4.1 Excretion

Excretion is the removal of chemical waste from the body which are produced by the metabolic processes within cells. The body cells form a variety of wastes, and if these substances are allowed to accumulate, their effects are likely to be toxic.

Definition

Excretion: Excretion is the removal of chemical waste from the body which are produced by the metabolic processes within cells.

Metabolism: Metabolism produces a number of toxic by-products, particularly the nitrogen-containing compounds.

Main Idea

• The body cells form a variety of wastes.
• If these substances are allowed to accumulate, their effects are likely to be toxic.
• The body fluids, such as the blood and lymph, carry wastes away from the tissues that produce them.
• Other parts remove these wastes from the blood and transport them to the outside.
• The respiratory system, for example, removes carbon dioxide from the blood.
• The urinary system removes various salts and nitrogenous wastes.

Core Concept

• Excretion = removal of metabolic waste
• Metabolism = produces toxic by-products
• Nitrogenous wastes = ammonia, urea, uric acid
• Body fluids = blood and lymph carry wastes
• Respiratory system = removes carbon dioxide
• Urinary system = removes salts and nitrogenous wastes
• Urinary system = maintains water and electrolyte balance
• Urinary system = regulates pH and volume of body fluids
• Urinary system = controls red blood cell production and blood pressure

Detailed Explanation

Excretion

Metabolism produces a number of toxic by-products, particularly the nitrogen-containing compound. The excretion is the removal of chemical waste from the body which are produced by the metabolic processes within cells.

Nitrogen as Excretory Product

Animals excrete a variety of nitrogenous waste products, but ammonia, urea and uric acid predominate. A major factor in determining the mode of nitrogen excretion is the availability of water in the environment.

Relationship Between Excretory Products and Habitats

• Ammonia — aquatic
• Urea — aquatic and terrestrial
• Uric acid — terrestrial

Ammonia

• Ammonia is highly toxic because it tends to raise the pH of body fluids and interferes with membrane transport functions.
• It is highly soluble in water and diffuses rapidly across cell membrane.
• It is therefore excreted rapidly.
• One gram of nitrogen, in the form of ammonia, requires five hundred ml of water to dissolve it to nontoxic level.
• Such plenty of water can only be afforded by many aquatic organisms, particularly those in freshwater.
• Examples: most fishes, protozoans, sponges, cnidaria, echinoderms, and crustaceans.
• Animals which excrete ammonia as their major nitrogenous waste product are called ammonotelic.

Urea

• Organisms with less freshwater available, such as some marine organisms and all terrestrial organisms, remove most of their nitrogenous waste in the form of urea.
• They will often invest some energy to convert the ammonia into urea, which is 100,000 times less toxic than ammonia.
• One gram of nitrogen, in the form of urea, requires 50 ml of water to dilute it to nontoxic level.
• Animals which excrete urea as their major nitrogenous waste product are called ureotelic.
• Examples include mammals e.g. man, many amphibians and some marine animals like shark.

Uric Acid

• Uric acid is a byproduct of breakdown of purine and is less toxic than urea.
• It can precipitate from solution, allowing the 4 nitrogen atoms per uric acid molecule to be excreted.
• One gram of nitrogen, in the form of uric acid, requires just 1 ml of water for its excretion.
• It has evolved in two groups with major water shortage problems:
• Terrestrial invertebrates e.g., land snail, many insects
• Egg-laying vertebrates e.g., reptiles (snakes, lizards) and birds
• These animals are called uricotelic.
• Examples include many terrestrial insects, reptiles, and most birds.

Critical Thinking

Where do you think the carbon dioxide used in the formation of urea comes from? Where does the remainder of excess carbon dioxide go to be excreted?

Science Titbits

Humans excrete small quantities of uric acid but this is produced from the breakdown of nucleic acid and not from breakdown of proteins. Approximately one gram of uric acid is excreted in urine per day.

Urinary System of Man

The excretory system (urinary system) consists of kidneys, ureters, urinary bladder, and a tubular urethra. The kidneys lie on either side of the vertebral column between the twelfth thoracic and third lumbar vertebrae.

• Each ureter is a tubular organ about 25 cm long, which begins as the funnel-shaped renal pelvis.
• It extends downward parallel to the vertebral column to join the urinary bladder.
• It transports urine from the kidney to the urinary bladder.
• The urinary bladder is a hollow and distensible muscular organ.
• It is located within the pelvic cavity.
• It serves as urine reservoir.
• The urethra is a tube that carries urine from urinary bladder to the outside of the body.

Table of Nitrogenous Wastes and Habitats

Intermediate Level

Types of Nitrogenous Wastes

Relationship Between Excretory Products and Habitats

Urinary System

Functions of Urinary System

• Removes various salts and nitrogenous wastes.
• Maintains normal concentrations of water and electrolytes.
• Regulates pH and volume of body fluids.
• Controls red blood cell production.
• Controls blood pressure.

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Excretion is the removal of metabolic waste.
• 2. Metabolism produces toxic by-products.
• 3. Nitrogenous wastes: ammonia, urea, uric acid.
• 4. Ammonia is highly toxic; requires 500 ml water/g nitrogen.
• 5. Urea is 100,000 times less toxic than ammonia; requires 50 ml water/g nitrogen.
• 6. Uric acid is less toxic than urea; requires 1 ml water/g nitrogen.
• 7. Ammonia is excreted by aquatic animals (ammonotelic).
• 8. Urea is excreted by mammals and amphibians (ureotelic).
• 9. Uric acid is excreted by terrestrial animals (uricotelic).
• 10. Respiratory system removes carbon dioxide.
• 11. Urinary system removes salts and nitrogenous wastes.
• 12. Urinary system maintains water and electrolyte balance.
• 13. Urinary system regulates pH and volume of body fluids.
• 14. Urinary system controls red blood cell production and blood pressure.
• 15. Kidneys, ureters, urinary bladder, and urethra are parts of urinary system.

Remember

• - Excretion = removal of metabolic waste
• - Ammonia = aquatic; highly toxic; 500 ml water/g N
• - Urea = aquatic and terrestrial; 100,000× less toxic; 50 ml water/g N
• - Uric acid = terrestrial; least toxic; 1 ml water/g N
• - Ammonotelic = ammonia excreters
• - Ureotelic = urea excreters
• - Uricotelic = uric acid excreters
• - Urinary system = kidneys, ureters, bladder, urethra
• - Kidneys = filter blood
• - Ureters = 25 cm long; carry urine to bladder
• - Urinary bladder = stores urine
• - Urethra = carries urine out of body

Common Mistakes

• 1. Confusing excretion and elimination — Excretion = removal of metabolic waste; elimination = removal of undigested food.
• 2. Thinking ammonia is less toxic than urea — Ammonia is more toxic.
• 3. Mixing up the terms — Ammonotelic (ammonia), ureotelic (urea), uricotelic (uric acid).
• 4. Forgetting the water requirements — Ammonia: 500 ml; urea: 50 ml; uric acid: 1 ml.
• 5. Confusing the habitats — Ammonia = aquatic; urea = aquatic and terrestrial; uric acid = terrestrial.
• 6. Thinking kidneys are the only excretory organs — Lungs, skin, and liver also excrete wastes.
• 7. Mixing up ureter and urethra — Ureter carries urine to bladder; urethra carries urine out of body.
• 8. Forgetting the role of urinary system in blood pressure — It controls blood pressure.
• 9. Thinking urea is more toxic than ammonia — Urea is less toxic.
• 10. Mixing up the examples — Fishes = ammonotelic; mammals = ureotelic; birds = uricotelic.

Quick Revision

Excretion is the removal of chemical waste from the body which are produced by the metabolic processes within cells. Metabolism produces a number of toxic by-products, particularly nitrogen-containing compounds. Animals excrete a variety of nitrogenous waste products, but ammonia, urea, and uric acid predominate. The exact nature of excretory product is determined mainly by the availability of water to the organism, which is based upon its habitat. Ammonia is excreted by aquatic animals (ammonotelic), urea by aquatic and terrestrial animals (ureotelic), and uric acid by terrestrial animals (uricotelic). The urinary system consists of kidneys, ureters, urinary bladder, and urethra. It removes various salts and nitrogenous wastes, maintains normal concentrations of water and electrolytes, regulates pH and volume of body fluids, and controls red blood cell production and blood pressure.

60-Second Revision

• Excretion = removal of metabolic waste
• Ammonia = aquatic; highly toxic; 500 ml water/g N
• Urea = aquatic and terrestrial; 100,000× less toxic; 50 ml water/g N
• Uric acid = terrestrial; least toxic; 1 ml water/g N
• Ammonotelic = ammonia excreters
• Ureotelic = urea excreters
• Uricotelic = uric acid excreters
• Urinary system = kidneys, ureters, bladder, urethra
• Kidneys = filter blood
• Warning: Ammonia is the most toxic, uric acid is the least toxic

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• 4. Urinary System of Man

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• 4.2 URINARY SYSTEM OF MAN

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