Excretion

Excretion

  • The process by which waste products are released from the system of an organism.

  • Waste products arise from metabolic reactions, which include:

    • Carbon dioxide

    • Excess water

    • Salts

    • Nitrogenous wastes: urea, uric acid, creatinine

Importance of Excretion

  • Accumulation of waste products can be toxic and damage body tissues.

  • Although water is not toxic, excess accumulation disrupts homeostasis, leading to complications.

Key Definitions

  • Metabolic reactions: All chemical reactions in the body.

  • Homeostasis: Maintaining internal body environment for proper cellular function.

  • Internal environment: Made up of tissue fluid and blood.

Organs of the Excretory System in Humans

Organ

Function

Lungs

1. Removal of excess CO2, water vapor, and heat

Liver

2. Produces urea and uric acid from protein breakdown

3. Urea filtered by kidneys, excreted in urine

4. Bile excreted as bile salts in feces

Skin

5. Removal of excess water, salt, urea, and uric acid via sweat

Kidneys

6. Filter blood to form urine

7. Urine contains excess water, salt, urea, and uric acid

Identification of Substances Excreted

  • Carbon dioxide: Product of cellular respiration.

  • Water vapor: Product of cellular respiration, found in food and water.

  • Urea: Formed in the liver through deamination of amino acids.

  • Uric acid: End product of nucleic acid metabolism.

  • Creatinine: Formed from creatinine phosphate in cells.

  • Bile: Formed in liver during breakdown of hemoglobin.

Structure of the Urinary System

  • Composed of:

    • 2 kidneys

    • 2 ureters

    • 1 bladder

    • 1 urethra

  • Blood Supply: Renal arteries & veins.

Kidneys

  • Reddish-brown organs located on either side of the spinal column.

  • Anchored against the dorsal body wall by connective tissue.

Ureters

  • Muscular tubes that move urine from each kidney to the bladder through peristalsis.

Bladder

  • Muscular sac that expands and contracts.

  • Stretch receptors indicate fullness to notify the brain.

Urethra

  • Tube connecting the bladder to the outside world.

  • Sphincter muscles control urine flow.

Structure of the Kidney

External Structure

  • Bean-shaped, brown organs about the size of a fist.

  • Covered by a renal capsule of fibrous connective tissue.

  • Layered with fat for cushioning.

Internal Structure (Macroscopic)

  • Renal capsule: Protects the kidney.

  • Cortex: Reddish-brown area underneath the renal capsule.

  • Medulla: Lighter inner area containing grouped tubes.

  • Renal pyramids: Apex forms renal papilla, which opens into renal calyx.

Internal Structure (Microscopic)

  • Each kidney contains approximately one million nephrons, the functional and structural units.

Function of a Nephron

  • Removes excess water, wastes, and substances from blood.

  • Returns essential substances like sodium, potassium, or phosphorus when needed.

Processes in Kidney Function

1. Glomerular Filtration

  • Takes place in the renal corpuscle; blood in glomerulus is under high pressure.

  • Substances filtered: water, salts, glucose, urea, vitamins, amino acids.

  • Proteins and blood cells cannot pass through.

2. Tubular Reabsorption

  • Reabsorption of useful substances via osmosis, diffusion, and active transport.

  • Takes place mainly in the Proximal Convoluted Tubule, Loop of Henle, and Distal Convoluted Tubule.

  • Areas of Reabsorption:

    • Proximal Convoluted Tubule: largest volume, absorbs water, glucose, minerals, and vitamins.

    • Loop of Henle: reabsorbs water and sodium actively; important for maintaining sodium content in blood.

    • Distal Convoluted Tubule: further reabsorption of sodium and water.

3. Tubular Excretion

  • Active process adding non-filterable wastes to tubular fluid.

  • Occurs mainly in the distal convoluted tubule and includes excretion of chemicals like drugs and ions.

Urine Formation

  • Urine flows from collecting ducts to the renal pelvis, then to the ureter.

  • Urine transported from the bladder via the urethra during urination.

Composition of Urine

  • Yellowish fluid with composition affected by health, hydration, diet, and activity.

  • Main components include:

    • Water

    • Inorganic salts: chlorides, sulfates, phosphates

    • Metabolic wastes: urea, uric acid, creatinine

    • Foreign substances: preservatives, colorants, medications.