Nutrition and Excretion
4. Excretion and Waste Removal
Learning outcomes
- I can define excretion and distinguish it from egestion.
- I can identify common metabolic wastes produced by animals.
- I can explain the role of excretory organs.
- I can describe how waste products are removed from the body.
- I can explain why excretion is important for homeostasis.
What Is Excretion?
Every living cell carries out chemical reactions. Together, these reactions make up an organism's metabolism.
Metabolism produces useful substances, but it also produces waste products. Some of these wastes can become harmful if they accumulate.
Excretion is the removal from the body of waste products produced by metabolism, as well as substances present in excess of the body's requirements.
Animals must continually remove wastes in order to maintain stable internal conditions.
Important metabolic wastes include:
- Carbon dioxide
- Urea and other nitrogenous wastes
- Excess water
- Excess mineral ions
Different organs are responsible for removing different wastes.
Excretion Is Not the Same as Egestion
Excretion and egestion are often confused, but they describe different processes.
Excretion
Removes substances produced by metabolism or substances present in excess.
Examples include:
- Carbon dioxide produced during cellular respiration.
- Urea produced from the breakdown of excess amino acids.
- Excess mineral ions.
- Excess water.
Egestion
Removes undigested and unabsorbed food from the digestive tract as faeces.
This material has passed through the digestive system but has not entered the body's tissues and undergone cellular metabolism.
Therefore:
Excretion = removal of metabolic wastes
Egestion = removal of undigested material
Why Must Metabolic Wastes Be Removed?
Waste products are not necessarily harmless.
If they accumulate, they can change the conditions inside the body and interfere with normal cellular processes.
For example:
- Too much carbon dioxide can alter blood pH.
- High concentrations of nitrogenous wastes can damage cells.
- Too much or too little water can disrupt cell function.
- Incorrect concentrations of mineral ions can affect nerves and muscles.
Excretion therefore contributes directly to homeostasis.
What Is Homeostasis?
Homeostasis is the maintenance of relatively stable internal conditions despite changes inside or outside the organism.
The body regulates factors such as:
- Temperature
- Blood glucose concentration
- Water balance
- Ion concentrations
- Carbon dioxide concentration
- pH
Excretion helps regulate several of these conditions.
It does more than simply "get rid of waste." It helps maintain the chemical environment in which cells function.
Major Excretory Organs
Several organs contribute to excretion.
Important examples include:
| Organ | Important substances removed or processed |
|---|---|
| Kidneys | Urea, excess water, excess mineral ions and other substances |
| Lungs | Carbon dioxide and water vapour |
| Skin | Water, mineral ions and small amounts of urea in sweat |
| Liver | Processes amino acids and converts toxic ammonia into urea |
These organs perform different but connected roles.
The Lungs and Carbon Dioxide
During aerobic cellular respiration, cells release energy from nutrient molecules.
A simplified relationship is:
glucose + oxygen → carbon dioxide + water + energy transferred
Carbon dioxide is therefore a metabolic waste product.
It must be removed from the body.
How Carbon Dioxide Is Removed
Carbon dioxide produced by cells enters the blood.
The circulatory system transports it toward the lungs.
In the lungs:
Blood → carbon dioxide diffuses into alveoli → carbon dioxide is exhaled
Much of the carbon dioxide is transported in the blood in the form of bicarbonate ions, rather than simply as dissolved carbon dioxide gas.
At the lungs, reactions are reversed and carbon dioxide is released into the alveoli.
It then leaves the body during exhalation.
Why Carbon Dioxide Must Be Removed
Carbon dioxide can react with water and influence acidity.
If too much carbon dioxide accumulates in the blood, blood pH can decrease.
Because enzymes and other cellular processes function properly only within suitable conditions, carbon dioxide levels must be carefully regulated.
The respiratory system therefore contributes to both:
- Gas exchange.
- Excretion.
- Acid-base homeostasis.
Nitrogenous Waste
Proteins are digested into amino acids.
Amino acids are important for producing proteins and other biological molecules.
However, excess amino acids cannot simply be stored in large quantities in the same way as glycogen or fat.
They can be broken down.
This produces nitrogen-containing waste.
The Liver and Urea Formation
The liver plays an important role in processing excess amino acids.
The nitrogen-containing part of amino acids is removed in a process called deamination.
This initially produces ammonia, which is highly toxic.
The liver converts ammonia into the less toxic substance urea.
A simplified pathway is:
Excess amino acids → deamination → ammonia → urea
The urea then enters the bloodstream.
Blood transports it to the kidneys.
The Kidneys
The kidneys are major excretory organs.
Humans normally have two kidneys located toward the back of the abdominal cavity.
Their functions include regulating:
- Water balance.
- Mineral ion concentrations.
- Blood composition.
- Acid-base balance.
They also remove:
- Urea.
- Other nitrogen-containing wastes.
- Some drugs and their metabolites.
- Various substances present in excess.
The Urinary System
The main structures of the urinary system are:
- Kidneys
- Ureters
- Urinary bladder
- Urethra
The pathway of urine is:
Kidneys → ureters → bladder → urethra → outside the body
Each structure has a different function.
Kidneys
The kidneys produce urine by filtering and processing blood.
They remove selected substances while retaining substances that the body still needs.
This is important.
The kidneys do not simply remove everything from the blood.
They carefully regulate what is retained and what is excreted.
Ureters
The ureters are muscular tubes that carry urine from the kidneys to the bladder.
Muscular contractions help move urine along the ureters.
Urinary Bladder
The urinary bladder temporarily stores urine.
Its muscular walls allow it to expand as urine accumulates.
Urethra
The urethra carries urine from the bladder to the outside of the body.
Do not confuse the ureter with the urethra.
Ureter: kidney → bladder
Urethra: bladder → outside
Inside the Kidney
Each kidney contains approximately a million microscopic functional units called nephrons.
A nephron filters blood and modifies the resulting fluid to produce urine.
Important nephron structures include:
- Glomerulus
- Bowman's capsule
- Renal tubule
- Loop of Henle
- Collecting duct
- Surrounding blood capillaries
The nephron allows the kidney to control the composition of urine.
Filtration of the Blood
Blood enters the kidney and reaches clusters of tiny capillaries called glomeruli.
High pressure causes small substances to leave the blood and enter the nephron.
This process is called ultrafiltration.
Small substances entering the filtrate include:
- Water
- Glucose
- Mineral ions
- Urea
Large blood cells and most large plasma proteins normally remain in the blood.
Selective Reabsorption
Filtering the blood removes useful substances as well as wastes.
The kidney therefore needs to recover substances the body still needs.
This is called selective reabsorption.
Useful substances move from the nephron back into the blood.
Examples include:
- Glucose
- Amino acids
- Much of the water
- Required mineral ions
The kidneys therefore work through a combination of:
Filtration + selective reabsorption + secretion → urine
What Is in Urine?
Normal urine contains mostly water, along with dissolved substances.
These commonly include:
- Urea
- Excess mineral ions
- Other nitrogenous wastes
- Various metabolites
The exact composition of urine changes according to the body's condition.
For example, urine can become more concentrated when the body needs to conserve water.
Water Balance
Water enters the body through:
- Drinks.
- Food.
- Metabolic reactions.
Water leaves through:
- Urine.
- Sweat.
- Exhaled air.
- Faeces.
The kidneys help balance water input and water loss.
If the body needs to conserve water, the kidneys can produce a smaller volume of more concentrated urine.
If the body contains excess water, a larger volume of more dilute urine can be produced.
This is an important example of homeostasis.
ADH and Water Regulation
Water reabsorption by the kidneys is influenced by a hormone called antidiuretic hormone, or ADH.
When the body needs to conserve water:
- ADH levels increase.
- Collecting ducts become more permeable to water.
- More water is reabsorbed into the blood.
- Less water remains in the urine.
- Urine becomes more concentrated.
When the body contains excess water, ADH levels decrease and less water is reabsorbed.
This is an example of hormonal control maintaining stable internal conditions.
Mineral Ion Balance
The kidneys also regulate concentrations of ions such as:
- Sodium ions
- Potassium ions
- Chloride ions
These ions are essential for processes including:
- Nerve impulses.
- Muscle contraction.
- Fluid balance.
- Cell function.
However, concentrations must remain within appropriate ranges.
The kidneys can adjust how much of these substances is reabsorbed or excreted.
The Skin as an Excretory Organ
The skin contains sweat glands.
Sweat commonly contains:
- Water
- Sodium chloride and other ions
- Small amounts of urea
Sweating has an important role in temperature regulation.
When sweat evaporates from the skin, thermal energy is transferred away from the body.
Although sweat removes some metabolic waste, waste removal is not its primary physiological role.
The Liver as an Excretory Organ
The liver has several important roles connected with waste removal.
These include:
- Deamination of excess amino acids.
- Conversion of ammonia into urea.
- Breakdown of old red blood cells and haemoglobin components.
- Processing of drugs and toxins.
- Modification of substances so they can be eliminated.
The liver therefore works closely with the kidneys.
A useful pathway is:
Protein → amino acids → excess amino acids processed by liver → urea → blood → kidneys → urine
Different Animals Remove Nitrogen in Different Ways
Not all animals excrete nitrogenous waste in the same form.
Three important nitrogenous wastes are:
- Ammonia
- Urea
- Uric acid
Ammonia
Many aquatic animals, including many fish, can release nitrogen largely as ammonia.
Ammonia is highly toxic but dissolves readily in water.
Aquatic animals have abundant surrounding water in which ammonia can be diluted and removed.
Urea
Mammals, including humans, convert much of their nitrogenous waste into urea.
Urea is less toxic than ammonia.
It can therefore be transported in the blood before being removed by the kidneys.
Producing urea requires energy, but it reduces the danger associated with transporting ammonia.
Uric Acid
Birds, reptiles, and some other animals excrete much of their nitrogenous waste as uric acid.
Uric acid can be eliminated with relatively little water.
This is useful for animals that need to conserve water.
It is also useful for animals that develop inside shelled eggs because relatively insoluble uric acid can accumulate with less toxicity than ammonia.
Excretion Is Adapted to Habitat
Waste-removal strategies reflect an animal's environment.
Consider three animals:
Aquatic Fish
Water is abundant.
Ammonia can diffuse into surrounding water.
Mammal
Water must be conserved more carefully.
Nitrogen is commonly converted into urea.
Desert Reptile
Water conservation is extremely important.
Producing uric acid allows nitrogenous waste to be eliminated with relatively little water.
This demonstrates a relationship between:
Habitat → physiological challenge → excretory adaptation
Excretion and Homeostasis
Excretion contributes to homeostasis in several interconnected ways.
Carbon Dioxide Regulation
The lungs help regulate carbon dioxide concentration and therefore blood pH.
Water Regulation
The kidneys adjust water loss.
Ion Regulation
The kidneys regulate mineral ion concentrations.
Nitrogenous Waste Removal
The liver and kidneys prevent potentially harmful nitrogenous compounds from accumulating.
Acid-Base Regulation
The lungs and kidneys both contribute to maintaining suitable pH.
The excretory system is therefore fundamentally a regulatory system.
Worked Example: Exercise
Imagine a student runs for 30 minutes.
Muscle cells increase their rate of cellular respiration.
This produces more carbon dioxide.
The student therefore:
- Breathes more rapidly and deeply.
- Transports more carbon dioxide to the lungs.
- Exhales carbon dioxide more rapidly.
At the same time, sweating may increase to help control body temperature.
Several organ systems therefore cooperate to maintain homeostasis during exercise.
Worked Example: A Hot Day
Suppose someone spends several hours outside on a hot day.
They lose considerable water through sweating.
The body must prevent excessive water loss.
The kidneys respond by conserving more water.
As a result:
- More water is reabsorbed.
- Urine volume decreases.
- Urine becomes more concentrated.
This shows how excretion can be adjusted according to the body's changing needs.
Worked Example: Protein Breakdown
Suppose excess amino acids are broken down.
The pathway can be followed:
Excess amino acids
↓
Deamination in the liver
↓
Ammonia produced
↓
Ammonia converted to urea
↓
Urea enters bloodstream
↓
Blood carries urea to kidneys
↓
Kidneys remove urea
↓
Urea leaves the body in urine
This pathway demonstrates cooperation between the digestive, circulatory, liver, and urinary systems.
What Happens If the Kidneys Fail?
If kidney function becomes severely impaired, wastes and excess substances can accumulate in the blood.
Problems can include:
- Accumulation of urea and other wastes.
- Abnormal ion concentrations.
- Disrupted water balance.
- Changes in blood pH.
One treatment for severe kidney failure is dialysis.
Dialysis removes selected wastes and excess substances from the blood using movement across a partially permeable membrane.
Another possible treatment for some patients is a kidney transplant.
Excretion vs Egestion: A Worked Example
Imagine someone eats an apple.
Some material is digested and absorbed.
The absorbed glucose reaches cells and is used in respiration.
Respiration produces carbon dioxide.
The carbon dioxide is carried to the lungs and exhaled.
That carbon dioxide is excreted because it is a metabolic waste.
Some fibre from the apple passes through the digestive tract without being digested.
It eventually leaves in faeces.
That fibre is egested because it was not produced by cellular metabolism.
This distinction is fundamental.
Common Mistakes
Saying Faeces Are the Main Excretory Product
Faeces consist largely of undigested material, microorganisms, water, and other substances.
Their removal is mainly egestion, not excretion.
Saying Urea Is Produced by the Kidneys
Urea is produced mainly in the liver.
The kidneys remove urea from the blood.
Saying Urine Is Stored in the Kidneys
Urine is produced by the kidneys but stored in the bladder.
Confusing the Ureter and Urethra
Ureter: kidney → bladder
Urethra: bladder → outside
Saying Sweat Is Only for Excretion
Sweat contains some excretory substances, but its major role is thermoregulation.
Thinking the Kidneys Simply Filter Out Everything
The kidneys filter small substances and then selectively reabsorb substances the body needs.
Saying All Animals Excrete Nitrogen in the Same Form
Different animals may primarily excrete ammonia, urea, or uric acid depending partly on their physiology and environment.
Check Your Understanding
1. Define excretion.
2. Explain the difference between excretion and egestion.
3. Give three examples of metabolic wastes.
4. Why must carbon dioxide be removed from the body?
5. Describe the pathway of carbon dioxide from a respiring cell to the outside of the body.
6. Where is urea produced?
7. Explain why ammonia is converted into urea in humans.
8. Name the four main structures of the urinary system.
9. Describe the pathway of urine from the kidney to outside the body.
10. What is a nephron?
11. Explain the difference between filtration and selective reabsorption.
12. Describe how the kidneys contribute to water balance.
13. Explain how excretion contributes to homeostasis.
14. Why might an aquatic animal excrete ammonia while a desert animal benefits from producing uric acid?
15. A person becomes dehydrated after exercising in hot weather. Predict how the volume and concentration of their urine should change and explain why.
Key Terms
- Excretion – removal of metabolic wastes and substances present in excess of the body's requirements.
- Egestion – removal of undigested and unabsorbed material from the digestive tract.
- Metabolic waste – unwanted substance produced through metabolic reactions.
- Homeostasis – maintenance of relatively stable internal conditions.
- Urea – nitrogen-containing waste produced mainly in the liver and removed by the kidneys.
- Ammonia – highly toxic nitrogen-containing waste produced during amino acid metabolism.
- Uric acid – nitrogenous waste excreted by many birds, reptiles, and other animals.
- Deamination – removal of the nitrogen-containing part of an amino acid.
- Kidney – organ that regulates blood composition and produces urine.
- Nephron – microscopic functional unit of the kidney.
- Ultrafiltration – pressure-driven filtration of small substances from blood into a nephron.
- Selective reabsorption – recovery of required substances from kidney filtrate into the blood.
- Ureter – tube carrying urine from a kidney to the bladder.
- Bladder – muscular organ that temporarily stores urine.
- Urethra – tube carrying urine from the bladder out of the body.
- ADH – hormone involved in regulating water reabsorption by the kidneys.
- Osmoregulation – regulation of water and solute balance.
- Dialysis – artificial removal of selected wastes and excess substances from the blood when kidney function is inadequate.
Key Takeaways
- Excretion removes metabolic wastes and substances present in excess.
- Excretion is different from egestion, which removes undigested material.
- Carbon dioxide, urea, excess water, and excess mineral ions are important excretory substances.
- The lungs excrete carbon dioxide produced during cellular respiration.
- The liver converts toxic ammonia from amino acid metabolism into urea.
- The kidneys remove urea and regulate water and ion concentrations.
- Urine travels kidneys → ureters → bladder → urethra.
- Nephrons are the functional units of the kidneys.
- Kidney function involves filtration followed by selective reabsorption and other transport processes.
- The kidneys adjust urine concentration to help regulate the body's water balance.
- ADH helps control water reabsorption.
- Sweat contains water, ions, and small quantities of urea, although sweating is especially important for temperature regulation.
- Different animals use different nitrogenous wastes, including ammonia, urea, and uric acid.
- Excretory adaptations are often related to an animal's habitat and need to conserve water.
- Excretion is essential for homeostasis because it prevents harmful wastes from accumulating and helps regulate the body's internal chemical environment.