Excretory System: Processes, Adaptations and Health Implications

The excretory system is vital for removing waste substances like ammonia, urea, and uric acid from animals’ bodies, maintaining their health. Different animals excrete different types of waste, depending on their habitat and evolutionary adaptations. In humans, the excretory system comprises kidneys, ureters, a urinary bladder, and a urethra, ensuring the elimination of waste and maintaining internal balance.

Understanding Animal and Human Excretion: Processes and Adaptations

1. Diverse Excretory Strategies in Animals

• Accumulation of Excretory Substances: Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na+, K+, Cl–, phosphate, sulphate, etc., either by metabolic activities or by other means like excess ingestion. 
  • These substances have to be removed totally or partially.
• Ammonotelism: The process of excreting ammonia is Ammonotelism. 
  • Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature.
• Terrestrial Adaptation and Nitrogenous Waste: Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water.
• Ureotelism: Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals.
• Conversion of Ammonia to Urea in Mammals: Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys.
• Uricotelism in Reptiles, Birds, and Insects: Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water and are called uricotelic animals.

2. Human Excretory System

• Components of the Human Excretory System: In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.

• Anatomy of the Kidneys: Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness, with an average weight of 120- 170 g.
• Structural Zones within the Kidney: Inside the kidney, there are two zones, an outer cortex and an inner medulla.
• Structure and Function of Nephrons: 
  • Functional Units: Nephrons: Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units.
    • Each nephron has two parts- the glomerulus and the renal tubule.
  • Bowman’s Capsule and Glomerulus: The renal tubule starts with Bowman’s capsule, a double-walled cup-like structure enclosing the glomerulus.
    • Together, the glomerulus and Bowman’s capsule form the malpighian body or renal corpuscle.
    • From there, the renal tubule coils into the proximal convoluted tubule (PCT), followed by Henle’s loop, a hairpin-shaped segment with descending and ascending limbs. 
    • After Henle’s loop, it continues as the distal convoluted tubule (DCT). Multiple DCTs converge into the collecting duct, draining into the renal pelvis. 

• • Components of Nephron: Malpighian corpuscle, PCT, and DCT are in the kidney’s cortex, while the loop of Henle extends into the medulla. 
    • Cortical nephrons have shorter loops, while juxta medullary nephrons have longer ones. 
  • Peritubular Capillaries and Vasa Recta: The efferent arteriole forms peritubular capillaries around the renal tubule.
    • Vasa recta, which runs parallel to Henle’s loop, is primarily found in juxta medullary nephrons.
    • Cortical nephrons typically lack or have highly reduced vasa recta
• Urine Formation and Excretion Process
  • Transport of Urine to the Bladder: Urine formed by the nephrons is ultimately carried to the urinary bladder, where it is stored till a voluntary signal is given by the Central Nervous System (CNS).
  • Storage and Signaling in the Urinary Bladder: This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. 
    • In response, the stretch receptors on the walls of the bladder send signals to the CNS.
  • Micturition: The process of release of urine is called micturition, and the neural mechanism causing it is called the micturition reflex.
  • Average Daily Urine: An adult human excretes, on average, 1 to 1.5 litres of urine per day.
    • The urine formed is a light yellow coloured watery fluid which is slightly acidic (pH-6.0).
  • Consequences of Kidney Malfunction: Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called uremia, which is highly harmful and may lead to kidney failure.
    • In such patients, urea can be removed by a process called hemodialysis.
  • Renal Calculi: stone or insoluble mass of crystallised salts (oxalates, etc.) formed within the kidney.

Conclusion

• The human excretory system, with its intricate network of kidneys and associated structures, plays a crucial role in maintaining bodily functions by eliminating waste products. 
• Understanding this system’s function is essential for comprehending health issues like kidney malfunction and the methods used to address them, such as hemodialysis.

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