EXCRETORY SYSTEM

Excretory products are substances that are eliminated from the body as waste products, primarily through the excretory system. The excretory system, consisting of organs such as the kidneys, ureters, urinary bladder, and urethra, plays a crucial role in removing metabolic waste, excess ions, and other undesirable substances from the body.

The major excretory products include:

• Urea: Formed in the liver during the breakdown of amino acids. A Main nitrogenous waste product in humans. It is transported to the kidneys for excretion in urine.
• Creatinine: Produced during the breakdown of creatine phosphate in muscles. Excreted by the kidneys as a waste product. Elevated levels may indicate kidney dysfunction.
• Ammonia: Generated during the breakdown of proteins and amino acids. Converted to urea in the liver; excreted by the kidneys.
• Uric Acid: Result of the breakdown of purines found in nucleic acids (DNA, RNA). Excreted by the kidneys; excess accumulation can lead to conditions like gout.
• Water: Ingested through food and beverages; produced during metabolic processes. Essential for maintaining hydration and eliminating dissolved waste products.
• Electrolytes (Sodium, Potassium, Calcium, etc.): Ingested through food; released during cellular metabolism. Excreted by the kidneys to maintain electrolyte balance in the body.
• Bilirubin: Produced during the breakdown of haemoglobin in aged red blood cells. Excreted by the liver as a component of bile; excess in the bloodstream can lead to jaundice.
• Carbon Dioxide: Produced during cellular respiration. Excreted by the lungs through exhalation; a waste product of energy metabolism.
• Sweat (contains various waste products): Released through sweat glands in the skin. Cooling the body and eliminating certain waste products, including salts.
• Toxins and Drugs: Result from the metabolism or breakdown of ingested substances, including medications. Processed by the liver and excreted through the kidneys or eliminated through other routes.
• Fecal Waste: Undigested food, bacteria, and other waste materials in the digestive system. Eliminated from the body as solid waste through the rectum and anus.

The excretory system, particularly the kidneys, plays a central role in filtering blood and selectively reabsorbing essential substances while excreting waste products in the form of urine. The efficient removal of excretory products is vital for maintaining homeostasis, preventing the accumulation of harmful substances, and supporting overall health.

In plants, excretion involves the removal of waste products and metabolic by-products. While plants do not have a dedicated excretory system comparable to animals, they employ various structures and processes to eliminate waste and maintain cellular homeostasis. 

• Respiration: During cellular respiration, plants produce carbon dioxide as a metabolic by-product. Carbon dioxide is released through small pores called stomata on the surfaces of leaves and stems.
• Photosynthesis: During photosynthesis, oxygen is produced as a by-product. Oxygen is released into the atmosphere through stomata.
• Transpiration: Water is taken up by plant roots and transported to leaves, where it is lost to the atmosphere as water vapour. Waste minerals and salts dissolved in water are left behind in the plant or deposited in older leaves.
• Metabolic Waste: Plants produce various metabolic waste products during cellular processes.  Waste products are stored in vacuoles or eliminated through the shedding of leaves, bark, or other plant parts.
• Laticifer Cells: Specialized cells known as laticifers contain latex, a milky fluid rich in secondary metabolites. Some plants excrete toxic or bitter substances into latex, deterring herbivores and protecting against pathogens.
• Senescence: Aging and death of plant tissues. During senescence, plants may mobilize nutrients from older tissues to younger parts, and waste products may accumulate in older leaves.
• Root Exudates: Plant roots release various organic compounds, such as organic acids and enzymes, into the soil. These compounds can alter the soil environment, facilitate nutrient uptake, and influence microbial activity.
• Accumulation in Vacuoles: Plant cells have large vacuoles. Waste products and secondary metabolites can accumulate in vacuoles, isolating them from the rest of the cell.
• Leaf Abscission: Shedding of leaves, often in response to seasonal changes. Before shedding, plants reabsorb nutrients from leaves, and waste products may remain in the discarded foliage.
• Guttation: Exudation of water from the tips of leaves, especially in the morning. Dissolved minerals and waste products may be expelled along with water during guttation.

While plants lack specialized excretory organs like kidneys or lungs, they have evolved diverse mechanisms to manage waste products and maintain cellular functions. These processes contribute to the overall health and survival of plants, helping them adapt to environmental conditions and interact with other organisms in their ecosystems.

3. Excretion in Animals

Excretion in animals is the process by which metabolic waste products and excess substances are removed from the body to maintain internal balance and prevent the buildup of toxic substances. Different animal species have evolved specific excretory structures and mechanisms adapted to their environments. 

• Kidneys: Paired organs that filter blood and produce urine. Remove metabolic waste products, excess ions, and water from the bloodstream. Regulate electrolyte balance and blood pressure.
• Ureters: Tubes that carry urine from the kidneys to the urinary bladder. Transport urine to the bladder for storage before elimination.
• Urinary Bladder: A muscular organ that stores urine. Expands to store urine and contracts to expel it through the urethra.
• Urethra: Tube that carries urine from the bladder to the external environment. Eliminates urine from the body.
• Skin: The largest organ of the body. Sweating removes water, salts, and some metabolic waste products (urea and ammonia) through sweat glands.
• Lungs:  Respiratory organs. Exhaling removes carbon dioxide, a waste product of cellular respiration.
• Liver: Important metabolic organ. Converts toxic ammonia to urea, which is then excreted by the kidneys. Also involved in the breakdown of various metabolic waste products.
• Gills (in aquatic animals): Respiratory organs in aquatic organisms like fish. Remove ammonia and other waste products from the bloodstream into the surrounding water.
• Malpighian Tubules (in insects): Filamentous tubules in the insect digestive system. Remove nitrogenous waste (uric acid) and excess ions from the hemolymph (insect blood).
• Rectum and Anus: Terminal part of the digestive system. Eliminate solid waste (feces) from the body.
• Specialized Adaptations: Some animals have developed unique adaptations for excretion, such as salt glands in marine animals to eliminate excess salts or special scent glands for chemical communication and marking territory.
• Homeostatic Mechanisms: Hormonal regulation (e.g., antidiuretic hormone) and feedback loops help maintain the balance of water, electrolytes, and waste products in the body.
• Dietary Influence: The type of diet influences excretion. For example, herbivores may excrete large amounts of water and plant fibre, while carnivores produce more concentrated urine.
• Evolutionary Diversity: Excretory systems vary widely across animal taxa, reflecting adaptations to different environments, lifestyles, and physiological demands.

In the wondrous world of animals, maintaining a stable internal environment, known as homeostasis, is crucial for survival. One key aspect of homeostasis is osmoregulation, the process of regulating the concentration of dissolved solutes (salts, sugars, etc.) in the body fluids. This intricate dance between water and solutes is where the fascinating roles of ADH (antidiuretic hormone) and urine formation come into play. 

Osmoregulation in Action

Animals face diverse challenges to their internal environment, from fluctuating water intake to changes in salt content. Osmoregulation ensures that the concentration of solutes in their body fluids remains within a narrow range, despite these external variations. This is crucial for maintaining proper cell function, enzyme activity, and overall health.

Different Strategies for Different Environments

The specific strategies animals use for osmoregulation vary depending on their habitat:

• Aquatic Animals:
  • Marine fish: Lose water due to osmosis and actively take up salts from the surrounding seawater. Gills and specialized cells help excrete excess salts.
  • Freshwater fish: Gain water due to osmosis and actively lose salts through their gills and urine.
• Terrestrial Animals:
  • Mammals and birds: Conserve water by producing concentrated urine and minimizing water loss through sweat and respiration.
  • Insects: Eliminate nitrogenous waste as uric acid, which requires minimal water for excretion.

ADH: The Conductor of the Water Symphony

Antidiuretic hormone (ADH), produced in the hypothalamus, plays a critical role in regulating water balance by influencing urine production:

• High blood osmolality: When the concentration of solutes in the blood increases (dehydration), ADH release is stimulated.
• ADH action: ADH increases water reabsorption in the kidneys, leading to the production of less concentrated urine (less water excreted).
• Low blood osmolality: When the concentration of solutes in the blood decreases (excess water intake), ADH release is inhibited.
• Reduced water reabsorption: This results in the production of more dilute urine (more water excreted).

The Magic of Urine Formation

Urine formation in the kidneys occurs through three main steps:

1. Glomerular filtration: Blood is filtered through tiny structures called glomeruli, removing water, wastes, and some small molecules.
2. Tubular reabsorption: Most of the filtered water, essential nutrients, and some salts are reabsorbed back into the bloodstream in the renal tubules.
3. Secretion and excretion: Waste products, excess salts, and some unwanted substances are secreted into the tubules and eliminated as urine.

The Significance of Osmoregulation

Maintaining proper osmoregulation is essential for animals in several ways:

• Enables proper cell function: Stable internal solute levels ensure optimal cell function and prevent dehydration or swelling.
• Regulates body temperature: Water loss through sweating or urination helps animals regulate their body temperature.
• Eliminates waste products: Urine helps remove harmful metabolic byproducts from the body.

 

 

5. Physiological and Behavioural Adjustments

 

The ability to adapt to changing environments is crucial for survival in the animal kingdom. Animals achieve this through a combination of physiological adjustments, internal changes to their body and metabolism, and behavioural adjustments, modifications in their actions and responses. 

Physiological Adjustments

Physiological adjustments involve internal changes that occur at the cellular and organ level, enabling animals to respond to environmental challenges:

• Temperature regulation: Hibernation in bears, sweating in humans, and panting in dogs are examples of adjustments to maintain body temperature in different environments.
• Metabolic adaptations: Certain animals, like hummingbirds, have high metabolic rates for sustained activity, while others, like snakes, have slower metabolisms for energy conservation.
• Respiratory adaptations: Diving mammals can hold their breath for extended periods, while aquatic animals like fish have gills for efficient oxygen uptake from water.
• Sensory adaptations: Bats use echolocation to navigate in darkness, while eagles have keen eyesight for hunting prey.

Behavioural Adjustments

Behavioural adjustments involve modifications in an animal's actions and responses to environmental cues:

• Migration: Birds migrate long distances to find food and breeding grounds, while some fish migrate seasonally to avoid harsh conditions.
• Food selection: Animals adjust their diet based on available food sources, with some species being opportunistic feeders and others specialists.
• Hibernation and estivation: Some animals enter periods of dormancy to conserve energy during harsh winters (hibernation) or hot summers (estivation).
• Social behaviours: Animals may form groups for protection, foraging, or breeding, adapting their social interactions based on environmental pressures.
• Learned behaviours: Some animals, like primates, can learn new skills and adjust their behaviour based on experience.

The Interplay of Physiology and Behaviour

These adjustments often work in concert, with physiological changes influencing behaviour and vice versa:

• Hibernation involves both physiological changes in metabolism and body temperature, as well as behavioural changes like reduced activity and seeking shelter.
• Migration requires physiological adaptations for long-distance travel but also involves behavioural decisions about timing and route selection.
• Food selection is influenced by both the animal's physiological needs and its ability to recognize and locate suitable food sources in the environment.

The Significance of Adjustments 

The ability to make both physiological and behavioural adjustments is crucial for several reasons:

• Increased survival: Adaptations allow animals to cope with environmental challenges like extreme temperatures, food scarcity, and predators.
• Improved reproductive success: Finding mates, raising offspring, and ensuring their survival often rely on adaptations to specific environments.
• Resource utilization: Adjustments help animals efficiently utilize available resources and avoid wasting energy in unsuitable conditions.
• Evolutionary advantage: Over generations, adaptations that enhance survival and reproduction become more common, leading to the evolution of new species.

 

Previous Year Questions 1. Which of the following adds/add nitrogen to the soil? (upsc 2013) Excretion of urea by animals Burning of coal by man Death of vegetation Select the correct answer using the codes given below: (a) 1 only       (b) 2 and 3 only         (c) 1 and 3 only          (d) 1, 2 and 3 Answer: C