5.0     DEFINITION OF EXCRETION

It is the process by which organisms eliminate metabolic waste products and excess substances to maintain internal balance. It is essential for preventing toxicity and ensuring the proper functioning of cells and organs.

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TYPES OF WASTE PRODUCTS

• Carbon Dioxide: Produced during cellular respiration; excreted through lungs.
• Urea: Formed from the breakdown of proteins; less toxic than ammonia; excreted by kidneys.
• Ammonia: Highly toxic; primarily excreted by aquatic organisms.
• Uric Acid: Less soluble; excreted by birds and reptiles to conserve water.
• Excess Substances: Such as salts and water, which are regulated to maintain osmotic balance.

5.1     ORGANISMS AND THEIR EXCRETORY SYSTEM

Respiratory organs and surfaces are specialized structures in organisms that facilitate the exchange of gases (oxygen and carbon dioxide) with the environment. The following are some organisms and their excretory systems:

• Humans
• Kidneys: Main organs for filtering blood, regulating water, electrolytes, and producing urine.
• Skin: Excretes sweat, which contains water, salts, and urea.
• Lungs: Excrete carbon dioxide during respiration.
• Fish – Gills: Excrete ammonia directly into the water.
• Amphibian – Kidneys: Excrete urea; skin also plays a role in excretion.
• Insects – Malpighian tubules: Excrete uric acid, which conserves water.

5.2    FUNCTIONS OF THE KIDNEY

• Filter blood to remove waste and excess substances.
• Regulate blood pressure, electrolyte balance, and pH levels.
• Produce urine, which is composed of water, urea, salts, and other metabolites.

• URINARY SYSTEM

Urine is stored in the bladder and excreted through the urethra. Urination is controlled by sphincter muscles. Key structures of the kidney include:

• Nephrons: The functional units of the kidney responsible for filtering blood and forming urine.
• Cortex: The outer region of the kidney where the renal corpuscles and convoluted tubules are located.
• Medulla: The inner region containing the Loops of Henle and collecting ducts.
• Pelvis: The funnel-shaped structure that collects urine from the collecting ducts and channels it to the ureter.

5.4      DIAGRAM OF THE KIDNEY

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          5.5     STAGES OF URINE FORMATION

• Ultra-filtration: Occurs in the glomerulus, a network of capillaries within the Bowman’s capsule. Blood pressure forces water, ions, and small molecules (e.g., glucose, urea) from the blood into the Bowman’s capsule, forming the filtrate.
• Selective Reabsorption: Takes place primarily in the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
• Hormonal Secretion: Involves the active transport of additional waste products and excess ions from the blood into the renal tubules. Occurs mainly in the distal convoluted tubule and collecting duct.

•    EXCRETORY PRODUCTS OF PLANTS
• Oxygen – A by-product of photosynthesis, released through stomata.
• Carbon Dioxide – Released during respiration through stomata.
• Water – Expelled through transpiration via stomata and lenticels.
• Gum – A sticky substance excreted by some plants to seal wounds or as a defense mechanism.
• Resin – Excreted in response to injury or as a protective measure, particularly in coniferous trees.
• Tannin – A bitter compound produced by some plants to deter herbivores.
• Alkaloids – Toxic compounds (e.g., caffeine, nicotine) produced by certain plants for defense.
• Essential Oils – Aromatic compounds produced in some plants, particularly in flowers and leaves.
• Saponins – Detergent-like compounds that help plants protect themselves from pests.
• Calcium Oxalate Crystals – Excreted as waste products in some plants like Philodendron.
• Uric Acid – Some plants excrete uric acid, particularly in species that grow in arid environments.
• Mucilage – A gelatinous substance that helps retain water and defend against pests.
• Oxalates – Some plants excrete oxalates as waste; it can form insoluble crystals (e.g., calcium oxalate).
• Phenols – Compounds like flavonoids and lignans, which may be excreted for protection against UV radiation or herbivores.
• Amino Acids and Peptides – Some plants excrete excess amino acids and peptides into the soil or surrounding environment.