply

How Artificial Kidney Works (Dialysis)

Process (Flow Chart):

Blood taken from patient

        ↓

Passed through semi-permeable tubes

        ↓

Waste diffuses into dialysing fluid

        ↓

Clean blood returned to body

Key Points:

• Dialysing fluid:
  • Same osmotic pressure as blood
  • No nitrogenous waste
• No reabsorption occurs (unlike real kidneys)
• Healthy kidney:
  • Filters ~180 L/day
  • Excretes only 1–2 L urine/day

EXCRETION IN PLANTS

Plants do not have special excretory organs.

Methods Used by Plants:

 Gaseous Wastes

• Oxygen (from photosynthesis)
• Carbon dioxide
• Removed through stomata

Excess Water

• Removed by Transpiration

Other Waste Products

• Stored in:
  • Vacuoles
  • Leaves (which fall off)
• Stored as:
  • Resins
  • Gums (in old xylem)
• Some wastes are:
  • Released into the soil

Summary Table:

Humans	Plants
Kidneys remove waste	No special organs
Urine formed	Waste stored or released
Dialysis possible	Leaves fall off