Excretory Products and Their Elimination

Learning Outcomes:

1. Understand various excretory products in animals and the importance of their elimination.
2. Learn about different forms of nitrogenous waste and excretory systems in various organisms.
3. Grasp the mechanisms of urine formation and kidney function regulation in humans.
4. Comprehend disorders related to excretory systems and their clinical management.

The process of eliminating metabolic wastes is essential for maintaining homeostasis in animals. Animals generate several waste products like ammonia, urea, uric acid, carbon dioxide, water, and various ions such as Na+, K+, Cl-, phosphate, and sulphate.** These need to be removed from the body to avoid toxicity. The elimination of these substances varies across species depending on their habitat and evolutionary adaptations. In this section, we will explore the different excretory products, mechanisms of their removal, and the physiological importance of excretion.

Major Nitrogenous Wastes

1. Ammonia: The most toxic nitrogenous waste; it requires large amounts of water for removal. Animals that excrete ammonia are known as ammonotelic. This includes bony fishes, aquatic amphibians, and aquatic insects. Ammonia diffuses out through body surfaces or gill surfaces (in fish) as ammonium ions.
2. Urea: Terrestrial animals often convert ammonia to urea, which is less toxic. Mammals, terrestrial amphibians, and marine fishes are primarily ureotelic. Urea is synthesized in the liver and then filtered and excreted by the kidneys.
3. Uric Acid: The least toxic form, excreted with minimal water loss, is seen in reptiles, birds, land snails, and insects. These animals are termed uricotelic, and their nitrogenous waste is excreted as a paste or pellets.

Important Note: Terrestrial adaptation has led to the evolution of urea and uric acid as the major forms of nitrogenous waste due to water conservation needs.

Excretory Structures in Animals

Animals have evolved various excretory structures based on their complexity:

1. Protonephridia (Flame Cells): Found in Platyhelminthes (e.g., flatworms), rotifers, some annelids, and Amphioxus. These structures mainly regulate ion balance and fluid volume (osmoregulation).
2. Nephridia: Present in earthworms and other annelids, nephridia remove nitrogenous wastes and help in fluid regulation.
3. Malpighian Tubules: In insects (e.g., cockroaches), these tubules are involved in nitrogenous waste removal and osmoregulation.
4. Antennal (Green) Glands: Excretory structures in crustaceans like prawns that eliminate waste products.

Human Excretory System

The human excretory system consists of two kidneys, ureters, a urinary bladder, and a urethra. The kidneys are the primary organs responsible for filtering blood and forming urine.

1. Kidneys: These are bean-shaped organs located between the last thoracic and third lumbar vertebrae. Each kidney is about 10-12 cm in length and weighs 120-170 g. The kidneys have an outer cortex and an inner medulla, which is divided into medullary pyramids. These pyramids project into the calyces.
2. Nephrons: The functional units of the kidney are nephrons. Each kidney contains about 1 million nephrons. Each nephron consists of two main parts:

• Glomerulus: A tuft of capillaries formed by the afferent arteriole. It filters blood under pressure.
• Renal Tubule: Begins with Bowman’s capsule, continues as proximal convoluted tubule (PCT), followed by Henle’s loop, and ends as distal convoluted tubule (DCT).

Important Concept: The arrangement of nephrons in the kidney allows efficient filtration, reabsorption, and secretion processes, crucial for urine formation.

Urine Formation

Urine formation involves three main processes: glomerular filtration, reabsorption, and secretion.

1. Glomerular Filtration: This is the process of filtering blood through the glomerulus into Bowman’s capsule. Approximately 1100-1200 ml of blood is filtered per minute. Glomerular Filtration Rate (GFR) is about 125 ml/min or 180 liters/day.
2. Reabsorption: Nearly 99% of the filtrate is reabsorbed by various segments of the nephron. Essential nutrients like glucose and amino acids are reabsorbed by active transport, while water is reabsorbed passively.
3. Secretion: Certain substances like H+, K+, and ammonia are secreted into the filtrate. This process helps in maintaining ionic balance and the pH of body fluids.

Table: Reabsorption and Secretion in Nephron Segments

Function of the Renal Tubules

1. Proximal Convoluted Tubule (PCT): The PCT is lined with cuboidal epithelium. About 70-80% of electrolytes and water are reabsorbed here. The PCT also plays a role in maintaining pH by selectively secreting H+ and ammonia.
2. Henle’s Loop: The descending limb is permeable to water, concentrating the filtrate, while the ascending limb is impermeable to water but allows the passage of electrolytes.
3. Distal Convoluted Tubule (DCT): Involved in the conditional reabsorption of Na+ and water. It also plays a role in the secretion of H+ and K+ ions to regulate blood pH.
4. Collecting Duct: It allows the reabsorption of water and passage of small amounts of urea into the medullary interstitium to maintain the osmolarity gradient.

Mechanism of Filtrate Concentration

Mammals possess a countercurrent mechanism between the limbs of Henle’s loop and the vasa recta (the blood vessels running parallel to Henle’s loop). This mechanism helps maintain a gradient of increasing osmolarity from the cortex to the medulla (300 mOsmol/L to 1200 mOsmol/L).

Important Note: The countercurrent system is critical for water conservation in mammals, allowing the production of highly concentrated urine.

Regulation of Kidney Function

The kidneys are regulated by hormonal feedback mechanisms involving the hypothalamus, juxtaglomerular apparatus (JGA), and the heart.

1. ADH (Antidiuretic Hormone): Released from the hypothalamus in response to low blood volume or high osmolarity, ADH increases water reabsorption in the collecting ducts, reducing urine output.
2. Renin-Angiotensin Mechanism: When GFR falls, the JGA releases renin, which triggers the conversion of angiotensinogen to angiotensin II. This powerful vasoconstrictor increases GFR and stimulates the release of aldosterone from the adrenal cortex, leading to increased Na+ and water reabsorption.
3. Atrial Natriuretic Factor (ANF): Released by the heart in response to increased blood pressure, ANF reduces blood pressure by promoting vasodilation and inhibiting renin and aldosterone.

Micturition

The process of urination involves the storage of urine in the urinary bladder. Once the bladder stretches to a certain level, stretch receptors signal the central nervous system (CNS) to initiate the micturition reflex. This results in the contraction of bladder muscles and the relaxation of the urethral sphincter, allowing urine to be expelled.

MCQ: What hormone is responsible for increasing water reabsorption in the kidney?
Answer: ADH (Antidiuretic Hormone).

Role of Other Organs in Excretion

Other organs also contribute to excretion:

1. Lungs: Expel large amounts of CO2 and water vapor during respiration.
2. Liver: Metabolizes harmful substances and excretes bilirubin, biliverdin, and steroid hormones via bile.
3. Skin: The sweat glands excrete NaCl, small amounts of urea, and lactic acid, aiding in thermoregulation and waste elimination.

Disorders of the Excretory System

1. Uremia: Accumulation of urea in the blood due to kidney failure. It can be treated through hemodialysis.
2. Kidney Stones (Renal Calculi): Formation of insoluble masses of crystallized salts like oxalates.
3. Glomerulonephritis: Inflammation of the glomeruli, often caused by infections or autoimmune disorders.

Important Note: Kidney transplantation is the ultimate treatment for acute renal failure, but the donor organ should ideally come from a close relative to minimize rejection.

In conclusion, the excretory system is vital for maintaining body homeostasis, regulating fluid and electrolyte balance, and removing toxic wastes. Disorders of this system can lead to severe health issues, which can often be managed by medical interventions like dialysis or surgery. The integrated hormonal control over kidney function ensures efficient waste elimination while conserving water and electrolytes.