OCR Specification focus:
‘Excretion removes metabolic wastes, including carbon dioxide and nitrogenous waste, maintaining efficient metabolism and homeostasis.’
Excretion is vital for maintaining the internal balance of organisms by eliminating toxic metabolic wastes. Without it, waste accumulation disrupts enzyme function, metabolism, and overall homeostasis.
The Concept of Excretion
The Meaning of Excretion
Excretion is a fundamental biological process essential for the survival of living organisms. It ensures that the internal environment remains stable by removing metabolic waste products generated during normal cellular activity.
Excretion: The removal of metabolic waste products from the body of an organism.
It is crucial to distinguish excretion from egestion. Excretion refers only to the removal of waste products formed by metabolism inside the body’s cells, whereas egestion refers to the elimination of undigested food material from the digestive tract.
Types of Metabolic Waste
Metabolic wastes arise from different biochemical processes occurring within cells. The most significant wastes that must be excreted in humans are carbon dioxide (CO₂) and nitrogenous waste.
1. Carbon Dioxide
Produced during aerobic respiration when glucose and other substrates are oxidised to release energy.
Excess CO₂ dissolves in body fluids forming carbonic acid, which lowers blood pH.
Labeled cross-section of an alveolus with surrounding capillaries, showing diffusion of CO₂ from blood to air and O₂ from air to blood. This illustrates how the lungs excrete CO₂, helping stabilise blood pH. The figure includes cell-type labels (e.g., type I pneumocytes), which are additional to the syllabus but aid orientation. Source.
This pH change interferes with the structure and function of haemoglobin, reducing oxygen transport efficiency.
CO₂ is mainly excreted through the lungs by the process of exhalation.
2. Nitrogenous Waste
Produced from the breakdown of excess amino acids in the liver.
The amino group (–NH₂) is removed in deamination, forming ammonia.
Ammonia is highly toxic and is converted into urea through the ornithine cycle before being transported to the kidneys for excretion.
Simplified urea cycle showing conversion of ammonia (NH₃) and CO₂ into urea in the liver, reducing toxicity prior to renal excretion. This underpins the importance of excretion in maintaining safe metabolite levels. The diagram includes enzyme names and intermediates beyond OCR depth; focus on the overall conversion to urea. Source.
Urea is the principal nitrogenous waste excreted in urine.
Importance of Excretion
Maintaining Efficient Metabolism
The accumulation of metabolic wastes interferes with the biochemical pathways that sustain life. Enzymes, which control metabolic reactions, operate efficiently only under specific conditions of pH and ionic concentration.
Carbon dioxide accumulation leads to a drop in blood pH, altering enzyme shape and reducing their catalytic activity.
Ammonia and other nitrogenous wastes are toxic, causing cell damage even at low concentrations.
By removing these substances, excretion maintains the internal chemical stability required for efficient metabolism.
Homeostatic Balance
Homeostasis is the maintenance of a constant internal environment despite external changes. Excretion contributes to homeostasis by regulating the composition of blood and tissue fluids.
Homeostasis: The maintenance of a stable internal environment within an organism.
Without proper excretion, the body would be unable to regulate:
pH levels, due to accumulation of carbon dioxide and organic acids.
Water potential, since urea and ions affect osmotic balance.
Electrolyte concentrations, which are crucial for nerve impulse transmission and muscle function.
Excretory Organs and Their Roles
Several organs contribute to excretion in humans, each specialising in removing different types of waste:
Lungs: Remove carbon dioxide and some water vapour produced during respiration.
Liver: Converts excess amino acids into urea and detoxifies harmful substances.
Kidneys: Filter blood to remove urea, excess ions, and water, producing urine.
Skin: Excretes water, salts, and small amounts of urea through sweat glands.
Each organ functions in coordination to ensure that metabolic wastes are continually eliminated, supporting the stability of the internal environment.
The Liver’s Role in Waste Processing
The liver plays a key intermediary role between metabolism and excretion.
It processes amino acids, converting toxic ammonia to urea.
It detoxifies harmful substances such as alcohol and drugs.
It breaks down old red blood cells, with bilirubin excreted in bile.
These activities ensure that potentially harmful metabolic by-products are transformed into less toxic forms suitable for removal by the kidneys or intestines.
The Kidneys and Urine Formation
The kidneys are the main organs responsible for excreting nitrogenous waste. Their functional units, the nephrons, perform several crucial processes:
Ultrafiltration: Removal of small molecules like urea, glucose, and ions from the blood into the nephron.
Selective reabsorption: Reabsorption of essential substances such as glucose and water into the bloodstream.
Urine formation: Remaining waste products and excess water form urine, which is excreted via the ureters, bladder, and urethra.
This process ensures that toxic substances are removed while retaining essential nutrients and maintaining water balance.
The Relationship Between Excretion and Other Systems
Excretion interacts closely with other physiological systems to maintain overall equilibrium.
Respiratory System
The lungs eliminate carbon dioxide, a by-product of cellular respiration.
The rate of breathing adjusts to maintain acid–base balance.
Circulatory System
The blood transports metabolic wastes to excretory organs.
The renal artery supplies unfiltered blood to the kidneys, while the renal vein carries cleaned blood away.
Nervous and Endocrine Systems
Control mechanisms in the hypothalamus and pituitary gland adjust excretory activity based on body needs, for instance, by regulating antidiuretic hormone (ADH) secretion to control water balance.
Consequences of Excretory Failure
Failure of excretory systems can have severe effects:
Carbon dioxide build-up leads to respiratory acidosis and impaired oxygen delivery.
Urea accumulation causes uraemia, damaging cells and tissues.
Water imbalance disrupts blood pressure and cellular function.
These effects highlight the central importance of excretion to both metabolic efficiency and homeostatic control.
Summary of Key Processes
Excretion removes harmful metabolic wastes such as carbon dioxide and urea.
It prevents disruption to enzyme activity and pH balance, maintaining metabolic efficiency.
Excretory organs — lungs, liver, kidneys, and skin — function collectively to sustain homeostasis.
Without excretion, toxic accumulation would impair cell function and threaten organismal survival.
FAQ
Failure to excrete carbon dioxide leads to its accumulation in the blood, forming carbonic acid and causing a decrease in blood pH. This condition, known as respiratory acidosis, disrupts the function of enzymes and proteins that depend on stable pH levels.
The result is impaired haemoglobin binding to oxygen, reducing oxygen transport efficiency. Prolonged CO₂ buildup can cause dizziness, confusion, or even respiratory failure, showing how excretion of this gas is vital for maintaining homeostasis.
Ammonia is extremely soluble and highly reactive, rapidly altering pH and ion balance within cells. Even small amounts can denature enzymes and interfere with nervous system function.
The liver converts ammonia into urea through the ornithine cycle, producing a compound that is far less toxic and easily excreted by the kidneys. This conversion allows the body to safely remove excess nitrogen without damaging tissues.
Excretory processes help maintain the correct balance of ions such as sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻).
The kidneys selectively reabsorb or excrete ions depending on the body’s needs.
This maintains electrical neutrality, nerve transmission, and muscle contraction.
Without regulation, ion imbalances could lead to arrhythmias, muscle weakness, or cellular dysfunction.
Thus, excretion indirectly supports metabolic stability and cellular homeostasis.
While the general purpose—removing metabolic wastes—is the same, mechanisms vary by habitat and physiology.
Mammals excrete urea, conserving moderate amounts of water.
Fish excrete ammonia directly into water through gills, avoiding the need for conversion.
Birds and reptiles excrete uric acid, which is insoluble and allows water conservation in dry environments.
These differences demonstrate evolutionary adaptations to conserve water while maintaining efficient excretion.
The lungs regulate carbon dioxide levels, while the kidneys control the concentration of hydrogen ions (H⁺) and bicarbonate (HCO₃⁻) in the blood.
When blood becomes too acidic, the lungs increase ventilation to expel more CO₂.
The kidneys excrete more H⁺ and reabsorb HCO₃⁻ to buffer pH.
In alkalosis (high pH), the opposite occurs — less CO₂ is exhaled and fewer H⁺ are excreted.
This partnership between the two organs ensures a stable blood pH of around 7.4, essential for enzyme function and metabolic control.
Practice Questions
Question 1 (2 marks)
Explain the difference between excretion and egestion in humans.
Mark Scheme:
Excretion: removal of metabolic waste products from the body (1 mark)
Egestion: removal of undigested food material from the digestive tract (1 mark)
Allow 1 mark for clear distinction using correct terminology (e.g. excretion is from cell metabolism; egestion is not).
Question 2 (5 marks)
Describe how the removal of metabolic waste products such as carbon dioxide and nitrogenous waste helps maintain homeostasis in the human body.
Mark Scheme:
Carbon dioxide produced by respiration must be removed to prevent formation of carbonic acid (1 mark)
Carbonic acid lowers blood pH, affecting enzyme activity and oxygen transport by haemoglobin (1 mark)
Ammonia produced from deamination in the liver is toxic and must be converted into urea (1 mark)
Urea is excreted by the kidneys, maintaining safe concentrations of nitrogenous waste in the blood (1 mark)
Together, removal of CO₂ and urea maintains stable pH, ion balance, and metabolic efficiency — key aspects of homeostasis (1 mark)
